Corby Strategic Flood Risk Assessment - Stage 2 Final Report

Corby Borough Council August 2006

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Table of Contents

Lists of Tables & Figures...... iii Executive Summary ...... v 1 Introduction ...... 1 2 Flood Risk...... 5 Planning Policy Guidance Note 25 ...... 5 Circular 30/92 Maps ...... 6 Indicative Floodplain Maps...... 6 3 Causes of Flooding...... 9 Overflowing of Watercourses...... 9 Localised Flooding ...... 10 Mechanical, Structural or Operational Failure...... 11 4 Flooding in Corby ...... 13 Classes of Flooding...... 13 Records of Flooding ...... 14 Corby Borough Council...... 14 Environment Agency...... 15 Storm Drainage Studies Report by John Taylor & Sons ...... 15 Anglian Water Services ...... 18 Flood Alleviation Measures...... 19 Gretton Brook...... 19 Willow Brook - North Arm ...... 20 Willow Brook - Central Arm...... 22 Willow Brook - South Arm...... 25 Harpers Brook...... 28 Operational and Emergency Planning ...... 30 5 Strategic Assessment of Flood Risk...... 33 General Methodology...... 33 Topographic Divisions...... 35 Anthropogenic Influences...... 36 The River Nene Model ...... 39 Flood Risk Mapping ...... 41 Rapid Inundation Hazard Zones ...... 43 6 Planning Policy and Flood Risk...... 45 Milton Keynes South Midlands Strategy ...... 45 Northamptonshire County Structure Plan ...... 48 Corby Local Plan...... 49 Catalyst Corby (North Northamptonshire Development Company)...... 50 Guidance for Developers ...... 52

Continued over ….

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Continued ….

7 Assessment of Flood Risk in Development Areas ...... 53 8 Potential Development Areas in Corby...... 57 Western Extension ...... 59 Southern Extension...... 63 Northern Extension ...... 65 Oakley / Stanion Extension ...... 67 Priors Hall & Weldon Extensions ...... 71 Great Oakley Extension ...... 75 Weldon North Industrial Estate ...... 77 St James Industrial Estate ...... 79 Corby East...... 81 Weldon South Industrial Estate and Max Park ...... 83 9 Conclusions...... 85 10 Recommendations ...... 87 References ...... 89

APPENDICES

Appendix 1 Figures

Appendix 2 Photographs

Appendix 3 Environment Agency – Regional Flood Defence & Land Drainage Operational Plan for the Corby Borough Council Area

Appendix 4 ISIS Model – Input Data and Results

Appendix 5 Flood Risk Guidance Notes to Developers

Appendix 6 Flood Risk Standing Advice Matrix (and Explanatory Notes)

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 iii Lists of Tables & Figures

LIST OF TABLES

Table 1 PPG25 Flood Risk Zones Table 2 Properties Flooded by the Storm of 30 th July 1982 Table 3 Soot Banks Flood Storage Reservoir - Design Criteria Table 4 Oakley Vale Ponds - Storage Volumes Table 5 Responsibilities & Routine Actions for Flood Defence Table 6 Operational Emergency Response Arrangements Table 7 Future Housing Allocation in Northamptonshire Table 8 Areas Proposed for Industry in the Corby Local Plan Table 9 Indicative Standards for Fluvial Flood Defence Table 10 High Risk Category – Flood Risk Sub-Divisions

LIST OF FIGURES

Figure 1 Location Plan Figure 2 Potential Development Areas Figure 3 Critical Ordinary Watercourses Figure 4 Extent of Flooding, 30 th July 1982 Figure 5 Principal Hydraulic Installations in Corby (Two sheets) Figure 6 Strategic Flood Risk Maps (Six sheets) Figure 7 ISIS Model Sub-Catchments (Existing Urbanisation) Figure 8 ISIS Model Sub-Catchments (Proposed Urbanisation) Figure 9 ISIS Model Channel Cross-Section Locations Potential Development Area Plans Figure 10.1 Western Extension Figure 10.2 Southern Extension Figure 10.3 Northern Extension Figure 10.4 Oakley / Stanion Extension Figure 10.5 Priors Hall and Weldon Extensions Figure 10.6 Great Oakley Extension Figure 10.7 Weldon North Industrial Estate Figure 10.8 St James Industrial Estate Figure 10.9 Corby East Figure 10.10 Weldon South Industrial Estate & Max Park

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Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 v Executive Summary

The Department of Transport, Local Government and the Regions' Planning Policy Guidance Note 25 entitled "Development and Flood Risk" (PPG25) outlines how flood risk issues should be addressed in regional planning guidance and Local Authorities' development plans.

Bullen Consultants Ltd were commissioned by Corby Borough Council (CBC) in October 2003 to undertake a Strategic Flood Risk Assessment of Corby based on a Brief issued by the Borough Council. The Brief divided the study into two phases. Phase 1 involved the collation and evaluation of data and information relating to the hydrological and hydraulic aspects of flooding in Corby. Phase 2 of the study, based on the findings and conclusions of Phase 1, was to include the following main topics, including agreed work supplementary to the Brief:

• Identifying and mapping the areas of flood risk over the whole of the Borough within the categories defined in Planning Policy Guidance Note 25 (PPG25).

• Detailed flood risk assessments for a number of potential development areas within the Borough. (Ten specific development areas were subsequently defined by Catalyst Corby.)

• Assessment of the effect on flood risk in Corby of large scale urban development in the Gretton, Willow and Harpers Brook catchments.

• Preparation of guidance notes for developers, recommending procedures to be followed by prospective developers for the assessment of flood risk and measures to be adopted to minimise that risk and mitigate the effects of increased runoff from the development on flood risk elsewhere.

Phase 1 of the study was completed in March 2004 and a Report detailing the results of the data and information evaluation exercise and the conclusions reached by Bullen Consultants was presented to and accepted by the Council.

In undertaking Phase 2 of the project, Bullen Consultants have carried out a general Flood Risk Assessment for the whole of the Borough of Corby and detailed Flood Risk Assessments for ten potential development areas within the Borough that were identified by Catalyst Corby as areas within which significant urban and industrial / commercial development was under active consideration. The Strategic Flood Risk Assessment in Phase 2 of the Study has been carried out in line with the Environment Agency (Anglian Region)'s Guidance Notes to Local Authorities.

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Halcrow Engineering Consultants were recently engaged by the Environment Agency (EA) to construct a computer model of the hydrology and hydraulics of the whole of the River Nene and its principal tributaries. In Stage 1 of this study it was revealed that this model contains partial coverage of the Borough and therefore the results of the Halcrow modelling exercise is of importance to this study and flood risk assessment in Corby.

The results of the general Flood Risk Assessment of the Borough are presented in this Stage 2 Report as a set of six 1/10,000 scale maps covering the whole of the Borough and showing the flood risk at any point in one of three categories of flood risk.

Corby’s present flood defences are essentially those constructed as a direct result of the flooding which occurred in the town in July 1982 and are intended to provide protection for those areas of the town flooded during that event. It was hoped that the flood risk maps would enable the actual flood risk to be compared with the Environment Agency's estimate of flood risk without defences as shown on the Agency's Indicative Floodplain Maps. Unfortunately the coverage of the Halcrow model does not extend far enough upstream to include any of the flood storage reservoirs. There is also limited information on many of the reservoirs as regards design standards and storage capacities, as well as the areas they are intended to serve. It would therefore seem that a further study is required, which should involve the extension of the Halcrow model to give better coverage within the Borough. This would be a major undertaking involving extensive physical surveys of existing infrastructure to obtain the data required to construct the model. It should also be noted that the coverage of the Indicative Floodplain maps within the Borough is limited.

A separate Flood Risk Assessment was also presented for each of the ten potential development areas. For each area the principal flood risk source was identified and the extent of each of the three flood risk categories within that area described in detail. These assessments showed that, apart from marginal riparian land, all ten of the potential development areas are not at significant risk of flooding (i.e. PPG25 Zone 3). It is however, recommended that appropriate measures should be taken to attenuate surface water runoff from all ten potential development areas to avoid increasing flood risk to areas downstream.

This report includes recommendations for guidance notes for prospective developers. These have been drafted within a framework put forward by the Environment Agency and incorporate the Agency's Flood Risk Standing Advice Matrix

The project was carried out in close collaboration with CBC and the EA’s Anglian Region and utilised detailed sewerage plans of the Borough provided by Anglian Water. The study was completed and a draft of the Strategic Flood Risk Assessment Report was submitted to both the Council and the Agency for their comments in September 2004.

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In April 2005 Bullen Consultants were merged with Faber Maunsell Ltd. Pending receipt of the EA’s comments on the draft Report, a meeting between Faber Maunsell, CBC and the EA was held at Corby in July 2005. Because of other related studies that had commenced since September 2004 it was agreed that Faber Maunsell would undertake additional work to update Bullen’s draft SFRA Report. The extant and scope of the additional work required was agreed by CBC, FM and the EA in August 2005, although the results of one important related study did not become available until February 2006. This Final Report now incorporates the results of the extended study.

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Faber Maunsell Corby Strategic Flood Risk Assessment 1 1 Introduction

1.1 Bullen Consultants Limited were appointed by Corby Borough Council on 6 th October 2003 to undertake a Strategic Flood Risk Assessment of the Borough of Corby. The scope of the study was described in a Brief issued by the Borough Council. A location plan showing Corby in relation to the River Nene catchment is given in Figure 1.

1.2 The Brief divided the study into two stages. The first stage of the study was to consist of the collation and evaluation of all available data and information relevant to the strategic assessment of flood risk within the Borough. The Brief detailed the tasks associated with Stage 1 of the study and also outlined the scope and content of the Stage 1 Report.

1.3 Stage 1 of the Study was undertaken by Bullens between December 2003 and January 2004. This resulted in the publication of a Stage 1 Draft Report (Ref.1) which was submitted to Corby Borough Council on 23 rd January 2004. The conclusions and recommendations contained in that Report were accepted, with minor modifications, at a meeting held at the Borough Council on Wednesday, 3rd March 2004 and work on Stage 2 of the Study then commenced.

1.4 Planning Policy Guidance Note 25 ("PPG25") (Ref.2) issued in July 2001 by the Department for Transport, Local Government and the Regions expects local planning authorities to apply a risk-based approach to the preparation of their development plans in respect of possible flooding. Appendix F of PPG25 contains specific guidance for planning authorities and those working on their behalf, on the methodology to be used in undertaking a flood risk assessment. PPG25 and its implications are discussed in greater detail in the next Section.

1.5 PPG25 is due to be superseded later this year by Planning Policy Statement 25, also entitled Development and Flood Risk (“PPS25”). PPS 25 was issued as a draft document for consultation purposes earlier this year. The consultation period has now ended and publication of PPS25 is understood to be imminent, although at the time of writing PPG25 is still the definitive document. However, when PPS25 is published it will supersede PPG25 and all references in this Report to PPG25 should then be taken to refer to PPS25.

1.6 The findings of the data collation and evaluation phase of the Study (Stage 1) were intended to enable the scope and detailed methodology of the principal part of the Study, the flood risk assessments (Stage 2), to be formulated. This second stage of the Study consists of two distinct components, summarised as follows :- a) To classify all land within the Borough of Corby into three flood risk categories corresponding numerically to the three flood risk zones defined in PPG25.

b) To produce an assessment of the actual flood risk for ten potential development areas within Corby.

1.7 The Regional Spatial Strategy for the East Midlands (RSS8) seeks to promote major growth across North Northamptonshire. In the case of Corby, target figures for 16,800 new homes in the period 2001 to 2021 have been set. In the emerging Local Development Framework it is proposed that this growth will be accommodated both within the existing urban area and within sustainable urban extensions. The Regeneration Framework for Corby (Ref.3), prepared by Catalyst Corby (the government-driven specialist urban regeneration company set up to produce a regeneration framework for the town) outlined a number of ‘greenfield’ sites that may be suitable for housing development in the following areas :-

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a) Northern Extension 700 dwellings. b) Western Extension 6,500 dwellings. c) Southern Extension 1,400 dwellings. d) Prior’s Hall and Weldon Extension 6,300 dwellings. e) Oakley / Stanion Extension 4,500 dwellings.

1.8 These areas are shown in outline in Figure 2. While the Indicative Floodplain maps show that flooding of the proposed areas from primary sources (i.e. channel overtopping on Main Rivers) would be relatively minor within those areas, the effects that any development will have on downstream areas will need to be considered. It is only the Western and Northern Extensions which are upstream of the town centre and therefore impermeable area runoff from these areas could, unless attenuated, have an impact on flood risk within in the town. The increased runoff caused by the extensions on the downstream side of the town will also need to be assessed as they could have potential effects on any urbanised areas outside the Borough.

1.9 Provision is also made as part of the Framework for large scale commercial and industrial development on "greenfield" sites at the following locations :- a) Great Oakley b) St James Industrial Estate c) Corby East d) Max Park e) Weldon South Industrial Estate f) Weldon North Industrial Estate

1.10 The Environment Agency's Indicative Floodplain maps (see Section 2) have been used as a starting point for this study, supplemented by other information supplied by the Borough Council, the Environment Agency, Anglian Water Services, and other sources. Due consideration has also been given to the Environment Agency's guidance notes on flood risk assessment issued to Local Planning Authorities.

1.11 The methodologies used to carry out the strategic flood risk assessment and the individual flood risk assessments for the development areas, together with the results obtained, are described in detail in this study. The results are presented in a series of six 1/10,000 scale Strategic Flood Risk Maps covering the entire Borough (as well as areas outside the Borough boundary but within the Catalyst Corby development areas), together with plans of the area showing the principal hydrological and drainage features affecting flood risk.

1.12 The study was completed and a drafts of the Strategic Flood Risk Assessment Report were submitted to both the Council and the Agency in stages for their respective comments. A complete draft had been submitted by the end of September 2004 and the Borough Council’s comments were received soon after.

1.13 In April 2005 Bullen Consultants were merged with Faber Maunsell Ltd. Pending receipt of the Agency’s comments on the draft Report, a meeting between Faber Maunsell, Corby Borough Council and the Agency was held at Corby on 18 th July 2005. Because of other related studies that had commenced since September 2004 it was agreed that Faber Maunsell would undertake additional work to update Bullen’s draft Strategic Report.

1.14 The extant and scope of the additional work required was agreed by CBC, FM and the EA in August 2005, taking into account the following :

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• Recent extensions to the Agency’s Main River network in Corby Borough. • The introduction of the Agency’s Flood Zone Maps and the corresponding extensions of Flood Risk Zones up tributaries of the Harpers, Willow and Gretton Brooks. • The results of two technical reports due to be published by Messrs Halcrow later in 2005. • Recent changes to the Environment Agency’s Guidance Notes to Developers, and • Recent revisions to the Milton Keynes South Midlands Sub-regional Strategy.

1.15 This document, which now incorporates the results of the extended study, has been prepared solely as a Strategic Flood Risk Assessment of the Borough of Corby and as a Flood Risk Assessment for a number of potential development areas in Corby for the Corby Borough Council. Faber Maunsell Ltd accept no responsibility or liability for any use which is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

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Faber Maunsell Corby Strategic Flood Risk Assessment 5 2 Flood Risk

2.1 Flood risk can arise from both fluvial and tidal sources. Fluvial flooding occurs as a result of the overflowing or breaching of river or stream banks when the flow in the watercourse exceeds the capacity of the river channel to accommodate that flow. Tidal flooding occurs when an exceptionally high tide, almost always accompanied by a storm tide surge, overtops and/or breaches the tidal defences along a coastline or tidal estuary. There are no areas within the Borough of Corby that are subject to tidal flooding so all flooding and flood risk considered in this Report relates only to fluvial flooding.

Planning Policy Guidance Note 25

2.2 Planning Policy Guidance Note 25 (PPG25) defines three distinct zones of flood risk. These zones are based on the quantified degree of flood risk to which an area of land is subject at the time at which a land allocation decision is made or a planning application submitted. The PPG25 flood risk zones and their associated fluvial flood risk characterisations are summarised in Table 1 below :-

Zone Character of Risk Assigned Annual Flood Risk Probabilities

1 Little or No Risk Less than 0.1% (above 1 in 1000 years)

2 Low to Medium Risk 0.1% to 1% (from 1in100 to 1in1000 years)

3 High Risk Greater than 1% (under 1 in 100 years)

Table 1 - PPG25 Flood Risk Zones

2.3 The PPG25 flood risk zones give a broad indication of flood risk. However, most areas which fall within the High Risk zone (Zone 3) are on fluvial floodplains and many such areas already enjoy some degree of protection from established flood defences. The actual degree of flood risk to which these areas are subject may well be significantly less than that implied by their PPG25 classification.

2.4 PPG25 requires Local Planning Authorities to adopt a risk-based approach to development in areas at risk of flooding, and to apply a "sequential test" to such areas. This means that, other factors being equal, the planning authority would favour development in areas with a lower flood risk. It is obvious that potential development areas within the PPG25 "High Risk" zone may be at very different risks of flooding. For example, whereas the probability of flooding in one area may be as high as 10% (1 in 10 years) the probability in a neighbouring area may be as little as 2% (1 in 50 years), yet both are within PPG25 Zone 3. The planning authority must therefore be able to rank potential development areas according to actual flood risk.

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2.5 PPG25 sub-divides the "High Risk" Zone 3 as follows :-

Zone 3a - Developed Areas

Areas which may be suitable for residential, commercial and industrial development provided the appropriate minimum flood defence standard can be maintained for the lifetime of the development.

Zone 3b - Undeveloped & Sparsely Developed Areas

Areas generally not suitable for residential, commercial and industrial development unless a particular location is essential for a specific use and an alternative lower risk location is not available.

Zone 3c - Functional Flood Plains

Areas possibly suitable for some recreation, sport, amenity or conservation uses. Built development in these areas should be wholly exceptional and limited to essential transport and utilities infrastructure.

2.6 A functional floodplain can be either an area of floodplain which is known to flood frequently and where flooding is tolerated, as it may prevent or ameliorate flooding elsewhere, or an area within a floodplain that can be deliberately inundated during a flood event to provide temporary retention storage for flood water.

Circular 30/92 Maps

2.7 Under Section 105 of the Water Resources Act 1991 the Environment Agency, having undertaken a nationwide study, produced a series of maps covering the whole of England and Wales ("Circular 30/92 Maps") showing areas of land considered to be at risk of fluvial and tidal flooding and the likely extent of that flooding.

2.8 These maps were issued to Local Planning Authorities by the EA in September 2000. Circular 30/92 maps are issued as 5km square "tiles" based on Ordnance Survey National Grid squares. Each region of the Agency was responsible for preparing the Circular 30/92 maps for its own area.

2.9 The criterion adopted by the Agency to define those areas considered to be at risk of fluvial flooding was an annual risk of flooding of 1% or more, i.e. could expect to be flooded at least once in a hundred years. This criterion was numerically the same as that subsequently adopted for the PPG25 "High Risk" Zone 3.

Indicative Floodplain Maps

2.10 "Indicative Floodplains" are defined by the Environment Agency as being those areas which would naturally (i.e. without flood defences) be subject to flooding on average at least once every hundred years. All areas within the Indicative Floodplain are therefore automatically deemed to be within the PPG 25 "High Risk" flooding zone.

2.11 In 2001 the Environment Agency reissued the Circular 30/92 maps in electronic format under the name of Indicative Floodplain maps. These maps were distributed to all Local Planning Authorities in the form of compact discs (CD). The information was also placed in the public domain on the internet. Except where minor local corrections had been made, the floodplain envelope shown on the Indicative Floodplain maps was the same as that which appeared on the Circular 30/92 maps. The Indicative Floodplain maps do not, however, indicate the areas considered to be defended to a 1 in 100 year standard against fluvial flooding.

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2.12 Bullen Consultants have been supplied with a complete set of Indicative Floodplain maps on CD. These maps are the updated (2002) version issued by the Agency's Anglian Region. The possession of these maps has effectively removed the need for the Circular 30/92 maps.

2.13 On the Indicative Floodplain maps, floodplains are shown to extend up river and stream valleys only to the upstream limit of Main River or where the catchment area above that point falls below 10 km 2. This arbitrary limit can sometimes result in the abrupt truncation of a floodplain and give the potentially misleading impression that significant flood risk ceases at the edge of the envelope shown on the map.

2.14 The shape and extent of the floodplain shown on the Indicative Floodplain maps should not be regarded as definitive. Detailed local studies of the floodplain sometimes reveal anomalies and inaccuracies in the position of the floodplain envelope as shown on the maps. The Environment Agency readily admit that such anomalies will appear from time to time and have indicated their willingness to modify the maps in such cases.

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Faber Maunsell Corby Strategic Flood Risk Assessment 9 3 Causes of Flooding

3.1 The Borough of Corby straddles the watershed between the Welland and Nene catchments. The River Welland forms the northern boundary of the Borough and the northern edge of the Borough is in the Welland valley. The majority of the Borough is, however, is situated within the catchments of a number of major tributaries of the River Nene. The actual town of Corby, along with the majority of the proposed development areas, are all within River Nene catchment. The town is situated towards the upper end of three of the Nene’s major tributaries, namely the Gretton, Willow and Harpers Brooks. Only a small portion (12.3ha) of the proposed Northern Extension development area falls within the catchment of the River Welland.

3.2 The Borough is situated on the Lower Lincolnshire Limestone with a capping of Boulder Clay. This combination has produced the landforms typical of the central English Midlands; broad, relatively flat river valleys separated by expanses of upland characterised by a rolling landscape of ridges and steeper, tributary stream valleys. Flooding in such areas can therefore occur in distinct ways, as described in the following sub-Sections. It should also be noted that in the Corby area the underlying geology contains extensive ironstone beds which has resulted in large scale mineral extraction in the area. This has now ceased, and most of these opencast quarries have since been restored, either to mixed agricultural use or to woodland.

Overflowing of Watercourses

3.3 When the flow in a river or stream exceeds the capacity of the channel to convey that flow, either because of limited cross-sectional area, limited fall, or a restricted outfall, then the water level in that channel will rise until the point is reached where the banks of the channel are overtopped. Water will then spill over the channel banks and onto the adjoining land. With an upland river the adjoining land is its natural flood plain, which will generally be of limited extent and fairly well defined.

3.4 In the case of a major river, such as the Welland, the floodplain may be a kilometre or more in width, though it may not be equally distributed on either side of the river channel. However, due to local variations in geomorphology, the width of the floodplain may vary considerably from point to point along the river valley. Floodplains are characterised by flat, riparian land along the valley floor. In pre-industrial England, such land was regarded as liable to flooding and was traditionally reserved for grazing and stock rearing and human settlements were almost always established beyond the edge of the floodplain. In the industrial age and more recent times with different priorities, pressures for development have resulted in the widespread colonisation of floodplains, often with steps taken to mitigate the associated risks of flooding.

3.5 The town of Corby originated as a market town with a history going back as far as the eighth century. The original settlement would have been situated approximately 4.5 km south of the River Welland which is by far the major watercourse in the area. The town is in fact out of the Welland’s catchment and is separated from the river by an escarpment adjacent to the southern edge of the river's floodplain and running parallel to it. Because of its distance from the river there has been no need for any urban encroachment into the Welland floodplain as the town has developed.

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3.6 However, as mentioned above, Corby has several tributaries of the River Nene running through it, and these watercourses have been progressively entrained and culverted as the urban area has encroached on their valley floors. Needless to say there are still many stretches of these tributaries which flow in natural, open channels and are therefore prone to overtopping during low probability storm events. However, due to the relatively small flood flows any overspilling into the floodplains is likely to have relatively localised consequences. The Borough Council has stated that they are not aware of any plans for development within the Welland floodplain.

3.7 When overtopping of an embanked watercourse occurs, the depth of water flowing over the floodwall or embankment will probably be small, a few centimetres at most. The bank will act like a weir and the rate of flow per unit length will be relatively modest and this, combined with the limited duration of the overtopping, will limit the volume of water cascading over the defences to cause flooding. If overtopping does occur and the protected area is of considerable extent, any flooding that results will often be disruptive rather than disastrous. The situation becomes far more critical if overtopping of an earth embankment erodes its crest, leading to a breach in the embankment.

3.8 There are no known areas in Corby where rivers in flood are contained within raised embankments or within any type of "hard" defence (e.g. concrete floodwalls) and therefore the risks and consequences of embankments breaching need not be considered any further. However it should be noted that several of the storage reservoirs within Corby are either raised or partly raised within bunded embankments and could therefore present a potential flood risk to the area downstream in the event of a failure of these embankments.

Localised Flooding

3.9 Almost all localised flooding of a serious nature occurs as a result of a severe convective storm, localised in extent and duration and generally during the summer. This flooding can, however, be exacerbated by two factors, blockages in the local surface water drainage system or by "floodlocking". Each of these factors is considered separately below. In some instances, in what would otherwise have been a relatively moderate rainstorm, these factors can themselves be the cause of flooding.

3.10 Intense storm rainfall, particularly in urban areas, can create runoff conditions which temporarily overwhelm the capacity of the local sewer and drainage systems to cope with the sudden deluge. Localised “flash” flooding then occurs. In upland areas with small, relatively steep, impermeable catchments, this may result in quite severe flooding over a limited area, often with a considerable depth and velocity of flood water. The duration of such flooding is usually relatively short but this does not mitigate its impact for those affected, especially when the flooding may have developed suddenly and unexpectedly.

3.11 Localised flooding can also occur in urban areas where a stream or watercourse has been extensively culverted, as is the case in Corby. In its natural state, if the channel capacity of a stream is exceeded the channel will overflow along a considerable length and the resultant flooding is distributed over a wide area. If, however, the stream runs through a long culvert and the hydraulic capacity of that culvert is exceeded under flood conditions the culvert becomes surcharged at its upstream end. Water levels will then rise rapidly and localised flooding upstream of the culvert, often quite serious, can occur. The floodwater, in attempting to follow the natural line of the culverted watercourse, may also flow through the built-up area above the line of the culvert. This applies equally to many larger surface water sewerage systems in urban areas which are, in effect, culverted watercourses.

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3.12 Local flooding is often exacerbated by deficiencies in the local surface water drainage system, but these can usually be remedied by relatively minor works once they have been exposed by a flooding event. Local flooding can also be caused by temporary blockages or obstructions in a drainage system, especially one that has been extensively culverted. Such flooding can therefore be virtually random in its occurrence, although the prevalence of blockages at a particular location would suggest a systematic problem, justifying action to modify the drainage system at that location in order to resolve it. In recent years some watercourses considered to be particularly at risk from such blockages have been designated "Critical Ordinary Watercourses" (COWs) although this designation does not, as yet, have any statutory status. Within Corby there are four COWs. These are indicated in Figure 3.

3.13 In inland areas, all local surface water drainage systems discharge to a major stream or river. Except where pumps have been installed, this discharge is by gravity. If the receiving stream or river is in flood, especially where that watercourse is contained within raised floodwalls or banks, the flow in the local drainage system can no longer drain to the river and is impounded behind the defence line for the duration of the flood. This is known as "floodlocking". This can result in secondary flooding within the defended area, even though the defences may not have been breached or overtopped. There are no known areas in Corby where rivers in flood are contained within flood walls or embankments and therefore the risks and consequences of floodlocking need not be considered any further.

Mechanical, Structural or Operational Failure

3.14 Although less common than overtopping, flooding can also be caused by the mechanical or structural failure of engineering installations such as land drainage pumps (or their power supplies), sluice gates (or the mechanism for raising or lowering them), lock gates, outfall flap valves etc. Such failures are, by their nature, more random and thus unpredictable than the failures described in the previous sub-Sections, and may occur as a result of any number of reasons. These include poor design, faulty manufacture, inadequate maintenance, improper operation, unforeseen accident, vandalism or sabotage. Examples of these types of problems occurring in Corby might be the failure of the pumps which drain the Gretton Book Reservoir, or of the hydrobrake on the control outlet of the Great Oakley Reservoir.

3.15 Structural failure, in this context, is also taken to include the failure of "hard" defences in urban areas such as concrete floodwalls. "Hard" defences are most unlikely to fail by the overtopping / erosion / breaching sequence experienced by earth embankments. Their failure tends to be associated with the slow deterioration of structural components, such as rusting of steel sheet piling and concrete reinforcement, or the failure of ground anchors. Such deterioration is often difficult to detect and failure, when it occurs, may well be sudden and unforeseen. Structural failure of "hard" defences is most likely to happen at times of maximum stress, when water levels are at their highest during a flood. There are no known areas in Corby where rivers in flood are contained within floodwalls and therefore the risks and consequences of structural failure need not be considered any further.

3.16 Flooding can also be caused or exacerbated by the untimely or inappropriate manual operation of sluices, or by the failure of the person or organisation responsible to open or close a sluice at a critical time. Responsibility for the operation of sluices rests with various public bodies as well as riparian landowners. Operational failures of this nature generally occur during a flood event and their results are to exacerbate rather than to cause flooding, and their impact is normally limited in extent. There are no such manually operated mechanisms known to exist in Corby, and therefore the risks and consequences of this type of operational failure need not be considered any further.

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3.17 The risks associated with the types of failure described above are almost impossible to quantify, especially as experience has shown that there is a joint probability relationship between this class of failure and flooding resulting directly from extreme meteorological events. It can of course be argued that if a risk of this type was quantifiable and found to be finite then action should already have been taken to alleviate the risk. Even an assessment of relative risk for failures of this type must depend on a current and detailed knowledge of the age and condition of plant, its state of maintenance, operating regime etc at a significant number of disparate installations.

Faber Maunsell Corby Strategic Flood Risk Assessment 13 4 Flooding in Corby

Classes of Flooding

4.1 Because of the absence of any expanses of pump-drained land within the Borough, potential flooding in Corby can be limited to three general classes determined by the predominant landforms in the area. These three classes are outlined below.

Lowland Plains – Class 1

4.2 This type of flooding results from the overflowing of relatively large, slow moving rivers (and tributaries with which they are in hydraulic continuity) onto a wide, extensive flood plain. Flooding of this will vary in depth but arises from continuous, frontal rainfall, usually on an already saturated catchment, and it develops over a period of hours or even days. The duration of flooding will, however, be correspondingly prolonged. Away from the river itself the flood flow velocities are low. The land adjacent to the main river may, in certain places, be protected by floodbanks but these will be of modest height and, even if breached, the consequences will be very much less severe than in fenland areas.

Upland River Valleys – Class 2a

4.3 Close to their headwaters, rivers in these areas will be smaller but faster flowing. Flooding will occur by overtopping of the banks but as the valley, and hence its floodplain, is relatively narrow the flooded area will be of limited extent. Nevertheless, the depth of flooding may be considerable, especially where the river flow is impeded by obstructions. The velocity of the water over the floodplain may be considerable, although with the gentler topography of Corby it is unlikely to reach "life and limb" hazard velocities encountered, for example, in steep, narrow Pennine valleys. This type of flooding is typically "flashier" than that experienced in Class 1 and, especially with smaller watercourses, arises from exceptionally heavy, but shorter duration and more intense rainstorms.

Upland River Valleys – Class 2b

4.4 This has the same origin as Class 2a flooding but occurs in urban areas where the watercourse has been extensively culverted, as described in paragraph 3.11.

Localised Flooding – Class 3a

4.5 In theory this results from a local urban drainage system being unable to cope with the rate of runoff from a particularly heavy, intense storm larger than that for which it was designed. In practice, the problem is usually exacerbated by an obstruction or blockage of the drainage system, either by a long term accumulation of silt or debris, or by larger debris carried along with the storm runoff. In such cases the flooding can occur almost at random, and on a relatively moderate storm event and hence comes under the heading of "residual risk" flooding in PPG25. Unless the problem is chronic, due to under-design, once the obstruction has been removed the flood risk can be drastically reduced. This type of flooding is normally associated with the local surface water or combined (surface and foul) sewerage systems and its effects are generally limited to part of a street or small cluster of properties.

Faber Maunsell Corby Strategic Flood Risk Assessment 14

Localised Flooding – Class 3b

4.6 This occurs at the downstream end of small local drainage systems, either urban or rural, as the result of "floodlocking" of the local drainage outfall by flood levels in the river or watercourse into which the local system discharges. This has been described in some detail in paragraph 3.13.

4.7 Within Corby, Class 1 flooding will only occur along the River Welland. Class 2a flooding could occur along the Central and Southern Arms of Willow Brook and on Harpers Brook, upstream from the town. Class 2b flooding might be expected along all three arms of Willow Brook, as they flow along a series of open channels and culverts through the urbanised areas. Class 3a flooding could occur almost anywhere within the Corby urban area but is most likely along the routes of large surface water sewers. The risk of Class 3b flooding within Corby is negligible as there are no areas protected by any form of banks or walls. Class 3b flooding could possibly occur downstream of the Borough at Deene Lake. The Indicative Floodplain maps show that Deene Hall may be at risk and that if this area is in fact defended, then it may become prone to becoming floodlocked during a low probability flooding event.

Records of Flooding

4.8 Corby Borough Council, the Environment Agency and Anglian Water were all contacted to obtain information on flooding records and drainage problems in the Borough. The responses received are summarised below.

CORBY BOROUGH COUNCIL

4.9 The Borough Council maintain no specific set of records of flooding incidents which have occurred in Corby. It is likely that individual flooding reports have, over the years, been kept on files relating to specific streets or properties but are not readily available for collation. Many of these reports would, in any case, relate to minor localised flooding of roads, gardens etc. As a result, any information of flooding that has been obtained from the council has been put together from the knowledge and experience of the council staff based on past events. The main points are:

Flooding of Gretton Brook Road in the vicinity of the Gretton Reservoir has occurred in the past, however no properties have been flooded. A larger diameter pipe has been installed under the road linking the reservoir to the head of the open channel section of Gretton Brook. This seems to have alleviated this problem although it is not known to what extent this has been tested by a low probability storm event.

Flooding of the Willow Brook Central Arm has occurred along Cottingham Road. Reshaping of the channel in conjunction with installation of weirs in Thoroughsale Woods was undertaken in the 1980s to add an online buffering zone upstream of Studfall Avenue.

The grilles on the inlet to the culverted section of the Willow Brook Central Arm at Studfall Avenue has in the past been prone to contributing to the problems in this area if they are not maintained properly.

Occasional flooding of the Willow Brook Central Arm at Bridge Street in Weldon has occurred in the past.

Occasional house flooding of the Willow Brook South Arm at Oakley Road has occurred in the past.

Faber Maunsell Corby Strategic Flood Risk Assessment 15

Occasional flooding of Willow Brook South Arm at Hillside Crescent in Weldon has occurred in the past, however this has been restricted to garage and garden flooding only.

Occasional garden flooding at Brayford Avenue, Burleigh Drive and the end of Cecil Close.

Occasional flooding of the area to the west of the A427 / A6003 junction.

Flooding of the area around the entrance to the culverted section along Gainsborough Road downstream of Sower Leys Road. This seems to occur approximately once every two years. Further downstream on the South Arm, just after the confluence of the upper and lower branches in the area of the school at the Oakley Road / Westcott Way junction, flooding occurred approximately eight years ago. This was probably the result of a surcharging manhole, although this area does seem to be relatively low lying and therefore this water could have been runoff from the over topping of an upstream open channel.

ENVIRONMENT AGENCY

4.10 A search of the Environment Agency’s files revealed only one report which contained any documented information of past flood events within Corby Borough.

4.11 Although very limited, there is some information included in a 1984 report by John Taylor and Sons (Ref.4) undertaken for the Corby District Council. The purpose of the study was to quantify the effects of previous flooding with particular reference to the floods which occurred in 1982. Causes of particular problem areas were then identified, along with economically feasible methods for alleviating them. A summary of this report is given below.

STORM DRAINAGE STUDIES REPORT BY JOHN TAYLOR & SONS

4.12 The storm event which occurred on the 30 th July 1982 was estimated by the Meteorological Office as being an event with a probability of 1 in 72 years. The flooding was generally limited to the main valleys with secondary flooding occurring in isolated low lying spots. The damage cost approximately £40,000 (1982) to repair, spread over 80 separate properties.

4.13 Mathematical models of the storm sewer system were developed using already available information in conjunction with extra survey information obtained for the purposes of the models. Although rather simplified, the models were run through a Wallingford simulation method in order to highlight any constrictions in the system and to calibrate the sewer performance with the drainage standards set by the Council. Since any given storm event is unlikely to correlate exactly with the statistically calculated design storm of a given probability and duration because of the uneven distribution of rainfall over the town, this will lead to differences between the theoretical predictions and the observed flows.

4.14 The flooding observed in July 1982 was the result of what was estimated to be a 1 in 72 year, 2-hour duration storm event. Because of the limited size of the catchment, both upstream and within Corby, the area is more sensitive to shorter duration storms. In fact the report stated that a 1 in 10 year, 1-hour duration storm has a higher peak flow than the event observed in 1982. Therefore briefer flooding will occur for more frequent events of short duration.

4.15 Modelling was undertaken for the North, Central and Southern Arms of Willow Brook for the 2, 10 and 50 year 1-hour storm events, along with a 72 year 2-hour storm.

Faber Maunsell Corby Strategic Flood Risk Assessment 16

4.16 The study concluded that simply increasing the hydraulic capacities of sewers which were identified as causing upstream surcharging would not be an appropriate solution as this would simply transfer the problem downstream. It would also have been an expensive option with the benefit-cost ratio making such works unfeasible. Instead the use of balancing reservoirs was recommended. The recommendations for alleviation works to be carried out on Willow Brook as a result of the modelling were as follows.

4.17 Southern Arm

From the modelling a problem of surcharging of pipes upstream of Hazel Leys School was identified. This causes flood water to run down Gainsborough Road to collect at the roundabout of Gainsborough Road, Sower Leys and Jubilee Avenue.

The best solution would be to divert this flood water to the area to the south of Gainsborough Road, upstream from the roundabout. However, this area has been raised and landscaped and therefore regrading would be unfeasible.

Therefore the suggested solution was to construct a flood storage reservoir with a capacity of 1,500 m 3. Two suggested locations were given, the first being the section of open channel along the front of Hazel Leys School where embankments could be constructed to increase the capacity of the Brook, in conjunction with lowering the general ground levels in the downstream area bounded by Blake Road and Beardsley Court. Improvements would be made to the drainage at the roundabout to divert the surface flood water to the channel.

An alternative location was further downstream where the channel is bounded by Brangwyn Walk and Bonnington Walk. However this location is further from the problem area which would result in a longer flow path for the surface flood water to have to travel.

4.18 Central Arm

From the modelling it was recommended that a 4,000 m3 flood storage reservoir would be required on the Central Arm of Willow Brook and that its location should be to the south of Thoroughsale Woods, immediately upstream of Studfall Avenue. Also recommended was a small degree of subsidiary balancing immediately upstream of Willowbrook Road.

It was also proposed that flooding of the area upstream of The Jamb could be alleviated by upgrading the embankments in this area in order to create an additional 1,400 m 3 of online storage.

4.19 The following additional recommendations were also made as a result of the study :-

Screens: Regular maintenance and clearing is necessary to ensure that the risk of blockages is limited. This also means that the watercourses upstream of the screens need to be kept clear. Enlargement of some existing screens was suggested, in particular the point where the Central Arm is diverted to the Southern Arm to give enhanced protection against constrictions.

The Jamb Bridge: The central wall of the bridge is a significant obstruction to flow and should ideally be removed. However this will probably mean complete reconstruction of the bridge making it unfeasible and instead this constriction point should be compensated for in the design of the works suggested for the channel immediately upstream.

Faber Maunsell Corby Strategic Flood Risk Assessment 17

Stephenson Way: The channel which runs behind Stephenson Way spills over its banks as a result of a 1200mm diameter culvert which constricts the flow. This problem could be alleviated by replacing the existing 1200mm pipe culvert with a 2m wide × 1.2m high box culvert.

Rockingham Road/Studfall Avenue Roundabout: This area is shown to flood during a 1 in 10 year simulation of the model. However, any works to increase the size of the sewers would be impractical. It would be better to enhance the drainage of the roundabout itself in conjunction with any improvements made to the channel behind Stephenson Way as mentioned above.

Individual Drainage Problems: There are a number of properties in various locations that are subjected to flooding which can be partly attributed to the inadequate capacity of the storm water drains. These problems can often be solved by the construction of kerbs, regrading of pavements etc to divert flows.

4.20 The Appendix to the John Taylor & Sons Report includes a list of the properties flooded by the July 1982 storm. However, the extent of the flooding in each location was not indicated so it is not known whether in fact this involved the flooding of houses. The list of properties affected is given in Table 2 below.

Willow Brook - Northern Arm Number of Properties Flooded Location 5 123 to 131 Shire Road 6 74 to 84 Studfall Avenue 2 1 & 2 Kane Walk 10 232 to 250 Stephenson Way Total 23 Willow Brook - Central Arm Number of Properties Flooded Location 6 110 to 120 Kingsthorpe Avenue 1 292 Studfall Avenue 1 22 Rosedale Avenue 8 175 to181 Cottingham Road 20 Boon Walk 4 1, 8, 11 & 15 Chapel Lane Total 40 Willow Brook - Southern Arm Number of Properties Flooded Location 1 39 Tower Hill Road 3 482, 492 & 494 Gainsborough Road 2 47 & 49 Gainsborough Road 5 1, 3, 5, 7, & 9 Brayford Ave 6 1, 3, 5, 7, & 9 Burghley Drive 2 Danesholme Total 19

Table 2 - Properties Flooded by the Storm of 30 th July 1982

Faber Maunsell Corby Strategic Flood Risk Assessment 18

4.21 The search of Environment Agency files also revealed a plan showing the extent of flooding during this event, although it is not known if this plan was produced in conjunction with the John Taylor & Sons report. Either way, the flooded areas indicated on the plan have been reproduced in Figure 4 with the properties mentioned in the report as listed below.

4.22 Details of the main problem areas are given below:

39 Tower Hill Road From the modelling, the observed problem area during the 1982 event was shown not to be the result of flooding or surcharging of the main surface water sewer adjacent to the property as initially thought. It was concluded that the flooding of this property was caused by the runoff from Tower Hill Road in conjunction with the paved and roofed areas of the school, and water from nearby fields. During an extreme event this runoff is not picked up by road drainage, but instead runs down the driveway of 39 Tower Hill whose front door step is 1m lower than the road level.

47 & 49 Gainsborough Road The flooding of these properties is unlikely to have been caused as a result of an inadequate main surface water sewer. The extent of flooding caused by surcharging has generally been limited to the boundaries of Holbein, Brangwyn and Bonnington Walks which are a least 2m lower than these two properties. It was therefore concluded that flooding of these properties is the result of inadequate local drainage.

110 to 120 Kingsthorpe Avenue As concluded from the modelling, the observed flooding along Kingsthorpe Ave cannot be attributed to the main storm- water sewer because the manhole covers in the field on the opposite side of the road are approximately 1 m lower than the door steps of these houses. The most likely reason for the flooding of these properties was stated to be due to the runoff from areas behind the properties and that the observed flooding of the fields is due to a combination of low spots and poor drainage.

Studfall Avenue / Tanfields Roundabout Flooding of properties flooded in this area was stated to be a result of the fact that these properties are located within a natural hollow combined with the inadequate capacity of the main surface water sewer. Upgrading the hydraulic capacity of the main sewer would be expensive and it was suggested that the surface water could be diverted while yard drainage systems could be examined to ensure no backflow from surcharging sewers could occur.

123 to 131 Shire Road This stretch of Shire Road is lower than the junction of Shire Road and Shetland Way. These houses have their doorsteps below the level of the road while the actual cross-fall of the road means that the runoff from the houses opposite crosses the road, enters through the driveway entrances, and floods these properties.

4.23 Some further information on flooding in the Borough was obtained from the knowledge and experience of the EA staff based on past events. However the issues raised were all included in those raised by CBC as discussed in paragraph 4.9.

ANGLIAN WATER SERVICES

4.24 Anglian Water were contacted by letter in February 2004 for details of their records of sewer flooding in Corby but no response was received. Anglian Water have, however, supplied a full set of 1/1,250 scale sewer record plans (surface water, foul and combined) for the Corby urban area.

Faber Maunsell Corby Strategic Flood Risk Assessment 19

Flood Alleviation Measures

4.25 In the recent past Corby Borough has been relatively unscathed by the effects of serious flooding. This can be put down to the fact that it is situated near the head of a cluster of small catchments and therefore the accumulated quantity of runoff is still relatively small by the time it reaches any urbanised areas. Therefore the only form of flood defence works in Corby consists of several flood storage reservoirs, while the lack of any threat from a major river has meant that concrete or brick floodwalls and earth embankments have not been seen as necessary. The town itself underwent major development during the early 1930s when Corby’s steel industry was established and the open stream channels were progressively entrained and culverted as development encroached on the valley floors. As mentioned in paragraph 4.7, the majority of the flooding problems in Corby are likely to be fairly localised as a result of the urban drainage systems being unable to cope with rate of runoff, either through design limitations and/or from the physical obstruction of the channels and culverts. Two early studies of the hydrology of the Northern and Central arms of Willow Brook (Refs.5 & 6) were undertaken for Corby Borough Council by Rodney Environmental Consultants in the early 1980s.

4.26 Stage 1 of the study identified all the information available (and that which was not available) on watercourses, surface water sewers, flood storage reservoirs etc. This information came from a number of sources including archive reports, Anglian Water records, design calculations, planning consent applications, and the knowledge of both CBC and EA staff. To help put all this information from different sources together, and to aid in the understanding of how this infrastructure is hydraulically linked, detailed plans of the area (Figure 5) were put together showing all the details of the various information gathered.

4.27 As mentioned in paragraph 4.25, flood defence works in Corby consist principally of a number of flood storage reservoirs. Several of these reservoirs were constructed during the 1980s, presumably to cope with the increased urban runoff as the town developed. However it was not until the town was exposed to a storm event of major significance, such as that which occurred in July 1982, that shortcomings in flood defence standards were highlighted. This prompted several further studies to be carried out. Details of the flood storage reservoirs (FSRs) in Corby are listed below and photographs of these reservoirs have been included in Appendix 2.

GRETTON BROOK

Gretton Brook Flood Storage Reservoir

4.28 Gretton Brook FSR (Photos 1 to 5) is located adjacent to the intersection of Gretton Brook Road and Earlstrees Road. The EA are responsible for the operation and maintenance of this reservoir and therefore most of the information on this reservoir has been sourced from them. A report has recently been written for the EA entitled “Corby Area Flood Storage Reservoirs – Gretton Brook” (Ref.7). This study details a description of the reservoir including the contributing drainage area and operation of the reservoir; the findings of an inspection which included a functional test of the pumps; an assessment of the standard of protection; and the recommendations of the study. The findings of this study are summarised as follows.

4.29 Gretton Brook FSR is described as being partially on-line and partially off-line. There are four storm water outfalls into the reservoir, namely a 300mm dia pipe in the NE corner, a 300 mm dia pipe along the southern perimeter, a 600mm dia pipe in the SW corner, and a 600mm dia pipe in the NW corner. The main inlet to the reservoir is via a steel sheet pile weir within a reinforced concrete chamber. Flows exceeding the capacity of a 815mm dia pipe in Gretton Brook overtop the weir and flow into the reservoir via a pipe under Gretton Brook Road.

Faber Maunsell Corby Strategic Flood Risk Assessment 20

4.30 The reservoir is located within a gullet which was formed by the extraction of ironstone. A dam was constructed across the gullet to give a storage capacity of 25,230m 3. This volume of storage is estimated to be able to provide the necessary attenuation required during a flood event with of probability of occurring once in 10 years. For fluvial flooding the indicative standard of protection is between 50 and 200 years for Land Use Band A (intensively developed urban area) and between 25 and 100 years for Land Use B (less intensive urban areas). Therefore the industrial catchment would not seem to be protected to an acceptable standard. There is no overflow, and therefore if the capacity of the reservoir is exceeded, the surrounding area (which is predominately industrial) may incur some damage, but there would be no direct risk to life or limb.

4.31 The reservoir is emptied by pumping the stored water back to the diversion structure on Gretton Brook. Because the pump cuts in automatically as the water rises, pumping will occur while water is still being diverted from Gretton Brook. The pumps were tested as part of the EA's study and generally found to be in good running order with only minor remedial works recommended. However the Borough Council consider that the system does not operate efficiently.

Car Yard Flood Storage Reservoir

4.32 There is a small reservoir (Photo 7) located within a vehicle breaker's yard which is assumed to discharge into Gretton Brook. The reservoir is located adjacent to Gretton Brook Road approximately 1km east of the Gretton Brook FSR. It is presumed that the reservoir serves the purpose of temporary attenuation for the surface water runoff from any paved, roofed or compacted “brownfield” areas of the car yard. This presumption should be taken with caution however as no information or details of this reservoir were revealed during Stage 1 of this study. This reservoir has not been included in the Environment Agency’s Flood Defence and Land Drainage Operational and Emergency Plan (see paragraph 4.85) and therefore it can be assumed that it is of minor significance to a study carried out at a strategic scale.

WILLOW BROOK - NORTH ARM

Stanier Road Flood Storage Reservoir

4.33 The Stanier Road FSR, or Rockingham Road FSR as it has been referred to in the past, (Photos 7 to 9) is an online reservoir stretching along a 30m length of channel extending approximately 15m both upstream and downstream of the Stanier Road Bridge. Little information has been found on this reservoir. It was, however, constructed during the late 1980s and was sized to balance the increased runoff created by the development of the area bounded by Rockingham Road, Willow Brook North Arm, and Stephenson Way in conjunction with the Hubble Road / Crick Close development area. Design drawings show that the original channel was reconstructed to form a new two-stage channel with a main channel approximately 0.5 m deep and 1.5 m wide and a trapezoidal shaped floodplain approximately 10m wide on each side with bank heights of approximately 1m. The Borough Council are responsible for the operation and maintenance of the Stanier Road FSR.

Pen Green Lane Balancing Pond & Pen Green Flood Storage Reservoir

4.34 Pen Green FSR (Photos 10 to 12) is an offline reservoir on the North Arm of Willow Brook. The reservoir is located adjacent to Pen Green Lane immediately upstream of the Corby- Oakham railway line. Anglian Water are responsible for the operation and maintenance of the reservoir. A search of the EA’s files revealed various short reports and notes on the Pen Green Reservoir which were written in the early to mid 1980s when various options were looked at for increasing the capacity of the reservoir. Construction of the original reservoir took place in 1976 in order to provide temporary storage of the runoff created by the development within the catchment upstream of this point.

Faber Maunsell Corby Strategic Flood Risk Assessment 21

4.35 The basic layout of the system is a box culvert in the channel which acts as a constriction, pushing water up over a side spillway into the reservoir. Downstream of the control is a length of open channel before the watercourse flows over the railway line in an aqueduct (Photos 15 & 16). Immediately downstream of the aqueduct there was a siphon under the redundant British Steel railway sidings but this siphon has subsequently been replaced with a pipe culvert. The results of studies undertaken as a result of flooding of the railway line suggested that remedial works would need to be undertaken. The capacity of the reservoir was estimated to be 15,500m 3, although it was calculated that to cater for a 1 in 10 year storm 28,000m 3 would be required.

4.36 As a result of this additional storage requirement, the Pen Green Lane Balancing Pond (Photos 13 & 14) was constructed during the late 1980s on land between the original reservoir and the railway line. However a plan obtained from the EA’s files states that the capacity of this pond is 4,000m 3 and therefore it would still not be adequate to provide the total 28,000m 3 required for a 1 in 10 year storm. From the plans, it would seem that the arrangement of the existing reservoir was relatively unchanged and that the hydraulic connection to the new balancing pond is via a 1,500mm × 1,200mm box culvert set at a level such that it acts as a control, thus creating permanent wet areas in the new pond.

4.37 There is no separate outlet from the balancing pond to the North Arm. Instead the water levels subside as water exits back through the box culvert as the original reservoir is drained by pumping back into the channel when flow in the channel falls below its capacity. It should be noted however that the pump is not currently operating automatically and therefore the reservoir is not emptied after a storm event (although the pump can be operated manually by Anglian Water). Thus the effective capacity of not only the reservoir, but also the balancing pond is reduced, as the water level is greater than that of the control level between the two.

4.38 As mentioned in paragraph 4.34, both the Pen Green Lane Balancing Pond and the Pen Green FSR are, strictly speaking, offline storage reservoirs. There are two sewer discharges to the North Arm in the vicinity of the reservoirs. The first is a 1,500mm dia pipe which, after going through an oil interceptor, discharges into the channel immediately upstream of the control structure. From the Anglian Water sewer plans, this would seem to drain part of the Earlstrees Industrial Estate. A second drain from the Earlstrees Industrial Estate discharges immediately downstream of the channel control structure. This is a 450mm dia pipe which is marked on the Anglian Water plans as a foul sewer.

Phoenix Parkway Flood Storage Reservoir

4.39 The Phoenix Parkway FSR is an online reservoir on the North Arm of Willow Brook adjacent to the junction of Phoenix Parkway and Genner Road. (No photos available due to restricted access.) CBC are responsible for the operation and maintenance of the reservoir, but were, however, unable to supply any information or details in the form of reports etc. A search of the EA’s files revealed a report completed for the Corby District Council titled “Willowbrook Central and West Storm Water Retention Facility” (Ref.8). This study was undertaken by George Crowder Associates in August 1988 and appears to be a summary report of the detailed design.

4.40 It is assumed that the reservoir was constructed to the design summary given in the report as no information was found to suggest otherwise but this should be taken with caution. Discussions between the Corby District Council and the British Steel Corporation resulted in a request for a facility to be designed to accommodate the runoff from both the Willowbrook Central and West sites and part of the British Steel site. The design was completed in August 1988 and it was anticipated that construction would commence in October 1988. Attention should also be drawn to George Crowder’s “Willowbrook East Stormwater Retention Facility” study of March 1987 (Ref.9).

Faber Maunsell Corby Strategic Flood Risk Assessment 22

4.41 The design of the reservoir involved the estimation of the quantities of runoff at the time prior to development, and then after the completion of the proposed development. The estimated peak pre-development flow was 2.7m 3/s for a 1 in 10 year, 1-hour duration storm upstream of the Pen Green Railway Aqueduct, based on an area of 221ha. The development area of 62ha was then used to determine the required storage by taking into account the additional inflow and the permissible outflow.

4.42 The inlet to the lake is via two 1,200mm dia culverts located at the western end of the site. The lake comprises of a central channel for dry weather flows and frequent storm conditions, while floodplains on both sides of the channel accommodate less frequent storms. The central channel is maintained by concrete-grass bocks infilled with angular stone while the floodplains are lined with geotextiles. French drains are used to prevent waterlogging of the floodplains. The control is a 1,050mm dia pipe designed for a 1 in 10 year storm return period while the spillway has been fixed for a 1 in 50 year water level (101.75mOD), thereby giving a storage volume of 9,500 to 10,000m 3.

4.43 The estimated peak pre-development flow and therefore the permissible outflow is 2.7m 3/s. However, the peak discharge from the head / discharge relationship is slightly larger, at 2.9m 3/s. The Willowbrook Central and West site outfall is located at the western end of the site with a lined basin constructed between the outfall to the central channel to prevent erosion of the floodplain. The Anglian Water sewer plans show that this outfall is 1200mm diameter. Indicated on the OS plans are two large pond areas to the north of the Phoenix Parkway Reservoir. These are disused British Steel Corporation sludge beds and play no role in flood storage.

WILLOW BROOK - CENTRAL ARM

Hazel Leys (upstream of the Willowbrook Road culvert) Studfall Avenue and The Jamb

4.44 The 1984 report by John Taylor and Sons mentioned in paragraph 4.11 included recommendations as to the works required to alleviate the problem areas identified by both the hydraulic modelling and the July 1982 floods. These recommendations included three balancing ponds; one in the Hazel Leys area (1,500m 3, Photo 17), one immediately upstream of Studfall Ave (4,000m 3 , Photos 18 & 19) and the third located immediately upstream of The Jamb (4,000m 3 , Photos 20 & 21). Off-line ponds were suggested, with surplus flows being diverted into them, with a restricted outlet slowly bleeding that surplus back into the system further downstream. The report stated that where gradient and available land precluded off-line storage, then an on-line pond might be chosen, with a restriction of the outlet from the pond leading to progressive inundation of designated low lying areas at times of high flows.

4.45 The designated areas already provided a degree of balancing and therefore only minor earthworks were required to form bunds to limit the spread of the impounded water. As no details of the works carried out have been revealed, it is assumed that the storage capacities resulting from the channel works are similar to those recommended in the study. It should, however, be noted that the reservoirs are in fact on-line rather than off-line as recommended in the report. These reservoirs are not included in the Environment Agency’s Flood Defence and Land Drainage Operational and Emergency Plan (see paragraph 4.85) and therefore it can be assumed that these reservoirs are of minor significance to a study carried out at a strategic scale. It should be noted, as mentioned in paragraph 5.32, that the Boating Lake (Photo 22) is for amenity and recreational purposes and was not designed to provide any sort of flood storage. However there is the facility to lower water levels in this lake if required.

Faber Maunsell Corby Strategic Flood Risk Assessment 23

Crucible Road Flood Storage Reservoir

4.46 The Crucible Road FSR is an off-line reservoir on the Central Arm of Willow Brook located between Crucible Road and Lloyds Road (Photos 23 to 25). CBC are responsible for the operation and maintenance of this reservoir although they were unable to supply any information or details in the form of reports etc. A search of the EA’s files revealed a report completed for the Corby District Council titled “Central Stream of Willow Brook Retention Facility, Corby” (Ref.10). This study was undertaken by George Crowder Associates in April 1987 and is a submission to Anglian Water of the proposed design of the reservoir. It is presumed that the reservoir was constructed to the design summary given in the report as no information was found to suggest otherwise, but this assumption should be taken with caution.

4.47 The reservoir was required to cope with the increased runoff and changes in drainage pattern caused by the redevelopment of the British Steel Corporation site, BOS Plant & Heavy Rolling Mills area. The reservoir is situated immediately upstream of a diversion where the majority of the flow in the Central Arm is transferred to the Southern Arm of Willow Brook. However because of the hydraulic limitations of the diversion culvert, a reservoir is required for the temporary attenuation of the runoff upstream of this point so that the flows do not exceed the hydraulic capacity of the diversion culvert.

4.48 Adjacent to Crucible Road, the Central Arm divides into two culverts. The first is a continuation of the stream through the British Steel Corporation's operational works site and has an estimated intake capacity of 0.3m 3/s. The second culvert diverts the majority of the flow to the Southern Arm and has a capacity of 7m 3/s. The new (at the time) development of the British Steel Corporation site, BOS Plant & Heavy Rolling Mills area is drained via two 1,200mm dia pipe outfalls. The Phoenix Centre / Heavy North End outfall discharges into the Central Arm not far upstream of the diversion, while the 1,200mm dia pipe from the BOS Plant & Heavy Rolling Mills area runs along parallel to the diversion culvert and discharges directly into the Southern Arm.

4.49 Because of the design and operation of the Weldon Reservoir, the Environment Agency has stipulated that the maximum allowable flow diversion to the Southern Arm is 10.1m 3/s. The reservoir was designed for a 1 in 50 year flood which sets a precedent the design of new works within the upstream catchments. Taking into account that the capacity of the main diversion culvert is 7m 3/s, while the BOS sewer capacity is 2.6m 3/s, the total 9.6m 3/s is under the maximum stipulated by the Environment Agency. However it should be noted that this total capacity cannot be realised as the peak flow in the BOS sewer passes before the peak flow in the culvert due to the fact that the time of concentration for the BOS Plant & Heavy Rolling Mills area is far less than that of the combination of the upper catchment, the urbanised area upstream of the diversion, and the Phoenix Centre / Heavy North End.

4.50 Runoff hydrographs were produced for storm durations of 3 to 20 hours (note that return period was not stated) and showed that the runoff from the upstream catchment exceeded the 7m 3/s capacity of the diversion culvert and that storage was therefore required.

4.51 The present channel between an existing weir and the diversion culvert was reformed such that it was uniform in both section and gradient. The reservoir was constructed adjacent to the reformed channel with a 20m long side-spillway weir from the channel into the reservoir. The water level in this section of channel is controlled by a weir with a crest level of 96.4 m located immediately downstream of the side-spillway weir. Although not given, the level of the side-spillway weir is such that the flow over the downstream weir and into the diversion culvert would not exceed 7m 3/s. The minimum storage volume of the reservoir was calculated to be 2,500m 3. It was, however, stated that a larger design volume of approximately 3,740m 3 would provide for less frequent storms (i.e. 1 in 100 years). The reservoir was designed so that it holds a small amount of water on a permanent basis in order that it is aesthetically more pleasing.

Faber Maunsell Corby Strategic Flood Risk Assessment 24

4.52 The outflow from the reservoir is to the BOS Plant 1,200mm dia sewer which runs parallel to the diversion culvert directly to the Southern Arm of Willow Brook. Because the time of concentration of the BOS Plant area is far less than that of the rest of the catchment, the peak flows in the sewer will have diminished long before the capacity of the reservoir is reached. Therefore at this point there is capacity in the sewer to take the outflow of the attenuated runoff. This outlet control is a 4m wide broad crested weir with a 750mm wide rectangular notch to control the flow and increase the outflow gradually as the flows in the surface water sewer from the BOS plant decrease.

Weldon Lagoon

4.53 Weldon Flood Storage Lagoon (Photos 26 to 29) is an on-line reservoir on the Central Arm of Willow Brook, located adjacent to the junction of Stamford Road, Bangrave Road and Steel Road. Although Anglian Water are responsible for the operation and maintenance of the reservoir they were unable to supply any information or details in the form of reports etc. A search of the EA’s files revealed a report (Ref.11) completed for a flood risk assessment of a proposed development downstream of the reservoir. This report, along with file notes and correspondence associated with the risk assessment and in conjunction with correspondence between the EA and Anglian Water with regards to upgrades of the lagoon, revealed a limited amount of information on the lagoon.

4.54 Construction of the lagoon was undertaken during the 1980s by the Commission for New Towns to ensure that development of the areas upstream of this point did not increase the flow rates in the stream. The lagoon was designed for a catchment area of 0.92km 2 which gave a total capacity of 29,857m 3 at the level of the dam spillway crest. Because the lagoon is a raised reservoir and had a total storage volume greater than 25,000m 3 it should have been registered under the Reservoir Act 1975. This seems not to have been the case, and we understand that the capacity has therefore been reduced to below this figure. The control outlet structure of the lagoon is set at a level to ensure that the lagoon is partially full (9,029m 3) at all times, to ensure that it is more aesthetically pleasing. Therefore the active storage volume was 20,827m 3 at the dam crest or 18,038m 3 when allowing for a 300 mm freeboard. This volume suggests that the lagoon was probably designed for a return period of less than 30 years.

4.55 The outflow from the lagoon is through a 600mm dia pipe which discharges into the Central Arm of Willow Brook. According to an inspection carried out on the lagoon (see paragraph 4.56), There are five surface water sewer discharges into the lagoon. The pipe diameters of these discharges are 1,050mm, 900mm, 450mm, 300mm, and an unknown size. However, the Anglian Water sewer plans however show two 900mm discharges and an unknown size pipe discharge. It is assumed that the discharges reported as a result of the inspection are in fact correct.

4.56 After concerns about the structural integrity of the lagoon, an inspection was carried out in February 2001. This resulted in a warning that the dam was at risk of failure and that a spillway which would reduce the retained volume should be constructed immediately. Upgrading the embankment was not considered to be a viable option due to the restrictions on building outwards posed by the location of the Weldon Ponds. A spillway was therefore constructed which reduced the active storage volume to 8,621m 3. Although the EA realised the need to mitigate against failure of the dam they are unhappy about the reduced level of storage which has resulted as this could produce an increased extent and depth of flooding in the area downstream of the lagoon. As yet nothing has been done since construction of the spillway to mitigate this impact of the loss of flood storage.

4.57 Adjacent to the Weldon Lagoon are the Weldon Ponds which are used to settle solids in the effluent water from the British Steel Corporation's Tube Works prior to discharge into the Central Arm. The effluent flows through the ponds, but any excessive flow bypasses the ponds by means of a storm overflow channel, leading directly into the stream. The ponds were not designed to have any significant flood storage capacity.

Faber Maunsell Corby Strategic Flood Risk Assessment 25

WILLOW BROOK - SOUTH ARM

Snatchill Flood Storage Reservoir

4.58 The Snatchill FSR (Photos 30 to 35) is an on-line reservoir on the South Arm of Willow Brook located adjacent to Oakley Road, between Bankside Close and Cecil Close. CBC are responsible for the operation and maintenance of the reservoir but the only information which was obtained came from the Planning Department of the CBC. Although extremely limited, this information (which was on microfiche) gave the basic details of the reservoir submitted as part of the planning application. It is presumed that the reservoir was constructed to the specifications of the information submitted, but this should be taken with caution.

4.59 The reservoir seems to have been constructed during the early 1990s to cope with the increased runoff created by the development of upstream areas. It is not known for what probability storm event the reservoir was designed, nor its capacity. It is, however, known that when the water depth in the FSR is 5m the inundated area is 1.73ha. The outflow control is a 675mm dia orifice set at an invert level approximately that of the bed of the reservoir. While it seems that the majority of the runoff is discharged from surface water sewers into the open channel upstream of the reservoir there are 225mm and 325mm dia surface water sewer discharges into the reservoir itself.

Eurohub Balancing Pond

4.60 The Eurohub Balancing Pond is an online reservoir discharging to the Southern Arm of Willow Brook, and is located adjacent to the Stanion Lane Plantation at the end of Long Croft Road. (No photos available due to restricted access.) Eurohub are responsible for the operation and maintenance of the reservoir. A search of the EA’s files revealed the results of a flood risk study completed as part of a proposed development (Ref.12). This study was undertaken by THDA Consulting Engineers in June 2003 as requested by the Environment Agency. The pond was originally constructed in 1995 as part of the original infrastructure associated with the Wincanton Logistics building. It was designed to balance flows for a 100 year storm event to pre-development greenfield runoff rates. The outfall of the pond is to a drainage ditch which in turn discharges to the South Arm of Willow Brook opposite the industrial units on St Mark's Road. The recent assessment was carried out for the development of a site next to the Wincanton Logistics building.

4.61 The original design was for a 25.8ha drainage area. However, when the recent assessment was carried out the total area (including the existing and proposed areas) was estimated to be 25.9ha. It can therefore be assumed that the original pond was designed with future development in mind. To be conservative, a 20% increase in rainfall/runoff was included in the assessment, presumably to account for climate change, and the results showed that the total outflow (87.5m 3/s) would not exceed the pre-development "greenfield" runoff rate (129.5m 3/s) for a 100-year storm.

4.62 The predicted water level of 102.62mOD during the 100-year event would effectively mean that there would be a freeboard of 1.5m, however at the time of the study, this freeboard could not be utilised due to a 450mm dia overflow pipe in the control structure at a level of 101.8 mOD. It was therefore suggested that this overflow pipe be removed in order to increase the capacity of the pond and therefore to reduce the downstream flood risk. The Borough Council think that there might be difficulties in implementing this as the pond is privately owned.

Faber Maunsell Corby Strategic Flood Risk Assessment 26

Soot Banks Flood Storage Reservoir

4.63 The Soot Banks FSR (Photos 36 to 38) is an on-line reservoir discharging to the Southern Arm of Willow Brook and is located adjacent to Stanion Lane. Prologis Kingspark Developments Ltd (PKDL) are responsible for the operation and maintenance of the reservoir and the information on this reservoir came from a report (Ref.13) obtained from a search of the EA’s files. This report contained the agreement between the EA and PKDL on the conditions for the construction of the reservoir and its operation. It is presumed that the reservoir was constructed to the design summary given in the report as no information was found to suggest otherwise, although this should be taken with caution. The reservoir was required to cope with the increased runoff and changes in drainage pattern caused by the redevelopment of the site to the south of Willow Brook South Arm. This included the construction of the PDI building , ancillary buildings and storage areas for motor vehicles.

4.64 The storm-water runoff is collected through some form of onsite network and is discharged into the reservoir through a single inlet which is a 675 mm dia pipe. Any inflow is discharged from the inlet pipe into an open drainage gully (within the reservoir) which runs across the reservoir to the opposite bank where the outlet structure is located. This control structure is a concrete overflow weir within a concrete chamber. The design of the outlet structure and earthworks carried out to create the reservoir were such that the following (Table 3) specifications are met with regards to the maximum allowable discharge rates and the minimum levels of storage.

Return Period Discharge Rate Pond Volume Water Level 5 yrs 52 l/s 1,091 cu.m 93.2mOD 25 yrs 78 l/s 1,591 cu.m 93.4mOD 50 yrs 88 l/s 1,835 cu.m 93.5mOD 100 yrs 106 l/s 2,187 cu.m 93.7mOD

Table 3 - Soot Banks Flood Storage Reservoir Design Criteria

4.65 The outlet pipe is through the reservoir's northern embankment, discharging into a 15m length of open drainage channel which in turn discharges to the South Arm of Willow Brook. An overspill weir is also located on the northern embankment of the reservoir and it is assumed that any discharge over the weir flows down the embankment and straight into the South Arm.

4.66 On the plans which accompany the Agreement Report is a pond located upstream from the Soot Banks Reservoir. This is an old clay pit into which water is diverted from the Willow Brook. The plans indicate that this pond serves the purpose of a backup water supply for British Steel and is therefore likely to be kept filled to capacity on a permanent basis. If this is no longer the case, this pond could possibly be modified to provide a limited volume of flood storage.

Weldon Flood Storage Reservoir

4.67 Weldon FSR (Photos 39 to 47) is both an on-line and off-line reservoir on the South Arm of Willow Brook. The reservoir is located along the length of channel between the sewage treatment works and the western edge of Weldon. The Environment Agency are responsible for the operation and maintenance of this reservoir and therefore the information has come from records and documents contained in their files combined with the personal knowledge of Agency staff.

Faber Maunsell Corby Strategic Flood Risk Assessment 27

4.68 Construction of the reservoir was undertaken during 1976/7 by the then Welland and Nene River Division of Anglian Water as part of a flood relief scheme for Weldon village that was funded by a MAFF grant. The reservoir lies on both sides of the A43 road, under which it is connected via two large arched culverts, approximately 3m high and wide. Upstream of the A43 the reservoir seems to be off-line with a basin stretching along side the southern bank of the South Arm. The inflow/outflow between this storage area and the open channel is via several pipe culverts through the embankment which separates them (Photograph 40).

4.69 To the south of this section of reservoir are a further two balancing ponds which are situated on an upper terrace. The more upstream of these ponds has two inflow structures (Photo 41) which would presumably discharge surface runoff collected from the area between Geddington Road and the A43. This pond has a spillway structure (Photos 42 & 43) which overflows to the area to the south of the main reservoir, which in turn flows into this main reservoir via a similar spillway structure.

4.70 The second of the ponds on the upper terrace to the south of the reservoir is located adjacent to the A43 (Photo 44). It would seem that the purpose of this reservoir is the temporary attenuation of runoff from the A43 itself. A collector drain runs along the eastern side of the road and discharges into the pond. A spillway on the northern side of the pond then discharges into a concrete-lined channel which in turn discharges to the South Arm, immediately upstream of the A43 culverts.

4.71 The section of reservoir on the downstream side of the A43 is a large on-line storage area. At the downstream end of the reservoir is an earth embankment with a concrete-lined spillway (Photos 45 to 47). The outlet is via two 48 inch dia pipe culverts through the embankment at the bed level of the channel. The right side pipe has a penstock attached to its downstream end. Normally both culverts remain fully open until capacity is reached in the downstream channel. At this point a level sensor control starts to close the penstock and the reservoir begins to fill. As the water level in the reservoir increases the penstock closes further until it is fully shut. The increasing head of the reservoir increases the flow through the unrestricted culvert thus keeping a constant flow rate within the downstream channel. Since construction, the reservoir capacity has been exceeded twice, and this resulted in flow over the spillway and the consequent minor flooding of downstream gardens.

4.72 The reservoir was designed for an event with a return period of 50 years. The original capacity was 50,000m 3, but this has subsequently been upgraded to 54,000m 3 to cope with runoff from adjacent development and also from the A43 itself. The reservoir is subject to the Reservoirs Act 1975 and therefore the control equipment is tested biannually to maintain operational efficiency.

Quarry Road Flood Storage Reservoir

4.73 The Quarry Road FSR (Photos 48 & 49) is an on-line reservoir discharging to the South Arm of Willow Brook and is located adjacent to Hillside Crescent. CBC are responsible for the operation and maintenance of the reservoir but were not able to supply any technical details in the form of reports etc. apart from information from the Council's Planning Department which, although extremely limited and on microfiche, gave the basic details of the reservoir from the original planning application. It is assumed that the reservoir was constructed to the specifications of the information obtained but this should be taken with caution.

4.74 The reservoir seems to have been constructed during the early 1990s presumably to balance the extra runoff created by the development of the area of land to the east of the A43 road. It is not known for what probability storm event the reservoir was designed, nor its capacity. The outflow control structure is located within the embankment on the northern side and discharges into the adjacent South Arm of Willow Brook, as does the emergency spillway.

Faber Maunsell Corby Strategic Flood Risk Assessment 28

HARPERS BROOK

Great Oakley Flood Storage Reservoir

4.75 The Great Oakley FSR (Photos 50 to 53) is located immediately downstream of Brooklyn Farm on Woodlands Lane. The EA are responsible for the operation and maintenance of this reservoir and therefore most of the information on this reservoir has been sourced from them. A report recently written for the EA entitled “Corby Area Flood Storage Reservoirs – Great Oakley” (Ref.14). This report details a description of the reservoir including the contributing drainage area, the operation of the reservoir, the findings of an inspection, an assessment of the standard of protection, and the recommendations of the study. The findings of this study are summarised as follows.

4.76 Great Oakley Reservoir is an on-line reservoir formed by the construction of an embankment across Harpers Brook. This embankment is 110m long, 3m wide at the crest, and has upstream and downstream slopes of 1 in 2.5. The outflow is via a 3.3m × 1.2 m reinforced concrete box culvert laid through the base of the embankment at stream bed level. The inlet and outlet works on the upstream and downstream side of the embankment are substantial concrete structures which include head walls and wing walls. The outflow is uncontrolled, with an estimated flow rate of 10.6m 3/s when the water level is that of the spillway. The spillway is a trapezoidal channel constructed with concrete slabs, located on the left side of the embankment. The length of the spillway channel is 23m, its base width is 5.5m and its side slopes are approximately 1 in 6.

4.77 The discharge from the spillway is across fields back to Harpers Brook. However, as the nearest properties are at Little Oakley, approximately 1.5km downstream, and not immediately adjacent to Harpers Brook, it is unlikely that any overflow would cause significant flooding. The storage capacity below spillway crest level has been calculated as 15,827m 3. This would provide a standard of protection against events with a return period of between 30 and 50 years which is considered to be adequate based on the downstream land use (i.e. Land Use Band C – typically large areas of high grade agricultural land and/or environmental assets of national significance requiring protection with some properties also at risk, including caravans and temporary structures for which the indicative level of protection is 5 to 50 years). Most components of the dam, spillway and culvert were considered to be in good condition with only some minor maintenance recommended. The reservoir storage area is open fields which are used mainly for grazing.

CTC Pond and the Oakley Vale Ponds

4.78 The CTC Pond (Photos 54 & 55) is an on-line reservoir located next to the Brooke Western City Technology College. Information of this reservoir came from a report by Haiste Ltd entitled “Alfred McAlpine Homes, Snatchill Development, Corby – Catchment Study Report” (Ref.15). This report was obtained from the EA’s files and examines the effects of the development and the recommendations of a balancing facility. Information on the CTC Pond and the Oakley Vale Ponds also came from a report entitled “Oakley Vale Flood Risk Assessment” (Ref.16) carried out by JBA Consulting Engineers for Cameron Taylor Bedford and from various correspondence found in the EA’s files.

4.79 The CTC Pond was constructed during the mid 1990s in order to balance the increased runoff from the construction of the college and Phases 2 and 3 of the McAlpine Homes development. The pond was designed to take into account that the area is likely to undergo continued development for a number of years. It is not known to what specifications the pond was originally constructed, and as part of the Oakley Vale development the pond has been modified, presumably to the specifications found in the EA’s files. It is assumed that the reservoir was constructed to the design summary given in the report as no information was found to suggest otherwise, though this should be taken with caution.

Faber Maunsell Corby Strategic Flood Risk Assessment 29

4.80 The pond is an on-line reservoir constructed within the natural watercourse and has a capacity of 6,047m 3 below spillway crest level. The outflow of the pond is through the embankment into the natural watercourse, controlled by the 600mm dia outlet pipe which limits the outflow to 200 l/s. The maximum overflow rate would be 7m 3/s. It is not known for what probability event these parameters have been designed.

4.81 For the Oakley Vale development, JBA Consultants carried out a flood risk assessment as required by the EA to address the possibility of flooding resulting from that development. A hydrological assessment was carried out to determine the permitted outflow from the area, based on the mean annual "greenfield" runoff rate during events up to a 200-year return period. Three storage ponds were proposed with capacities as required by the EA, i.e. 80% of the 10-year event in addition to the 200 year "greenfield" event, with a 20% increase in flood flows to allow for climate change. These volumes are given below in Table 4. Ponds 2 and 3 have already been constructed and it is assumed that they were built to the storage specifications given in Table 4.

3 Development Pond Volume (m ) Areas Served 1 6,575 Areas 2 to 5 2 13,922 Area 1 3 18,361 Area 1

Table 4 - Oakley Vale Ponds, Storage Volumes

4.82 The Oakley Vale Ponds (Photos 56 to 58) are on-line, constructed upon the same watercourse which runs through the CTC Pond and then into Harpers Brook. It should however be noted that this section of the drainage path has been replaced with a watercourse at a higher level, as a result of raising of the land associated with the development. These reservoirs are not included in the Environment Agency’s Flood Defence and Land Drainage Operational and Emergency Plan (see paragraph 4.85). The reason for this is certainly not because these reservoirs are of minor significance, but is more likely to be due to the fact that they are relatively new and have yet to be taken fully into consideration.

Longcroft Road Balancing Pond

4.83 The Longcroft Road Balancing Pond (Photos 59 & 60) is located adjacent to the junction of Longcroft Road and Stamford Road. W S Atkins and Northampton County Council are responsible for the operation and maintenance of the reservoir. Most of the information on this reservoir has however been sourced from a study found in the EA files carried out by Scott Wilson Kirkpatrick Consulting Engineers (Ref.17). The purpose of this reservoir is for the drainage of the Southern Distributor Road and the associated development of the area which was proposed in 1995. The proposal was for two balancing ponds connected in series on the western verge of Stamford Road (A43). All runoff is drained to a tributary of Harpers Brook via road gullies and carrier pipes or filter drains, and runs through an oil interceptor before being discharged into the ponds. This tributary is the drain which is culverted under Stanion.

4.84 The balancing pond design was based on the 100-year return period, which was a National Rivers Authority requirement at that time. However the contributing drainage network was only designed to accommodate a one-year storm in-bore, and a five-year storm with surcharging but without flooding of the chambers, and therefore full utilisation of the pond will never occur.

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Operational and Emergency Planning

4.85 Within the Anglian Region, the EA has issued Flood Defence and Land Drainage Emergency Operational Plans in conjunction with the local authorities. These documents are intended to clarify areas of responsibility for the operation and maintenance of flood defence structures within the local authority's area and to summarise the agreed joint emergency response by each of the public bodies involved. This Plan is included as Appendix 3 with the main points being listed below in Tables 5 and 6. Reservoirs and COWs in the Borough have been identified as have the organisations responsible for their maintenance and these are also given in Table 5 below.

Organisation Defence Element Action Required Responsible

Main River EA Inspect raised flood defences every six (Flood Defences) months and implement any necessary remedial works. Advise defence owners of any deficiencies and agree action to rectify.

Main River EA Undertake regular maintenance to ensure (River Channel) an effective drainage is maintained.

Flood Storage Resrs (EA) EA Inspect routinely and implement any (Gretton Bk., Weldon, necessary remedial works. Gt.Oakley)

Ordinary CBC, AWS, EP & Undertake regular maintenance of the Watercourses Riparian Owners watercourse, banks and outfalls to ensure effective drainage and flood defence.

Critical Ordinary Environment Agency Now maintained by the Environemtn W'courses (COWs) (Previously Corby BC, Agency as Main Rivers - see notes (Willow Brook North - Pen Anglian Water, EP & under Main River (River Channel) above. Green to Studfall Avenue. Riparian Owners) Willow Brook Cent - Crucible Rd to the Jamb. Willow Brook Cent – Weldon Village. Willow Brook South – u/s Stanion Lane)

Surface Water AWS CBC & Riparian Undertake routine maintenance of outfall Sewer Outfalls Owners structures.

Surface Water AWS & Riparian Undertake regular maintenance to ensure Pumping Stations Owners that effective drainage is maintained.

Table 5 - Responsibilities & Routine Actions for Flood Defence

Table continued over ....

Faber Maunsell Corby Strategic Flood Risk Assessment 31

Table continued ….

Organisation Defence Element Action Required Responsible

Culverts / Grilles on CBC & Riparian Undertake regular maintenance to ensure Ordinary Watercourses Owners that effective drainage is maintained. (Pen Green Aqueduct, Hazel Leys, Crucible Rd)

Flood Storage Reservoirs CBC Inspect routinely and implement any and Balancing Lakes Phoenix Parkwy FSR necessary remedial works. (non EA) Stanier Road FSR Crucible Road FSR Quarry Road FSR Snatchill FSR Oakley Vale Ponds

AW Pen Green FSR Weldon Lagoon (subject to the Reservoirs Act 1975)

EP Pen Green Ln Pond

PKDL Soot Banks FSR

Eurohub Eurohub Lake

WSA / NCC Longcroft Rd Pond

Highway Drainage WSA / NCC Undertake Routine maintenance of drains, culverts etc.

Table 5 - Responsibilities & Routine Actions for Flood Defence (Contd.)

KEY: EA Environment Agency CBC Corby Borough Council AWS Anglian Water Services EP English Partnerships PKDL Prologis Kingspark Developments Ltd WSA/NCC W S Atkins / Northamptonshire County Council

Note : Table 5 is summarised from the Environment Agency's Anglian Region Flood Defence & Land Drainage Operational Plan for the Corby Borough Council Area.

4.86 The response procedures to be undertaken by each organisation in the event of an emergency flood situation as stated in the plan are summarised as follows in Table 6.

Faber Maunsell Corby Strategic Flood Risk Assessment 32

Organisation Operational Response Responsible

EA Flood Warning and Operational Liaison Operational Response - open Incident Room at Kettering at warning threshold of 85.94mOD in Weldon FSR, contact CBC, AW, EP, and deploy patrols to check defences etc and respond to any damage.

CBC Deploy patrols to clear grilles and respond to any breaches / damage. CBC supplies filled sandbags to properties that are in locations known to be subject to flooding. It is then the residents responsibility to deploy the sandbags during times of high flow. If requests are received from residents in other locations then the Council will try to assist to the best of its ability and resources.

AW OPERATIONAL LIASON & RESPONSE

WSA / NCC OPERATIONAL CONTACT React to incidents as they are reported.

EP OPERATIONAL CONTACT

Table 6 - Operational Emergency Response Arrangements

KEY : As Table 5.

Note: Table summarised from the Environment Agency's Anglian Region Flood Defence & Land Drainage Operational Plan for the Corby Borough Council Area.)

Faber Maunsell Corby Strategic Flood Risk Assessment 33 5 Strategic Assessment of Flood Risk

General Methodology

5.1 For the purposes of this study, the quantitative limits of the flood risk categories used will correspond exactly with the three zones defined in PPG25 (Table 1). Since the EA's Indicative Floodplain maps represent an important initial attempt to define the limit of PPG25 Flood Risk Zone 3 (and hence the boundary between PPG 25 Zones 2 and 3) they will be used as the basis for the detailed strategic (i.e. Borough-wide) assessment of flood risk within Corby.

5.2 A recent study (Ref.18) has shown that although they are generally accurate and reliable, close inspection of the Agency's Indicative Floodplain maps reveals various anomalies in the plotting of the envelope. These anomalies can be grouped into six types, as follows: 1) Where the flood level on one side of a floodplain is significantly different from that on the other.

2) Where the flood envelope does not follow a closely adjacent contour line where "ponded" flooding is known or can be assumed to occur.

3) Where the edge of the flood envelope indicates that the flood level at a point downstream is higher than the level a significant distance upstream.

4) Where the presence of an "island" in the floodplain has been overlooked.

5) Where the water level gradient implied by the flood envelope boundary is clearly at variance with the general land level gradient along the valley floor (thalweg) in that area, except where due to an obvious obstruction to flow.

6) Where the presence of an obvious obstruction to overbank or channel flows (artificial embankment, restricted waterway at bridge, etc) has been overlooked.

5.3 A number of sources of data and information are available which can be used to check and, where necessary, refine the EA's Indicative Floodplain maps and thus the outer limit of the high risk zone (Zone 3). These are -

LIDAR Data

5.4 LIDAR data is topographical data that has been obtained by aerial survey using radar-based techniques. The EA has established a national database of topographical spot-level data derived from a satellite imaging process, many of which have been undertaken by the EA in connection with their "Section 105" surveys of floodplains. At present the LIDAR data coverage does not extend over the whole country. Contoured plots of LIDAR data are found to be of greatest use in open country as the presence of buildings is found to give rise to clearly anomalous results in built-up areas.

5.5 In Stage 1 of the Study, a small scale plan made available by the Agency suggested that there was a relatively small amount of LIDAR data coverage within the Borough. This plan revealed that LIDAR coverage in Corby Borough was limited to the Welland floodplain, the Willow Brook South Arm downstream from Geddington Road, the whole of Gretton Brook, and Harpers Brook as far upstream as Oakley Road. No LIDAR data exists for the Central or Northern Arms of Willow Brook.

Faber Maunsell Corby Strategic Flood Risk Assessment 34

Ordnance Survey Maps

5.6 1/25,000 scale OS maps are contoured at 5m intervals which is adequate to give an indication of the shape of the floodplain at any location. The contours are supplemented by spot heights to the nearest 1m on roads. It should, however, be noted that road levels can, particularly in floodplains, be significantly higher than adjacent land levels.

5.7 A complete 1/2,500 scale OS map coverage of Corby Borough has been provided by the Borough Council on CD which can be accessed using "Mapinfo" software. These maps are not contoured but include spot heights on roads to the nearest 0.1m (though some of these metric spot heights are conversions from earlier imperial units and are therefore only accurate to the nearest 1ft / 0.3m).

Ordnance Survey "Profile" Data

5.8 During Stage 1 of the Study it was found that OS "Profile" data, large scale contoured plans, was available to Local Authorities through SLA agreements with the Ordnance Survey. However many, including Corby, had not availed themselves of the opportunity to acquire this data as Profile Data contouring is less precise than LIDAR data (reportedly +1.0m compared with +0.15m) and requires verification using ground control points. Bullen Consultants have also been given to understand that "Profile" data is in fact derived from the spot heights and contour lines on existing large scale OS maps and provides no more accurate information than can be inferred or deduced from the OS maps.

Circular 30/92 and Indicative Floodplain Maps

5.9 These maps (see Section 2) are based on OS 1/10,000 scale maps. These maps are not contoured but show spot heights to the nearest 1m. In many cases these are at the same locations as the spot heights on the 1/25,000 scale maps but additional spot heights are shown on the 1/10,000 maps.

Flood Records

5.10 Records of past flooding, where available, (see Section 4) can be used to verify or amend the floodplain envelope. However due to the fact that what little information is available for Corby is mainly localised flooding within an urban area, this is likely to be of comparatively little use at the strategic level.

5.11 In order to define the extent of all three actual flood risk categories (corresponding numerically to the PPG25 flood risk zones) it is necessary only to delineate the boundaries between Categories 2 and 3 and between Categories 1 and 2 - i.e. the 1% (1 in 100 year) and 0.1% (1 in 1000 year) flood envelopes. The delineation of a more accurate 1% line by the removal of anomalies has been described above. The delineation of the 0.1% line presents more of a problem.

5.12 Research has demonstrated that the channel capacity of a natural river or stream is approximately equivalent to the mean annual flood flow which can be shown statistically to 1 have a return period of 2 /3 years (Ref.19). In other words, the natural river channel will start to spill over onto its floodplain nearly every other year. It is therefore reasonable to assume that there will also be a natural return period at which the floodplain is completely covered.

Faber Maunsell Corby Strategic Flood Risk Assessment 35

5.13 If a 1 in 1000 year flood were to occur there is no reliable means of determining whether or not it was the 1 in 1000 year flood, rather than (say) a 1 in 500 year or a 1 in 10,000 year flood. In an area of sedimentary rocks, such as Northamptonshire, which was glaciated during the ice ages, the landforms are relatively recent. Since the last ice age there will, on the balance of probabilities, only have been a single 10,000 year flood and about a dozen floods in the 1,000 to 5,000 year return period range. It is therefore reasonable to assume that the fluvial floodplains have, since the ice ages, been shaped by a relatively small number of floods with return periods not greatly in excess of 1,000 years. This suggests that the 1000-year flood would cover the entire flood plain but not to such a depth that the flood plain would be radically reshaped. Conversely, the outer limit of the flood plain roughly defines the 1000 year (0.1%) flood envelope.

5.14 Since there is often a marked discontinuity in ground slope between the floodplain and the land on either side, it is quite likely that in many places the 1000-year flood envelope is not significantly wider than the 100 year flood envelope. The respective depths of flooding over the flood plain could, of course, be significantly different.

5.15 The EA's Indicative Floodplain maps of Corby have been studied in detail looking for any anomalies in the six types listed in paragraph 5.2 above, in conjunction with the data sources described in paragraphs 5.4 to 5.10. Any obvious anomalies identified have been corrected and the 1 in 100 year flood envelope amended accordingly. The methodology outlined in para.5.13 has then been used to add an estimate of the 1 in 1000 year flood envelope to the Indicative Floodplain maps. This process has resulted in the production of the set of six 1/10,000 scale Strategic Flood Risk maps of Corby included as Figure 6 in this report.

Topographic Divisions

Lowland Areas

5.16 The river valley of the Welland is the only lowland area identified within Corby Borough. Lowland areas are expanses of relatively flat land which, unlike fenland, has sufficient slope to drain by gravity but are characterised by broad floodplains and wide, slow flowing, often embanked rivers. Only a small part of the Borough lies within the Welland catchment, and an even smaller and entirely rural area on the northern margin of the Borough is situated within the Welland floodplain.

Upland Areas

5.17 The majority of Corby Borough falls within the upland areas which are associated with the numerous tributary valleys of the River Nene. The fact that many of these valleys are now heavily urbanised does not affect their classification as upland areas.

5.18 The floodplains in these upland valleys follow the course of the streams but are also narrow, rarely extending for more than much more than 50m on either side of the watercourse. Although the coverage within the Borough of the floodplain envelope shown on the EA’s Indicative Floodplains maps is somewhat limited (probably due to the reasons given in paragraph 2.13) the floodplain that is marked has been transposed onto the six flood risk maps presented as Figure 6. At the scale of 1/25,000 generally used for Strategic Flood Risk maps any differences between the floodplain defined by the Agency and that defined in this study would be too small to appear, but at the 1/10,000 scale used for the Corby Strategic Flood Risk maps the difference can reasonably be represented.

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5.19 The 1 in 1000 year flood envelope in upland areas has been assumed to extend across the whole of the floodplain, but for reasons given in paragraph 5.13 the difference between the 1 in 100 year and 1 in 1000 year flood envelopes in these areas will be fairly small. In upland areas the 1in1000 year (Flood Risk Category 2) envelope is therefore only shown in Figure 6 where it extends a significant distance beyond the 1 in 100 year (Category 3) envelope.

Anthropogenic Influences

5.20 These are large man-made structures in river catchments and along the course of the rivers and streams themselves which could potentially have a significant impact on flood risk by virtue of the manner in which they are liable to increase or decrease flood levels during a given rainfall event. These structures include impounding reservoirs, canals, flood alleviation works and flood storage reservoirs. Each of these categories, where relevant to Corby, is discussed separately below.

Impounding Reservoirs

5.21 The industrial water supply for Corby is taken from Eyebrook Reservoir. (We understand form the Borough Council that the public supply now comes from Anglian Water’s Rutland Water / Grafham / Pitsford system.) Eyebrook is a medium sized impounding reservoir situated on the Eye Brook, a north bank tributary of the Welland. The Eye Brook catchment lies wholly in Leicestershire and Rutland but its confluence with the Welland is 1km east of Caldecott, on the Corby Borough boundary.

5.22 An impounding reservoir may have a number of different and opposing impacts on flood risk downstream. If the reservoir is partially empty when the flood occurs, it will act as a flood storage reservoir for water entering the reservoir from its own catchment until it is full and overflowing. Even when overflowing, it continues to act in this manner. If, for example, there is a depth of water of one metre over the reservoir spillway, there will be a corresponding one metre depth of water held temporarily in the reservoir above spillway level. The volume of this stored water will equal the depth over the spillway times the surface area of the reservoir - a substantial volume.

5.23 On the other hand, whereas only a proportion of the rain falling on agricultural land will appear as runoff, even in a major flood event, virtually all the rain falling on the surface of an overflowing reservoir during a storm will soon flow over the reservoir spillway as runoff. A large impounding reservoir will therefore, when full, increase the amount of runoff from its catchment for a given rainfall event. This has the opposite effect of the phenomenon described in paragraph 5.22 above. The net effect - whether the increased volume of runoff is counterbalanced by the reduction of the peak runoff due to the temporary storage of flood water in the reservoir - will depend upon the surface area of the reservoir and the geometry of the spillway.

5.24 In the absence of any conclusive evidence as to whether the effect on the River Welland of flood discharges from Eyebrook Reservoir is negative or beneficial, the net effect must be assumed neutral. In any case, whatever the effect it would only be felt along the Welland downstream of Caldecott and its impact within the Borough 4km length of floodplain along the south bank of the river between Caldecott and Thorpe by Water. This section of floodplain is entirely agricultural in nature and is not scheduled for development. The effect of Eyebrook Reservoir on flood risk in Corby can therefore be disregarded.

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Flood Alleviation Schemes

5.25 As mentioned in paragraph 4.25, Corby Borough has been largely unscathed by the effects of severe flooding and therefore the only type of flood defence works provided in Corby have been a number of relatively small flood storage reservoirs. The lack of any threat from a major river has meant that concrete floodwalls and earth floodbanks have not been regarded as necessary.

5.26 In recent years a number of flood storage reservoirs, lagoons and runoff retention ponds have been constructed within the Borough. In some cases the role of these reservoirs has been strategic in scale, whereas others have been site-specific. The flood storage reservoirs listed and discussed in Section 4 can therefore be categorised as either being a single strategically planned installation or minor runoff retention works built as an integral part of a specific development. Although the information obtained on the reservoirs has been rather limited, they have been categorised as being either strategically planned or site specific installations, as follows :-

Strategically Planned : Gretton Brook Flood Storage Reservoir Pen Green Lane Balancing Pond Pen Green Flood Storage Reservoir Phoenix Parkway Flood Storage Reservoir Hazel Leys, Studfall Ave & The Jamb Weldon Lagoon Snatchill Flood Storage Reservoir Weldon Reservoir Great Oakley Flood Storage Reservoir

Site Specific : Car Yard Reservoir Stanier Road Flood Storage Reservoir Crucible Road Flood Storage Reservoir Eurohub Balancing Pond Soot Banks Flood Storage Reservoir Quarry Road Flood Storage Reservoir CTC Pond & Oakley Vale Ponds Longcroft Road Balancing Pond

5.27 The function of the minor installations has been to attenuate the peak rates of additional runoff generated by the impermeable areas created by the development of "greenfield" sites or derelict land such as abandoned quarries. This is intended to limit the runoff from the development area to the receiving watercourse to what it would have been prior to the development in order to minimise any increase in downstream flood risk along the receiving watercourse.

5.28 In an ideal situation, strategic reservoirs are constructed in advance of widespread development in order to ensure that the potentially adverse effects of the additional runoff expected from the developed area are mitigated before they arise. This type of strategic reservoir can obviate the need for a multiplicity of site-specific flood storage in the development area. On the other hand, some strategic reservoirs are constructed only after development has taken place, quite often as the result of a flood event which has occurred. Hence these reservoirs are designed more to resolve a current problem caused by large scale development whereas the site-specific reservoirs are created as part of the development to mitigate any effects before they occur.

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5.29 The provision of runoff retention reservoirs in connection with "greenfield" development is a fairly recent phenomenon and their efficiency, either as individual installations or collectively at a strategic level, has yet to be fully evaluated. Much will depend on whether they have been constructed correctly to the design criteria (spillway levels, pipe diameters etc) and how well they are maintained. Whilst the incorporation of runoff attenuation facilities into "greenfield" developments of any significant size is clearly essential if downstream flood risk is to be controlled, it cannot be assumed that the collective effect of large numbers of small individual installations in a catchment will achieve the same end result as a single strategically planned installation.

5.30 Flood storage reservoirs are of two distinct types, on-line and off-line. Under normal conditions both types of reservoir should remain effectively empty in order to maximise the volume of flood storage available in the reservoir. The on-line reservoir is created by the damming of a watercourse. A culvert through the base of the dam will have a limited capacity, less than the anticipated flood flow in the stream. If the flow in the stream exceeds the capacity of the culvert, water will be impounded in the reservoir and the level of water in the reservoir rise until the inflow falls below the culvert's capacity. In an extreme event, larger than that for which the reservoir was designed, the reservoir will fill and excess water will flow over an emergency spillway and back into the watercourse downstream of the reservoir.

5.31 An off-line reservoir is constructed alongside a watercourse. When the flow in that watercourse reaches a critical level it overflows into the reservoir by means of a spillway weir or sluice set into the bank of the watercourse. The reservoir is connected to the watercourse at its downstream end by a culvert fitted with a non-return valve which permits the reservoir to empty when the flood has passed but prevents premature backflow to the reservoir from the watercourse. It is of critical importance to ensure that the reservoir does not start to fill too early so that flood storage is no longer available at the peak of the flood.

5.32 On-line amenity lakes such as the Boating Lake in Hazel Wood are normally kept full of water and thus have a relatively small storage volume available for the retention of storm runoff. They are therefore of limited effectiveness for peak flow attenuation. A large on- line lake will, nevertheless, provide a limited amount of runoff attenuation - see para.5.22.

5.33 Only limited information was found on the flood storage reservoirs in Corby and therefore for many it is not known for what storm event probability they were designed. It was only the Eurohub Pond, Sootbanks Reservoir, and the CTC & Oakley Vale Ponds which were found to be designed to a standard of at least a 1 in 100 year event. Due to the limited coverage of the ISIS catchment model (see below) within the Borough, with lumped inputs from upstream sub-catchments within which these reservoirs are included, it is not possible to take them into account within the scope of this study.

5.34 This is due to the limited information on the exact extent of the areas these reservoirs serve in conjunction with the uncertainties involved with the standard of protection they provide. It should be noted that all reservoirs, no matter what period storm event they were designed for, will offer some degree of alleviation during an event with a return period of 100 years. Whilst they may be overtopped at the height of a storm greater than that for which they were designed (i.e. outflow from the reservoir will approach or equal inflow, whether it be through the outlet structure or over the spillway), the reservoir will still provide buffer storage and attenuation of flood flows during the early stages of the storm and even, to some extent, during its peak.

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5.35 However to take these effects of flood alleviation into account would require the extension of Halcrow’s ISIS model (see below) beyond its present upstream limits. This would involve considerable surveying of watercourses, culverts and hydraulic structures within the Borough for the data input to allow the construction of such a model. This is outside the scope of this study and would be a major undertaking at considerable cost. However, if in the future large areas of development look as if they are likely to proceed, then hydraulic modelling of those watercourses and reservoirs directly affected may be required in order to gain a more precise assessment of the actual flood risk within the Borough. The Borough Council understand that the Environment Agency have recently submitted an application for funding for this project.)

Flood Alleviation Works Upstream of Corby

5.36 As Corby is near the head of the Willow Brook and Harper's Brook catchments, there are no flood alleviation schemes upstream of the town. The diversion of the headwaters of the South Arm of Willow Brook into the headwaters of Harpers Brook along the east side of the A6014 road in the 1970s is not, strictly speaking, regarded as a flood alleviation scheme as it merely transfers flood flows from one watercourse to another without diminishing those flows.

The River Nene Model

5.37 The consulting engineers, Halcrow, were engaged by the Environment Agency in 2002 to produce a large catchment-scale computer model of the Nene from its tidal limit at Dog-in- a Doublet Sluice below Peterborough, to the river's headwaters above Northampton. The model which combined distributed hydrological inputs based on the Centre for Hydrology & Ecology's (formerly the Institute of Hydrology) Flood Estimation Handbook (FEH) procedures combined with hydraulic modelling of the river channel and its principal tributaries based on HR Wallingford's ISIS modelling techniques.

5.38 The FEH and ISIS procedures used in the Nene Model are both currently regarded as "state of the art" techniques. Both have now been thoroughly tested in a wide range of applications, separately and in combination, and found to be accurate and reliable. Thus the model results may be accepted with confidence. (Recent research, as yet uncorroborated, (Ref.20) has suggested that flood flows generated using FEH may be over-estimates but, if so, this would make the Nene Model results conservative.)

5.39 The River Nene Model was completed, commissioned, tested and handed over to the EA in September 2003 and installed on an Agency computer system to be operated by Agency staff. The size and complexity of the model necessitated the dedicated use of a powerful computer system and specialist staff by the Agency to run the model to simulate the flow of flood water through the catchment under flood events of pre-determined return periods.

5.40 The River Nene ISIS model included the Willow Brook and Harpers Brook tributaries and the modelled lengths of both tributaries extended into Corby Borough. The modelled length of the Willow Brook commenced just downstream of the outlet from the Weldon Washland and the Harpers Brook from outlet from the Great Oakley Washland. It was hoped that the existing ISIS model could be used to assess the effect on flood flows in these two watercourses resulting from future large scale urban development within and around the Borough as proposed by Catalyst Corby.

5.41 The Willow Brook and Harpers Brook components of the Nene model were provided for this purpose by the EA in electronic format to enable Bullen Consultants to run them on their own computer system, thus enabling the 100-year and 1000-year flood events, both and without climate change effects, to be simulated under both present and future development conditions.

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5.42 The River Nene model was intended primarily as a strategic model of the whole catchment and the River Nene itself. A considerable reliance on "lumped" inputs was therefore to be expected, especially in the tributaries. Close examination of the Willow and Harpers Brook components of the model revealed that the sub-catchments defined by Halcrow for the headwaters of the two Brooks (i.e. within the Corby urban area) were not entirely accurate representations of the shape, size or extent of urbanisation of the actual sub-catchments. The location and boundaries of the relevant sub-catchments and the present extent of urbanisation in each are shown in Figure 7. A standard Average Annual Rainfall (AAR) figure had also been used for the whole of each Brook and this did not reflect variations in AAR across the catchment, and notably the higher AAR values in the headwater sub- catchments.

5.43 More precise values of these ISIS model input parameters were evaluated for the Corby sub-catchments and details of the modified parameters used in this study, together with the corresponding Halcrow figures for comparison, are presented in Appendix 4. The inaccuracies in Halcrow parameter values can be explained by the fact that these two Nene tributaries are relatively insignificant in the context of the overall Nene model and that project's resources would have been more focused on other areas. However for the purposes of this current study, a higher degree of accuracy of the inputs into this section of the model is required.

5.44 The Willow Brook / Harpers Brook models were then re-run for the 100-year and 1000- year events with the input parameters modified to reflect the possible future extent of urbanisation in each sub-catchment in order to determine the potential effect of that development on flows in the two Brooks and flood risk downstream. The proposed development areas in relation to the sub-catchments are shown in Figure 8. It should be noted that for these modelling runs, the model is over conservative in assuming no effective on-site attenuation of the increased urban runoff. In order to investigate the potential effect of future climate change, the model was run again for both the existing and proposed development scenarios with the 100-year and 1000-year flood flows increased by 20% in order to investigate the impact of climate change on flood levels for an event of a given return period.

5.45 The results of the Willow & Harpers Brook modelling are summarised in Appendix 4. They include the water levels at each of the surveyed channel cross sections used in the model within the Borough. A location plan of these cross sections is included as Figure 9. These water levels not only aid in the production of the flood risk maps but they also allow for a comparison between the existing development and proposed development scenarios. It can be seen that for a 100-year event the difference in water levels between the existing and proposed development scenarios is generally 100mm, whilst for the 1000-year event the difference is generally an increase of 100mm for Willow Brook and 200mm for Harpers Brook.

5.46 Also given in Appendix 4 are the maximum outflows from each sub-catchment as estimated by the ISIS model, based on the hydrological and catchment characteristic inputs from FEH in conjunction with the estimates of both the existing and proposed extent of the urban areas. The results for Harpers Brook show an increase in the outflows from each of the sub-catchments which could be expected as a result of the various scenario changes (i.e. the addition of the proposed development and the effects of climate change). However the same cannot be said for the Willow Brook sub-catchments. Even though the existing and the proposed extent of urbanisation is higher for Willow Brook than it is for Harpers Brook, the outflows from the sub-catchments are much lower. For the Willow Brook catchments the increase in outflow is also significantly lower than what would be expected when going from the existing to the proposed urban extent scenarios.

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5.47 This would tend to suggest that the results for Willow Brook (and possibly also for Harpers Brook) should be taken with caution. Some time was spent trying to determine possible reasons why the model would produce such results, but no plausible explanation was found. It was felt that it wasn’t within the scope of this study to start pulling the model apart to the extent that would seem to be required. It should however be noted that even though the increase in flows was a lot lower for Willow Brook than for Harpers Brook, the rise in the corresponding waters levels were still quite similar, i.e. 10 cm for the 100-year event and 20 cm for the 1000-year event.

Flood Risk Mapping

5.48 The flood risk mapping of Corby Borough and the preparation of the set of six Strategic Flood Risk maps (Figure 6) has been based principally upon topographical information and the use of engineering judgement in conjunction with the results of the Willow Brook / Harpers Brook ISIS model runs carried out specifically for the purposes of this study as described in paragraphs 5.41 to 5.47 above. The mapping shows the flood risk under present conditions and extent of urban development. The flood risk mapping has been extended outside the Borough Boundary in the Pipewell, Little Oakley and Kirby Hall areas to include all of the potential development areas defined by Catalyst Corby.

5.49 The existence of the various flood storage reservoirs and other flood flow attenuation measures will all have an impact, as intended, on flood flows and hence flood risk. However, the Category 3 / Category 2 boundary corresponds to the 100-year flood envelope. Even though the design parameters of most of the FSRs etc is not known, it is fairly certain that only a minority of them will have been designed to a 100-year defence standard. The great majority, especially the smaller ones, will have been designed to a significantly lower standard. Although some degree of flood flow attenuation is achieved with storms lmore severe than that for which the reservoir was designed, this limited attenuation diminishes with increasing storm severity to the point where in a major event the reservoir is physically overwhelmed by the flood water. This situation occurs before the peak of the flood and the flood peak will pass virtually without any attenuation at all.

5.50 For the reasons given above, the extent of the 100-year flood envelope has been determined on the conservative assumption that the effect of the various FSRs and other flood flow attenuation installations can be disregarded for floods of this magnitude or greater. It should nevertheless be pointed out that the attenuation effect in less severe flood events (say 10, 20 or even 50-year events) may well be very significant.

5.51 Where small tributary streams are involved, such as the headwaters of the Willow and Harpers Brooks, the flood risk assessment is made on the assumption that the full width of the floodplain will be affected by all floods in excess of the 100-year return period. In reality, the difference in width between the 100-year and 1000-year flood envelopes on these narrow floodplains is too small to be represented on strategic-scale maps, even though the corresponding difference in flood depths may be significant.

5.52 During a flood the water level in a river will rise above the ground level in the defended area. The surface water sewer outfalls which discharge through the flood defence line will, of course, be fitted with a non-return flap valve to prevent flood water entering the defended area from the river through the sewer. However, if there is heavy rainfall over the defended area while the river is in flood, all surface water runoff from the defended area will be impounded behind the flood defences until such time as the river level falls and gravity discharge can recommence. This phenomenon is known as "floodlock" and can give rise to secondary flooding within the defended area, even though the defences may not have been overtopped or breached. Since there are no defended areas of this nature within Corby, the problem of "floodlock" does not generally arise, although it is understood to occur on a small scale in Old Corby village.

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5.53 There is no indication on the large scale Anglian Water sewer plans that there is any physical connection between two small streams on the western side of Uppingham Road (A6003) and the culverted headwaters of Willow Brook (North & Central Arms) within the built-up area of the town (see A-A, B-B and C-C on the Corby Strategic Flood Risk Map, Sheets 3 & 5). A connection can, however, be inferred from circumstantial evidence on the sewer plans, and this has been confirmed by the Borough Council. A similar situation exists in the headwaters of Gretton Brook, where the existence of a culverted section of the Brook under Gretton Brook Road can also be inferred.

5.54 Lack of suitable LIDAR or other topographical data within the heavily built up residential areas west of Corby town centre had prevented the quantitative assessment of flood risk category envelopes in those areas in the draft Strategic Flood Risk Maps submitted with the Draft Report in September 2004.

5.55 In August 2005 an extension of the Study was agreed between Corby Borough Council, the Environment Agency and Faber Maunsell in which the flood risk envelopes shown on the Strategic Flood Risk Maps would be extended to the western edge of the built up area, reflecting the upstream extension of Main River along the Willow Brook through this area in 2005. These revised maps are incorporated in this Report.

5.56 The upstream extension of the flood risk envelopes were carried out using the best available qualitative methodologies and with reference to the Agency’s Flood Risk Maps, published in Autumn 2004. Detailed physical inspections and on-site assessments were carried out along the lines of all the urban watercourses involved. Although the first of the two reports mentioned in paragraph 1.14 which were be used to inform the extension of the Strategic Flood Risk Assessment, the Corby Water Cycle Strategy Study (Ref.21), became available in draft form in September 2005 the more relevant second report, the Nene Tributaries Pre-Feasibility Studies (Willow Brook Pre-Feasibility) Report (Ref.22), did not become fully available until the latter half of February 2006.

5.57 The February 2006 report gave the results of hydraulic modelling undertaken on the upper reaches of the North, South and Central Arms of Willow Brook. This modelling extended further upstream than the earlier hydraulic modelling and the new upstream limits of the modelling are now as follows: Willow Brook (North Arm) Rockingham Road (N end of Studfall Avenue) Willow Brook (Central Arm) a) 60m upstream of Corby – Oakham railway line. b) 100m upstream of Water Lane, Weldon. Willow Brook (South Arm) Footbridge S of St Mark’s Road.

5.58 The availability of these additional modelling results, which included both 100-year and 100-year plus climate change flood level estimates, enabled the Strategic Flood Risk Maps to be further revised to incorporate these results. However, as the newly-modelled reaches of these streams were largely within urban areas it was found that the significant anomalies in the LIDAR coverage of these urban areas created considerable difficulties in reconciling flood levels with local topography in many places. The errors of estimate in the flood envelopes thus derived were found to exceed the relatively minor differences between the 100-year, 100-year plus climate change and the 1000-year flood envelopes and the deficiencies in the data prevented any attempt to produce these additional flood envelopes.

5.59 There are a number of other locations on Sheet 3 where there appear to be isolated and discontinuous lengths of surface water sewers and open watercourses within the town. Since it is highly unlikely that there are discontinuous lengths of sewer or watercourse, these anomalies may be because either :-

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a) the information on the Anglian Water sewer plan is incomplete, or b) the missing links exist, but are not regarded by Anglian Water as their responsibility or part of their surface water sewer system. In the latter case, the missing links could be the responsibility of the Borough Council or of a private owner. The Council, however, consider that these missing links are Anglian Water’s responsibility and it is therefore recommended that steps are taken by the Council to persuade Anglian Water to locate and identify these missing links in the town's arterial surface water drainage system and to confirm their ownership.

Rapid Inundation Hazard Zones

5.60 In a major flood event where a river is confined within flood defences, there may be an appreciable difference between the water level on one side of the flood defence and the ground level in the defended area behind that defence. If that defence were then to fail, whether through the collapse of a floodwall or the breaching of an embankment, there would be a sudden inrush of flood water into the defended area. The velocity and depth of water cascading through the breach could, initially at least, be sufficiently great to sweep a person off their feet resulting in their death by injury or drowning. The area within which this could occur is known as the Rapid Inundation Hazard Zone.

5.61 The premature failure of a flood defence structure is by its nature a residual risk, but its potentially fatal consequences dictate that it be given serious consideration in flood risk assessment. As flood water pours through a breach it will fan out, and its velocity and depth will decrease with distance from the breach. At some distance from the breach the velocity and depth of water will have diminished to a point where an adult is capable of standing upright in the flow. This is deemed to be the outer edge of the rapid inundation hazard zone. The distance of this point from the defence line, and hence the width of the hazard zone, will be determined by the flood level / ground level difference (head of water) and the width of the breach.

5.62 The absence of any raised flood defences such as floodbanks or floodwalls in Corby means that the identification and determination of conventional hazard zones need not be considered. It should, however, be borne in mind that during a flood event, when flood storage reservoirs are full to capacity, there is a residual risk of collapse of a reservoir embankment, releasing the flood water stored in the reservoir.

5.63 In such an event a surge of water would flow down the already flooded stream valley, resulting in a relatively rapid increase in flood levels along the valley. This would not necessarily present a "life and limb" hazard downstream. The magnitude of the hazard and associated risk will depend on the volume of water in the reservoir and height of the breached embankment, as well as the presence and type of other hydraulic structures immediately downstream. These could either act as ad-hoc relief reservoirs, limiting the effect of the initial breach, or could breach themselves in a "domino" effect, thereby magnifying the impact of the initial breach. With this type of event the actual envelope of the hazard zone will be impossible to determine with any precision and will be unique to the particular circumstances prevailing at any given time or flood event.

5.64 The safety of all raised reservoirs is governed by the Reservoirs Act 1975, which lays down strict rules for their inspection and maintenance. The size of reservoirs covered by the Act is subject to a de-minimis capacity of 25 million litres (25,000 cu.m), below which it is considered that a breach would not give rise to an unacceptable risk downstream. The largest FSRs will be subject to the provisions of the Act. Given the absence of information on so many of the FSRs in Corby, the Borough Council should, however, ascertain whether there any reservoirs in the Borough which should be subject to the provisions of the Reservoirs Act but which are not currently registered, inspected and maintained accordingly.

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Faber Maunsell Corby Strategic Flood Risk Assessment 45 6 Planning Policy and Flood Risk

6.1 Several documents have been consulted from which to establish the relevant planning policies from a strategic level down to a local level. This has been undertaken in order to gain an understanding of the areas within the Borough which have been designated for potential growth, in conjunction with the issues identified in the plans concerning the association of flood risk with development. These documents are:

Milton Keynes & South Midlands Sub-Regional Strategy

Northamptonshire County Structure Plan

Corby Borough Local Plan

Catalyst Corby Regeneration Framework

Milton Keynes South Midlands Strategy

6.2 In March 2001, the Milton Keynes & South Midlands (MKSM) area was identified by the Government as one of four potential major growth areas in South East England and a sub- regional study of the potential urban development in the MKSM area was commissioned in July 2001. This study involved the East Midlands, East of England and South East England regional planning bodies. The MKSM area included the whole of Northamptonshire (as well as much of Bedfordshire and Buckinghamshire) and hence involved the Nene and Welland catchments.

6.3 The Final Report of the MKSM Study was published in September 2002 and concluded that the area has considerable potential for economic growth, and therefore urban development, over the next thirty years. A Consultation Draft document setting out proposals for a sub-regional strategy for Milton Keynes & South Midlands was subsequently published in July 2003 (Ref.23). Following consultation, a number of amendments to the MKSM sub-regional strategy were proposed in October 2004 (Refs.23a and 23b). In March 2005 the Regional Spatial Strategy for the East Midlands (RSS8) was published (Ref.23c), incorporating parts of the Milton Keynes & South Midlands Sub-Regional Strategy that refer to Northamptonshire.

6.4 The Part A Statement in RSS8 asserts that "Corby / Kettering / Wellingborough will grow in a complementary way while retaining their separate identities. At Corby the emphasis will be on the regeneration of the town centre. At Kettering and Wellingborough the emphasis will be on managing growth and job creation in a sustainable way that realises their potential.” Strategic Policy 1 states that the majority of the development in the MKSM Sub- Region will be focused in certain designated growth towns. One of these is Corby / Kettering / Wellingborough where an additional 34,100 dwellings will be required by the year 2021.

6.5 The Part B Statement for Northamptonshire in RSS8 (Policy 1 – The Spatial Framework) includes the following allocation (Table 7) for housing provision in Corby in the wider Northamptonshire context. The numbers given are the total allocations in the 10-year periods shown in each of the successive reports (Refs.23, 23a, 23b & 23c) issued in 2003, 2004 and 2005 respectively. Local authorities are listed from upstream to downstream in the Nene catchment.

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Local Authority 2001/11 2011/21 Total

Daventry District 2005 5,400 5,400 10,800 2004 5,400 5,400 10,800 2003 5,400 3,900 9,300 South Northants District 2005 3,300 3,300 6,600 2004 3,300 3,300 6,600 2003 4,800 3,300 8,100 Northampton Borough 2005 12,500 17,500 30,000 2004 12,500 17,500 30,000 2003 12,550 13,850 26,400 Wellingborough Borough 2005 5,950 6,850 12,800 2004 5,950 6,850 12,800 2003 5,400 9,600 15,000 Kettering Borough 2005 6,800 6,300 13,100 2004 6,800 6,300 13,100 2003 8,120 7,080 15,200 Corby Borough 2005 6,200 10,600 16,800 2004 6,200 10,600 16,800 2003 6,200 9,800 16,000 East Northants District 2005 5,200 4,200 9,400 2004 5,200 4,200 9,400 2003 3,600 3,600 7,200 TOTALS 2005 45,350 54,150 99,500 2004 45,350 54,150 99,500 2003 46,070 51,130 97,200

Table 7 - Future Housing Allocation in Northamptonshire

Notes :- 1) Developments in East Northamptonshire District are all downstream of Corby and thus are not relevant to this study, but have been included in Table 7 for completeness. 2) Not all the allocated development in South Northamptonshire will take place in the River Nene catchment. 3) The 2004 and 2005 figures for Northampton and Corby include provision made in urban extensions across neighbouring local authority boundaries, and are additional to the figures given for the ‘receiving authorities’.

6.6 Northamptonshire Policies 2 and 3 detail the proposed development in the Northampton principal urban area and propose the rates of housing development for the Northampton implementation area respectively and are therefore not relevant to this study.

6.7 Northamptonshire Policy 4 concerns the development of the Corby, Kettering and Wellingborough principal urban areas. In all three towns the capacity for additional development within the existing built-up area is limited and therefore the majority of the additional development must occur through sustainable urban extensions. The following areas of Corby are proposed for development of approximately 8,000 to 10,500 dwellings:

Faber Maunsell Corby Strategic Flood Risk Assessment 47

North East of Corby

North West of Corby

South of Corby

South West of Corby

West of Corby

6.8 The strategy proposed that there should be an increase in employment of 43,800 jobs in North Northamptonshire (i.e. Corby, Kettering, Wellingborough and East Northamptonshire) in the period to 2021. The priority for Corby will be the renewal and redevelopment of the town centre which will include high quality retail, housing and employment development. This will also include a new railway station.

6.9 The strategy also proposed that the following social infrastructure should be developed to meet the needs of all three existing towns (i.e. Corby, Kettering and Wellingborough) and the new sustainable urban extensions :- Education expand and develop educational provision from primary to higher education in order to improve educational attainment and skill levels across Northamptonshire. Healthcare substantial investment in expanding and modernising primary community health and social care, and secondary healthcare services and facilities.

6.10 There is a general statement at the end of the Part A Statement in RSS8 headed “Utilities” (Section 2, paras.53 to 55) which states that “the growth areas will require a strategic approach and investment programme for waste water and surface water drainage management which takes a coordinated approach to land drainage, nature conservation, landscape management and open space provision so that catchment flood risk is not increased and water quality does not deteriorate as a result of the cumulative impacts of development.”

6.11 In implementing the strategy it is probable that the Local Planning Authorities concerned will as a matter of course ensure that these major urban developments incorporate sustainable urban drainage systems (SUDS) and that appropriate physical features, such as adequate runoff retention storage and flow retarders, are included in the design of these systems. These features are necessary in order to limit the surface water runoff from the newly impermeable areas created by the urban developments to the rates and volumes of runoff which would have been generated by those "greenfield" areas prior to urbanisation.

6.12 Although SUDS are now routinely included in the design of new urban drainage systems in Britain, they have not been in use for long enough or widely enough to ascertain how effective they are at the catchment scale in controlling runoff from urban areas. Even if SUDS were to become a mandatory feature of all urban development, they are designed to attenuate runoff from storm events of a specific return period, or less. The attenuation of runoff from storms greater than that for which the drainage system was designed will only be partial. It is therefore inevitable that, even with the universal use of SUDS, future urban development will result in greater runoff volumes and higher flood peaks than was hitherto the case.

6.13 This phenomenon, combined with the potential increase in storm runoff and flood flows resulting from climate change, will inevitably have an adverse effect on flood risk in river catchments subject to large scale urbanisation, particularly where that urbanisation takes place in the headwaters of the catchment. This situation is likely to arise as a result of MKSM-related development in the catchments of the tributaries of the River Nene as a consequence of which there will almost certainly be an increase of flood risk downstream.

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6.14 Corby Borough Council should therefore take an active and concerned interest in all MKSM-related development proposals in the tributaries of the River Nene in its area, particularly the Willow, Harpers and Gretton Brooks. For the same reason, the Environment Agency should closely monitor the hydrological response of these major tributaries to identify any significant changes in the shape and magnitude of the unit hydrographs of these watercourses resulting from urbanisation, though it is appreciated that these changes may take a considerable time to manifest themselves conclusively. As soon as any such changes and their associated trends are positively identified by the Agency, their implications for future flood risk both in Corby and downstream should be evaluated and, if necessary, appropriate measures taken to maintain the present flood defence standards in Corby and downstream areas.

Northamptonshire County Structure Plan

6.15 The Northamptonshire Structure Plan published in March 2001 sets out the land-use strategy in the county for the period up until the year 2016 and beyond. Its main role is :-

To provide a framework of strategic policies and proposals for local planning and development control decisions.

To ensure that the provision for development at the local level is realistic and consistent with national and regional policies.

To secure consistency between local plans for neighbouring districts.

6.16 This Structure Plan, in conjunction with the Northamptonshire Minerals Local Plan, the Northamptonshire Waste Local Plan, and the district-wide plans prepared by each of the seven Borough and District Councils in the county, all combine to form the Development Plan for Northamptonshire. Within the Structure Plan, Corby has been designated as a Special Policy Area with the objectives of the strategic policy framework to :-

Build a vibrant and balanced community that is economically, socially and environmentally sustainable.

Enhance the role of Corby in a regional and sub-regional context.

6.17 Despite the loss of the town's steel-making industry during the early 1980s, Corby has since experienced significant economic growth and job creation. However this growth has been concentrated on new industry and employment near the fringes of town and has not been balanced by population growth as many of those who work in the town's industries commute from neighbouring areas. Unemployment levels in Corby itself are still higher than the County average, as a number of residents have not been able to share in the town’s employment success. The challenge is to make Corby a more sustainable location that no longer has these imbalances. Policy C01 identifies Corby as a Special Policy Area due to its need for regeneration.

6.18 Provision is made in the Structure Plan for 7,000 houses and 190ha of industrial and commercial development within the existing urban area of Corby. This reflects the identified need for net inward migration to the County. This would involve the following main points :-

Regeneration of Corby town centre to support its role as the primary focus for shopping, leisure, culture, entertainment and an increase in the amount of housing. This would include a multi-modal transport interchange.

Encouragement of mixed-use developments.

The utilisation of "brownfield" land, contaminated land, existing buildings and infrastructure. Priority will be given to the regeneration of existing urban areas where major economic, social and environmental problems exist, in order to achieve a balanced and sustained growth for Corby.

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6.19 As part of the County Structure Plan’s environmental assets and natural resources objectives there would be an overall reduction in flood risk. Flooding on river floodplains is a natural process but, unless carefully located and designed, urban development can be at risk of flooding as well as increasing the flood risk in downstream areas. Policy AR8 states that development will not be permitted in areas at direct risk from flooding or where it is likely to increase flood risk to any extent elsewhere unless adequate measures are taken to mitigate these effects. Local Planning Authorities will also seek to negotiate with developers wherever possible to achieve developments which provide for an overall reduction in existing levels of flood risk. All development proposals must incorporate Sustainable Urban Drainage Systems and provision for their future maintenance. Flood issues should not, however, be confined to just the floodplains as development throughout a catchment can have an impact on flooding by increasing runoff.

6.20 The Northamptonshire County Structure Plan will eventually be replaced by the emerging North Northamptonshire Local Development Framework which is being jointly produced by Corby, Kettering, Wellingborough and East Northamptonshire, together with Northamptonshire County Council.

Corby Local Plan

6.21 The Corby Borough Local Plan, adopted on 10 th June 1997, is an important document in determining the extent of any proposed development areas in the Borough. As stated in the Plan, the overall development for the Borough is set by the Northamptonshire County Structure Plan which was originally approved in 1989. Alteration No. 1, which deals with housing and employment provision for the period 1988 to 2006, received the Secretary of State’s approval in 1992. The same period of 1988 to 2006 is also adopted for the Local Plan which has set the following targets for Corby:

250 hectares of land for employment.

Provision for 6,000 new houses. The Local Plan is currently under review and will eventually be replaced by the emerging Local Development Framework which will contain provision for the 16,800 new dwellings set out in RSS8 and 14,000 new jobs.

Employment Land

6.22 During the early 1980s the ending of steel making in Corby and the partial closure of the steelworks resulted in a loss of 7,500 jobs over a two year period and took the number of jobs within the town to below 16,000. This figure had however risen to over 23,000 by 1993 which could be partly attributed to the development of 180ha of land since 1982 for light industrial use. Based on projected employment densities, the proposed development of a further 250ha for employment would create 9,000 jobs by 2006. The sites allocated for employment uses that have planning permission are listed in Table 1 of the Corby Local Plan. These sites total 252ha and where possible have been assigned to reclaimed land which is often not of high agricultural or amenity value. The four largest areas make up over half of the total proposed employment areas. These are as indicated in Table 8 on the next page.

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Local Plan Location Area Type Reference J 23 N of Birchington 23.1 ha Business / Industry / Distribution

J 20 Barn Close 28.0 ha Rail-served development Stanion Lane / J 35 25.0 ha Rail-served development Soot Banks J 37 Priors Hall 51.6 ha Business / Industry / Distribution

Table 8 - Areas Proposed for Industry in the Corby Local Plan

Housing

6.23 The increased employment opportunities resulting from the business and industrial development of 250ha would result in an increase in population, both from natural population growth and inward migration. The council predicts that the Borough’s population will rise to 63,000 (or 25,400 households) by 2006. The County Structure Plan provides for an additional 5,500 houses in Corby with 500 in the local villages. Taking into account those either already built or under construction, land for a further 3,165 houses is required. The following sites have been designated in the Local Plan for these houses:

Snatchill – 2,550 houses

West of Stanion – 560 houses

Corby (various small sites of 0.4 to 2.5 ha scattered throughout Corby) – 229 houses

Catalyst Corby (North Northamptonshire Development Company)

6.24 Catalyst Corby was the first of the government-driven specialist urban regeneration companies and was set up to produce a Regeneration Framework for the Borough. Catalyst Corby's proposed Regeneration Framework for the town (Ref.3) “…sets out an exciting strategy to transform Corby into a vibrant and successful place where people will want to live, work, visit and do business”. Residential, employment, town centre and transportation proposals are all interlinked and will provide the Borough with a range of new opportunities, facilities and services. Catalyst Corby recently merged with the North Northants Together Partnership to create a new Urban Regeneration Company known as the North Northants Development Company.

6.25 The implementation of the framework will require a population in excess of 100,000, resulting in an increase of 47,000 people. Based on an average household size of 2.3 persons, this would requires the provision of housing land for 23,500 dwellings (including an allowance for vacancies). The Northamptonshire Structure Plan has a provision for 7,500 dwellings of which a minimum will be concentrated within the urban area by 2016. Existing designated sites will provide over 4,000 dwellings with the two most significant developments being Oakley Vale and West of Stanion. Planning consent for 2,940 houses for Oakley Vale was granted in July 2001 while 560 houses have been allocated for West of Stanion. The results of a recent study have indicated that the urban capacity of under- utilised or brownfield sites for housing development would be 1,400 houses and as part of the regeneration and redevelopment of the town centre, it is expected that space for 800 residential units would be available.

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6.26 Parts of the existing residential areas suffer from poor urban design and layout, and problems of under-investment, vacancy etc are evident. These areas will be restructured such that they are more attractive places to live and would be given an equal priority with the new developments in the regeneration of Corby. However it should be noted that this is not likely to increase the total number of dwellings.

6.27 The Regeneration Framework concludes that there is a net need for a further 17,300 houses to achieve a population of 100,000. These will be provided within new development areas surrounding the town. The greenfield sites which have been designated for housing development are in the following areas, shown in outline in Figure 2.

Northern Extension 700 dwellings.

Western Extension 4,500 dwellings.

Southern Extension 1,400 dwellings.

Prior Hills and Weldon Extension 6,300 dwellings.

Oakley / Stanion Extension 4,500 dwellings.

6.28 In order to achieve the balanced and sustainable growth desired in the Framework, a suitable amount of employment land will be required for the population growth to reach its target of 100,000. A study has shown that approximately 150ha of employment land is available, but based on an employment land uptake rate of 15ha per year over the 25 year Regeneration Framework timescale this would need an additional 225ha which would provide for over 30,000 new jobs.

6.29 Provision is also made as part of the Framework for large scale commercial and industrial development at the following locations:

Oakley Hay (a) Headquarters Business Park

Great Oakley (a) Technology Cluster

Compass Point (b) Town centre mixed use development

St James Industrial Estate (b) Town centre mixed use development

Corby East (b) Mix of high quality business locations, processing parks and logistics uses

Max Park (c) Mix of high quality business locations, processing parks and logistics uses

Weldon South Ind. Estate (a) Mix of high quality business locations, processing parks and logistics uses

Phoenix Parkway Ind. Est. (a) General industrial uses and expansion of retail park

Weldon North Ind Estate (d) Motor Cluster with associated development including high quality business locations

Willow Brook Ind Estate (a) General industrial uses

Earlstrees Industrial Estate (a) General industrial uses

Notes: (a) Corby Borough Local Plan employment sites (b) Regeneration Framework employment sites (c) Local Plan employment site in-filled with Regeneration Framework employment sites. (d) Local Plan employment site with additional Regeneration Framework employment site.

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6.30 The Regeneration Framework also details the transport strategy which includes improvements to the road, rail, walking and cycling networks. The majority of the works would simply involve upgrades to existing infrastructure and therefore have negligible adverse affects on the existing drainage systems. However, the construction of the proposed A43 Corby Link Road would cross the floodplain of Harpers Brook and could have an impact on flood risk upstream of the road to a greater or lesser extent.

6.31 At the time when this study began, the Local Development Framework (LDF) process was not well advanced. The analysis of housing and employment figures have therefore been based on the Regeneration Framework (Ref.3) prepared by Catalyst Corby in 2003. Whilst the LDF process is now further advanced and options for urban extensions to Corby have been refined, for the purposes of completeness the Strategic Flood Risk Assessment covers all development areas originally outlined in the Catalyst Corby Regeneration Framework.

Guidance for Developers

6.32 Included in the Brief for this Study was the requirement to prepare Guidance Notes for Developers. These notes would provide prospective developers of land in the Borough with guidance on the Council's policies on flood risk and the development of land deemed to be at risk of flooding. These notes would also give guidance to developers on the design of surface water drainage systems to minimise the generation of storm runoff from paved and impermeable areas in accordance with best practice, as exemplified by Sustainable Urban Drainage Systems ("SUDS") (Ref.24).

6.33 In preparing these Guidance Notes, care was taken to ensure that they are in full accordance with the Environment Agency's draft flood risk policies issued in October 2003. The full text of the suggested Guidance Notes will be found in Appendix 5 at the end of this Report.

6.34 The Environment Agency has also prepared a Flood Risk Matrix to enable Local Planning Authorities to carry out an initial evaluation of development applications and, according to the location, size and scale of the proposed development, decide on the procedure to be adopted by the planning authority in determining that application. The procedures suggested in the matrix focus on the degree of consultation between the planning authority and the Environment Agency appropriate to the application. Version 8.1 of the Agency's Flood Risk Matrix, together with the accompanying explanatory notes, is included as an appendix (Appendix 6) to this Report.

6.35 In anticipation of the publication of Planning Policy Statement 25 (PPS25) in place of PPG25 later this year (2006) the Environment Agency has suggested that a minimum general design standard of the 100-year return period storm or flood event should be adopted for flood defence and runoff alleviation works undertaken by developers in the Borough. It may, however, be necessary for a higher standard (i.e. greater than 100 years) to be imposed on certain developments where the Borough Council, advised by the Agency, consider that the Flood Risk Vulnerability of the proposed development, or land and property likely to be affected by the development, requires a higher standard. Developers are therefore advised to consider the use and vulnerability of the proposed development in accordance with the latest guidance in order to determine appropriate standards at an early stage in the planning process.

Faber Maunsell Corby Strategic Flood Risk Assessment 53 7 Assessment of Flood Risk in Development Areas

7.1 Any useful assessment of flood risk within the ten potential development areas identified by Catalyst Corby and the Borough Council requires an evaluation of actual flood risk over the whole of the development area. This will enable the Council to apply the sequential test required by PPG 25, both as regards the variation of flood risk within a development area and also for the purposes of ranking development areas in accordance with their respective overall degree of flood risk.

7.2 As the PPG 25 flood risk zones are based on the situation that would obtain in the absence of any flood defences, it follows that within any PPG 25 flood risk zone the actual level of flood risk will depend on the existence of flood defences and the standard of protection provided by those defences. Assuming that its envelope has been correctly drawn, within any PPG 25 Zone 3 (High Risk) there will be areas that are protected to a higher standard than 1% (1 in 100 years). In these cases their actual flood risk classification will be Zone 2 (Medium to Low Risk). In practice, this will only apply to Zone 3 since flood protection to a higher standard than 0.1% (1 in 1,000 years) is unheard of.

7.3 In order to be able to apply the sequential test within the PPG 25 High Risk zone (Zone 3) it will be useful to establish sub-divisions of actual flood risk within Zone 3. This has been done using the indicative fluvial flood defence standards laid down in the Department for the Environment, Food, and Rural Affairs' Flood & Coastal Defence Project Appraisal Guidance (Economic Appraisal) ("FCDPAG 3") (Ref.25). These are summarised in Table 9 below.

Land Return Annual Use Period Probability Typical Development Band (Years) of Failure

A 50 to 200 0.5% to 2% Intensively developed urban areas.

Less intensive urban areas with some B 25 to 100 1% to 4% high-grade agricultural land

Large areas of high-grade agricultural C 5 to 50 2% to 20% land with some properties at risk.

Mixed agricultural land with occasional D 11/ to 10 10% to 80% 4 properties at risk.

Low-grade agricultural land with E < 2 1/ > 40% 2 isolated agricultural properties at risk.

Table 9 - Indicative Standards for Fluvial Flood Defence

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7.4 Based on the indicative standards shown in Table 9 the following flood risk sub-divisions within PPG25 Zone 3 have been derived. As these sub-divisions refer to actual (as opposed to unprotected) flood risk they will be referred to as "categories" in this report to avoid confusion with the PPG 25 flood risk zones. Similarly, upper case letters will be used for the Category 3 sub-divisions to distinguish them from the lower case letters used in Table 1 of PPG 25 to define the appropriate planning response for land in PPG 25 Zone 3.

Range Sub-Division Return Period Type of Development Ann. Prob. (Years) Category 3A less than 20 > 5% Mainly agricultural.

Category 3B 20 to 50 2% to 5% Less intensive urban.

Category 3C 50 to 100 1% to 2% Intensive urban.

Table 10 - High Risk Category – Flood Risk Sub-Divisions

The sub-divisions of Flood Risk Category 3 used in this study are given in Table 10 above. Food Risk Categories 1 (Little or No Risk) and 2 (Low to Medium Risk) are for all practical purposes equivalent (as well as being numerically equivalent) to the PPG 25 Flood Risk Zones 1 and 2.

7.5 Flood risk sources considered in the assessment include all open watercourses (rivers, streams, arterial drains and riparian drains) and, where applicable, principal surface water sewers. Possible flooding from foul sewers is not included in the assessments as this can occur from a variety of causes, often with no direct or quantifiable relationship to extreme rainfall events. Combined sewers (i.e. those conveying both foul sewage and surface water runoff) are also excluded from this study because of a lack of information on the amount of surface water entering combined sewers. In any case, the volume of surface water conveyed by combined sewers is usually small compared with that conveyed by surface water sewers and the fact that there are few of them in Corby means that their influence on flood risk is negligible.

7.6 The degree of actual flood risk throughout each of the ten potential development areas, expressed in terms of actual Flood Risk Categories, has been assessed from a combination of factors, sources of information and engineering judgment. Flood risk is assessed as current flood risk - no allowance can be made for enhanced flood risk within the development area which could arise as a result of inappropriate future development. If the predicted effects of climate change over the next fifty years are considered to be sufficient to transfer a development area to a higher Flood Risk Category this will be stated in the assessment.

7.7 The risk of flooding of a development site is not the only consideration. The potential increased flood risk posed by the urbanisation of a "greenfield" development site to other areas downstream of the development site also has to be evaluated. This risk can arise not only from the additional runoff volumes and higher peak runoff rates generated by newly impermeable areas, but also from the reduction in natural floodplain storage capacity if the development takes place in a floodplain. Some of the ten potential development areas are situated at the upper ends of small catchments with extensive built- up areas downstream. Development in such areas could have a significantly adverse impact on flood risk for existing urban developments further downstream. It is, however, assumed that no development would be permitted that would so restrict the capacity of any watercourse that flooding would be caused or exacerbated upstream of the development.

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7.8 The ten individual flood risk assessments will be presented in a common format, under the following headings: a) General description of the development area b) Hydrology of the development area (including hydraulic structures etc) c) Flood risks within the development area d) Flood risks to downstream areas.

7.9 The flood risk assessments of potential development areas made in Section 8 for strategic planning purposes do not preclude the necessity for site-specific flood risk assessments of individual development sites within the wider development areas. The flood risk assessments of development areas should nevertheless be used as a general framework within which site-specific flood risk assessments are undertaken.

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Faber Maunsell Corby Strategic Flood Risk Assessment 57 8 Potential Development Areas in Corby

8.1 Specific assessments of flood risk were made for five areas identified for potential residential development and five areas identified for potential industrial development within the Borough. These areas correspond with those identified in the Catalyst Corby Regeneration Framework. The Framework incorporates areas designated for development within the Corby Local Plan but considerably extends the Local Plan proposals. The development areas proposed in the Framework therefore include the majority of the land designated for future development in the Local Plan, and hence the flood risk assessments will include land allocated for development under the Local Plan.

8.2 It should, however, be noted that several development areas proposed in the Local Plan have not been included in the Framework or the Development Areas assessed in this Report. These are mainly small areas scattered throughout the existing urban area and therefore would not involve the reclamation of large areas of “greenfield” land. This means that their development is likely to have minimal potential for adverse impact on flood risk at the strategic level. There are also several development areas within the Local Plan which have been altered, either through boundary changes or changes in their land use, in the Framework. These boundary changes are generally minor, and for the purposes of a flood risk assessment at a strategic scale no differentiation between residential and industrial land use is required.

8.3 It should be noted that the following analysis of housing and employment figures has been based on the Regeneration Framework (Ref.3) prepared by Catalyst Corby in 2003, as at the time this study began the Local Development Framework process was not well advanced. Whilst the LDF process is now further advanced and options for urban extensions and employment sites in Corby have been refined, for the puroses of completeness the Strategic Flood Risk Assessment covers all development areas originally outlined in the Catalyst Corby Regeneration Framework.

8.4 The ten potential development areas for which flood risk assessments are required are summarised below. They are also shown in Figure 2.

Western Extension residential development

Southern Extension residential development

Northern Extension residential development

Oakley / Stanion Extension residential development

Priors Hall and Weldon Extensions residential development

Great Oakley light industrial development (Technology Cluster)

Weldon North Industrial Estate light industrial dev'ment (Motor Business Cluster)

St James Industrial Estate town centre mixed use development

Corby East mix of high quality business, processing parks and logistics uses

Weldon South Industrial Estate and Max Park mix of high quality business, processing parks and logistics users

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8.5 The ten potential development areas listed above are all relatively large "greenfield" areas on the outskirts of the town, ranging in size from 3 to 453 hectares, all of which represent expansion on the fringes of the existing urban area. It should be noted that the Western Extension, Southern Extension, Oakley / Stanion Extension, Priors Hall / Weldon Extensions and the Weldon North Industrial Estate development areas all extend onto land outside the Borough Boundary.

8.6 As listed in para.6.27, provision has been made as part of the Regeneration Framework for a further 17,300 houses to be provided within the five proposed "greenfield" residential development areas surrounding the town. The main part of this Section commences with the flood risk assessments for each of these major housing development areas. As mentioned in para.6.25, planning consent for 2,940 houses for Oakley Vale was granted in July 2001 and Phases 1 and 2 of this development are currently under construction. A flood risk assessment has already been undertaken for this development and as planning requirements stipulate that on-site drainage systems are designed to limit the rates of urban runoff to a "greenfield" rate, then this development area need not be considered as part of the current study.

8.7 Provision is also made in the Regeneration Framework for large scale industrial and commercial development on "greenfield" and "brownfield" areas around Corby, and the flood risk assessments for the five industrial and commercial development areas follow those of the housing development areas. For the purposes of the Strategic Flood Risk Assessment it is only those developments which will take place on "greenfield" land (or on areas of restored ironstone quarries that, although technically "brownfield", exhibit the hydrological characteristics of "greenfield" land) that need to be considered. The development proposed for normal "brownfield" areas within the existing urban area need not be considered further. This is because those areas are already considered to be developed and urbanised, and that redevelopment is very unlikely to have an appreciable effect on the rate and amount of surface water runoff. The following areas are therefore not considered any further :- Oakley Hay Compass Point Phoenix Parkway Industrial Estate Willow Brook Industrial Estate Earlstrees Industrial Estate.

8.8 The flood risk assessment for each development area is presented in a standard format, commencing with a general description of the development area followed by a detailed description of the hydrology of the area. The assessment itself evaluates the existing flood risk within the development area but does not evaluate the potential effect on flood risk downstream resulting from the proposed development of the area. It is therefore assumed that all urban development in these areas will have surface water drainage systems designed to incorporate appropriate measures for the attenuation of surface water runoff from the newly created impermeable surfaces in these areas to pre-development rates and volumes.

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Western Extension

General Description of the Potential Development Area

8.9 The Western Extension potential development area as outlined in the Regeneration Framework is situated on the western edge of Corby, approximately 2km from the town centre. The large (453 hectare) development area runs for 6km along the western side of Uppingham Road (A6003) from Rockingham Top Farm in the north to Storefield Wood in the south. The development area is almost entirely farmland with the occasional small patch of woodland plantation. The layout of the development area is shown in Figure 10.1.

8.10 The potential development area is irregular in shape. As mentioned above, the area’s eastern boundary is formed by Uppingham Road from Rockingham to just beyond the A6003 / A6014 roundabout. The area’s southern boundary follows approximately the line of Oakley Road, but deviating south of Oakley Road between Storefield Wood and the roundabout. The long, irregular western boundary of the development area is formed (from south to north) by the eastern edges of Hedgerow Spinney, Swinawe Wood, Broad Angle, Ash Coppice, Great Cottage Wood, Blackthorn Wood and Rockingham Park. This edge of the Park runs along the watershed between the headwaters of Willow Brook (North Arm) and the River Welland catchment. Land levels across the area vary between 136mOD in Ash Coppice to less than 95mOD where the A6003 crosses Harpers Brook.

8.11 The great majority of the development area is at present pastoral farmland. There are several areas of woodland, namely Swinawe Barn Plantation and about half a dozen small unnamed plantations, within the area. The farmhouses and farm buildings at Lower Lodge Farm, Middleton Lodge Farm, Beanfield Lawns Farm Cottages and Forest Lodge are also included in the development area, together with the large public water supply service reservoirs and twin water towers at Beanfield.

8.12 The southern end of the development area, south of Swinawe Wood and east of Pipewell village, falls within Kettering District. The northern end of the area is severed from the remainder by the A427 (Corby to Market Harborough) road.

Hydrology of the Potential Development Area

8.13 The Western Extension development area spans the headwaters of the Harpers Brook, the Southern, Central and Northern Arms of Willow Brook, and Gretton Brook. Only small sections of the upper ends of the Gretton Brook and Willow Brook (North Arm) catchments are situated within the development area and therefore no distinct stream channels exist at this point. Any runoff from this part of the area will probably drain within the soil layers towards Uppingham Road or the A427 where it is would be intercepted by the roadside drainage systems. Any contribution that this area is likely to make to the flood hydrology in these two catchments is likely to be via shallow groundwater or interflow, rather than surface runoff. This is due to the rolling, rather than steep, topography and the land use which is pastoral farmland. The extents of the Gretton Brook and Willow Brook (North Arm) catchments within the development area are 8ha and 6ha respectively.

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8.14 The Willow Brook (Central Arm) has two separate small valleys within the development area which contribute to its flow. The more northerly valley originates within the development area and the amount of surface runoff at this point is not sufficient to create any significant open channels. The area of this northerly sub-catchment within the development area is 86ha and, as before, any contribution this area makes to the flood hydrology of the Willow Brook is likely to be via interflow or groundwater. The more southerly valley of the Willow Brook (Central Arm) catchment has a drainage ditch which runs eastwards from its initial point in the centre of the development area to Uppingham Road, downstream of which it is culverted under the urban area, although its precise route is unknown. Unlike the more northerly sub-catchment, the Central Arm’s southerly sub- catchment extends for a significant distance beyond the western boundary of the development area, and this sub-catchment covers some 55ha upstream and 67ha within the development area respectively. The topography of both these small Central Arm valleys is rolling rather than steep.

8.15 The Willow Brook (South Arm) has two tributary valleys within the development area contributing to its flow. In the more northerly valley a stream rises to the west of the development area and flows east across the area to Uppingham Road, from which point it is culverted under the urban area west of the main road, although the culvert’s exact route is uncertain. This sub-catchment of the Southern Arm covers an area of 38ha upstream of the development area and 78ha within the development area. Much of Ash Coppice and four small plantations are included within this sub-catchment.

8.16 The more southerly valley of the Willow Brook (South Arm) was diverted south to Harpers Brook in 1963 in a culvert. The precise line of this diversion is uncertain. It does not appear on the Anglian Water sewer plans, though there are indications that it could follow a dogleg route, parallel with but to the north of Copenhagen Road then south along Oakley Road. For simplicity the stream diversion culvert is shown diagrammatically on the Strategic Flood Risk map along the western side of Uppingham Road (A6003).

8.17 This valley also contains a stream that rises to the west of the development area and runs to Uppingham Road. The areas of the Southern Arm’s southerly sub-catchment upstream of and within the development area are 82ha and 100ha respectively. Once again, the topography is rolling rather than steep. Land use is pastoral, although the northern slopes of this sub-catchment are heavily wooded, and include Askershaw Wood, Bar Coppice, Broad Angle, Swinawe Wood and Swinawe Barn Plantation.

8.18 The southern (Kettering District) portion of this development area lies entirely within the upper end of the Harpers Brook catchment. Harpers Brook itself rises at Dob Hall, north of Desborough and flows through Pipewell before entering the development area just west of Lower Lodge Farm. Harpers Brook is culverted beneath Uppingham Road before continuing on through the southern tip of the Corby urban area in open channel. Harpers Brook has a 710ha catchment upstream of the development area and 108ha within the development area. The topography of this catchment is very similar to that of the Willow Brook headwaters. A small tributary of the Harpers Brook crosses the development area between Storefield Wood and the A6003/A6014 roundabout.

8.19 A desk study of large scale OS maps show four small ponds within the development area. The first is adjacent to the junction of the A427 and A6003 and is probably a natural farm pond. It is possible that this pond may have a highway drainage attenuation function though it is not clear how or where the runoff would be discharged. Another three ponds are situated at the southern end of the development area adjacent to Lower Lodge Farm, in the Harpers Brook catchment. None of these three ponds appear to be natural features and it is likely that they were created for stock watering or amenity purposes. None of these ponds can be expected to provide significant flood flow attenuation.

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8.20 The Anglian Water sewerage plans show no surface water sewers whatsoever within the Western Extension development area. Nor do the sewer plans give any indication as to the means of disposal of road drainage from the section of Uppingham Road which borders the development area, though this is most probably via roadside drains discharging to the Central and Southern Arms of Willow Brook and to Harpers Brook.

8.21 As far as can be ascertained, no flood risk assessments have yet been carried out for the Western Extension by prospective developers.

Flood Risk within the Potential Development Area

8.22 In theory, all the surface watercourses identified within the Western Extension constitute potential flood risk sources for the development area. In practice, because of the pronounced topography of the area and the limited flood flows in all these watercourses except Harpers Brook, the flood risk within the development area from these sources is likely to be very local and confined to the margins of the watercourses. Hence there are no significant flood risk sources within the development area, apart from Harpers Brook. The great majority of the Western Extension development area will therefore be within Flood Risk Category 1.

8.23 The Environment Agency's Indicative Floodplain map shows no areas of floodplain within the Western Extension, but this will be because floodplains are shown to extend up river and stream valleys only to the upstream limit of Main River or where the catchment area above that point falls below 10 sq.km. None of the catchments west of Uppingham Road exceed this size, the largest being the Harper's Brook (8.2sq.km).

8.24 The Strategic Flood Risk map of Corby (Fig.6, Sheets 2&5) indicates an area of flood risk in the narrow floodplain along the valley of Harpers Brook between the A6003 and Pipewell. A small strip of land along both banks of the Brook falls within Flood Risk Category 3. (The marginal area in Flood Risk Category 2 is too narrow to appear on the strategic maps.) The Strategic Flood Risk map also shows a small piece of land on the south bank of the Storefield Wood tributary immediately upstream of the A6003 in Flood Risk Category 3.

8.25 The effect of climate change will not have any appreciable effect on the severity or extent of flood risk described in paras.8.22 and 8.24 above. There is no provision in the Kettering Borough Council's Local Plan for significant urban development within the headwaters of Harpers Brook that would be likely to increase flood flows in this stream above their present “greenfield” levels.

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Southern Extension

General Description of the Potential Development Area

8.26 The Southern Extension potential development area as outlined in the Regeneration Framework is situated at the southern tip of the Corby urban area, approximately 3km from the town centre. The 76.8 ha development area runs along the valley of a tributary of the Harpers Brook from the A6003 / A6014 roundabout in the west as far as the Kettering to Corby railway line in the east. The development area is shaped roughly like an arrowhead, with the point towards the east, as shown in Figure 10.2.

8.27 The development area’s western boundary is formed by the A6003 (Uppingham Road). Its northern boundary follows The Headway, Woodlands Lane and then the Harpers Brook itself east of Woodlands. The area’s southern boundary follows the line of a farm track ENE from the A6003 along a ridge as far as the Borough boundary at the sharp bend in Mill Hill Lane, from where it continues along Mill Hill Lane and the Borough boundary and past Geddington Station until it reaches the railway line.

8.28 The development area is at present entirely agricultural land, mainly pastoral. Ash Plantation is excluded from the development area, as is an industrial site south of The Headway. From its maximum elevation of just above 110mOD at the A6003 opposite Storefield Wood the land falls away to the east to around 80mOD where the railway crosses Harpers Brook.

Hydrology of the Potential Development Area

8.29 The Southern Extension development area is located mainly within the catchment of the Storefield Wood tributary of Harpers Brook. This south bank tributary meets Harpers Brook just east of Brooklyn Farm. Downstream of this confluence Harpers Brook forms the northern boundary of the development area. There is a small area of land near Geddington Station which would fall naturally within a catchment of another small tributary of Harper's Brook east of the railway but it is likely that any surface runoff from this small area would be intercepted by drains running along the edge of the railway line and diverted into the main branch of Harpers Brook just inside the development area.

8.30 The tributary of Harper's Brook which runs through the development area is the only watercourse within the Southern Extension. This stream initiates approximately 2 km west of the development area, and runs eastwards to its confluence with the main branch of Harper's Brook at Great Oakley. The areas of the Harper's Brook tributary catchment both upstream of, and within the development area are 818ha and 77ha respectively. The topography of this area tends towards steep rather than rolling.

8.31 A desktop study using large scale OS maps has revealed two small ponds within the development area. One of these is adjacent to Ash Plantation, the second is close to Geddington Station. Both appear to be natural farm ponds and are too small to provide any attenuation of flood flows. Just upstream of the development area, Harpers Brook itself has been enlarged to form a small off-line ornamental lake SE of Oakley Hall but this may give little, if any flood flow attenuation. The small Oakley Grange tributary which enters Harpers Brook from the north 300m upstream of the railway has four small on-line runoff storage ponds to attenuate runoff from recent urban development in its catchment.

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8.32 There is, however, a significant on-line flood storage reservoir, Great Oakley FSR, on Harpers Brook immediately downstream of the confluence of Harpers Brook and its Storefield Wood tributary. This reservoir, which is operated and maintained by the Environment Agency, has a storage capacity of 15,827cu.m below spillway level which is intended to attenuate flood flows in Harpers Brook (and, presumably, the Willow Brook diversion) from a 30 to 50 year return period flood event. The area of new development for which this FSR was designed is not known, but it is very unlikely that was intended to accommodate any development beyond that allowed for in the Local Plan.

8.33 The Anglian Water sewer plans show no significant surface water sewers either within or in the immediate vicinity of the development area.

Flood Risk within the Potential Development Area

8.34 The Harpers Brook and its Storefield Wood tributary constitute the principal sources of flood risk within the Southern Extension development area. In practice, because of the pronounced topography of the area, the narrow floodplain, and the modest flood flows in the tributary (its catchment area is only about 2sq.km), the flood risk within the development area from these sources is likely to be very local and confined to the margins of those watercourses. The great majority of the Southern Extension development area will therefore be within Flood Risk Category 1.

8.35 The Environment Agency's Indicative Floodplain map shows no floodplain along the Harpers Brook tributary which runs through the Southern Extension. However, a narrow floodplain is shown along the Harper's Brook itself, with a slight area of inundation along the northern boundary of the development area downstream of Brooklyn Farm.

8.36 The Strategic Flood Risk map of Corby (Fig.6, Sheet 5) also indicates areas of flood risk in the narrow floodplain along the valley of Harpers Brook between the A6003 and the Kettering to Corby railway line, although only about 700m along the south bank of the of the Brook is in the development area. A narrow strip of land along the Brook falls within Flood Risk Category 3 and here, again, the marginal area in Flood Risk Category 2 is too narrow to appear on the strategic maps. An even narrower strip of land along the Storefield Wood tributary as far upstream as Ash Plantation is also in Flood Risk Category 3.

8.37 There is provision in the Corby Borough Council's Local Plan for urban growth within this development area. It should be noted that in the local Plan an area (S26) within the Southern Extension has been proposed for commercial development while an area (P16(E)) has been designated for nature conservation. There would be little difference (either increase or decrease) in the additional runoff generated if part of this area was in fact commercial, as indicated in the Local Plan, rather than residential as proposed by Catalyst Corby. However, if some of this area was used for conservation rather than residential use then the amount of additional runoff is likely to be significantly less. Note that an employment area (J10&P5(J)) designated in the Local Plan is immediately adjacent to the proposed urban area and therefore has been included within the assessment area.

8.38 It is assumed that if any large scale development (such as that proposed by Catalyst Corby in the Western Extension area) takes place upstream of the Southern Externsion development area at some future date then appropriate steps will be taken to ensure that storm water discharges from that upstream development are limited to their "greenfield" rates and volumes. Even so, in an extreme event with a greater return period than that for which the drainage system has been designed, the upstream development could still give rise to increased flood risk within the Southern Extension development area.

8.39 The possible future effects of climate change are not expected to have an appreciable effect on the extent of areas at risk of flooding within this development area, though the degree of risk within those small flood risk areas along Harpers Brook could increase significantly, especially if there were to be substantial urban development upstream.

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Northern Extension

General Description of the Potential Development Area

8.40 The Northern Extension potential development area as outlined in the Regeneration Framework is a 58.3 hectare irregularly shaped but compact area located on the northern tip of the Borough, 2.5 km north west of the town centre. The development area (see Figure 10.3) straddles the watershed between the River Welland and Nene (Gretton Brook) catchments.

8.41 The Northern Extension’s northwestern boundary follows the edge of woodlands which lie along the steep, scarp slope of the Welland valley west of Rockingham. Rockingham Wood, situated on the crest of the ridge, forms the northeastern boundary of the development area. The south eastern boundary is formed by the edge of the industrial area (the northern part of Earlstrees Industrial Estate) to the north of Princewood Road. The development area’s southwestern boundary is the A6003 (Uppingham Road) NW of the water tower and the A6116 (Rockingham Road) SW of the water tower.

8.42 The development area is situated on the relatively level crest of the ridge which runs from SW to NE along the southern edge of the Welland valley. To the north of the ridge the scarp slope falls away steeply to the floodplain of the River Welland. The crest of the ridge rises to a maximum height of about 132mOD in the centre of the development area, and no point in the area is much below 125mOD.

8.43 At present the great majority of the development area consists of pastoral farmland and wasteland, although the Seymour Plantation (8.1ha) on the SW edge of the area is also included. Adjacent to the A6003 / A6116 junction the area includes Shire Farm and the adjoining Anglian Water service reservoir and water tower. A wide strip of land inside the southeastern boundary of the development area appears at one time to have been quarried for ironstone and part of this area has subsequently been used for landfill. Under the Corby Local Plan, Seymour Plantation is to become the site of a business park and the ironstone quarry / landfill area will, after restoration and with appropriate precautions against landfill gases, be allocated for commercial development and a crematorium.

Hydrology of the Potential Development Area

8.44 The Northern Extension development area is situated partly within the catchment of the River Welland, although most of its area is in the Gretton Brook catchment. Because of its location on the watershed, there are no natural watercourses within the development area.

8.45 The only surface watercourses which have been identified within the development area are small drains on its SW and SE boundaries. The ditch along the SW boundary appears to collect storm runoff from Rockingham Road (A6116). The open ditch is only 225m long and disappears into a culvert at the western end of Gretton Brook Road. This culvert appears to be upstream end of Gretton Brook.

8.46 The drain along the SE boundary of the development area appears to originate in an old ironstone gullet in the centre of the area and follows the line of the gullet northeastwards for about 700m before turning SE towards Princewood Road. This drain is culverted from where it leaves the development area and is a tributary of the culverted upstream section of Gretton Brook which runs beneath Gretton Brook Road.

8.47 The line of the culverted section of Gretton Brook is not known for certain - it is not shown on the Anglian Water sewer plans - but its route can be inferred with reasonable certainty from the name, Gretton Brook Road, and the downstream termination points of isolated lengths of AW surface water sewers along the line of the road. The Gretton Brook FSR is situated to the south of Gretton Brook Road, just east of its junction with Earlstrees Road.

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8.48 The Anglian Water sewer plans show no surface water sewers within the development area. There is however, a surface water sewer running east along Rockingham Road which appears to discharge to the open ditch near the S corner of the development area. There is also a network of surface water sewers draining the Industrial Estate between Princewood Road and the development area boundary.

Flood Risk within the Potential Development Area

8.49 The position of the Northern Extension development area astride the Welland / Nene watershed clearly indicates that major flood risk within the development area will be non- existent. The Environment Agency's Indicative Floodplain map shows the development area to be well clear of any significant floodplain. Similarly, the Corby Strategic Flood Risk map, Fig.6 (Sheet 3), shows the whole of the development area in Flood Risk Category 1.

8.50 In theory, the two surface watercourses identified within the Northern Extension development area constitute a potential flood risk source for the area, but in practice, because these two drains are so small and have such small contributory catchments, they pose no flood risk other than that associated with on-site drainage. Any risk from this source would be very localised and confined to the margins of those watercourses, if it exists at all, confirming that the whole of the Northern Extension development area will be in Flood Risk Category 1.

8.51 The effect of climate change will not have any appreciable effect on the level of flood risk described above, although all surface water drainage systems associated with future urban development in the area should be designed to accommodate potential increases in storm runoff resulting from climate change to avoid the risk of minor flooding from on-site drainage systems.

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Oakley / Stanion Extension

General Description of the Potential Development Area

8.52 The Oakley / Stanion Extension potential development area as outlined in the Regeneration Framework is situated approximately 2.5 km SE of Corby town centre, located between the Oakley Vale development area and Stanion village. The large (313.7 hectare) development area, at present almost entirely agricultural land, is compact but irregular in shape as shown in Figure 10.4.

8.53 The development area is bounded by the Kettering to Corby railway line to the west and an area of mixed woodland plantations to the north (Bandy Slade, Oakley Purlieus, South Wood and, to the north of Long Croft Road, Stanion Lane Plantation). The main A43 (Stamford Road) forms the development area’s eastern and southeastern boundaries. The southern boundary follows the minor road from the A43 to Little Oakley, skirts Little Oakley village, and then runs west along the public footpath and old millstream from the village to the railway.

8.54 The Oakley / Stanion development area lies along the north side of the valley of the main branch of Harpers Brook. The area’s rolling topography with a handful of minor tributary valleys leading down to Harpers Brook, has a pronounced slope from north to south. Elevations range from about 116mOD at the NW corner of the area on the edge of the woodland at Bandy Slade to 72mOD in the small valley west of Stanion. The development area’s southern and southeastern boundaries coincides roughly with the edge of the Harpers Brook floodplain.

8.55 On land to the west of the Oakley / Stanion Extension on the opposite side of the railway is the Oakley Vale development which is currently under construction, with the whole development likely to be completed in five phases over the next ten years. On the north of the development area Long Croft Road leads to a modern warehousing and storage complex.

8.56 The presence of disturbed ground, a few small abandoned industrial remains and the routes dismantled railway lines indicates that much of the southern half of the development area has at some time in the mid twentieth century been quarried for ironstone. On the north side of Long Croft Road there is a substantial railway cutting running the whole length of the road, the route of the branch railway that served the extensive ironstone quarries east of the A43 and north of Stanion, as well as those in the development area itself.

Hydrology of the Potential Development Area

8.57 Although ironstone quarrying in the Stanion Lane Plantation area has made the watershed between Harpers Brook and the Willow Brook (South Arm) somewhat indeterminate, it is clear that the Oakley / Stanion Extension development area lies entirely within the Harpers Brook catchment. It is, however, likely that ironstone quarrying, both within the development area and beyond its northern boundary, has considerably disrupted the natural hydrology of the local sub-catchments.

8.58 Three small surface watercourses have been identified within the development area, all tributaries of Harpers Brook. The first of these rises west of the railway within the Oakley Vale area and is culverted beneath the railway embankment where it enters the Oakley / Stanion Extension. At some time it has been diverted into an old millstream along the north side of Harpers Brook which now discharges to Harpers Brook just upstream of Little Oakley. It is possible that in a major flood event this stream could overflow the south bank of the millstream and resume its natural north - south course across the floodplain and straight into Harpers Brook.

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8.59 The second tributary rises to the north of Little Oakley and runs south to its confluence with Harpers Brook at Little Oakley, picking up flow from another small drain along the way. The third small tributary rises just within the development area’s eastern boundary, close to New Grange Farm, is partially culverted under the A43 and Stanion village.

8.60 A study of large scale Ordnance Survey plans has revealed that there are as many as five small ponds within the development area. These are likely to be natural used as stock water reservoirs from which water is pumped to troughs on the property. There are two artificial “tanks” in the eastern half of the area which are a legacy of quarrying activities. None of these ponds are likely to provide significant attenuation of flood flows.

8.61 The Long Croft Road Balancing Pond is located in the NE corner of the proposed development area. Runoff from the Southern Distributor Road development would be drained into the pond via road gullies and sumps, carrier pipes or filter drains before being discharged to the New Grange Farm tributary of Harpers Brook.

8.62 There are no Anglian Water surface water sewers either within, or in the immediate vicinity of the development area.

Flood Risk within the Potential Development Area

8.63 The principal flood risk source within the development area would at first appear to be Harpers Brook. However, both the Environment Agency’s Indicative Floodplain map and the Corby Strategic Flood Risk Assessment maps Figure 6 (Sheets 5 & 6) show that the southern boundary of the development area is clear of the Brook’s floodplain and the Category 3 (High Risk) flood risk envelope. As was explained earlier in this report, land in small upland valleys subject to Category 2 (Low to Medium) flood risk is limited to narrow strips of land between Flood Risk Categories 1 and 3, usually too narrow to be represented at the strategic scale. The only land in the development area likely to fall within Category 2 is a tiny area, (probably less than 0.5ha) west of Manor Farm, Little Oakley. Harpers Brook can therefore be discounted as a flood risk source as regards the Oakley / Stanion Extension development area.

8.64 In theory, the three small tributary waterourses within the Oakley / Stanion Extension also constitute a potential flood risk source for the development area. In practice, because of the pronounced topography of the area, their virtually non-existent floodplains and their tiny catchments (and equally insignificant flood flow rates), the flood risk within the development area from these minor watercourses is likely to be very local, confined to the margins of the watercourses. There is therefore no significant flood risk within the development area, and virtually the whole of the Oakley / Stanion Extension development area will be in Flood Risk Category 1 (Little or No Risk).

8.65 Because the Longcroft Road Balancing Pond has been constructed to deal with impermeable area runoff from urban development outside the Oakley / Stanion Extension area it has an associated flood risk, though this is likely to be fairly minor and of limited extent. The drainage network that feeds into the pond is designed for a 5-year storm event and thus localised flooding of the Southern Distributor Road development area will occur before the pond reaches its full storage potential.

8.66 In a more severe event, the outlet drain from the Long Croft Road balancing pond could become surcharged and any flooding from the balancing pond in this development area is likely to appear as shallow overland flow from the pond to the Harpers Brook Tributary, approximately 300m south of the pond. It should, however, be noted that New Grange Farm could be within the flow path between the pond and the tributary. The 300m strip of land between the pond and the tributary should therefore be regarded as in Flood Risk Category 3, though the impact of the flooding would be relatively minor.

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8.67 There is provision in the Borough Council's Local Plan for urban development within the Oakley / Stanion Extension area. In the Local Plan an area (R8, P12(R) & S21) within this development area has been allocated for housing, an area (L8) for recreation and leisure, while an area (P16(E)) has been designated for environment, nature and conservation. If some of this area were to be used for recreation / leisure or environment / conservation, as proposed in the Local Plan, rather than for residential development, as proposed in the Regeneration Framework, then the amount of additional runoff generated within the development area is likely to be significantly less.

8.68 Although a site-specific flood risk assessment has been undertaken by the developers in connection with the existing development at Oakley Vale (Ref.15), as far as can be ascertained no assessments have yet been carried out by prospective developers for land in the Oakley / Stanion Extension.

8.69 The effect of climate change is not expected to have any appreciable effect on the flood risk assessment made in paragraphs 8.64 and 8.66 above.

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Priors Hall & Weldon Extensions

General Description of the Potential Development Area

8.70 The Priors Hall and Weldon Extensions as outlined in the Regeneration Framework are situated on the eastern edge of the Borough, almost surrounding the village of Weldon. The area, shown in Figure 10.5, is very irregular in shape and extends over 413.8 hectares. The Priors Hall Extension to the north and the Weldon Extension to the south are separated by the main A43 (Corby to Stamford) road.

8.71 The Priors Hall Extension is bounded on the south by the A43 and A427 roads. The development area’s western boundary follows the eastern edge of the Weldon North Industrial Estate as far as Shire Cottage and then doglegs east before continuing north on a line parallel with Gretton Road but 400m to the east. The area’s northern boundary runs along Kirby Lane. The eastern boundary runs along the edge of a belt of woodland (The Rookery) as far as the northern end of Priors Hall Plantation where it heads west to follow a meandering line between Priors Hall Golf Course and the old Priors Hall ironstone quarry, reaching the A43 opposite Little Cot Farm.

8.72 The northwestern boundary of the Weldon Extension is formed by the Old Stamford Road as far as the village. It skirts the southern outskirts of Weldon to the A427 (Oundle Road) then follows that road north to Manor House where it turns to skirt the northern end of the village, finally turning NE to follow the A43 to the northern tip of the Weldon Extension. The southern and eastern boundaries of this part of the development area follow a curving line starting from a track heading south from the Old Stamford Road, across open country along the 100m contour to the point where that contour line crosses the A427, and then north across open country to the A43.

8.73 At present the Priors Hill Extension area is entirely either farmland or waste land, although it includes a belt of woodland along the valley of the Willow Brook (North Arm) and plantations along its eastern boundary with Priors Hall golf course. Much of this area has been extensively quarried for ironstone and this is reflected by the broken topography, with a deep north - south gullet extending for some 1.5 km down the centre of the area. Ground levels north of the A43 vary between 80m and 100mOD. The northern part of the Priors Hall Extension includes a section through the valley of the North Arm of Willow Brook.

8.74 The Weldon Extension development area is similar in character to Priors Hall, almost entirely farmland or waste land, though it includes the Manor House, some residential and miscellaneous property south of Church Street in Weldon, and a caravan site and playing field on the southern edge of the village off Oundle Road. An area at the western end of the development area has been quarried for ironstone. The Weldon Extension lies on the southern slopes of the valley of the South Arm of Willow Brook and therefore slopes appreciably from south to north. From its maximum elevation of 100mOD in the SE corner of the area the land falls away to around 75mOD at the point where Willow Brook (South Arm) crosses the area’s eastern boundary.

Hydrology of the Potential Development Area

8.75 The combined area is situated within the catchments of the South, Central and North Arms of Willow Brook, although as it lies well downstream of the point where the Central Arm has been diverted to the South Arm only the North and South Arms are of hydrological significance in this area.

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8.76 The Priors Hall Extension includes a 1.5 km length of Willow Brook (North Arm). Most of the Priors Hall quarry area lies within the Willow Brook (South Arm) catchment although the line of the watershed between the North and South Arms cannot be determined with any precision in this area. Upstream of the development area the North Arm has a catchment of 93 hectares. The channel of the North Arm through the development area is unnaturally straight and appears to have been realigned and regraded at some time.

8.77 The South Arm of Willow Brook and the downstream end of the truncated Central Arm of Willow Brook (now a Critical Ordinary Watercourse) run on closely parallel courses through the village of Weldon, but as their confluence is just upstream of the development area’s boundary only the South Arm actually runs through the Weldon Extension. The combined catchment area of the South / Central Arm where it enters the development area is 321 hectares.

8.78 Three smaller surface watercourses have also been identified within the development area, all in the vicinity of Weldon. Two of these enter the South Arm of Willow Brook from the south just downstream of Weldon village. Their downstream ends across the floodplain are assumed to be culverted. Another originates at the southern end of the Weldon North Industrial Estate and runs east, parallel with the A427 road, before crossing under the road to join the Central Arm of Willow Brook at the A43 / A427 roundabout. This ditch may receive some surface water runoff from the Industrial Estate, but see para.8.80 below.

8.79 Within the development area large scale OS maps show one small pond in the Central Arm catchment, adjacent to the stream, and around a dozen ponds in the Southern Arm catchment area, scattered throughout the woodland along the eastern boundary. These are almost certainly water-filled hollows in the floor of the old Priors Hill quarry left after the cessation of ironstone extraction. The largest of these ponds is about 1ha in area. The old quarry ponds may provide a certain amount of runoff interception storage and therefore provide some minor attenuation of flood flows.

8.80 There are no Anglian Water surface water sewers within the development area. The Anglian Water sewer plans show surface water drainage from the Weldon North Industrial Estate going to the Anglian Water overflow pond at the W end of Larratt Road. The only Anglian Water surface water sewerage in Weldon village is the small local system in the Manorfield (Bramblewood Road) area on the north side of the village which drains to the Willow Brook (Central Arm) via a ‘hydrobrake’ flow retarder.

Flood Risk within the Potential Development Area

8.81 The North and South Arms of Willow Brook constitute the primary flood risk source in the Priors Hall and Weldon Extensions development area, but as will be seen from the Corby Strategic Flood Risk map, Figure 6 (Sheet 4), the flood risk area is limited mainly to the narrow floodplains of these two streams.

8.82 The Environment Agency's Indicative Floodplain map shows floodplain on either side of the Willow Brook South Arm through the development area, but not along the Willow Brook’s North or Central Arms or any other watercourses. The Strategic Flood Risk map shows small areas of Category 3 (High Risk) not only along the South Arm, but also along both banks of the North Arm through the development area but also Category 3 flood risk along the Central Arm of Willow Brook and at the southern end of the deep gullet in Priors Hall quarry.

8.83 The great majority of the development area is, however, in Flood Risk Category 1 (Little or No Risk), albeit with narrow areas of Flood Risk Category 3 (High Risk) land along the North, Central and South Arm floodplains, together with their corresponding marginal strips of Flood Risk Category 2 (Low to Medium Risk) land. The penetration of the Flood Risk Category 3 area some 200m into the northern end of Priors Hall quarry should be noted.

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8.84 It should also be noted that the distribution of flood risk across the floor of the old quarry will be heavily influenced by the manner in which the land in the quarry is regraded, landscaped and drained before development commences. If this is done carefully then it is quite probable that the area of Flood Risk Category 3 land at the southern end of the quarry can be eliminated and the whole quarry area brought within Flood Risk Category 1.

8.85 There is provision in the Borough Council's Local Plan for urban development within this area. Within the Priors Hall Extension an area (J37) has been allocated for employment, and other areas (L26, NC49) designated as recreation and leisure. It should, however, be noted that there would be little difference (either increase or decrease) in the amount of runoff generated if part of this area was in fact used for employment purposes rather than residential purposes as indicated in the Regeneration Framework. If some of this area was used for recreation and leisure as indicated within the Local Plan rather than residential, as proposed under the Framework, then the amount of additional runoff is likely to be significantly less. Within the Weldon Extension, the Local Plan has allocated an area (R17) for housing and area P13(V) has been designated as an environment and nature conservation area.

8.86 Because of the small extent of the flood risk area, the effect of climate change should not have any appreciable effect on the overall flood risk assessments made in paras.8.83 and 8.84 above, with the proviso that the design of the regrading and drainage of Priors Hall quarry takes the potential effects of climate change on storm runoff fully into account. It should also be noted that because the Willow Brook catchments upstream of the development area are already heavily urbanised, the potential increase in storm runoff due to climate change in such a catchment could have a disproportionate effect on future flood risk downstream, especially during high return period events. The standard of protection of existing flood attenuation schemes in the North and South Arms of Willow Brook should therefore be checked and re-evaluated within the next ten years.

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Great Oakley Extension

General Description of the Potential Development Area

8.87 The Great Oakley potential development area as outlined in the Regeneration Framework is situated within the Oakley Vale area of Corby which is currently under going residential development. The Great Oakley development area (Figure 10.6), which covers an area of only 15.4 hectares, is located 2.5km south of the town centre between the Brooke Weston City Technology College (CTC) and Harpers Brook.

8.88 The proposed development area is roughly rectangular in shape, aligned on a NNW to SSE axis. The whole of the area currently consists of agricultural land and includes Oakley Grange farm near the northern end of the area. The development area has been designated as a Science Park (J25) in the Borough’s Local Plan.

8.89 Harpers Brook forms the southern boundary of the development area. The area’s western boundary is the track from Geddington Station Lane to Oakley Grange and, at the north end, the edge of the CTC’s playing fields. The development area’s eastern and northern boundaries are a line running NW across open country from the bridge which carries Harpers Brook over the Ketteirng - Corby railway. The SE corner of the development area abuts the foot of the railway embankment at this point.

8.90 The development area has an appreciable slope from NE to SW from a maximum elevation of about 103mOD north of Oakley Garage down to a level of about 80mOD at the point where Harpers Brook passes under the railway.

Hydrology of the Potential Development Area

8.91 The Great Oakley development area is situated in the Harpers Brook catchment. Harpers Brook, which runs for 200m along the southern boundary of the development area is the only watercourse either within or bordering the area.

8.92 Large scale OS maps show one small pond within the development area, in a hollow 100m E of Oakley Grange. This pond appears natural and is too small to have any attenuation effect on storm runoff.

8.93 The topography of the development area suggests that its natural drainage path is down a dry valley the lower end of which meets Harpers Brook just upstream of the railway. There are two small streams on either side of the development area, both north bank tributaries of Harpers Brook.

8.94 The larger of the two streams, the Oakley Vale Stream, runs down a valley to the west of Oakley Grange and the CTC and receives the runoff from the Snatchill (Oakley Vale) housing area which is currently being developed. Four small on-line flood storage reservoirs, the CTC pond and the Oakley Vale ponds, have been constructed in series along the Oakley Vale Stream. The other, smaller stream rises NE of Lyveden Lodge and is culverted under the railway embankment 800m NW of the development area, eventually joining Harpers Brook at Little Oakley (see para.8.58).

8.95 The topography of the development area indicates that neither of these two streams has any connection with the hydrology of the development area, which appears to be associated solely with the dry valley described in para.8.93. This dry valley can be clearly seen just east of Oakley Grange and will intersect the downslope flow from the hillside between Oakley Grange and the Harpers Brook railway bridge.

8.96 The development area does not seem to have been affected by ironstone quarrying, although there are the remains of extensive ironstone workings to the north of the development area in the Oakley Vale / Snatchill area.

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8.97 There are no surface water sewers either within the development area or in its immediate vicinity.

Flood Risk within the Potential Development Area

8.98 Harpers Brook constitutes the only significant source of flood risk for the development area but, because of the area’s topography, any flood risk will be confined to a relatively narrow strip of land along the north bank of the stream. The great majority of the Great Oakley development area will therefore be within Flood Risk Category 1.

8.99 The Environment Agency's Indicative Floodplain map (2002 edition) shows a narrow strip of floodplain along both banks of Harpers Brook along the southern boundary of the Great Oakley development area. The indicative floodplain map shows a much greater width of floodplain upstream of the railway bridge, suggesting that some degree of throttling of flow occurs at this point.

8.100 The Strategic Flood Risk map, Figure 6 (Sheet 5), corroborates the Indicative Floodplain map and shows a similar narrow strip of land within the development area along the north bank of Harpers Brook in Flood Risk Category 3, although with much less throttling effect at the railway bridge, where the comparatively modest restriction of the waterway does not appear to justify the impact attributed to it in the Indicative Floodplain map. Here again, there will be an even narrower marginal strip of land in Flood Risk Category 2 between the Category 1 and Category 3 flood risk land.

8.101 Because the area of land at risk of flooding in the Great Oakley development area is so small, the potential effect of climate change will have no appreciable effect on the extent of the flood risk area described above. Nevertheless, because of absence of natural drainage channels within the area, any future development will have to rely entirely on piped drainage networks for the disposal of surface water runoff and this should be designed to allow for climate change effects to avoid problems with localised flooding.

8.102 A flood risk assessment was carried out for the Oakley Vale development area in December 2001 by JBA Consulting Engineers. This study considered the hydrology and hydraulics of the local drainage system and assessed the likely effects of the development. However this was undertaken for only Phases 1 and 2 of the development and did not include the Great Oakley development area designated as a Technology Cluster in the Regeneration Framework. Whilst proposals for storm water discharges from the remaining phases were not presented in detail, an overall strategy was discussed.

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Weldon North Industrial Estate

General Description of the Potential Development Area

8.103 The Weldon North Industrial Estate potential development area as outlined in the Regeneration Framework is situated in an area of old ironstone quarries to the north of the existing Weldon North Industrial Estate. The development area, which is approximately 4.5 km NE of Corby town centre, is immediately adjacent to the northwestern boundary of the proposed Priors Hall Extension development area. The Weldon North Ind. Est. development area would be made up of two sections. The first, an irregular quadrilateral of 36.8 ha lies to the west of Gretton Road. The second is a 59.9 ha rectangular shape located on the eastern side of Gretton Road. These are shown in Figure 10.7.

8.104 The section of the development area to the west of Gretton Road occupies the site of a disused ironstone quarry. It is bordered by the North Arm of Willow Brook on the north, by Gretton Road on the east, by the existing Weldon North Ind.Est. to the south, and the Willowbrook East Industrial Estate to the west. The block of land to the north of the development area, between Willow Brook (North Arm) and Gretton Brook Road, has recently been developed as a motor racing circuit.

8.105 The section to the east of Gretton Road is pastoral farmland and waste land, but with evidence of ironstone quarrying, particularly in the Weldon Lodge area. Where there is also a 7 ha area of woodland plantation. Also contained within this section are two residential properties, Weldon Lodge and Shire Cottage. This section of the development area is bordered by Kirby Lane to the north, the Priors Hall Extension area to the west and the south, and Gretton Road on the west.

8.106 The section of the development area to the west of Gretton Road is relatively flat with an elevation of approximately 100mOD, although there is the remains of ironstone gullets along the western and northern boundaries. The section east of Gretton Road extends across the valley of the Northern Arm of Willow Brook, falling from a maximum level of 100 m OD on either side of the Brook to a level of between 85m and 90mOD along the valley bottom. The more level land south of Weldon Lodge rises to a maximum elevation of 106mOD near Shire Cottage.

Hydrology of the Potential Development Area

8.107 The Weldon North Industrial Estate development area is situated almost entirely within the catchment of the Northern Arm of Willow Brook, although at the northern extremity of the area a small triangle of land (about 1ha) at the junction of Gretton Road and Kirby Lane is in the Gretton Brook catchment and, at the southern extremity, a slightly larger area of land around Shire Cottage falls within the Willow Brook (South Arm) catchment.

8.108 West of Gretton Road the North Arm of Willow Brook runs along the northern border of the development area from west to east. East of Gretton Road it flows through the development area. For some 2km east of Gretton Road the North Arm follows an artificially straight course (see also para.8.76). This would appear to be a legacy of previous ironstone quarrying in the area which has at some stage necessitated the regrading and possibly the lowering of the bed of the watercourse to facilitate the drainage of the adjacent quarries. Less than 200m beyond the development area’s northern boundary in the Kirby Lodge area, Gretton Brook runs parallel to Kirby Lane but although close to the development area has no hydrological connection with it.

8.109 Within the western section of the development area a small open ditch from the interior of the old quarry flows east towards Weldon Lodge, from where it is assumed to be culverted northwards to Willow Brook (North Arm). In the eastern section of the area there is a small drain which links two tiny ponds across the southern boundary of the development area. The direction of flow in this short length of drain is unclear, as is the ultimate destination of its discharge.

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8.110 There are what appear to be two substantial lagoons in series on the north side of Willow Brook (North Arm) opposite Willowbrook East Industrial Estate, about 500m upstream of the development area. These are understood to be abandoned British Steel sludge ponds and have no runoff retention or flood flow attenuation function.

8.111 There are no Anglian Water surface water sewers either within the proposed development area. The southern half of the existing Weldon North Industrial Estate east of Steel Road (A6116) drains to the Central Arm of Willow Brook. The AW sewerage plans show no surface water sewers whatsoever in the northern half of this part of the industrial estate but the local topography suggests that this area also drains southwards towards to the Central Arm. The whole of the section of the industrial estate west of Steel Road drains to the Central Arm. North of Steel Road the Willowbrook East Industrial Estate drains to the North Arm of Willow Brook, with a large (1,350mm) surface water outfall to the brook just upstream of the development area boundary.

Flood Risk within the Potential Development Area

8.112 In theory, the North Arm of Willow Brook constitutes the principal flood risk source for the development area. In practice, because of the pronounced local topography and the narrow floodplains, the flood risk from this source within the development area is likely to be very localised. The actual flood risk has probably been further minimised by the regrading and possible lowering of the bed of the North Arm through the development area (see para.8.108).

8.113 The Environment Agency's Indicative Floodplain map (2002 edition) shows no floodplain along the North Arm of Willow Brook whatsoever, but as the catchment area upstream of the development area is only 6.5 sq.km it should not be expected that the Indicative Floodplain map would indicate a floodplain as far upstream as Gretton Road. However, the Gretton Brook floodplain is shown as far upstream as the eastern end of Princewood Road although it does not impinge on the Weldon North Ind. Est. development area.

8.114 The distribution of flood risk within the development area is shown on the Corby Strategic Flood Risk map, Figure 6 (Sheet 4). This shows a strip of Flood Risk Category 3 land along both banks of the North Arm within the development area east of Gretton Road, but only two much smaller, discontinuous areas of Flood Risk Category 3 land on the northern edge of the development area west of Gretton Road. The great majority of the development area will therefore be in Flood Risk Category 1. East of Gretton Road there will be marginal strips of Flood Risk Category 2 land along the interface between the Category 1 and 3 land, but none west of Gretton Road.

8.115 There is provision in the Borough Council's Local Plan for urban development within the Weldon North Industrial Estate development area. In the Local Plan this development area includes employment areas J23 and J37, and a corridor of land designated for transport (T10) which will provide access into the area. It should be noted that Area J37 differs significantly in extent from the corresponding area proposed in the Regeneration Framework.

8.116 The effect of climate change should not have any appreciable effect on flood risk within the development area because of the very limited extent of the flood risk areas. It should, however, be noted that because the Willow Brook (North Arm) catchment upstream of the development area is already heavily urbanised, the potential increase in storm runoff due to climate change in such a catchment could have a disproportionate effect on future flood risk downstream, especially during high return period events. The standard of protection of existing flood attenuation schemes in the North Arm should therefore be checked and re-evaluated within the next ten years.

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St James Industrial Estate

General Description of the Potential Development Area

8.117 The St James Industrial Estate potential development area as outlined in the Regeneration Framework is proposed for the 102.6 hectare area currently occupied by the Stanion Lane Plantation, approximately 2.5 km southeast of the town centre and west of the junction of Geddington Road (A6086) and Stamford Road (A43). The area is shown in Figure 10.8.

8.118 The development area, approximately rectangular in shape, is bordered by Geddington Road to the north, Stamford Road to the east and Long Croft Road to the south. The western boundary of the area is formed by the short branch line to the Eurohub car distribution depot and steelworks from the main Kettering - Corby railway, and an old railway cutting which once linked the steelworks branch to the mineral railway line to the Stanion ironstone quarries.

8.119 Although the whole of the development area is now wooded, the irregular topography and some industrial remains suggest that the area has at one time been quarried for ironstone. The development area is situated on a generally flat ridge running from SW to NE which forms the watershed divide between the South Arm of Willow Brook and Harpers Brook. Ground levels across the development area vary between 96mOD and slightly above 105mOD.

8.120 The northern edge of the Stanion / Oakley Extension development area lies on the opposite side of Long Croft Road. To the north of the development area, the opposite side of Geddington road is lined with miscellaneous commercial premises, beyond which is the extensive Eurohub vehicle storage and distribution depot. The village of Stanion lies to the SW of the development area, beyond the A6116 / A43 roundabout.

Hydrology of the Potential Development Area

8.121 The St James Industrial Estate development area straddles the watershed between the catchments of the Willow Brook (South Arm) and Harpers Brook, though it is not possible to determine the precise line of the watershed because of the broken and disturbed nature of the ground. As far as can be ascertained, the majority (about 80%) of the development area lies in the Harpers Brook catchment, with only a 200m (approx) strip of land along the northern and northeast edges of the area within the South Arm catchment, despite the fact that the South Arm itself is only 200m beyond the development area’s western boundary.

8.122 The Willow Brook (Central Arm) diversion culvert discharges to the South Arm 300m north of the NW corner of the development area (“A” in Fig.10.8) but the increased flow in the south arm downstream of this point will have no impact on the area. The large ponds on the north bank of the South Arm (“B” in Fig.10.8) are old clay pits into which water in the brook can be diverted. They are used by Corus (British Steel) as an emergency water supply and provide no significant alleviation of flood flows.

8.123 There are no surface watercourses in the development area, but this is not surprising, given the area’s watershed location and the loss of the natural landforms as a result of quarrying. Neither are there any open watercourses in the immediate vicinity of the area, apart from the Willow Brook’s South Arm, which has an 11.5sq.km catchment area upstream of the area.

8.124 The Long Croft Flood Storage Reservoir is situated just outside the SE corner of the development area, adjacent to the A43 / AA6116 roundabout. It is understood that this FSR has been built to attenuate storm runoff from the Southern Distributor Road (incorporating Long Croft Road) and the associated urban development. The drainage from this FSR and associated flood risk downstream has been discussed in para.8.61 and paras.8.65 & 66.

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8.125 The Eurohub Balancing Pond (“C” in Fig.10.8) is a small private on-line flood storage reservoir built to serve the Wincanton storage and warehousing complex at the west end of Long Croft Road. It is understood to have been designed to attenuate runoff from the Wincanton site to “greenfield” levels for a 100-year return period storm.

8.126 There are three small ponds in the development area; one near the NW corner and the other two towards the SW corner. These ponds appear to be a legacy of ironstone quarrying, and may at present be used as a firefighting resource for the plantation. There is also a cluster of four disused sludge beds in the NW corner of the development area which may once have been associated with the town’s sewage treatment works on the north bank of the South Arm just downstream of the Central Arm diversion culvert outfall.

8.127 There are no surface water sewers shown on the Anglian Water sewerage plans within the development area or in its immediate vicinity. It must, however, be assumed that there is a private surface water sewer along Long Croft Road connecting with the FSR at the eastern end of the road.

Flood Risk within the Potential Development Area

8.128 The nearest flood risk source to the development area is the South Arm of Willow Brook, but the Environment Agency's Indicative Floodplain map (2002 edition) shows a very narrow floodplain terminating at the downstream side of the Corby sewage treatment works, some 400m short of the head of Main River at the upstream end of the sewage works. 8.129 The Strategic Flood Risk map, Figure 6 (Sheet 6) shows the Floor Risk Category 3 area extending along the South Arm as far upstream as the Corus (British Steel) ponds (“B”), but with the narrow flood risk envelope confined to the margins of the watercourse, well to the north of the branch railway. As there are no other significant flood risk sources within or close to the development area, the whole of the St James Industrial Estate development area may therefore be regarded as within Flood Risk Category 1.

8.130 Surface water runoff from much of the St James Industrial Estate development area could drain either to the South Arm of Willow Brook or to Harpers Brook. It is expected that most of the post-development runoff would drain to Harpers Brook via the New Grange Farm tributary (see para.8.59) but this would require the enlargement or duplication of the existing Long Croft Road FSR.

8.131 As regards runoff discharged to the South Arm of Willow Brook, the development area is situated upstream of the Weldon FSR on the South Arm. It cannot, however, be assumed that there would be any spare capacity left within the FSR to cope with any further upstream development (e.g. the St James Industrial Estate) and therefore this can not be relied upon as a means of balancing any increased runoff from the development area.

8.132 There is provision in the Borough Council's Local Plan for urban development within the St James Ind. Est. development area, though it should be noted that in the Local Plan the development area has been designated for recreation and leisure (NC19, NC43 and L20). If this area was used for recreation and leisure, as indicated in the Local Plan, rather than industrial use, as proposed under the Regeneration Framework, then the potential quantities of additional runoff generated are likely to be significantly less.

8.133 The potential effects of climate change will not have any appreciable effect on the level of flood risk in the development area described above. Surface water drainage systems within the development area should, however, be designed to accommodate the possible increases in runoff rates resulting from climate change to avoid creating local flood risk areas within the development area.

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Corby East

General Description of the Potential Development Area

8.134 The Corby East potential development area as outlined in the Regeneration Framework is situated approximately 3.5km SE of the town centre. Most (88.4ha) of the development area is situated to the east of the main A43 (Stamford Road) at its junction with the A6086 (Geddington Road), with a small (3.4ha) detached portion of the area west of the A43, in the angle between the A43 and the A6086, as shown in Figure 10.9.

8.135 The development area east of the A43 road is a compact but irregularly shaped area, bordered by the A43 dual-carriageway on the west, abandoned ironstone quarries to the north and east, and an extensive area of woodland, Cowthick Plantation, on the south. The detached and much smaller L-shaped part of the area west of the A43 is separated form the A43 by a small wedge of woodland, Ironpits Wood, except at its north eastern corner where it abuts the main road. On the NW the area shares a common boundary with the extensive Eurohub Central vehicle storage compound. Its SW boundary is the rear of the commercial premises along the north side of Geddington Road.

8.136 Much of the development area east of the A43 road is at present either pastoral farm land or waste land. The surrounding area is marked on the OS maps as disused quarries, and it is possible that some of the land within the development area has also been worked for ironstone and subsequently reclaimed for farmland. There is a 3.5ha extension of Cowthick Plantation into the south of the development area, and a much smaller (0.4 ha) isolated plantation block towards the east end of the development area. The small portion of the development area to the west of the A43 is at present the southern end of an abandoned ironstone quarry.

8.137 The development area to the east of the A43 is centred on a flat ridge running west to east which forms the divide between the Willow Brook (South Arm) and Harpers Brook catchments. The undulating land levels in this area are between 90m and 105mOD but the natural landforms in the area have been heavily modified by quarrying. Because the small detached portion of the development area west of the A43 has been extensively quarried, there is little definitive information that can be gained from the OS Plans with regards to its present landform or levels. Ground levels here are probably between 90m and 100mOD.

Hydrology of the Potential Development Area

8.138 The Corby East development area is located within the catchments of both the South Arm of Willow Brook and Harpers Brook, with slightly more in the former than the latter. The small detached part of the area west of the A43 lies wholly within the Willow Brook catchment.

8.139 Both Willow Brook (South Arm) and Harpers Brook are some considerable distance from the development area (>0.5km) and at a significantly lower elevation. There is a substantial north bank tributary of Harpers Brook which lies in a deeply incised valley along the western edge of Laudimer Wood, about 800m SE of the development area, but this has no hydrological effect on the development area. There are no significant watercourses within or adjacent to the development area.

8.140 OS plans show a small surface watercourse or ditch along the NW boundary of the detached portion of the development area west of the A43. It terminates at the A43 and presumably discharges to the road drainage system. It is probable that its purpose is to drain the old ironstone quarries in this area. The OS map also shows a small pond just outside the SE edge of the development area west of Harry’s Wood. This pond may be natural but could be associated with quarrying in the area. It is of no hydrological significance.

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8.141 There are no Anglian Water surface water sewers either within or in the immediate vicinity of either part of the development area.

Flood Risk within the Potential Development Area

8.142 The Environment Agency's Indicative Floodplain map (2002 edition) shows a narrow floodplain along both the South Arm of Willow Brook and Harpers Brook in the Weldon and Stanion areas, but these floodplains are only about 100m wide. The Corby Strategic Flood Risk map (Figure 6.0, Sheets 4 & 6) shows a very similar picture with Flood Risk Categories 2 and 3 land essentially coincident with the Indicative Floodplain.

8.143 As there are no secondary sources of flood risk within the development area, the whole of the Corby East development area can therefore be regarded as in Flood Risk Category 1. It should, however, be pointed out that the disturbed character of the landforms in the development area will make it necessary for surface water drainage systems to be configured and designed with considerable care if the creation of areas of local flood risk within the development area by inappropriate layout or design is to be avoided.

8.144 Because of the elevated watershed location of the development area, the effects of climate change will not have any effect on the flood risk assessment made above.

8.145 Much of the land within the development area to the east of the A43 has been allocated for recreation and leisure - a golf course - in the Borough Council's Local Plan (L21). Part of the small area to the west of the A43 has been designated for environment and nature conservation after landscaping (E7). If these areas were used for the purposes proposed within the Local Plan, rather than for industrial use as proposed under the Regeneration Framework, then the rates and volumes of surface water runoff from these areas are likely to be significantly less and close to their present ‘greenfield’ values.

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Weldon South Industrial Estate and Max Park

General Description of the Potential Development Area

8.146 The Weldon South Industrial Estate and Max Park potential development area as outlined in the Regeneration Framework are situated approximately 3.5km east of the centre of Corby, straddling the A43 road between its intersections with the A427 in the north and Old Stamford Road in the south. The Weldon South Industrial Estate (7.0ha) lies to the west of the A43, and the much larger Max Park development area (33.9ha) to the east of the main road, as shown in Figure 10.10.

8.147 The Weldon South Industrial Estate area on the western side of the A43 is a roughly rectangular area, rather elongated but widening out at its east end. The area is bordered on the north by the South Arm of Willow Brook (Weldon FSR), by the A43 dual carriageway road on the west, and an existing property boundary on the west. The whole of this area is at present a derelict industrial site, previously a civil engineering contractor’s depot located on the line of an abandoned mineral railway embankment.

8.148 The Max Park development area to the east of the A43 is much larger, and shaped like a truncated triangle. This area is bordered by the A43 to the west, the South Arm of Willow Brook (Weldon FSR) to the north, a disused ironstone quarry to the east and Old Stamford Road to the south. Most of the Max Park development area was previously a large ironstone quarry. The land in the northern half of the development area has recently been regraded and a major warehousing development is currently in progress (Summer 2004). The southern half of the area is still largely waste land though there is some existing commercial development along Old Stamford Road, a building stone depot, a bus depot and a caravan sales compound.

8.149 Because the area has been extensively quarried there are no contours or spot heights shown within the area on the OS map. From spot levels and contours in the vicinity it is estimated that ground levels in the development area are generally between 90m and 95mOD.

Hydrology of the Potential Development Area

8.150 The Weldon South Industrial Estate and Max Park development areas are both located within the catchment of the South Arm of Willow Brook. The principal flood risk source for both parts of the development area is the Willow Brook (South Arm) which runs along their northern boundary, upstream and downstream of the A43 road. There are no other significant flood risk sources in the development area, and the OS map shows no minor surface watercourses within or adjacent to either part of the area.

8.151 The Weldon Flood Storage Reservoir is located on the South Arm of Willow Brook, extending both upstream and downstream of the A43. The downstream section of the reservoir is on-line, with an embankment across the stream. West of the main road the upstream section of the reservoir is off-line, stretching along the south bank of the Brook. There is a smaller on-line reservoir, Quarry Road FSR, immediately downstream of Weldon FSR.

8.152 There is a pond within the smaller Weldon South development area adjacent to the A43 road. This pond, described in para.4.70, is for the attenuation of runoff from the main road and discharges into the Willow Brook (South Arm) immediately upstream of the culverts under the road.

8.153 The Anglian Water sewer plans show several surface water discharges into the South Arm of Willow Brook on the north (left) bank, opposite the development area. To the west of the A43 there is a 450mm and a 300mm diameter drainage outfall, and downstream of the A43 there is a 300mm and a 375mm diameter outfall. The AW plans show no surface water sewers within the development area, either east or west of the A43.

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Flood Risk within the Potential Development Area

8.154 The Environment Agency's Indicative Floodplain map (2002 edition) shows a narrow floodplain along both banks of the Willow Brook (South Arm) along the northern edge of the development area, both upstream and downstream of the A43 road bridge.

8.155 In theory, the South Arm of Willow Brook constitutes a potential flood risk source for the development area. In practice, because of the modified nature of the topography of the area, the narrow width of the natural floodplain and the presence of the flood storage reservoir, the flood risk within the development area is likely to be very localised and confined to the margins of the watercourse and reservoir.

8.156 The distribution of flood risk within the development area is shown on the Strategic Flood Risk Map, Figure 6 (Sheet 4). It is clear that, apart from a narrow strip of land in Flood Risk Category 3 (and a wholly insignificant strip in Category 2) the great majority of the Corby East development area will fall within Flood Risk Category 1.

8.157 Weldon FSR was designed to accommodate a flood event with a fifty year return period, but will still provide a diminishing degree of attenuation for events with progressively greater return periods. Even in the Category 3 flood risk areas in the development area, the actual flood risk will therefore be relatively small.

8.158 As with the Corby East development area, possible climate change will not have any appreciable effect on flood risk in this development area, except perhaps in the marginal land already in Category 3 where flood risk may increase.

8.159 Despite the fact that a considerable amount of development has already taken place in the Max Park development area, no flood risk assessments of this area appear to have been carried out by previous or prospective developers.

8.160 There is provision in the Corby Local Plan for significant urban development within this area. It should be noted that on the Local Plan the Weldon South Industrial Estate to the west of the A43 has been designated as an employment area (J19). To the east of the A43 the Max Park development area has been designated for employment (J12, J21 & J22) and a hotel development (S27). Also in the Local Plan there is a small Country Wildlife Site (NC44) in old quarry workings near the SE corner of the area, and a small area of land adjacent to the road junction in the SW corner proposed for landscaping. For the purposes of a strategic level study, the rates and volumes of runoff created by this pattern of land use will be approximately the same as if these areas were to be used for industrial purposes as proposed in the Regeneration Framework.

Faber Maunsell Corby Strategic Flood Risk Assessment 85 9 Conclusions

9.1 Taking the Flood Risk Zones defined in PPG25 (i.e. without taking existing flood defences into account) the Environment Agency has produced a series of maps covering England and Wales which show those areas of the country which the Agency consider to be within PPG25 Flood Risk Zone 3 - i.e. at high risk (>1% per annum fluvial or >0.5% tidal) of flooding. The Indicative Floodplain maps published by the Agency cover the Borough of Corby to a limited extent and show the flood risk as variously within PPG25 Flood Risk Zones 1, 2 and 3.

9.2 Leaving aside the question of whether this is an accurate or realistic assessment, these maps ignore the reduction in flood risk due to the presence of existing flood defences. Where the standard of protection provided by those defences is less than the 1 in 100 year fluvial flood event this does not invalidate the flood risk classification although it gives no indication of relative flood risk within Flood Risk Zone 3. However, in the case of Corby, where areas of the town that were subject to a fluvial flood risk are now defended to a standard of protection in excess of the 100-year event, the Agency's Indicative Floodplain maps are of no practical help to the Borough Council for planning purposes in determining the actual or even the relative degree of flood risk at different locations within the defended areas of the Borough. The use of these plans to the Borough Council is also hampered by the fact that the majority of the watercourses within the Borough are relatively minor and therefore have not been included by the Indicative Floodplain maps.

9.3 Studies and investigations undertaken in connection with this Report have identified the various potential sources of fluvial flood risk that could cause flooding in the Borough of Corby. The principal flood risk source for Corby is localised flooding along the minor watercourses running through the Borough. Taking natural and man-made topographical features into consideration, the areas of influence of the principal flood risk sources have been defined. Estimates of flood levels at these primary flood risk sources for events of different return periods have been obtained from both the assessment of the available topographical information and, where available, the results of modelling studies.

9.4 This has enabled a set of six 1/10,000 scale maps covering the whole Borough (Figure 6) to be prepared, showing an estimate of flood risk at any point within the Borough. These maps should enable the Borough Council to determine more accurately the relative degree of flood risk to which different areas of the Borough may be subject, thereby enabling informed planning decisions, both strategic and site-specific, to be made and justified with confidence.

9.5 In addition to the general flood risk maps described above, this Report also contains detailed Flood Risk Assessments for ten potential development areas identified by Catalyst Corby. These development areas were used because when this study began the Local Development Framework process was not well advanced. Whilst the LDF process is now further advanced and options for urban extensions and employment sites in Corby have been refined, for the purposes of completeness this Strategic Flood risk Assessment covers all the potential development areas originally outlined in the Catalyst Corby Regeneration Framework. For each development area the flood risk assessment includes a description of the extent of those parts of the development area within each of the three flood risk categories defined in this Report.

Faber Maunsell Corby Strategic Flood Risk Assessment 86

9.6 The three flood risk Categories adopted for this study are, for consistency, numerically equivalent to the three flood risk Zones defined in PPG25. Although there are a number of areas in the Borough deemed to fall within flood risk Category 3, the definition of "high risk" derived from PPG25 as being any area with a level of flood risk greater than an annual probability of 1% (i.e. more than once in 100 years) is exceptionally severe, especially when it is realised that a only a small minority of fluvial flood alleviation schemes approved and grant-aided by DEFRA give protection to this standard. The results of this study suggest that there are now no significant built-up areas within the Borough that are at a serious risk of flooding.

9.7 This Report makes suggestions for flood risk alleviation policies to be adopted by the Borough Council in dealing with applications for planning consent from prospective developers. In this context, the Report also puts forward guidance notes for developers to ensure that their proposals incorporate measures to reduce not only any flood risk to their own development but also, by attenuating storm runoff from impermeable areas created by their development, the flood risk to property downstream of the development.

9.8 It is hoped that this Report will form a sound and reliable basis for Corby Borough Council to make informed and confident decisions on planning issues, both at the strategic and site-specific levels, thereby reducing the time taken to reach decisions and the resources employed in reaching those decisions.

Faber Maunsell Corby Strategic Flood Risk Assessment 87 10 Recommendations

10.1 In accordance with the findings of this study and the conclusions reached in Section 9 of this Report the following recommendations are made:

That Corby Borough Council utilise the set of six Strategic Flood Risk Maps prepared for the Borough (Figure 6 in this Report) in devising strategies and policies for incorporation in their forthcoming Local Development Documents.

That the flood risk assessments made for the ten proposed development areas identified in this Report be used to apply the sequential test advocated in Planning Policy Guidance Note 25 (PPG25) in deciding the most appropriate areas for development and the sequence in which those areas should be developed.

That at least a 100-year standard of flood protection be applied by the Borough Council as a common standard for all significant development within the Borough, both in respect of flood risk to that development and for measures taken by developers to attenuate additional runoff generated by that development.

That the Borough Council consider undertaking topographic surveys of the channels and floodplains of those principal tributaries of the River Nene within the Borough not included in the Agency's recently acquired computer model of the River Nene with a view to extending the Agency's model to include these watercourses. Also required will be accurate assessments of the current degree of protection of the reservoirs and the exact extent of the areas for which they attenuate.

That, in conjunction with Anglian Water, hydraulic modelling of the largest surface water sewers in the Borough be undertaken in order to determine their hydraulic capacity and the degree of flood risk to neighbouring property posed by those sewers in a major storm event. This could be integrated with the extension of the Nene model as mentioned above.

That the Borough or appropriate organisation (as stated in the EA’s Operational Plan – Appendix 3) assess their current maintenance programme for the assets for which they are responsible for. The number of open channels and / or inlet structures etc which have their hydraulic capacities reduced due to the build up of rubbish was highlighted during site visits - see Photographs 10, 23, 25, 30, 31 and 34 in Appendix 2.

That the Borough Council give initial consideration to measures that might be taken to maintain the present standard of flood protection afforded to the Borough by the existing reservoirs, in the event of future large scale urban development in the Upper Nene catchment. Such measures are likely to include on-site balancing facilities as part of any development.

That the Borough Council adopts the flood risk alleviation policies suggested in this Report for dealing with applications for planning consent from prospective developers, together with the proposed guidance notes for developers to assist developers in putting forward proposals in line with the Council's flood risk alleviation policies.

Faber Maunsell Corby Strategic Flood Risk Assessment 88

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Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 89 References

1. Corby Borough Strategic Flood Risk Assessment - Stage 1 Report Bullen Consultants, Peterborough (for Corby BC) February 2004

2. Planning Policy Guidance Note 25 – Development and Flood Risk Department for Transport, Local Government & The Regions July 2001

3. The Regeneration Framework Catalyst Corby Urban Regeneration Company September 2003

4. Corby District - Storm Drainage Studies Report John Taylor & Sons (for Corby District Council) May 1984

5. Northern Arm of Willow Brook Catchment Study – Report (Part 1) Rodney Environmental Consultants, Liverpool (for Corby DC & Commission for New Towns) April 1983

6. Central Stream of Willow Brook Catchment Study – Report Rodney Environmental Consultants, Liverpool (for Corby DC & Commission for New Towns) February 1984

7. Corby Area Flood Storage Reservoirs - Gretton Brook Black & Veatch Consulting (for EA, Anglian Region) November 2003

8. Willowbrook Central and West Stormwater Retention Facility George Crowder Associates (for Corby District Council) August 1988

9. Willowbrook East Stormwater Retention Facility George Crowder Associates (for Anglian Water Authority) March 1987

10. Central Stream of Willowbrook Retention Facility George Crowder Associates (for Anglian Water Authority) April 1987

11. Flood Risk Assessment for Proposed Development at Dash Farm Close, Weldon Abington Consulting Engineers (for Environment Agency) December 2001

12. Eurohub Reservoir – letter summarising study findings, with attachments THDA Consulting Engineers (submission to Environment Agency) June 2003

13. Soot Banks Reservoir - Operational Agreement Deutsche International Custodial Services Ltd, (for Prologis Kingspark Developments Ltd and the EA) October 1999

14. Corby Area Flood Storage Reservoirs – Great Oakley Black & Veatch Consulting (for EA, Anglian Region) November 2003

15. Snatchill Development, Corby – Catchment Study Report Haiste Group Ltd (for Alfred McAlpine Homes Ltd) September 1991

16. Oakley Vale Flood Risk Assessment – Final Report JBA Consulting Engineers (for Cameron Taylor Bedford) December 2001

Continued over ….

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 90

Continued ….

17. Longcroft Balancing Pond – letter summarising study findings, with attachments Scott Wilson Kirkpatrick (submission to National Rivers Authority) December 1995

18. North Kesteven Strategic Flood Risk Assessment Bullen Consultants, Peterboro' (for North Kesteven DC) September 2002

19. A Study of the Bankfull Discharges of Rivers in England and Wales M. Nixon, Proc. I.C.E. Vol.12 February 1959

20. Evaluation of the FEH Rainfall – Runoff Method for Catchments in the UK Ashfaq A & Webster P Journ. C.I.W.E.M. pp223/8 Vol.16 2002

21. Corby Water Cycle Strategy, Phase 1 - Outline Strategy Summary Report Halcrow (for EA, Corby BC and Catalyst Corby) September 2005

22. Nene Tributaries Pre-Feasibility Studies, 2004/05 - Willow Brook Pre-Feasibility Report Halcrow (for Environment Agency) February 2006

23. Milton Keynes & South Midlands Sub-Regional Strategy - Consultation Draft East Midlands, East of England & SE England Regional Assemblies July 2003

23a Milton Keynes & South Midlands Sub-Regional Strategy - Schedule of Secretary of State Proposed Changes Govt. Offices for the South East, E Midlands & E of England October 2004

23b Milton Keynes & South Midlands Sub-Regional Strategy - Alterations to the regional Spatial Strategy etc Govt. Offices for the South East, E Midlands & E of England October 2004

23c Regional Spatial Strategy for the East Midlands (RSS8) Government Office for the East Midlands (ODPM) March 2005

24. Framework for Sustainable Drainage Systems (SUDS) in England and Wales National SUDS Working Group (Draft document for consultation) May 2003

25. Flood and Coastal Defence Appraisal Guidance - Economic Appraisal (Vol 3) Ministry of Agriculture Fisheries & Food (now DEFRA) December 1999

Appendix 1 - Figures

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Appendix 2 - Photographs

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Appendix 3 Environment Agency Operational Plan for the Corby Borough Council Area

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Appendix 4 ISIS Model Input and Results

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Appendix 5 Flood Risk Guidance Notes f or Developers

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Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 I

Corby Borough Council

Flood Risk Guidance Notes For Developers

Introduction

Flooding issues have long been recognised as a material consideration in the development planning process, and in view of the apparent increase in frequency and severity of fluvial flooding in recent years, the Government has asked Local Authorities to give greater consideration to flood risk in the planning process by discouraging inappropriate development.

Recent Government advice is that a precautionary and risk-based approach should be taken in respect of decisions made by Local Planning Authorities on applications for development consent where flood risk is an issue.

This advice is contained in the Department for Transport, Local Government and the Regions' Planning Policy Guidance Note 25 - Development and Flood Risk ("PPG25") which requires land to be categorised according to the level of flood risk to which that land is considered to be subject. The Flood Zone maps recently published (July 2004) by the Environment Agency will assist this process, augmented by the more detailed analysis of actual flood risk contained in Corby Borough Council’s Strategic Flood Risk Assessment. For fluvial flooding the three flood risk categories are defined as follows:

Category 1 Little or No Risk (annual probability of flooding less than 0.1%, or, on average, once in every 1000 years)

Category 2 Low to Medium Risk (annual probability of flooding between 0.1% and 1% or, on average, between once in 100 and once in 1000 years)

Category 3 High Risk (annual probability of flooding greater than 1% or, on average, more than once in 100 years)

The above categories relate to the actual flood risk to which the land in question is subject, and they take into account the reduction in risk afforded by any effective flood defences by which that land is protected from flooding.

PPG25 recommends that in Flood Risk Zones considered to be at little or no risk of flooding, most forms of development are suitable. Similarly, most types of development will also be acceptable in areas with a low to medium risk of flooding, though some types of sensitive development may need special consideration. In areas subject to a high risk of flooding, non-essential developments will not normally receive planning permission in previously undeveloped areas. In high risk areas already developed, however, development may be permitted where measures already exist or can be taken to give adequate protection against flooding.

In considering options for land allocations for future development, consideration will be given by the Borough Council to both the Flood Zones mapped by the Agency and the degree of actual flood risk identified by the Strategic Flood Risk Assessment. Where a 100-year standard for the design of flood defence or runoff attenuation works is stated in these Guidance Notes, developers should be aware that the Borough Council, advised by the Environment Agency, may impose a higher standard in those cases where the Flood Risk Vulnerability of the proposed development, or of land or property likely to be affected by the development, is considered to require it. Developers are therefore advised to consider the use and vulnerability of the proposed development in order to determine appropriate standards at an early stage in the planning process.

Developers are additionally advised that any works proposed within the Agency’s bye-law distance of any Main River (9 metres), works affecting a watercourse, or works within the floodplain may require the formal consent of the Agency prior to commencement of those works. Developers are therefore advised to contact the Environment Agency to discuss the proposed works. On ordinary watercourses consideration should be given to the provision of access to inspect and maintain the watercourse.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 II

Flood Risk Policies

The following Flood Risk Policies will be applied by the Borough Council to all applications submitted to the Council for development consent.

POLICY 1 The Need for Flood Risk Assessment

Corby Borough Council may require the submission of an appropriate Flood Risk Assessment from the developer in connection with any application for development consent.

If the Borough Council considers that a proposed development is on land considered to be at risk of flooding or is likely to present a significant flood risk or increased flood risk to other land or property, they may require that the developer submits a Flood Risk Assessment of the development site in connection with the application for planning permission. It should be assumed that a Flood Risk Assessment may be required in most cases, though exceptions will normally be made for minor developments such as alterations or extensions to existing buildings. Developers are therefore advised to seek the advice of the Planning Officer before submitting an application as to whether the Council is likely to require a Flood Risk Assessment. The Borough Council will use the Environment Agency’s Flood Risk Standing Advice Matrix when deciding whether or not a flood risk assessment is required.

The Flood Risk Assessment must examine the flood risk issues and implications for the development over its whole lifetime, taking into account (where relevant) the possible impacts of climate change. The Assessment must be appropriate to the location, size, complexity and sensitivity of the development proposal and should address those matters outlined in Appendix F of PPG 25. The Assessment should consider the risks of flooding from open watercourses and, where relevant, from surface water sewers and piped drainage systems, and groundwater.

The Flood Risk Assessment should also address the implications of increased surface water runoff from paved and impermeable areas created by the development for flood risk to land and property downstream of the development. If the Assessment finds that additional surface water runoff is likely to be generated by the development at times of heavy rainfall, the development proposals should incorporate suitable measures to attenuate the additional runoff to levels that existed prior to the development taking place. Consideration of the use of Sustainable Urban Drainage Systems ("SUDS") is recommended. SUDS are dealt with in detail in Policy 5.

Where a substantial development is envisaged, the Borough Council encourages developers to consult the Environment Agency before making a formal application for planning consent to discuss the potential flood risks to their development, on the likely impact of their proposals on flood risk elsewhere, and what flood risk mitigation measures might be necessary, effective and acceptable. A For substantial developments, a Flood Risk Assessment carried out by a competent person can be an essential element in the overall evaluation of the proposed development and its approval by the Borough Council.

Where flood risk alleviation works form a necessary pre-condition of development consent, such works will normally be funded by the developer and controlled by the Planning Authority, probably through planning conditions or a Section 106 Agreement with the Borough Council. Where the proposed alleviation works are likely to require ongoing future maintenance, appropriate agreements shall be entered into prior to construction of to ensure the long term effectiveness of the works.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 III

POLICY 2 Development in areas deemed to be at Little-or-No Risk of flooding in the Corby Strategic Flood Risk Assessment.

The Borough Council's Strategic Flood Risk Assessment has classified all land within the Borough in one or other of the three Flood Risk Categories described in the Introduction to these guidance notes. This classification has been undertaken at the strategic level and is intended primarily for guidance purposes in the overall planning process. It should not therefore be regarded as definitive and does not remove the need for site-specific flood risk assessments.

Planning approval will not normally be refused in these areas on flood risk grounds provided that :

A) The development will not itself be at any appreciable risk of flooding. (It may itself be at risk from other, secondary sources of flooding such as surface water sewers.)

B) The development will not create an appreciably increased risk of flooding for other persons, land and property. (Even though a development outside the floodplain may not itself be at any appreciable risk of flooding, it may nevertheless increase the risk to others by increasing the rate and volume of surface water runoff from the development site.)

C) All flood risk mitigation measures shall be implemented in accordance with the implementation programme submitted with the approved Flood Risk Assessment before the development is brought into use.

Developers should therefore appreciate that a Flood Risk Assessment may still be necessary for developments in Little-or-No Risk or Low-to-Medium Risk areas

POLICY 3 Development in areas deemed to be at High Risk of flooding in the Corby Strategic Flood Risk Assessment.

Developments within the natural floodplain of a river or stream are inherently at risk of flooding and can also increase flood risks to others, not only by increasing surface water run-off rates but by obstructing or diverting flood flows and reducing flood storage.

Planning consent will be refused for developments in these areas unless:

A) It is considered either appropriate (in developed areas), or essential (in other high risk areas) for that location within the criteria set out in Table 1 of PPG25.

B) It is protected from flooding to an appropriate standard or is designed to cope with the risk of flooding.

C) Ground floor living accommodation is excluded in residential developments where that development is adjacent to a raised flood defence.

D) The development does not create an unacceptable obstruction to flow across a floodplain under flood conditions, and does not divert the flow of flood water towards or across adjacent land or property.

E) The development does not appreciably reduce the volume available for the retention of water on the flood plain in times of flood.

F) The development does not jeopardise the integrity of existing flood defences in any way, or obstructs the operational access thereto.

G) All flood risk mitigation measures shall be implemented in accordance with the implementation programme submitted with the approved Flood Risk Assessment before the development is brought into use.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 IV

POLICY 4 Development involving building in areas identified as Washland or Functional Floodplain.

A washland is an area of land within a floodplain which may be deliberately inundated in times of flood to reduce the risk or severity of flooding elsewhere in the river system. A functional floodplain is an area of undefended floodplain which is expected to flood on a frequent basis and which, by being allowed to flood, will reduce the risk or severity of flooding elsewhere.

Development involving building in areas identified as washland or functional floodplain will only be permitted in exceptional circumstances. Although no specific areas of washland or functional floodplain have been identified in the Corby Strategic Flood Risk Assessment it should not be assumed that such areas will not be established in the future.

This policy is intended to prevent development which might impede the flow of water onto a washland or functional floodplain, or reduce the volume available for the temporary storage of flood water in those areas.

POLICY 5 Sustainable Urban Drainage Systems (SUDS)

The Borough Council requires developers to demonstrate that their surface water drainage proposals, particularly for large sites, are appropriate and adequate for the development and will not increase to the flood risk to land and property either upstream or downstream of the development site. The Council considers that Sustainable Urban Drainage Systems (SUDS) are a desirable means of achieving this and encourages their use by developers.

Urban development, especially of "greenfield" sites, alters the existing drainage characteristics of an area with roofs, roads and other impermeable surfaces from which rainfall is more rapidly translated into runoff. The management and control of this increased surface runoff has a major role in sustainable development.

Sustainable drainage is the practice of controlling surface water runoff as close to its origin as possible before discharge to a watercourse or to a soakaway. It has many benefits relating to a variety of environmental issues such as reducing flood risk, minimising pollution of watercourses and groundwater, minimising soil erosion and damage to natural habitats, maintaining or restoring natural flow regimes in receiving watercourses, maintaining groundwater recharge and achieving environmental enhancements. The many and diverse benefits resulting from the use of SUDS justify the requirement for the widespread use of SUDS in development proposals.

A variety of types of SUDS are described in detail in the latest version of a document entitled "Framework for Sustainable Drainage Systems in England and Wales" published by the National SUDS Working Group in May 2003 and available from the Environment Agency.

Continued over (Policy 6) …..

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 V

POLICY 6 Culverting of Open Watercourses

The Environment Agency and Borough Council are in general opposed to the culverting of open watercourses because of the adverse ecological effect, potentially increased flood risk and other consequences that are likely to arise. Where practical the Council may seek to have existing culverted watercourses restored to open channels as part of the development proposals.

The Borough Council will therefore only approve plans to culvert an open watercourse if there is no reasonably practical alternative to culverting, or if the detrimental effects of culverting would be so minor that they would not justify a more costly alternative. In all cases where it is appropriate to do so adequate mitigation must be provided for damage caused to natural habitats and to animal, plant and other species by the culverting.

If culverting is approved, the size and material of the pipes used must be adequate to convey flood flows in the watercourse and appropriate to any vehicular or other load likely to be imposed upon the culvert. The developer may be required to demonstrate to the Borough Council with appropriate hydraulic calculations that the culvert will adequately convey the flood flow in the watercourse without exacerbating flooding upstream or along the line of the culvert. The Council will not normally approve the installation of a culvert of smaller size than one further upstream on the same watercourse.

Culverts, especially in urban areas, are liable to become obstructed or blocked by debris carried by flood waters or by illegally deposited rubbish. The Council may therefore require a screen of a suitable design to be erected at the entrance to the culvert. The design of the screen must permit safe and convenient access for the removal of debris and rubbish. Where the culvert is longer than twenty metres, the Council may require the installation of one or more intermediate access manholes on the line of the culvert for maintenance purposes.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 VI

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Appendix 6 Flood Risk Standing Advice Matrix (and Explanatory Notes)

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Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2

FLOOD RISK - INITIAL PLANNING RESPONSE MATRIX

Environment Agency Standing Advice - Development & Flood Risk (England) (July 2004 Edition)

Column B C D E F Within Flood Risk Within Flood Risk Within Flood Risk Within Main River Cell 5 Development Category Category 3 Category 2 Category 1 byelaw distance (High risk areas) (Low / medium risk areas) (No / low risk areas) No planning comment - 6 Domestic extensions Consult EA Standard response Standard Response see consent note Industrial / commercial No planning comment - 7 Consult EA Standard response Standard Response extensions less than 250m 2 see consent note Change of use to a more 8 Consult EA Consult EA Standard response No planning comment ‘flood risk sensitive’ use No planning comment - 9 Camping & caravan sites Consult EA Consult EA Standard response see consent note Operational development less General surface water 10 Consult EA Consult EA Standard response than 1ha drainage information Operational development 11 Consult EA Consult EA Consult EA Standard response between 1ha and 5ha Civil emergency infrastructure 12 Consult EA Consult EA Consult EA Standard response less than 5ha All operational development 13 Consult EA Consult EA Consult EA Consult EA greater than 5ha

Note: This Flood Risk Matrix must be read in accordance with the Agency’s accompanying General Guidance Note and Technical Guidance Notes on Flood Risk Assessment.

See overleaf for Procedure Key …….

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2

Procedure Key - Development and Flood Risk Standing Advice

Consult EA High Risk - LPA should consult Environment Agency, Agency to see Flood Risk Assessment before commenting.

Standard response Low Risk - LPA need not refer planning application to Agency, standard response applies, FRA required.

No planning comment Lowest Risk - LPA need not refer to Agency, outside scope of standing advice.

General surface water Land Drainage Act consent information applicable to development adjacent to Ordinary Watercourses in all locations. drainage information

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 I

Corby Borough Council

FLOOD RISK STANDING ADVICE MATRIX

Environment Agency Explanatory Notes

PROCEDURE KEY

Flood Risk Assessment - the Obligation to Assess Risk

The need for an appropriate assessment of flood risk is set out in paragraphs 20 and 57 to 60 of PPG25. Further guidance on the scope of issues a Flood Risk Assessment (FRA) might cover is provided in Appendix F of PPG25. In addition, the Agency has produced a series of four guidance notes for Local Planning Authorities (LPAs) and developers intended to indicate as far as possible which FRA issues should be addressed in which circumstances. PPG25 specifically acknowledges that failure to provide an assessment of flood risk that is appropriate to the scale and nature of the development and the flood risk associated with that development may constitute a reason for refusal of planning consent (PPG25, paragraph 60).

These Explanatory Notes should, where applicable, be read in conjunction with the Flood Risk Guidance Notes for Developers in Appendix 3 of this Report.

A) General advice to Corby Borough Council on development within the RED boxes of the Matrix

Status

Comments should be sought form the Environment Agency on planning applications and flood risk assessments where the proposed development and the site location are as described within the RED boxes in the Matrix. These boxes represent high flood risk combinations of development type and/or location where the Agency needs to continue to provide a bespoke response at the pre-application, application and post-application stages.

Process

Developments falling within the RED boxes will need a Flood Risk Assessment (FRA) before the Agency can comment on the planning application.

The need for an appropriate assessment of flood risk is set out at paragraphs 20 and 57 to 60 of PPG25. General advice on the range of factors to consider in FRAs is contained in Appendix F of PPG25. The Environment Agency has produced a series of four guidance documents to aid those involved in the development process to identify the factors which need to be considered, and in which circumstances.

The following process is recommended to enable a 'fast track' consideration of FRAs at the planning application stage.

1) The Local Planning Authority (LPA) should advise applicants to contact the Environment Agency for pre-planning discussions to scope the specific requirements of their FRA. 2) Applicants should then submit a preliminary FRA to the Agency. 3) If necessary, the Agency will then contact the applicant to discuss any final amendments required to their FRA.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 II

4) Where the FRA is acceptable to the Agency, the Agency will issue a Letter of Compliance to the applicant. 5) At the planning application stage, the applicant submits the Letter of Compliance to the LPA together with the planning application and FRA, which the LPA will forward to the Agency.

Further information on the Letter of Compliance procedure is given in Item (B) below.

Outcome

If a FRA has not been submitted to the LPA with the planning application then the Agency will object to the application. The FRA submitted must demonstrate to the Agency's satisfaction that the development can proceed without creating an unacceptable flood risk, either to future occupants of the site or elsewhere. If it cannot do this then the Agency will maintain its objection.

Where the FRA is acceptable the Agency will advise on flood risk conditions at the Outline, Full, or Reserved Matters stage of the process, or make such other recommendations as appropriate.

Where an LPA is considering granting planning permission contrary to Agency advice for development within the RED boxes (as per paragraph 65 of PPG25) the Agency should be consulted again, informed of the grounds for the intended grant of planning permission, and given an opportunity to make further representations.

B) General advice to Corby Borough Council on development within the GREEN boxes in the Matrix

Status

The Agency will not comment on individual planning applications or pre-planning consultations and any accompanying flood risk assessment where the combination of the development type/scale and the site location falls within the GREEN boxes of the matrix that indicate lower flood risk.

There is, however, an exception to this general rule which applies to development between 1.0 and 5.0 hectares outside Category 3 (cell F12) in relation to pre-planning consultations. For this limited category of development the Agency wishes to continue to have a technical input at pre-application stage to ensure that surface water issues are adequately addressed. A procedure to help ensure this is set out below (the Letter of Compliance procedure).

Process - General

The LPA should process planning applications in accordance with their development plan policies and their Strategic Flood Risk Assessment, the information presented in any Flood Risk Assessment (FRA), and the standing advice in the relevant cell of the matrix.

Process - Letter of Compliance

Where applicable, as set out above, the Letter of Compliance procedure should be applied as follows:

1) LPAs should advise applicants to contact their local Environment Agency Planning Liaison Team in the first instance for pre-planning discussions to scope the specific requirements of their flood risk assessment.

2) Applicants should then submit a preliminary FRA to the Agency.

3) If necessary, the Agency will then contact the applicant to discuss any final amendments required to the FRA.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 III

4) When the FRA is acceptable to the Agency, the Agency will issue a Letter of Compliance to the applicant.

5) At the planning application stage, the applicant submits the Letter of Compliance to the LPA together with planning application.

Outcome

If an appropriate FRA is not provided where the standing advice advises that this is necessary then LPAs should refuse or defer the application pending receipt of further information and direct the applicant to commence pre-application discussions with the Agency.

C) General advice to Corby Borough Council on development within the GREY boxes in the Matrix

The Agency will not comment on individual planning applications or pre-planning consultations, and any accompanying Flood Risk Assessment where the proposed development and the site location fall within the GREY boxes in the matrix that indicate the lowest scale of flood risk. Developments within these boxes falls outside the scope of standing advice.

The Agency's response in these cases is either 'no comment' because no significant surface water runoff issues are raised or 'General surface water drainage information', which sets out a few basic land drainage principles for information purposes only.

The Agency does not require submission of a formal Flood Risk Assessment for developments in these locations. The attention of LPAs is however drawn to paragraph 60 of PPG25 which states that a FRA appropriate to the nature and scale of the development should be carried out. Should LPAs wish to insist on the submission of a FRA for developments within these cells, advice on what this should contain is given in FRA Guidance Note 3.

D) Development adjacent to Ordinary Watercourses

The requirement for consent under the Land Drainage Act 1991

Under the terms of the Land Drainage Act 1991, the prior written consent of the Environment Agency is required for any proposal to divert, culvert or otherwise obstruct the flow in an Ordinary Watercourse, including the provision of a connection to a culvert. An Ordinary Watercourse is any watercourse not identified as a Main River held on maps by the Environment Agency and DEFRA.

Some Local Authorities have their own byelaws and planning policy restrictions and Internal Drainage Boards (where they exist) will have their own byelaw consenting arrangements.

The Environment Agency makes the following specific comments:

Diversion of a Watercourse

Any diversion of an existing watercourse should maintain the original watercourse in cross section area and gradient. Such works will require the prior consent of the Environment Agency under the Land Drainage Act 1991.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 IV

Culverting

Culverting increases the risk of flooding by restricting the capacity of the watercourse to cope with increased flows during flood events. It also presents significant maintenance problems and involves an increased risk of obstruction and blockage. The Environment Agency therefore has a policy on culverting that strongly discourages the creation of new culverts or extensions to existing culverts, except where required for essential access purposes.

Applicants considering culverting are advised to contact the Agency at the earliest possible stage to discuss the feasibility of their proposals and the likelihood of gaining Land Drainage Act consent.

The Agency is likely to refuse consent to proposals to develop over both existing and proposed culverts. The reason for this is that buildings over culverts will obstruct any overland flow route for flood water when the capacity of the culvert is exceeded, thereby increasing the likelihood of flooding to the development and its neighbours. The culvert may also need to be repaired or replaced in the future to maintain or increase its capacity.

Ensuring Access for Maintenance

It is important to ensure that access for maintenance purposes is retained or provided in relation to all watercourses, whether culverted or not. For Ordinary Watercourses the Agency makes the following recommendations: For watercourses with a top width less than 2 metres and for culverts less than 1 metre wide or 1 metre diameter the Environment Agency recommends that a minimum easement on both banks of 3 metres from bank top or culvert edge is left clear to allow access for maintenance works. Where appropriate, a 4 metre wide access route should be provided to gain access to this easement.

For watercourses larger than 2 metres wide and culverts larger than 1 metres wide or 1 metre in diameter, the Environment Agency recommends that a minimum easement on both banks of 5 metres from bank top or culvert edge is left clear to allow access for maintenance works. Where appropriate, a 4 metre wide access route should be provided to gain access to this easement.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 V

COLUMN B DEVELOPMENT CATEGORIES

B5 Which cell does the proposed development fit into?

In assessing which cell a development falls into, where it could fit into more than one cell, a precautionary approach should always be adopted so that the development is assessed against the higher risk category.

For example, a proposed camping and caravan site falls on the boundary between Flood Risk Category 3 ( RED cell response) and Flood Risk Category 2 (GREEN cell response). The precautionary approach should be adopted, i.e. treat as a RED cell response and consult the Agency with a flood risk assessment.

B6 Domestic extensions

Domestic extensions are defined as including all ‘householder’ development, e.g. sheds, garages, games rooms etc within the curtilage of the existing dwelling, in addition to physical extensions to the dwelling itself.

This definition excludes any proposed development that would create a separate dwelling within the curtilage of the existing dwelling. These proposed developments fall within the RED cells of the matrix, i.e. “Operational development less than 1ha” or “Change of use to a more ‘flood risk sensitive’ use”.

B7 Industrial / Commercial extensions less than 250 square metres.

This size threshold of 250 m 2 relates to the size of the increased footprint of the building itself rather than the overall site size.

B8 Change of use to a more ‘Flood Risk Sensitive’ use

This category relates to the need to identify changes of use within high flood risk areas (i.e. within Flood Risk Category 3) which may increase flood risks to occupants, and to ensure that any resulting additional risks are properly assessed and managed. For example, a change of use from office to any form of residential use, especially at ground floor level, would fall into this category.

The vulnerability of potential occupants to flood risk is also a key factor. Those occupants considered most vulnerable would be people who by virtue of their age or infirmity would be at particular risk during a flood event. A proposed school would therefore be classified as a 'sensitive' development. The risks to vulnerable occupants are likely to be increased if the proposed development is intended to be residential in nature. Examples would include residential care homes and hospices.

Under the Use Classes Order 1987 relevant changes of use would include change of use

FROM A1 (shops), A2 (financial and professional services) & A3 (food and drink), and B1 (Business) & B2 (general industrial)

TO any Part C use, i.e. C1 (hotels), C2 (residential institutions) & C3 (dwelling-houses), and part D1 (non-residential institutions with vulnerable occupants), i.e. D1(a) (non-residential medical facilities) & D1(b) (e.g. crèches, day nurseries and day centres), and class D2 (assembly and leisure) uses.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 VI

B9 Camping and caravan sites

Camping and caravan sites may give rise to particular problems in relation to flood risk - see paragraph 70 and Appendix G of Planning Policy Guidance Note 25 (PPG25).

B10 Operational development less than 1 ha

This broad category covers any development that involves building, mining or engineering operations, i.e. anything other than a material change of use.

The 1ha size threshold in this category will be defined in most cases by the area enclosed by the red outline on the site plan. This defines the boundaries of the site that is the subject of the planning application. In some cases, e.g. at outline application stage, this may be the only information available on which to base a decision as to which cell in the matrix a given development falls within.

This can give misleading results where the proposed development is a small part of a large site, in which case the Local Planning Authority (LPA) should consider the scale of new buildings or hard standing proposed.

B11 Operational development between 1 and 5 ha

This broad category covers any development that involves building, mining or engineering operations i.e. anything other than a material change of use.

The 5ha size threshold in this category will be defined in most cases by the area enclosed by the red outline on the site plan. This defines the boundaries of the site that is the subject of the planning application. In some cases, e.g. at outline application stage, this may be the only information available on which to base a decision as to which cell in the matrix a given development falls within.

This can give misleading results where the proposed development is a small part of a large site, in which case the LPA should consider the scale of new buildings or hard standing proposed.

B12 Civil emergency infrastructure less than 5 ha

Civil emergency infrastructure includes installations such as hospitals, police stations and emergency vehicle depots. These installations need to be operational and access maintained in all circumstances - see paragraph 34 of PPG25.

B13 All operational development greater than 5 ha

This broad category covers any development which involves building, mining or engineering operations i.e. anything other than material change of use.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 VII

COLUMN C DEVELOPMENT WITHIN MAIN RIVER BYELAW DISTANCE

C5 Main River Byelaw Distance

This applies to any land within the byelaw distance of Environment Agency Main Rivers as shown on Environment Agency and DEFRA maps of Main Rivers. Main Rivers are indicated by a red line on the strategic flood risk maps (Figure 6) in the Corby Strategic Flood Risk Assessment.

Agency byelaws vary from region to region across England and LPAs will be advised of the byelaw distances that apply to Main Rivers in their area. In the Borough of Corby the ‘byelaw distance’ is 9 metres and development is not permitted on land within 9 metres of a Main River. The 9 metre distance is measured horizontally from the landward foot of any raised flood defence or, where there is no raised defence, within 9 metres from the top of the river bank. Environment Agency byelaws apply to any development within this 9 metre strip.

Cells C6 to C13

Environment Agency byelaw consent is required in each case and is likely to be refused. An Agency objection to the granting of planning permission is therefore likely. Early contact by the applicant with the Agency at pre-application stage is strongly advised, and prospective applicants should contact the Development Control Team at the Agency's Kettering office.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 VIII

COLUMN D DEVELOPMENT IN FLOOD RISK CATEGORY 3

Cell D5

Flood Risk Category 3 is the high risk flood risk category as defined in the Strategic Flood Risk Assessment and shown on the associated flood risk maps. This is land where the indicative annual probability of flooding is greater than 1% from rivers or watercourses (i.e. at least once in 100 years, on average).

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

Cells D6 and D7

Flood Risk Assessment - minimum requirements

Planning applications must be accompanied by a Flood Risk Assessment (FRA). To be acceptable as a FRA the applicant must, as a minimum, confirm in writing that :-

EITHER 1) Floor levels within the proposed development will be set no lower than existing levels,

AND

2) Flood-proofing of the proposed development has been considered by the applicant and incorporated where appropriate.

OR 3) That floor levels within the extension will be set 300mm above the known or modelled 1in100 year (1% annual probability) river flood level. This must be demonstrated by a plan to Ordnance Datum showing finished floor levels relative to the known or modelled flood level.

Process

A simple FRA confirming, as a minimum, the criteria set out above must accompany planning applications submitted to the LPA. The LPA should determine the adequacy of the FRA. If the applicant does not provide a FRA which meets these minimum requirements the Agency recommends that the application be refused.

Background

For proposed domestic extensions within Flood Risk Category 3 areas, the main aspect of flood risk to be considered is that the development itself may be at risk of flooding. The most effective means of addressing this risk is through submission of a simple flood risk assessment (FRA). This should identify the flood risks and set out the measures proposed to mitigate that risk. In some cases submission of a site plan showing floor levels related to Ordnance Datum may confirm that the site is outside the high flood risk area (i.e. Flood Risk Category 3). This will suffice as the FRA. Where such a plan indicates otherwise, or is not provided, mitigation measures will need to focus on setting appropriate floor levels and incorporating flood-proofing into the design of the extension. Further guidance on what issues should be covered by the FRA, including an assessment of any residual risks, is contained in the Agency's guidance note FRA 3.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 IX

Floor levels

From a purely flood risk viewpoint, the ideal mitigation in terms of floor levels is to ensure that these are set to above the known or modelled 1 in 100 year fluvial flood level at that location. However, in the case of an extension it will often not be practical to raise floor levels, given the potential effects on other issues such as access (including that for disabled users), usability and visual amenity. Any proposal to raise floor levels must therefore be discussed and agreed with the Local Planning Authority at the earliest possible stage.

Flood proofing

The Environment Agency recommends that in areas at risk of flooding, consideration should be given to the incorporation of flood-proofing measures into the design and construction of the development. These measures may include removable barriers on building apertures, such as doors and air-bricks, and bringing electrical services into the building at a high level so that power sockets are located above possible flood level. Additional guidance, including information on kite-marked flood protection products, can be found on the Environment Agency’s web site (www.environment-agency.gov.uk).

Residual risks

It should be noted that if the existing building is situated in a hollow or depression the measures adopted above such as maintaining floor levels at existing levels and flood proofing may not necessarily eliminate risks during a flood event. If in doubt about this, applicants should be asked to check ground levels. Even where it is possible to ensure that floor levels are set above the known or modelled 1in100 year (1%) fluvial flood level, flood risks will remain for events that exceeds those magnitudes.

Cells D8 to D13

In these cases the Environment Agency should be consulted and will need to see the FRA before commenting on the proposed development.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 X

COLUMN E DEVELOPMENT IN FLOOD RISK CATEGORY 2

Cell E5

Flood Risk Category 2 is the medium-to-low flood risk flood risk category as defined in Table 1 (paragraph 30) of PPG25. Category 2 flood risk areas are shown on the Local Planning Authority’s Strategic Flood Risk Assessment maps. These areas are land where the indicative annual probability of flooding from rivers or watercourses lies between 1% and 0.1% (i.e. between once in 100 years and once in 1000 years, on average).

The outer limit of the Flood Risk Category 2 envelope (the ‘extreme flood outline’) is the greater of the 1in1000 year (0.1% annual probability) modelled outline and the maximum documented extent of historic fluvial or tidal flooding. The inclusion of historic data is based on a precautionary approach to ensure that inappropriate development is prevented in areas with a recorded history of flooding.

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

Cells E6 and E7

Flood Risk Assessment - minimum requirements

Planning applications must be accompanied by a Flood Risk Assessment (FRA). To be acceptable as a FRA the applicant must, as a minimum, confirm in writing that :-

EITHER 1) Floor levels within the proposed development will be set no lower than existing levels,

AND

2) Flood-proofing of the proposed development has been considered by the applicant and incorporated where appropriate.

OR 3) That floor levels within the extension will be set 300mm above the known or modelled 1in100 year (1% annual probability) river flood level. This must be demonstrated by a plan to Ordnance Datum showing finished floor levels relative to the known or modelled flood level.

Process

A simple FRA confirming, as a minimum, the criteria set out above must accompany planning applications submitted to the LPA. The LPA should determine the adequacy of the FRA. If the applicant does not provide a FRA which meets these minimum requirements the Agency recommends that the application be refused.

Background

For proposed domestic extensions within Flood Risk Category 2 areas, the main aspect of flood risk to be considered is that the development itself may be at risk of flooding. The most effective means of addressing this risk is through submission of a simple flood risk assessment (FRA). This should identify the flood risks and set out the measures proposed to mitigate that risk.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XI

For most developments within Flood Risk Category 2, submission of a site plan showing floor levels related to Ordnance Datum should be sufficient to confirm that the site is above flood level. Where such a plan indicates otherwise, or is not provided, mitigation measures will need to focus on setting appropriate floor levels and incorporating flood-proofing into the design of the extension. Further guidance on what issues should be covered by the FRA, including an assessment of any residual risks, is contained in the Agency's guidance note FRA 3.

Floor levels

From a purely flood risk viewpoint, the ideal mitigation in terms of floor levels is to ensure that these are set to above the known or modelled 1 in 100 year fluvial flood level at that location. However, in the case of an extension it will often not be practical to raise floor levels, given the potential effects on other issues such as access (including that for disabled users), usability and visual amenity. Any proposal to raise floor levels must therefore be discussed and agreed with the Local Planning Authority at the earliest possible stage.

Flood proofing

The Environment Agency recommends that in areas at risk of flooding, consideration should be given to the incorporation of flood-proofing measures into the design and construction of the development. These measures may include removable barriers on building apertures, such as doors and air-bricks, and bringing electrical services into the building at a high level so that power sockets are located above possible flood level. Additional guidance, including information on kite-marked flood protection products, can be found on the Environment Agency’s web site (www.environment-agency.gov.uk).

Residual risks

It should be noted that if the existing building is situated in a hollow or depression the measures adopted above such as maintaining floor levels at existing levels and flood proofing may not necessarily eliminate risks during a flood event. If in doubt about this, applicants should be asked to check ground levels. Even where it is possible to ensure that floor levels are set above the known or modelled 1in100 year (1%) fluvial flood level, flood risks will remain for events that exceeds those magnitudes.

E8 Change of use to a more ‘Flood Risk Sensitive’ use

Important Notes

For all areas protected by formal raised flood defences the Environment Agency should be consulted. Failure of such defences may occur in very exceptional circumstances.

If the proposed development is close to a raised flood defence it is likely to be in a flood hazard zone where failure of that defence could pose a significant risk to life and limb. Land within a flood hazard zone may not be suitable for certain uses. A Flood Risk Assessment in accordance with Appendix F of PPG25, including a detailed evaluation of the width of the hazard zone at the proposed development site, will be required.

Flood Risk Assessment - minimum requirements

Planning applications must be accompanied by a Flood Risk Assessment (FRA). To be acceptable as a FRA the applicant must confirm in writing that :-

1) An assessment has been made of the flood risks accruing from the proposed change of use. This assessment should include a level survey to Ordnance Datum showing all known or modelled flood levels.

AND

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XII

2) What mitigation measures are proposed to deal with any identified risks, e.g. ensuring that bedrooms are located upstairs, warning or evacuation plans, flood-proofing, etc.

Process

A Flood Risk Assessment confirming the two elements set out above must accompany planning applications submitted to the LPA. The LPA should determine the adequacy of the FRA. If the applicant does not provide a FRA which meets these minimum requirements the Agency recommends that the application be refused.

Background

In most cases change-of-use development within Flood Risk Category 2 will pose little or no significant flood risk. However, for changes of use qualifying as 'flood risk sensitive’ uses the risks may be significant and need to be assessed and dealt with. For example, a proposed change of use from residential accommodation to a care home close to the boundary with Flood Risk Category 3 may bring a significantly increased number of vulnerable users close to a high flood risk area.

E9 Camping and Caravan Sites

Important Notes

For all areas protected by formal raised flood defences the Environment Agency should be consulted. Failure of such defences may occur in very exceptional circumstances.

If the proposed development is close to a raised flood defence it is likely to be in a flood hazard zone where failure of that defence could pose a significant risk to life and limb. Land within a flood hazard zone may not be suitable for certain uses. A Flood Risk Assessment in accordance with Appendix F of PPG25, including a detailed evaluation of the width of the hazard zone at the proposed development site, will be required.

Flood Risk Assessment - minimum requirements

Planning applications must be accompanied by a Flood Risk Assessment (FRA). To be acceptable as a FRA the assessment must, as a minimum, include the following three elements:-

1) A level survey to Ordnance Datum showing the known or modelled 1in100 year (1% annual probability) river flood level relative to proposed site levels.

2) An assessment of the flood risks posed to the camping and caravan site, including those based on any documented historic flooding and those associated with any increase in surface water runoff from the site.

3) Proposed mitigation measures to control those risks, e.g. ensuring that the pitches for camping and caravan users are situated as high as possible within the site, the provision of a warning / evacuation system, etc.

Process

In all cases a Flood Risk Assessment will need to be submitted with the application. This should address both the risk to the camping / caravan site itself and the control of surface water runoff from the site. The LPA should determine the adequacy of the FRA. If the applicant does not provide a FRA meeting these minimum requirements the Agency recommends that the application be refused.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XIII

Background

PPG25 (paragraph 70) recognises that camping and caravan sites are especially vulnerable to flooding. Further guidance is given in Appendix G to PPG25.

E10 Operational development less than 1ha

Important Notes

For all areas protected by formal raised flood defences the Environment Agency should be consulted. Failure of such defences may occur in very exceptional circumstances.

If the proposed development is close to a raised flood defence it is likely to be in a flood hazard zone where failure of that defence could pose a significant risk to life and limb. Land within a flood hazard zone may not be suitable for certain uses. A Flood Risk Assessment in accordance with Appendix F of PPG25, including a detailed evaluation of the width of the hazard zone at the proposed development site, will be required.

Where a FRA is not submitted with the application or the FRA is not accepted by the LPA, the Agency OBJECTS to the application and requests that the LPA either defers the application or refuses planning permission.

Flood Risk Assessment - minimum requirements

In order to demonstrate that the operational development less than 1hectare in Flood Risk Category 2 is of low risk the FRA should show :-

1) A level survey to Ordnance Datum showing the known or modelled 1in100 year (1% annual probability) river flood level relative to proposed site levels.

2) An assessment of the flood risks posed to the site, including those based on any documented historic flooding and those associated with any increase in surface water runoff from the site.

3) Proposed mitigation measures to control those risks, e.g. setting appropriate floor levels, providing flood-proofing; providing suitable means of surface water disposal, etc.

Process

Planning applications submitted to the LPA must be accompanied by a FRA. The LPA should determine the adequacy of the FRA.

Background

In Flood Risk Category 2 there are two main flood risk considerations :-

1) The flood risk to the site resulting from an extreme flood event, i.e. a fluvial flood with an annual probability less than 0.5%, and

2) Increased flood risk to land or property downstream resulting from the change of use of ‘greenfield’ land to developed land. The newly paved and roofed areas within the development reduce the natural permeability of the land and thus increase the rate of surface water runoff from that land.

Each of these two considerations is dealt with in more detail below.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XIV

Flood Risk to the Development Site

Flooding of any site outside the high-risk (Category 3) area will only take place in either an extreme flood event or as a result of localised flooding from blocked or inadequate drainage.

Flood Risk Category 2 areas are suitable for most types of development. However, PPG25 (paragraph 30) states that a Flood Risk Assessment should be requested by the Local Planning authority to assist in their decision making, particularly where protection from an extreme flood event is required or where a site has recently been known to flood. The FRA should use available historic information, topographic surveys and local knowledge to establish what the impact of flooding would have been. This can then be used to decide on any mitigation measures necessary to protect the development from future extreme events.

It is possible that flooding may occur from a source other than that identified on the LPA’s Strategic Flood Risk Assessment map This may occur due to local drainage inadequacies and surface water runoff problems in the development area. The FRA will need to investigate the cause and effect of such local flooding as well as identifying appropriate mitigation measures.

Flood Risk from the Development Site

The second matter for consideration is the risk posed to others by the development of a ‘greenfield’ site causing increased surface water runoff. This will depend mainly on the size and nature of the proposed development. Development areas approaching 1ha in area can generate a significant increase in the surface water discharge rate. The impact and risks posed by this will vary according to the characteristics of the development itself, i.e. the extent of impermeable surfaces and the characteristics of the drainage system - catchment size and shape, distance from receiving watercourse, pipe sizes and gradients, attenuation devices, etc.

The risks posed should be reduced if the Environment Agency has been consulted and has agreed the surface water disposal proposals at the development brief / outline design stage. Proposed developments will also present a lower risk if a FRA has previously been developed in consultation with the Agency for inclusion with the planning application.

PPG25 (paragraph 60) notes that the requirement to assess the flood risks posed by development includes an assessment of the runoff implications of development appropriate to the nature and scale of that development. In some cases the risks will be small and the assessment will be correspondingly simple. In others the assessment will need to be more detailed, reflecting the greater risks.

Specific FRA requirements relating to surface water run off from two specific sizes of development a) under 0.5ha, and b) between 0.5ha and 1.0ha are set out in the background text below.

Specific FRA requirements - Flood Risk due to Runoff from Operational Development Areas up to 0.5 hectare in Flood Risk Category 2

Assess the runoff likely to be generated from the proposed development.

Assess the capacity of the surface water drainage systems already in place, including their state of maintenance and performance.

Where it is apparent that increased flooding could occur, the FRA must identify measures to mitigate or manage the impact of such flooding.

These can be any or all of the following :-

• Sustainable urban drainage systems (SUDS)

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XV

• On-site surface water management measures to restrict surface water discharge rates prior to its leaving the site. Water may then discharge either at a rate capable of being conveyed by the receiving drainage system or at a rate equivalent to the ‘greenfield’ runoff rate for the site.

• Maintenance or improvement of the receiving watercourse to allow additional rates of flow to be accommodated.

• Local modifications to the receiving watercourse such that the increased volume is stored and accommodated at, or adjacent to, the site.

Specific FRA requirements - Flood Risk due to Runoff from Operational Development Areas between 0.5 and 1.0 hectare in Flood Risk Category 2

At this scale the development is likely to have an impact on the surface water drainage system to which its surface water will discharge, especially where the proposal is for industrial or commercial development with large impermeable areas.

In addition to the requirements outlined above, the FRA should include an assessment of the likelihood of additional flooding elsewhere and its associated impacts. This will identify flood management measures which should, in the first instance, consider the inclusion of SUDS in the drainage design.

Cells E11 to E13

The Environment Agency should be consulted in these cases and will need to see the FRA before commenting on the proposals.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XVI

COLUMN F DEVELOPMENT IN FLOOD RISK CATEGORY 1

Cell F5

Flood Risk Category 1 areas are land outside the floodplain as shown on the LPA’s Strategic Flood Risk Assessment maps.

Flood Risk Category 1 is the little-or-no-risk category defined in Table 1 (paragraph 30) of PPG25. Category 1 areas have an indicative annual probability of flooding of less than 0.1% (less frequently than once in 1000 years) from either fluvial or tidal sources.

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

“Dry Islands”

There are some areas within Flood Risk Category 1 that are surrounded by areas at a higher risk of flooding, i.e. areas falling within Flood Risk Categories 2 and 3.

In certain cases development within 'dry islands' presents risks associated with maintaining safe access and egress for occupants during flood events. The distribution of dry islands and risks posed by them in terms of access / egress vary considerably across the country. If there is any doubt as to how flood risks associated with 'dry islands' may affect the LPA’s area, please contact your local Environment Agency Planning Liaison team.

Cells F6 to F9

These cells include the notes “No planning comment - see consent note” (Cells F6, 7 & 9) or simply “No planning comment” (Cell F8). The consent note referred to above is that repeated below.

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

F10 Operational development less than 1 ha

Developments on this scale in lower risk locations within Flood Risk Category 1 fall outside the scope of formal standing advice. The following advice is, however, offered for information purposes only to aid LPAs and developers in managing the surface water runoff issues and as a pointer towards best practice for surface water runoff disposal.

Surface water run-off should be controlled as near to its source as possible through a sustainable drainage approach to surface water management. This approach involves using a range of techniques including soakaways, infiltration trenches, permeable pavements, grassed swales, ponds and wetlands to reduce flood risk by attenuating the rate and quantity of surface water runoff from a site. This approach can also offer other benefits in terms of promoting groundwater recharge, water quality improvement and amenity enhancements. Approved Document Part H of the Building Regulations 2000 sets out a hierarchy for surface water disposal which encourages a SUDS approach.

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XVII

The SUDS approach

In accordance with Approved Document Part H of the Building Regulations 2000, the first option for surface water disposal should be the use of sustainable urban drainage methods (SUDS) which limit flows through infiltration, e.g. soakaways or infiltration trenches, subject to establishing that these are feasible, can be adopted and properly maintained, and would not lead to any other environmental problems. For example, using soakaways or other infiltration methods on contaminated land carries groundwater pollution risks and may not work in areas with a high water table. Where the intention is to dispose of surface water runoff to a soakaway, these should be shown to work through an appropriate assessment carried out under Building Research Establishment (BRE) Digest 365.

Flow balancing SUDS methods which involve the retention and controlled release of surface water from a site may be an option for some developments at this scale, provided that balanced surface water flows do not exceed the local ‘greenfield’ runoff rate. Flow balancing should seek to achieve water quality and amenity benefits as well as managing flood risk.

Further information on SUDS can be found in PPG25 (paragraphs 40 to 42 and appendix E), in the Construction Industry Research & Information Association (CIRIA) Document C522 “Sustainable Urban Drainage Systems - Design Manual for England and Wales”, and the Interim Code of Practice for Sustainable Drainage Systems. The Interim Code of Practice provides advice on design, adoption and maintenance issues and a full overview of other technical guidance on SUDS. The Interim Code of Practice will be made available electronically on both the Environment Agency's website at: www.environment-agency.gov.uk and CIRIA’s website at www.ciria.org.uk .

Where it is intended that disposal be made to a public surface water sewer, the Water Company or its agents should confirm that there is adequate spare capacity in the existing sewer system taking future development requirements into account.

F11 Operational development between 1 ha and 5 ha

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

The Need for a Flood Risk Assessment

Where a FRA is not submitted with the application or the FRA is not accepted by the LPA, the Environment Agency OBJECTS to the application and requests that the LPA either defers the application or refuses planning permission. The applicant should then be advised to contact the Agency with a view to obtaining pre-application technical advice and securing a Letter of Compliance from the Agency prior to progressing their planning application any further. Where the LPA is minded to refuse the application, the following standard response paragraphs may be applicable.

Environment Agency objection - failure to submit a FRA

“The Environment Agency OBJECTS to the proposed development on the grounds that the application may present a significant flood risk from the generation of surface water runoff but is not accompanied by a flood risk assessment as required by PPG 25.”

Environment Agency objection - inadequate FRA

“The flood risk information submitted in support of the application is not acceptable to the Environment Agency for the following reasons: [add omissions / areas for clarification / more detail etc as appropriate]. The Agency therefore OBJECTS to the proposed development on the grounds that a proper assessment of flood risk has not been undertaken as required by PPG 25.”

Faber Maunsell Corby Strategic Flood Risk Assessment - Stage 2 XVIII

Flood Risk Assessment - minimum requirements

The minimum requirements for a FRA for operational development between 1ha and 5ha in Flood Risk Category 1 areas is set out below. In order to demonstrate that the development is at low risk of flooding the FRA should show:

1) That it will be feasible to limit surface water runoff to the ‘greenfield’ runoff rate for all events up to the 1in100 year storm (including the effects of climate change over the next fifty years) by balancing storage, and set out how this will achieved.

2) How sustainable drainage techniques (SUDS) will be used, explaining how any obstacles to their use will be overcome.

Process

A Flood Risk Assessment must accompany planning applications submitted to the LPA. The FRA will focus on the control of surface water runoff generated by the development and the ability of any receiving watercourse to deal with it. The LPA should determine the adequacy of the FRA.

Background

Within Flood Risk Category 1, the primary flood risk issue generated by most new development is the risk posed to others by increased amounts of surface water runoff. Development within this category can generate a significant increase in the surface water discharge rate. The impact and risks posed by this will vary according to the characteristics of the development itself, i.e. the extent of impermeable surfaces and the characteristics of the drainage system - catchment size and shape, distance from receiving watercourse, pipe sizes and gradients, attenuation devices, etc.

PPG25 (paragraph 60) notes that the requirement to assess the flood risks posed by development includes an assessment of the runoff implications of development appropriate to the nature and scale of that development. In some cases the risks will be small and the assessment will correspondingly simple. In others the assessment will need to be more detailed, reflecting greater risks. Even at outline application stage the applicant needs to be able to demonstrate that surface water runoff balancing can be achieved.

Applicants are strongly advised to contact the Agency at the pre-application stage to discuss the scope and contents of the FRA.

Letter of Compliance

The risks posed by the proposed development should be reduced if the Environment Agency has been consulted and has agreed the means of surface water disposal at the development brief / outline design stage. Proposed developments will also present a lower risk if a Letter of Compliance issued by the Agency after pre-application negotiations with the developer accompanies the flood risk assessment (FRA). Further information on Letters of Compliance may be obtained from the Agency’s website.

Sustainable Urban Drainage Systems

Information on SUDS can be found in PPG25 (paragraphs 40 to 42 and appendix E), in the Construction Industry Research & Information Association (CIRIA) Document C522 “Sustainable Urban Drainage Systems - Design Manual for England and Wales” and the Interim Code of Practice for Sustainable Drainage Systems. This Interim Code of Practice provides advice on design, adoption and maintenance issues and a full overview of other technical guidance on SUDS. The Interim Code of Practice will be made available electronically on both the Environment Agency's website at: www.environment-agency.gov.uk and CIRIA’s web site at: www.ciria.org.uk. It should be noted that whilst the focus within the FRA must be on flood risk management, any SUDS should also seek to maximise opportunities for water quality and amenity benefits.

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F12 Civil emergency infrastructure less than 5 ha

Within Flood Risk Category 1, the primary flood risk issue generated by most new development is the risk posed to others by increased amounts of surface water runoff. Development within this category can generate a significant increase in the surface water discharge rate. The impact and risks posed by this will vary according to the characteristics of the development itself, i.e. the extent of impermeable surfaces and the characteristics of the drainage system - catchment size and shape, distance from receiving watercourse, pipe sizes and gradients, attenuation devices, etc.

For Civil Emergency Infrastructure, which can range in size from a small fire station to a major hospital, the response required to proposed development in respect of flood risk will depend on the scale of the proposed development as set out below. Please note that where these developments are proposed on ‘dry islands’ then any flood risk assessment should include consideration of how access will be provided and maintained during flood events.

The Agency’s involvement will depend upon the size of the proposed development, as follows.

Flood Risk due to runoff from Civil Emergency Infrastructure sites up to 1 hectare in Flood Risk Category 1

The GREY cell response for Operational Development of areas under 1ha applies. Do not consult the Agency as the response falls outside the scope of formal Standing Advice.

Flood Risk due to runoff from Civil Emergency Infrastructure sites between 1 and 5 hectares in Flood Risk Category 1

The GREEN cell response for Operational Development between 1 and 5 ha applies. This is subject to Standing Advice at the planning application stage but the Agency wishes to have a pre-application technical input.

F13 All Operational Development greater than 5 ha

The Environment Agency should be consulted where an operational development exceeds 5ha, and the Agency will wish to see the Flood Risk Assessment before commenting on the proposals.

Applicants should note that development which involves the culverting or any obstruction to flow on an Ordinary Watercourse will require the Agency’s consent under the Land Drainage Act 1991. An Ordinary Watercourse is defined as any watercourse not identified as a Main River on maps held by the Environment Agency and DEFRA. For further information on Ordinary Watercourses and Land Drainage Act Consent refer to the Agency’s website (www.environment-agency.gov.uk).

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Appendix 7 Development Area Runoff Sections

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