Kirklees Flood Risk Management Studies 2015/16 Kirklees Council
Mirfield and Ravensthorpe Fluvial Viability Study
B2088200/ MRFVS /00 October 16
Flood Risk Assessment Lancashire C ounty Council
Mirfield and Ravensthorpe Fluvial Viability Study
Mirfield and Ravensthorpe Fluvial Viability Study
Project no: B2088200 Document title: Mirfield and Ravensthorpe Fluvial Viability Study Document No.: B2088200/MRFVS/00 Revision: 00 Date: October 16 Client name: Kirklees Council Project manager: A Pearson Author: M Miles File name: B2088200_Kirklees_MirfieldRavensthorpe_FluvialViabilityStudy_D00_V00.docx
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Document history and status
Revision Date Description By Check Review Approved
00 25 October 2016 Initial Draft M Miles A Perryman C Isherwood A Pearson
B2088200/HVS_OTN/00 i Mirfield and Ravensthorpe Fluvial Viability Study
Contents 1. Introduction ...... 1 1.1 Background ...... 1 1.2 Location ...... 2 1.3 History of Flooding ...... 3 2. Problem Identification ...... 5 2.1 Existing Flood Risk Information ...... 5 2.2 Flood Mechanisms ...... 7 2.3 Flood Damages ...... 17 3. Flood Risk Management Options ...... 23 3.1 Long List ...... 23 3.2 Shortlist ...... 23 3.3 Economic Appraisal ...... 29 4. Conclusions and Recommendations ...... 34 4.1 Conclusion ...... 34 4.2 Recommendations ...... 35
Appendix A. Economic Methodology Appendix B. Options Appraisal
B2088200/HVS_OTN/00 ii Mirfield and Ravensthorpe Fluvial Viability Study
1. Introduction
1.1 Background
Kirklees Council and the Environment Agency have commissioned Jacobs to undertake a Viability Study to investigate opportunities to manage the flood risk from fluvial sources across Mirfield and Ravensthorpe.
In March 2014, Kirklees Council produced a prioritisation analysis of their administrative area and identified high-risk areas susceptible to fluvial and surface water flooding. The analysis ranked clusters of properties at risk of flooding in a priority order based upon the risk to property, life, transport links and critical infrastructure. One cluster covering Mirfield was ranked 20th in the prioritised list with an estimated 34 properties at risk around Parkfield Crescent. Three clusters covering Ravensthorpe were ranked 38th, 40th and 43rd with an estimated 36, 35 and 32 properties at risk in close proximity around Kingfisher Crescent, Walker Street and Victoria Street respectively.
Clusters within Mirfield and Ravensthorpe have been identified as being at risk of surface water flooding from a local Ordinary Watercourse and from pluvial flooding. From a desktop study of Mirfield and Ravensthorpe, further properties were identified as being at risk of fluvial flooding based on the Environment Agency Flood Zones and National Receptor Database (NRD).
This study has therefore been prepared to identify and assess the existing flooding mechanisms from fluvial sources to Mirfield and Ravensthorpe and the immediate surrounding area. If properties are confirmed to be at significant risk, the study will assess options to reduce the risk, which Kirklees Council can look to implement.
A parallel surface water study is being undertaken by Jacobs and can be read in conjunction with this study to provide a greater understanding of the interaction between the fluvial and surface water flood risks.
1.1.1 Aims and Objectives
The overall aims of this Viability Study are to: Improve the local understanding of fluvial flood risk; Evaluate the current risk to properties in Mirfield and Ravensthorpe; and Identify potential viable solutions to reduce local fluvial flood risk.
In order to meet these aims, the following objectives are to: Present the current fluvial flooding mechanisms; Present economic damages and Benefit Cost Ratio (BCR) values; Summarise the affordable options for Mirfield and Ravensthorpe; and Produce recommendations for Kirklees Council moving forward.
In order to achieve these aims and objectives, this Viability Study uses a number of flood risk datasets that have been made available by Kirklees Council and the Environment Agency. Whilst these datasets are appropriate given the level of this study, they may contain a number of limitations and assumptions, which may over or under estimate actual flood risks as a result. Where possible local information collected on site will be used to supplement these datasets, but where limitations or assumptions remain, these will be clearly identified.
Consequently, it is not the intention of this Viability Study to identify a preferable FRM solution. The study however seeks to provide Kirklees Council with sufficient information to confirm its ranking within the table of flood risk hotspots and to assess whether the area warrants further investigation (given other FRM priorities). Where this is the case, this report will set out clear recommendations and actions.
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1.2 Location
The Viability Study focuses on Mirfield and Ravensthorpe, which are two separate urban locations connected through the A644 Huddersfield Road Corridor located in the north east of the Kirklees Metropolitan Borough area. Dewsbury town centre is located approximately 1.5km to the east. Huddersfield town centre is located approximately 5km to the west and lies upstream of Mirfield and Ravensthorpe.
There are two Environment Agency classified Main Rivers, the River Calder and the River Spen, of which the River Calder is a source of flooding to Mirfield and Ravensthorpe, whilst the River Spen, in terms of flood risk, would only directly affect Ravensthorpe.
Mirfield lies to the west of Ravensthorpe and is built on the north side of the Calder valley, on a steep hillside. A section of the town has been constructed on the floodplain of the River Calder and these mostly include non- residential properties. Topography varies in Mirfield from 29m to 125m AOD and Ravensthorpe from 29-50m AOD. Ravensthorpe is less undulating and smaller than Mirfield and is situated at the confluence of the River Calder and the River Spen. Figure 1.1 shows the site overview map.
Mirfield and Ravensthorpe’s close proximity to one another means that fluvial flood events affecting one would likely affect the other and any flood risk management options shortlisted would have to consider its potential effects upstream or downstream of their implementation. Consequently, both locations are assessed as part of this Viability Study.
Figure 1.1 – Mirfield & Ravensthorpe Site Overview Map
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1.3 History of Flooding
There are numerous records of historical flooding in Mirfield and Ravensthorpe with the most recent reported event occurring on the 26th December 2015, which affected large parts of Kirklees. Table 1.1 provides a non- exhaustive list of historical records, which have affected Mirfield and Ravensthorpe. Figure 1.2 illustrates the recent 2015 Boxing Day floods.
Table 1.1 – Historical flooding references for Mirfield and Ravensthorpe
Flood Date River Description Areas Affected Source Mechanism
Cleckheaton and the Spen Valley – due to the rapid thaw, the low lying land at Cleckheaton, Liversedge, and the Spen River Valley districts were generally under water. Cleckheaton, Rapid snow Calder & Leeds Mercury 30/12/1878 Dewsbury – due to the rapid thaw, the Liversedge, Spen Valley, thaw & rainfall – River (31/12/1878) River Calder water levels rose and Dewsbury, Ravensthorpe Fluvial Flooding Spen overflowed its banks. This was especially the case in Ravensthorpe, Dewsbury and parts of Water Lane, Dewsbury.
Dewsbury – the Market Place in Dewsbury was flooded and several roads into the Market Place were inundated. A lot of flood water came from the beck which rises in Birstall and passes through Batley and Batley Carr. There were also floods on the Thornhill, Mirfield and Ravensthorpe side of the river. River Spen Valley – large tracts of land Calder; The Leeds bordering the Spen Beck were covered Dewsbury, Colne Valley, Sustained 10/ River Mercury with water. The River Spen overflowed its Cleckheaton, Spen rainfall – Fluvial 1892 Spen; (15 & banks in many parts. Liversedge suffered Valley & Mirfield Flooding River 17/10/1892) great flooding as the Spen Beck flooded its Colne banks over almost its entire length. At Watergate the main thoroughfare between Cleckheaton and Heckmondwicke was 2 to 3 feet under water. Mirfield – Mirfield along the banks of the River Calder suffered flooding and the village of Ravensthorpe suffered particularly.
River Dewsbury & Mirfield – the swollen River Colne; Calder flooded many areas on both banks. Spen Valley, Littletown, Sustained heavy The Yorkshire River Extensive tracts of land were under water Cleckheaton, 07/08/1922 rainfall – Fluvial Post Calder; at Ravensthorpe, Thornhill Lane, Huddersfield, Dewsbury, Flooding (08/08/1922) River Dewsbury and Sands Lane and Mirfield, Ravensthorpe Spen Earlsheaton.
River Sustained Mirfield – Steandard Lane along the River Dewsbury, Batley, The Yorkshire Calder; rainfall overnight 19/09/1946 Calder was flooded. Numerous fields and Mirfield, Lockwood, Post (20 & River – Fluvial the Ship Inn were also flooded in the area. Savile Town 21/09/1946) Holme Flooding
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Flood Date River Description Areas Affected Source Mechanism
Ravensthorpe, River Liversedge, The 2007 floods affected up to an Heavy and Calder; Cleckheaton, Kirklees Surface estimated 500 properties across the district prolonged Summer River Chickenley, Mirfield, Water and were described by many residents as rainfall – Fluvial 2007 Spen; Milnsbridge, Brockholes, Management the worst in living memory. The flooding and Surface River New Mill, Denby Dale, Plan was widespread across the district Water Flooding Colne Scissett and Clayton West.
Slaithwaite, River Mirfield – A62 at Mirfield and Steanard Thongsbridge, Golcar, The Calder; Lane along the River Calder flooded. Mirfield, Ravensthorpe, Heavy and Huddersfield River Homes escaped devastation but numerous Holmbridge, prolonged 26/12/2015 Daily Examiner Spen; roads, fields, allotments and small Mytholmroyd, rainfall – Fluvial (26, 27 & River businesses were flooded in the area. Milnsbridge, Lockwood, Flooding 28/12/2015) Colne Rivers rising to unprecedented levels. Bradley Mills Birkby and Liversedge
Figure 1.2 – Boxing Day 2015 Floods 1
2 Lower Hopton Central Mirfield 1
Central Mirfield Ladywood Lakes 2 2
1 Drone footage of the flooding in Mirfield, courtesy of Steve Benson- taken from The Huddersfield Daily Examiner website http://www.examiner.co.uk/news/west-yorkshire-news/watch-shocking-drone-footage-shows-10655403. 2 Image taken from The Huddersfield Daily Examiner, 28th Dec 2015.
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2. Problem Identification
2.1 Existing Flood Risk Information
2.1.1 Previous Studies
Calder Catchment Flood Management Plan
The Calder Catchment Flood Management Plan (CFMP) references recorded surface water drainage and sewer flooding incidents in Mirfield and indicates flood incidents along the River Calder. The CFMP highlights Mirfield as having approximately 100-500 properties at flood risk in a 1% AEP (1 in 100 year) event.
Mirfield sits within the Mid Calder sub area, which was classified as Policy Option 4 “Areas of low, moderate or high flood risk where we (the Environment Agency) are already managing the flood risk effectively but where we may need to take further actions to keep pace with climate change”.
Ravensthorpe is within the Spen sub area, which is classified as Policy Option 5 “Areas of moderate to high flood risk where we (the Environment Agency) can generally take further action to reduce flood risk”. Ravensthorpe has not been identified as a specific flood risk area within the CFMP.
Calder Valley Strategic Flood Risk Assessment
The Strategic Flood Risk Assessment (SFRA) provides a strategic view of flood risk along the River Calder valley. The assessment identifies that the topography of the valley around Mirfield and Ravensthorpe is not as steep as the upper catchment, however the floodplain broadens and land use changes to include large areas of heavy industry.
The SFRA states that some large housing areas are located in the high-risk zone in Mirfield. A review of flow gauge records in the middle catchment (River Calder downstream of Elland, River Colne downstream of River Holme – Dewsbury) shows that whilst the floodplain broadens there is no significant attenuation in peak flows and consequently floodwaters continue to rise and fall quickly.
Defences consist of a mixture of floodwalls and earth embankments. The quality of formal river defences along the Calder Valley is high and the probability of flooding in these areas is deemed low to moderate. However, the consequences of flooding are very large. Areas directly adjacent to the defences can be subject to high flow velocities, should the defence overtop or fail.
Kirklees Local Flood Risk Management Strategy
The Kirklees Local Flood Risk Management Strategy (LFRMS) mentions that Mirfield is a high flood risk area, notably from surface water flooding and fluvial risk attributed to the River Calder. The LFRMS states that approximately 500 properties in Mirfield and 2,000 properties in Ravensthorpe (Huddersfield Road) are at risk from a 0.5% AEP (1 in 200 year) event, with the main sources of flooding being the River Calder and surface water for Mirfield and the River Calder and River Spen for Ravensthorpe. Ravensthorpe was highlighted as an area affected during the 2004 and 2007 flood events and Mirfield was referenced as being affected during the 2007 summer floods.
2.1.2 Fluvial flood data
The main source of flooding through the study area is the Main River Calder, which flows west to east through the south of Mirfield and Ravensthorpe and the Main River Spen, which flows north to south, joining the River Calder close to Ravensthorpe.
Additional fluvial flood sources, as shown in Figure 2.1, include a secondary un-named Main River that is largely culverted through Mirfield before discharging into the River Calder and Canker Dyke which is an Ordinary
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Watercourse discharging into the River Spen, located approximately 500m upstream of the confluence with the River Calder.
According to the Environment Agency Flood Map, a number of residential and non-residential properties in Mirfield and Ravensthorpe are at risk of fluvial flooding, predominantly located within the floodplain of the River Calder and at the confluence with the River Spen. The existing Flood Zone 3 and 2 are shown in Figure 2.1.
Figure 2.1 – Environment Agency Flood Map
River Spen Dewsbury
Secondary Canker Dyke Main River RavensthorpeMirfield (Culverted)
Mirfield
River Calder
The current Environment Agency Flood Zones 3 and 2 are based on the undefended outputs of the River Calder 1D-2D hydraulic model developed by the Environment Agency in 2014. The River Calder model also includes a defended scenario, which has been used as the baseline model for this viability study (assumes existing defences are in place). A 1D hydraulic model was previously completed in 2011, which used ISIS and HEC- RAS to represent the undefended scenario and was assessed as part of the 2014 update.
The 2014 hydraulic modelling included the simulation of seven flood events listed Table 2.1. Table 2.1 also lists the total number of properties at risk within each of the defended modelled flood extents. The distributions of properties at risk are illustrated in Figure 2.2.
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Table 2.1 – Number of properties at risk of fluvial flooding during the defended fluvial scenario
Modelled Flood Event Residential Properties Non-Residential Properties
10% AEP (1 in 10 years) 1 14 4% AEP (1 in 25 years) 9 57 2% AEP (1 in 50 years) 18 69 1.33% AEP (1 in 75 years) 21 72 1% AEP (1 in 100 years) 72 106 1% AEP (1 in 100 years) + climate change 147 148 0.1% AEP (1 in 1000 years) 1,183 269
Figure 2.2 – Distribution of properties at risk during defended 0.1% AEP (1 in 1000 year) fluvial event
2.2 Flood Mechanisms
In order to ground truth the flood risk datasets discussed above, identify properties at risk and collect additional local information; site visits to Mirfield and Ravensthorpe were undertaken on the 19th November 2015 and the 16th January 2016. The site visits were also used to assess potential flood risk management options introduced later in this report.
Within the Mirfield and Ravensthorpe study area, six clusters have been identified based on flood mechanisms and the number of properties at risk. The use of “clusters” has been adopted to help structure this report and describe the mechanisms and consequences of flooding and the appraisal of potential FRM measures.
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These six clusters are shown in Figure 2.2 and are discussed individually throughout the report. They include: 1) Cluster 1 - West Mirfield, Mirfield 2) Cluster 2 - Central Mirfield, Mirfield 3) Cluster 3 - Lower Hopton, Mirfield 4) Cluster 4 - Ladywood Lakes, Ravensthorpe 5) Cluster 5 - West Ravensthorpe, Ravensthorpe 6) Cluster 6 - East Ravensthorpe, Ravensthorpe
Figure 2.3 – Properties at risk within identified fluvial flood risk clusters
2.2.1 Cluster 1: West Mirfield
According to the River Calder model, the river begins to overtop on the north bank in a 10% AEP (1 in 10 year) event affecting two non-residential properties at a depth of up to 1m. This increases to approximately 40 non- residential (industrial) properties in the 0.1% AEP (1 in 1000 year) event at a depth of up to 3m.
The river also overtops at the same location along the southern bank near to the confluence of the un-named tributary with the River Calder. The modelling suggests the elevated land and the railway obstructs flow and diverts it towards the canal. Furthermore, the modelling indicates water overtopping the southern bank upstream on the River Calder during a 1% AEP (1 in 100 year) event flows alongside the raised railway before joining the un-named watercourse, which consequently overtops and merges with the aforementioned ponded area. The combined effect results in 10 residential properties being at risk of fluvial flooding on the southern bank at a depth of up to 2m. Figure 2.4 illustrates the key locations where properties are a flood risk.
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There is a weir located approximately 950m upstream of the main risk area and another located approximately 1000m downstream (see Cluster 2). Downstream of the upstream weir on the left bank the Batteyford Sporting grounds and buildings are at risk from during the 10% AEP (1 in 10 year) event. The downstream weir could influence flood risks upstream and downstream. Furthermore, sedimentation and siltation observed around the downstream weir could increase the risk of flooding upstream as capacity is reduced. Photographs of the downstream weir and associated sedimentation are shown in Figures 2.5 and 2.6.
Figure 2.4 – Flood Mechanism for Cluster 1 (West Mirfield)
Figure 2.6 – Sedimentation associated with the Figure 2.5 – Downstream weir downstream weir
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2.2.2 Cluster 2: Central Mirfield
The hydraulic model indicates that the River Calder overtops its banks during a 4% AEP (1 in 25 year) event, potentially affecting one residential property along Newgate up to a depth of 0.5m. During the 1% AEP (1 in 100 year) event, two residential properties are affected to a depth of up to 1-2m. The weir raises levels and reduces the velocity upstream. One consequence of the weir is that sedimentation could build up and contribute to the overtopping of the lock gate and associated pathway.
Furthermore, the current lock gate located at the start of the Calder and Hebble Navigation Canal is shown to overtop during the 4% AEP (1 in 25 year) event or greater, resulting in floodwater bypassing the lock gate via the footpath.
Figure 2.7 - Overtopping of the lock gate and flows down the footpath from the Boxing Day floods 20153
Less than 50m downstream of the weir, a viaduct could provide a potential restriction in flow within the channel. These pinch points could increase the risk of fluvial flooding to the properties along Newgate. Figure 2.8 highlights the viaduct and the low wall on the northern bank where the modelling suggests the flows overtop affecting the properties along Newgate.
Figure 2.8 – Low walls along left bank of the River Calder, Central Mirfield.
Viaduct Low Walls
750m upstream of the weir, the modelling indicates that the southern bank of the River Calder begins to overtop during the 10% AEP (1 in 10 year) event, close to a development that has recently been built. Two non- residential properties downstream are at risk during a 1% AEP (1 in 100 year) event and approximately 32 properties in a 0.1% AEP (1 in 1000 year) event. Flood depths range up to 2m for the two non-residential properties during the 1% AEP (1 in 100 year) event.
3 Taken from Drone footage, Project Mirfield Boxing Day 2015 Floods: http://projectmirfield.co.uk/news
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Figure 2.9 – Flood Mechanism for Cluster 2 (Central Mirfield)
The modelled section of the River Calder further downstream indicates out of bank flows begin during the 4% AEP (1 in 25 year) event, flowing along Hopton New Road and Hurst Lane towards the industrial units to the north of the railway. The model predicts the canal to overtop during the same event affecting ten non-residential properties in the industrial units, flowing towards Hopton New Road and down Hurst Lane converging with the flood extents from the River Calder. There is a recognition that this mechanism may originate from the River Calder breaching in Lower Hopton. Water can potentially flow through the tunnels under the railway line, along Hurst Lane and Hopton New Road, flooding properties between the railway and canal. Detailed hydraulic modelling will be required to fully understand the flood mechanisms between the canal and the River Calder in Central Mirfield.
In the modelled 2% AEP (1 in 50 year) event, the flood flows from the River Calder are also conveyed west, along the southern side of the railway line towards Newgate. Flood depths are typically below 0.5m along the connecting routes and up to 2m around the non-residential properties.
2.2.3 Cluster 3: Lower Hopton
The hydraulic model indicates that both northern and southern floodplains of the River Calder through this section act as functional storage, which start to be inundated during the 10% AEP (1 in 10 year) event. According to the modelled flood extents, floodwater affects one residential property on the southern bank along Steanard Lane at a depth up to 0.3m and in close proximity to two non-residential units on the northern bank.
Flooding to all non-residential units along the northern bank onset during the 4% AEP (1 in 25 year) event and can reach depths of up to 1m. Approximately eight residential properties on the southern bank, near to the junction of Granny Lane and Hagg Lane are at risk of flooding from the 4% AEP (1 in 25 year) event at depths of up to 0.5m. The risk to these residential properties at this location is potentially exacerbated by Valance Beck,
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which could contribute to flood flows. The remaining properties along Granny Lane, as highlighted in Figure 2.10, are at risk of flooding only during the 0.1% AEP (1 in 1000 year) event.
Figure 2.10 – Flood Mechanism for Cluster 3 (Lower Hopton)
2.2.4 Cluster 4: Ladywood Lakes
Downstream of Lower Hopton lies Ladywood Lakes, which is currently used for fishing and is located directly upstream of Ravensthorpe. At the downstream extent of the Calder and Hebble Navigation Canal as it re-joins the River Calder, five residential properties and one non-residential property (Shepley Bridge Marina) are at risk during the 4% AEP (1 in 25 year) event with depths of up to 0.5m. An additional three residential properties are at risk in the 0.1% AEP (1 in 1000 year) event. All properties at risk in this location are within the natural floodplain and therefore would flood frequently.
Properties highlighted at risk in Figure 2.11 include a non-residential unit (Biffa Yorkshire and Dewsbury Tyres) located between the Calder and Hebble Navigation Canal and the River Calder, which contains several properties at risk of fluvial flooding as the River Calder overtops its southern bank during a 10% AEP (1 in 10 year) event reaching depths up to 0.5m. The modelling indicates that the area becomes completely flooded to a depth of up to 1m during a 4% AEP (1 in 25 year) event, increasing to 2m for the 1% AEP (1 in 100 year) event.
Ladywood Lakes located within the meander of the River Calder is also at risk of flooding during the 10% AEP (1 in 10 year) event. One property is at risk during this event, namely the Ship Inn along Steanard Lane, which is known to have flooded historically with records dating as far back as 1946 (see Figure 1.2). The Ship Inn was one of the properties to be affected by the Boxing Day 2015 floods.
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Figure 2.12 and Figure 2.13 illustrate the extent of the Boxing Day 2015 floods in this location. The modelled flood depths at this location are predicted to reach 1m during the 10% AEP (1 in 10 year) event, increasing to 2m during the 1% AEP (1 in 100 year) event.
Figure 2.11 – Flood Mechanism for Cluster 4 (Ladywood Lakes)
Figure 2.12 – Ship Inn and Ladywood Lakes during Boxing Day 2015 floods4 Figure 2.13 – Ladywood Lakes
4 The Huddersfield Daily Examiner, 28th December 2015: http://www.examiner.co.uk/news/west-yorkshire-news/watch-shocking-drone-footage-shows-10655403
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2.2.5 Cluster 5: West Ravensthorpe
Ravensthorpe has been divided into two clusters to differentiate between the flood risk associated with the River Calder (West Ravensthorpe) and the flood risk associated with the potential interaction between the River Calder and the River Spen (East Ravensthorpe).
Figure 2.14 – Flood Mechanism for Cluster 5 (West Ravensthorpe)
The risk of flooding in West Ravensthorpe comes from two sources; the Main River Calder and the Ordinary Watercourse known as Canker Dyke. Canker Dyke runs in culvert through the majority of the urban area. The risk of fluvial flooding from Canker Dyke is unknown at this stage, as it was not represented in the River Calder hydraulic model. At this stage, the risk if deemed low.
Along the River Calder, the hydraulic model predicts that the northern bank overtops the during the 10% AEP (1 in 10 year) event initially flooding an area covered by dense vegetation. During the 1% AEP (1 in 100 year) event, approximately 25 non-residential and six residential properties at predicted to be at risk of fluvial flooding to a depth of 1-2m. During the 1% AEP plus climate change event the risk of flooding increases with approximately 31 non-residential properties and 12 residential properties at risk at a depth of up to 3m.
The number of properties at risk in West Ravensthorpe at risk from the 0.1% AEP (1 in 1000 year) event is approximately 825, , with flood depths reaching 3.5m. Flood water cannot flow out of the right bank due to the steep local topography, directing peak flows over the left bank towards Ravensthorpe. During the site visit, a path along the left bank was impassable due to dense vegetation and because the privately owned land was built up to the river’s edge. This meant that an assessment of current defences and environmental conditions was limited. Figure 2.14 shows the fluvial flood risk to properties from the modelled flood extents.
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2.2.6 Cluster 6: East Ravensthorpe
The flood outlines shown in Figure 2.15 are taken from the River Spen hydraulic model and show up to 14 properties at risk of flooding along Heron Close during the 4% AEP (1 in 25 year) flood event and approximately 11 properties at risk along Northstead during the 2% AEP (1 in 50 year) event.
Figure 2.15 – Flood Mechanism for Cluster 6 (East Ravensthorpe)
The hydraulic modelling suggests the flood depths along Heron Close and Northstead are shallow, reaching 0.3m during the 2% AEP (1 in 50 year) event. During the 0.1% AEP (1 in 1000 year) event, a total of 60 properties are at risk of fluvial flooding, 40 of which are located along Heron Close and 20 along Northstead. The hydraulic modelling indicates that the depth of flooding around the properties remains relatively shallow below 0.3m, the exception being 8 of the properties in Heron Close where depths reach 1m.
The flood risk associated with the River Spen is not considered as part of this fluvial Viability Study; however, it will be considered as part of the parallel surface water Viability Study for Mirfield and Ravensthorpe. This study will address the flood risk at Heron Close and Northstead.
The River Spen is culverted 500m upstream of Ravensthorpe with the hydraulic model showing no flooding between the culvert inlet and outlet. At the culvert outlet, the neighbouring fields act as a natural floodplain, which begin to become inundated during the 10% (1 in 10 year) event. However, no properties are predicted to be at risk in this location.
The River Calder begins to overtop during a 10% AEP near the confluence with the River Spen, with no properties at risk. Four non-residential properties are at risk during the 4% AEP event, rising to 8 non-residential properties for the 1% AEP event at a depth of up to 2m.
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During a 1% AEP plus climate change event, flood risk significantly increases with the River Spen and River Calder flood extents merging, predicting an estimated 73 properties at risk of fluvial flooding to a depth of up to 2m. However, the majority of flood depths are below 0.5m.
Figure 2.16 illustrates the restrictions on the River Spen at the confluence, which could be contributing to the risk of flooding.
Figure 2.16 – Confluence between the River Spen and the River Calder
River Spen
River Calder
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2.3 Flood Damages
In order to develop a better understanding of the likely consequence of flooding in Mirfield and Ravensthorpe, a high-level economic analysis has been undertaken to calculate Present Value (PV) damages. Damages include direct damages resulting from floodwater inundating a property as well as ‘indirect’ damages that occur as a result of a flood event such as the cost of emergency response, providing temporary accommodation, and the loss of personal items.
As the main source of flooding is fluvial from Main Rivers, the calculation of flood damages has been undertaken using a standard methodology outlined in the Multi-Coloured Manual5 (MCM) over a 100-year appraisal period. The calculation of PV damages is based on maximum flood depths extracted from the hydraulic modelled representation of the defended scenario during the 10% AEP (1 in 10 year), 4% AEP (1 in 25 year), 2% AEP (1 in 50 year), 1.33% AEP (1 in 75 year), 1% AEP (1 in 100 year) and 0.1% AEP (1 in 1000 year) events. The full methodology of the economic analysis is presented in Appendix A.
The funding of flood risk management interventions is driven by the financial value of benefits derived from flood protection (damages avoided), against the cost of providing the protective measure. This is presented at a Benefit to Cost Ratio (BCR). Flood risk management options that provide a strong BCR are more likely to attract funding. By introducing the Benefit Cost Ratio (BCR) concept early on in the Viability Study, it is possible to assess the affordability of Flood Risk Management (FRM) options without the need for detailed option testing (to calculate PV benefits) and option PV costs. By doing so, it is possible to identify locations which are likely to merit substantive capital investment to reduce the flood risk and conversely, it will indicate areas where it is unlikely that significant expenditure can be justified due to the low PV damages estimated and high BCR required to attract Flood Defence Grant in Aid funding (FDGiA).
BCRs are also intrinsically linked to potential available funding that would be required to promote flood risk management projects. For example, options that return a BCR value of 3.0 or greater are more likely to secure central government funding than a score lower than 3.0.
2.3.1 Present Value Damages
To assess the economic benefits of any flood risk management measure, a present day economic damages assessment must be derived to be used as the present day baseline. For this study, the Do-Minimum (present day) damage situation has been used as the baseline position against which the benefits of further expenditure are evaluated. The following economic cases are explored in this study; 1) Do-Minimum - this baseline scenario assumes the minimum amount of action or intervention necessary to sustain the standard of service. In this scenario, the existing defended hydraulic model outputs have been used and it is assumed the existing SoP of these defences will be maintained over the appraisal period. 2) Do-Something (4% AEP Standard of Protection (SoP)) - this scenario assumes FRM measures are put in place to protect all properties remaining at risk up to the 4% AEP(1 in 25 year) event, effectively avoiding damages up to an including this event. In order to estimate the residual PV Damages under this scenario, flood depths generated during the 4% AEP(1 in 25 year) event were removed (assuming the property would no longer flood), leaving all flood depths generated during events greater than the SoP. 3) Do-Something (2% AEP SoP) - this scenario assumes FRM measures are put in place to protect all properties remaining at risk up to the 2% AEP (1 in 50 year) event, effectively avoiding damages up to an including this event. In order to estimate the residual PV Damages under this scenario, flood depths generated up to the 2% AEP (1 in 50 year) event were removed (assuming the property would no longer flood), leaving all flood depths generated during events greater than the SoP.
5 Flood Hazard Research Centre (2010) The benefits of Flood and Coastal Risk Management: A Manual of Assessment Techniques (Multi-Coloured Manual)
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4) Do-Something (1% AEP SoP) - this scenario assumes FRM measures are put in place to protect all properties remaining at risk up to the 1% AEP (1 in 100 year) event, effectively avoiding damages up to an including this event. In order to estimate the residual PV Damages under this scenario, flood damages generated up to the 1% AEP (1 in 100 year) event are discounted (assuming the property would no longer flood), leaving all flood depths generated during events greater than the SoP.
The Do Minimum represents the baseline case for this study as no appropriate data was available to estimate the PV Damages for the Do Nothing scenario. Consequently, the options detailed in this report should not be immediately dismissed if damages and benefits are marginal as the business case presented in this report is conservative.
The lack of a Do Nothing scenario means that Incremental Benefit Cost Ratios (IBCRs) have not been calculated and therefore the results will be referred to as benefits over and above the Do Minimum scenario. Going forward, it is recommended that a Do Nothing model run is produced which can then be used as the baseline for this study to conduct a more robust economic assessment.
The results in Table 2.2 were generated within the Jacobs ‘EcMap’ tool for ArcGIS.
Table 2.2 – Present Value Damages
Do-Minimum Do-Something ID Cluster Name Baseline 4% AEP SoP 2% AEP SoP 1% AEP SoP
1 West Mirfield £12,100k £5,900k £4,400k £2,600k
2 Central Mirfield £14,100k £6,300k £5,000k £3,300k
3 Lower Hopton £11,900k £4,500k £3,400k £1,800k
4 Ladywood Lakes £4,300k £1,700k £1,200k £700k
5 West Ravensthorpe £17,800k £17,800k £17,800k £14,700k
6 East Ravensthorpe £11,400k £10,500k £10,200k £8,400k
Total £71,600k £46,700k £42,000k £31,500k
2.3.2 Affordability Assessment
Table 2-3 shows the PV Costs which would result in a range of BCRs at each site. To generate these values, the PV Damages for ‘Do-Something’ scenarios are divided by the target BCR value. This gives an indication of whether it is economically viable to implement any measures at each site. If the costs associated with a favourable BCR are exceptionally low then it is unlikely that there will be any measures which could be afforded.
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Table 2.3 - Potential PV Costs associated with a range of FRM BCRs
Very Weak Weak/Marginal Strong Very Strong
2.0 3.0 6.0 8.0
A weak or marginal A very weak economic case and A strong economic A very strong Cluster Target SoP economic case. Very unlikely to attract case. Likely to attract economic case. Has unlikely to be viable central government some government a very high chance of or be able to attract funding. There is a funding although attracting 100% funding from any potential that local unlikely to cover the funding from the source. funding sources will whole costs. central government. aid e.g. the local levy.
4% AEP SoP £3,100k £2,000k £1,000k £700k 1. West 2% AEP SoP £3,800k £2,500k £1,300k £1,000k Mirfield 1% AEP SoP £4,700k £3,200k £1,600k £1,200k
4% AEP SoP £3,900k £2,600k £1,300k £1,000k 2. Central 2% AEP SoP £4,500k £3,000k £1,500k £1,100k Mirfield 1% AEP SoP £5,400k £3,600k £1,800k £1,300k
4% AEP SoP £3,700k £2,500k £1,200k £900k 3. Lower 2% AEP SoP £4,300k £2,800k £1,400k £1,000k Hopton 1% AEP SoP £5,000k £3,400k £1,700k £1,300k
4% AEP SoP £1,300k £900k £400k £300k 4. Ladywood 2% AEP SoP £1,500k £1,000k £500k £400k Lakes 1% AEP SoP £1,800k £1,200k £600k £400k
4% AEP SoP £0k £0k £0k £0k 5. West 2% AEP SoP <£20k <£15k <£10k <£5k Ravensthorpe 1% AEP SoP £1,600k £1,000k £500k £400k
4% AEP SoP £500k 300k £200k £100k 6. East 2% AEP SoP £600k £400k £200k £200k Ravensthorpe 1% AEP SoP £1,500k £1,000k £500k £400k
4% AEP SoP £12,500k £8,300k £4,100k £3,000k
Overall Site 2% AEP SoP £14,720k £9,715k £4,910k £3,705k
1% AEP SoP £20,000k £13,400k £6,700k £5,000k
2.3.3 Cluster 1 - West Mirfield
The economic damages calculated for West Mirfield are relatively high due to the presence of two non- residential properties on the northern bank, which are at risk of fluvial flooding from the 10% AEP (1 in 10 year) event at a depth of up to 1m. These two non-residential properties contribute to over £5,700k of the £11,800k PV non-residential damages. The remaining damages are associated with the residential properties on the northern bank, which onset during the 1% AEP (1 in 100 year) event.
Table 2.4 summarises the Present Value damages (PVd), the percentage contribution of the residential and non-residential damages and the number of properties at risk within West Mirfield for the Do-Minimum scenario.
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Table 2.4 - Present Value Damages - West Mirfield
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 25 £300k 2% Non-Residential 34 £11,800k 98% Total 59 £12,100k 100%
2.3.4 Cluster 2 - Central Mirfield
The economic damages calculated for Central Mirfield are relatively high due to the presence of two non- residential properties located between the railway and the canal, which are at risk of fluvial flooding from the 4% AEP (1 in 25 year) event at a depth of up to 2m. These two non-residential properties contribute up over £8,600k of the £14,100k PV damages in this area. The majority of the additional damages are associated with residential properties located along Newgate on the northern bank during the 4% (1 in 25 year) AEP event.
Table 2.5 summarises the damages and overall contribution of both residential and non-residential properties within Central Mirfield for the Do-Minimum scenario.
Table 2.5 - Present Value Damages - Central Mirfield
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 106 £500k 4% Non-Residential 39 £13,600k 96% Total 145 £14,100k 100%
2.3.5 Cluster 3 - Lower Hopton
The economic damages associated with Lower Hopton are relatively high due to the presence of one large isolated non-residential unit located on the northern bank opposite Granny Lane, which is at risk of fluvial flooding from the 4% AEP (1 in 25 year) event at a depth of up to 1m. This unit makes up over £10,100k of the £11,900k PV damages in this area. The majority of the remaining damages are associated with the non- residential units further downstream on the northern bank, which are at risk from the 4% AEP (1 in 25 year) event with smaller PV damage contributions associated with the residential properties along Granny Lane.
Flood risk management options for Lower Hopton were considered economically unfeasible at this stage. This was because 85% of the estimated PV damages were attributed to one large isolated non-residential unit, located in the floodplain on the northern bank of the River Calder. Due to the low flood depth and the high economic cost associated with just one property this cluster has been discounted and not considered further.
Table 2.6 summarises the damages and overall contribution of both residential and non-residential properties within Lower Hopton for the Do-Minimum scenario.
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Table 2.6 - Present Value Damages - Lower Hopton
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 24 £400k 3% Non-Residential 17 £11,500k 97% Total 41 £11,900k 100%
2.3.6 Cluster 4 - Ladywood Lakes
The economic damages associated with Ladywood Lakes are relatively high due to the presence of three non- residential properties, namely the Ship Inn, Biffa Yorkshire and Dewsbury Tyres. The Ship Inn is a known flood hotspot located on the southern bank of the River Calder and Biffa Yorkshire and Dewsbury Tyres are located between the River Calder and the canal, all of which are at risk of fluvial flooding from the 10% AEP (1 in 10 year) event. These properties contribute to over £2,900k of the £4,300k PV damages in this area. The remaining damages are associated with properties around Shepley Bridge Marina that onset during the 4% AEP (1 in 25 year) event at depths up to 0.5m.
Table 2.7 summarises the damages and overall contribution of both residential and non-residential properties within Ladywood Lakes for the Do-Minimum scenario.
Table 2.7 - Present Value Damages - Ladywood Lakes
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 8 £400k 9% Non-Residential 16 £3,900k 91% Total 24 £4,300k 100%
2.3.7 Cluster 5 - West Ravensthorpe
The economic damages in West Ravensthorpe are high due to the presence of numerous and densely packed non-residential properties at risk of fluvial flooding on the northern bank from the 1% AEP (1 in 100 year) event at a depth of up to 2m.
The extent of flooding significantly increases when considering the 0.1% AEP (1 in 1000 year) event, with approximately 825 properties at risk of fluvial flooding throughout West Ravensthorpe.
The properties at risk from the 1% AEP event make up over £8,100k of the £17,800k PV damages in this risk area. The remaining damages are distributed across West Ravensthorpe up to the 0.1% AEP (1 in 1000 year) event at depths up to 3.5m.
With the majority of the damages shown to occur during a 1% AEP (1 in 100 year) event or greater, flood risk management measures providing lower SoP’s will provide little benefit. The aim in this location would be to reduce the risk of flooding during extreme events greater than the 1% AEP (1 in 100 year) event or managing the risk of climate change over the long-term.
Table 2.8 summarises the damages and overall contribution of both residential and non-residential properties within West Ravensthorpe for the Do-Minimum scenario.
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Table 2.8 - Present Value Damages - West Ravensthorpe
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 813 £2,300 13% Non-Residential 134 £15,500 87% Total 947 £17,800k 100%
2.3.8 Cluster 6 - East Ravensthorpe
The economic damages in East Ravensthorpe are high due to the presence of numerous and densely packed non-residential properties at risk of fluvial flooding on the northern bank of the River Calder and the western bank from the River Spen with high PV damages estimated near to the confluence of both Main Rivers.
The extent of flooding significantly increases when considering the 0.1% AEP (1 in 1000 year) event, with approximately 675 properties at risk of fluvial flooding throughout East Ravensthorpe.
Two non-residential properties at risk from the 0.1% AEP (1 in 1000 year) event contribute to over £5,100k of the £11,400k PV damages in this area. The remaining damages are distributed across East Ravensthorpe up to the 0.1% AEP (1 in 1000 year) event at depths up to 2m.
With the majority of the damages shown to occur during a 1% AEP (1 in 100 year) event or greater, flood risk management measures providing lower SoP’s will provide little benefits. The aim in this location would be to reduce the risk of flooding during extreme events greater than the 1% AEP (1 in 100 year) event or managing the risk of climate change over the long-term.
Table 2.9 summarises the damages and overall contribution of both residential and non-residential properties within East Ravensthorpe for the Do-Minimum scenario.
Table 2.9 - Present Value Damages - East Ravensthorpe
Property type Properties at risk Do-Minimum
PV Damages Contribution
Residential 629 £1,000k 9% Non-Residential 50 £10,400k 91% Total 679 £11,400k 100%
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3. Flood Risk Management Options
3.1 Long List
Based on the flood risk mechanisms modelled, local knowledge of the catchment and historical flood records, a long list of structural, non-structural and adaptation options have been identified. A high-level multi-criteria scoring exercise has also been undertaken to assess each option against a range of technical, economic and environmental criteria. This scoring approach aims to identify viable options measured against the criteria listed above.
The simple assessment of each option has been made to determine the overall feasibility and whether they should be progressed to further assessment. Appendix B contains the results of the multi-criteria scoring exercise.
3.2 Shortlist
From the long list of options presented in Appendix B, three main options (and sub-options) have been shortlisted for further consideration to determine their technical and economic viability. The options include: 1) Option 1 – Upgrade to existing canal lock gates in Central Mirfield a) Increase the height of the existing lock gate b) Replace the existing lock gate with a penstock c) Permanent flood barrier d) Temporary flood barrier e) Pathway alterations 2) Option 2 - Permanent raised flood defences a) West Ravensthorpe b) East Ravensthorpe c) West Mirfield 3) Option 3 - Offline storage at Ladywood Lakes
3.2.1 Option 1 – Upgrade to existing canal lock gates - Central Mirfield
The hydraulic model predicts that the canal begins to overtop during a 4% AEP (1 in 25 year) event, which result in a new flood pathway being created and floodwater affecting industrial units to the north of the railway. However, it cannot be determined whether the increased flows during a flood event downstream of the canal along the River Calder is backing up and contributing to the flood risk at the industrial units without further reviewing the model in detail. Detailed hydraulic modelling is also required to fully understand the flood mechanisms between the canal and the River Calder in Central Mirfield.
Option 1 therefore aims to prevent this flood mechanism from occurring by implementing a number of individually or collective measures. Figure 3.1 shows the location of the proposed options.
It has been assumed that the existing lock gate overtops during the 1% AEP (1 in 100 year) event or greater, based on comparing the Boxing Day 2015 floods with the modelled flood extents. Option 1A and Option 1B include increasing the height of the existing lock gate by 0.5-1m in line with the base of the bridge or replacing the existing lock gate with a penstock that is raised by 0.5-1m. Both sub-options would ultimately provide the same benefits; however, the Canals and Rivers Trust would need to operate the penstock rather than the public for the passage of narrowboats, which may make sub-option 1B unviable. The proposed increase in height in sub-option 1A, assumes that the proposed lock gate will overtop during the 1% AEP (1 in 100 year) plus climate change event or greater.
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Figure 3.1 – Proposed penstock in Central Mirfield
The modelling also suggests that the footpath to the side of the lock gate conveys flow around the existing lock gate and into the canal during a 4% AEP (1 in 25 year) event or greater, see Figures 3.2 and Figure 3.3. The recent 2015 Boxing Day floods showed the lock gate being overtopped with flows down the footpath and into the canal, shown in the photograph in Figure 2.7.
Figure 3.2 – Existing lock gate and footpath entrance Figure 3.3 – Footpath beside existing lock gate
Option 1C (permanent flood barrier) and Option 1D (temporary flood barriers) have therefore been proposed to prevent this flow path from occurring. Alternately, Option 1E has been proposed, which increases the slope of the pathway leading up to the lock gate so that the path is at the same height once the lock gate is reached.
In all cases, the height of the flood barrier or pathway will need to be at the same height as the upgraded structure to maintain the same level of protection. In the case of Option 1E, only the pathway approaching the
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structure is raised. Once the lock gate is reached, the existing steps are kept in its current state and alignment to allow the same passage under the bridge. It is assumed that Options 1A and 1B will be a similar cost and provide a similar level of benefit. The same assumption is made for Options 1C, 1D and 1E. As a result Options 1A (raise existing lock gate) and 1E (land raise) will be taken forward to the economic appraisal and hereinafter referred to in combination as Option 1.
3.2.2 Option 2A – Permanent Raised Flood Defences - West Ravensthorpe
The hydraulic model indicates that the River Calder overtops of the northern bank during a 1% AEP (1 in 100 year) event, affecting residential and non-residential properties in West Ravensthorpe.
Option 2Ai therefore involves the construction of a 640m permanent raised flood defence running along the bank of the River Calder and the canal to provide a 1% AEP (1in 100 year) SoP to 25 non-residential and 6 residential properties. This would include the protection of three non-residential properties that contribute to approximately £5,000k of the £17,800k PV damages in this area.
Figure 3.4 – Proposed Flood Defences - West Ravensthorpe
Option 2Aii - The flood risk across West and East Ravensthorpe significantly increases between the modelled 1% AEP (1 in 100 year) flood event and the 1% CC AEP (1 in 100 year plus climate change) flood events indicating that the area is sensitive to future increases in flood flows resulting from our changing climate. The increase in flood extent anticipated over the next 100 years is shown in Figure 3.4 and indicates that a further 6 non-residential and 6 residential properties will be at risk from the 1% AEP event. A precautionary approach to the provision of defences would involve building any new defences to the level and extent required to protect against the 1% AEP event and to include the required increase in level and extent to cater for climate change from day one. This precautionary approach would increase the length of the proposed raised flood defence by 110m to 750m and provide protection to an additional six non-residential and six residential properties, equating to £3,100k in PV damages avoided. Figure 3.4 shows the location of the proposed flood defence options.
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The local ground conditions along the River Calder in this location look to be particularly unstable with the banks comprising of soft mud that is highly saturated with sparse vegetation, which reduces the cohesive binding of the banks. As a result, a geotechnical survey will be required to assess the buildability of the option.
There is also an existing wall (see Figure 3.5) at the confluence of the canal and the River Calder; however, the condition of the wall is poor and it would need to be demolished and replaced to tie in with the proposed flood defence. Consequently, an alternative route behind the dense vegetation has been assessed, which is away from the River Calder, less vegetated, more accessible and with better ground conditions. The alternative route is illustrated in Figure 3.4. Based on the potential benefits available, Option 2Aii has been considered during the economic appraisal. If this option is taken forward, the consequences will have to be carefully considered to decide if compensatory measures are required. This is due to the potential increase in flood risk upstream or downstream from a reduced floodplain at the location of the flood walls.
Figure 3.5 – Existing wall and left bank Figure 3.6 Local ground conditions
Existing Stone Wall – Unstable
3.2.3 Option 2B - Permanent Raised Flood Defences - East Ravensthorpe
The hydraulic model indicates that the earliest onset of flooding occurs because of overtopping of the northern bank of the River Calder at the confluence with the River Spen, which initially affects non-residential properties.
Option 2Bi includes the construction of a 400m raised flood defence running along the northern bank of the River Calder to provide a 1% AEP (1in 100 year) SoP to nine non-residential properties, that contribute £1,600k of the £11,400k PV damages in this area.
Option 2Bii - The flood risk across West and East Ravensthorpe significantly increases between the modelled 1% AEP (1 in 100 year) flood event and the 1% CC AEP (1 in 100 year plus climate change) flood events. Consequently a precautionary approach has been undertaken to highlight the additional number of properties that can be better protected against the 1% CC AEP flood event. There is the option to increase the length of the proposed raised flood defence by 220m to the west along the River Calder and 330m along the western bank of the River Spen to provide a 1% CC AEP (1 in 100 year plus climate change) SoP equating to £2,900k in PV damages avoided. The proposed flood defence extension would also extend away from the River Spen along a field boundary to potentially provide additional floodplain storage. The additional 550m of raised flood defence would provide protection to an additional 21 non-residential and 49 residential properties with the total length of walls totalling 950m. Figure 3.7 shows the location of the proposed flood defence options.
Similar to West Ravensthorpe, the local ground conditions along the River Calder are particularly unstable with the banks partially eroded and comprising of soft mud with sparse vegetation coverage leading to a greater susceptibility to further erosion. Access to the site is also restricted with no clearly defined access route along the River Calder. As a result, a geotechnical survey will be required to assess the buildability of the option. Detailed hydraulic modelling will also be required to ensure there are no negative impacts downstream in East Ravensthorpe and Dewsbury.
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Based on the potential benefits available, Option 2Bi and 2Bii have been considered during the economic appraisal.
Figure 3.7 – Proposed Flood Defences - East Ravensthorpe
3.2.4 Option 2C – Permanent Raised Flood Defences - West Mirfield
The hydraulic model indicates that the River Calder overtops both bank during a 10% AEP (1 in 10 year) event initially affecting two non-residential properties, rising to approximately 40 properties during the 0.1% AEP (1 in 1000 year) event.
Option 2C therefore includes the construction of a 660m raised flood defence along the northern bank of the River Calder to provide a 1% AEP (1 in 100 year) SoP to 12 non-residential properties and three residential properties, which contribute to £10,000k of the £12,100k PV damages in this area. Figure 3.8 shows the location of the proposed flood defences. Again, geotechnical surveys and detailed hydraulic modelling will be required to assess the buildability and technical feasibility of this option.
Based on the potential benefits available, Option 2C has been taken forward for detailed economic appraisal.
It should be noted that the area adjacent to Cooper Bridge Sewage Treatment Works was identified as a potential storage option in the long list of options, with the potential to use the football pitches opposite as an additional flood storage area. However, this was discounted at the initial assessment stage due to unknowns surrounding potential ground conditions, existing underground structures and environmental concerns.
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Figure 3.8 – Proposed Flood Defences - West Mirfield
3.2.5 Option 3– Offline Storage Area - Ladywood Lakes
The hydraulic model indicates that flooding of the Ship Inn and Ladywood Lakes, which are located within the southern meander of the River Calder, onsets during the 10% AEP (1 in 10 year) event. Figure 3.9 shows the location of the proposed flood storage area.
Upstream flood storage areas of Mirfield and Ravensthorpe were considered; see Section 4.2.4. However, due to the steep and urban nature of the River Colne and Calder floodplains storage options were limited. Ladywood Lakes was identified as a large open area, which could potentially attenuate 160,000m3 of flood water (based on a surface area of 80,000m2 and a depth of 2m). An indicative embankment approx. 1m above existing ground level has been estimated, with inlet and outlet structures to ensure the storage area comes into operation during a 4% AEP (1 in 25 year) event or less. As part of the storage option, the water level in Ladywood Lakes would need to be maintained 1m below its current level in order to generate additional capacity and limit the height of the new embankment to 1m AEGL. The implementation of the storage option aims to reduce risk to properties locally and downstream in Ravensthorpe.
There is a potential opportunity to extend the storage area to the land on the northern bank of the River Calder as highlighted in Figure 3.9. This provides approximately 68,000m3 of additional flood storage (based on a surface area of 34,000m2 and a depth of 2m). An indicative embankment approx. 1m above existing ground level has been estimated, with inlet and outlet structures to ensure that the storage area comes into operation during a 4% AEP (1 in 25 year) event or less.
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Figure 3.9 – Flood Storage Options
Implementation of both storage areas could provide an additional volume of approximately 228,000m3. It is recommended that the next phase of work would be an investigation of the Environment Agency’s modelled data to understand the river dynamics and conduct further analysis to determine whether the volume of stored water is adequate to deliver reductions in flood risk and to what SoP. Following this analysis, detailed hydraulic modelling might be required to generate a Do Nothing economic baseline and additional modelling of the proposed options. The hydraulic modelling will determine the viability of Ladywood Lakes as a storage option either on its own, or part of the compensatory measures required for the other options identified in this report, such as the flood walls in East and West Ravensthorpe.
3.3 Economic Appraisal
3.3.1 Costs
A high level costing exercise has been undertaken to provide indicative costs associated with implementing and maintaining each option shortlisted, including Options 1 (1A and 1B), 2A, 2B, 2C and 3.
Total costs include: Capital costs Risk contingency (10% of capital costs) Enabling costs (% of works costs based on total cost of scheme) Annual and intermittent maintenance costs Optimum bias (60% of total costs)
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Table 3.1 provides a summary of the cash costs for each of the flood risk management options. These has been shown as cash costs and by using whole life discount factors, also shown as Present Value Costs assuming a 100-year appraisal period.
As the flood risk across West and East Ravensthorpe significantly increases between the modelled 1% AEP and the 1% CC AEP flood events, a precautionary approach has been undertaken to highlight the additional PV costs that are required to protect up to the 1% CC AEP event. This is shown in Table 3.1.
Option 2Ai and 2Bi would provide protection against the 1% AEP event; however consideration should be given to the longer term view that an extension to the raised flood defences may be required in the future to address the potential impact of climate change.
Table 3.1 - Present value costs summary
West Ladywood Central Mirfield West Ravensthorpe East Ravensthorpe Mirfield Lakes
Option 1B Option 1E Option 2Ai Option 2Aii Option 2Bi Option 2Bii Option 2C Option 3* 1% AEP 1% CC AEP 1% AEP 1% CC AEP
Increase Extended Extended Permanent Permanent Permanent Offline the height Permanent Permanent Pathway raised flood raised flood raised flood storage at of the raised flood raised flood alterations defences defences defences Ladywood existing defences defences (750m) (950m) (660m) Lakes lock gate (640m) (400m)
Capital Costs £165k £21.4k £2,838.k £3,255k £1,892k £4,085k £2,838k £2,038.5k
Risk Contingency £16.5k £2.1k £283.8k £322.5k £189.2k £408.5k £283.8k £203.9k
Enabling Costs £58.1k £7.5k £255.4k £319.3k £170.3k £404.4k £281k £201.8k
Maintenance Costs £196.4k £0.0k £2,868.3k £3,585.4k £1,912.2k £4,541.5k £3,155.2k £4,260.5k
Total Cash Costs £435.9k £31.1k £6,245.5k £7,452.2k £4,163.7k £9,439.4k £6,841.7k £6,704.6k
Total PV Costs £273.6k £29.0k £3830.7k £4,788.3k £2,553.8k £6,065.2k £4,497.5k £3,607.1k
PV Cost + Optimum Bias £437.8k £46.4k £6,129.1k £7,661.3k £4,086.1k £9,704.4k £7,196.1k £5,771.3k *Option 3 considers the combined volume of both storage areas (228,000m3)
3.3.2 Benefits
Table 3.2 summaries the Present Value Benefits that have been estimated for each option, by calculating the residual damages with the option in place and subtracting this value from the Do-Minimum damages.
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Table 3.2 - Present value damages and benefits
West East Ladywood Central Mirfield West Mirfield Ravensthorpe Ravensthorpe Lakes Option 1B Option 1E Option 2A* Option 2B* Option 2C Option 3**
Increase the Permanent Permanent Permanent Offline storage height of the Pathway raised flood raised flood raised flood at Ladywood existing lock alterations defences defences defences (750m) Lakes gate (950m) (660m)
Residual Residential PVd £200k £3,000k £1,100k £200k £4,100k
Residual Non-Res. PVd £2,000k £3,900k £2,800k £300k £7,300k
Total PVd £2,200k £6,900k £3,900k £500k £11,400k
Residential PVb £400k £100k £300k £200k £400k
Non-Res. PVb £11,500k £10,800k £6,000k £11,400k £21,700k
Total PVb £11,900k £10,900k £6,300k £11,600k £22,100k *PVD and PVB was not calculated for the 1% CC AEP for Options 2A and 2B **Option 3 considers the combined totals of Ladywood Lakes, West and East Ravensthorpe
3.3.3 Summary
Table 3.3 summarises the monetised benefit cost assessment, which includes BCR and raw Partnership Funding Scores for each option shortlisted. The BCR indicates which option is most economically advantageous; whilst the raw Partnership Funding Score shows how much Flood and Coastal Risk Management (FCRM) Grant in Aid (GiA) funding the option is eligible for as a percentage of the total costs without any partnership contributions.
The Partnership Funding Score is intrinsically linked to the benefits the option provides, which include the reduction in flood risk to households. The Partnership Funding Score has been calculated with properties located within Ravensthorpe located in the 20% most deprived areas; and properties located within Mirfield in the 60% least deprived according to the Index of Multiple Deprivation 20106.
Table 3.3 shows that Option 1 is the most economically advantageous, with a BCR of 24.6 and a high Partnership Funding Score of 138%. Overall flood damages to residential and non-residential could be reduced by £11,900k, however, a large proportion of these benefits are associated with the non-residential properties. The overall scheme costs are low, compared to the other schemes, at £484k.
The uncertainty with Option 1 is in the absence of recorded flow data, the current flood outlines from the canal are based on an estimated flow into the canal. Anecdotal evidence from Kirklees Council suggested that the canal might not have overtopped during the 2015 Boxing Day floods and that the flood mechanism could be overtopping of the River Calder near Lower Hopton with flows moving north, uphill and under the railway line and into the industrial unit. The potential impact of a closed lock gate in a 1% CC AEP event or greater could increase risk downstream in Lower Hopton. However an open lock gate could act as a bypass channel storing excess flood waters. Detailed modelling is required to determine the impact of the various scenarios to determine the best course of action during a exceedance event.
Flood events of slightly increased magnitude; events over longer time periods; or structural stress on the current lock gate, compared to the Boxing Day 2015 floods could lead to flood issues in the future. It was noted on site that the current lock gate was potentially towards the end of its lifetime. As a minimum, if this option was not considered, an assessment of the structural integrity and condition would have to be made to make sure that
6 http://neighbourhood.statistics.gov.uk/HTMLDocs/AtlasOfDeprivation2010/index.html
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recent flood events have not compromised the integrity to the point where collapse or major leakage may occur. Therefore, given the BCR is very positive, this option is recommended.
Options 2A and 2B have been sensitivity tested against the costs of providing defences to the 1% AEP and the 1% AEP with climate change; see Table 3.3. By adopting this precautionary approach the BCRs can be compared to determine whether to protect to the 1% or 1% CC AEP event.
Table 3.3 - Economic appraisal summary
West Ladywood Central Mirfield West Ravensthorpe East Ravensthorpe Mirfield Lakes Option 1B Option 1E Option 2Ai Option 2Aii Option 2Bi Option 2Bii Option 2C Option 3* 1% AEP CC 1% AEP 1% AEP CC 1% AEP
Increase Permanent Permanent Permanent Permanent Permanent Offline the height Pathway raised flood raised flood raised flood raised flood raised flood storage at of the alterations defences defences defences defences defences Ladywood existing (750m) (640m) (950m) (400m) (660m) Lakes lock gate
Economics
Total PVd £2,200k £6,900k £6,900k £3,900k £3,900k £500k £11,400k
Total PVb £11,900k £10,900k £10,900k £6,300k £6,300k £11,600k £22,100k
Total PVc £484k £7,661k £6,537.7k £9,704k £4,086.1k £7,196k £5,771k
BCR 24.6 1.4 1.7 0.6 1.5 1.6 3.8
Residential properties better protected against flood risk compared to Do-Minimum**
Very Significant 1 0 0 0 0 1 5
Significant 0 0 0 0 0 2 2
Moderate 3 12 6 49 0 1 63
Prospect of FCRM GiA Funding
Raw Partnership Funding 137% 8% 9% 4% 9% 9% 22% Score *Option 3 considers the combined volume of both storage areas (228,000m3). **Very Significant has been classified as properties at risk up to the 5% AEP flood event. Significant has been classified as properties at risk between the 5% AEP and 1.33% AEP flood event. Moderate has been classified as properties at risk between the 1.33% AEP and 1% AEP flood event.
Options 2Ai and 2Aii have low BCR values of 1.4 and 1.7 respectively. These options result in minimal residential properties better protected, resulting in extremely low partnership funding scores. The benefits of £10,900k for Options 2Ai and 2Aii are associated with a reduction in flood depths rather than flood onsets. The overall scheme costs are high at £7,661k and £6,537k for Options 2Ai and 2Aii respectively. There is also the requirement for compensatory storage to mitigate an increase flood risk elsewhere. This is not currently costed in anticipation of a Do Nothing baseline being generated as part of future Do Nothing modelling.
Option 2Bi is not economically viable against the current Do Minimum baseline with a BCR 0.6. 49 residential properties are better protected against flood risk; the scheme costs are £9,704k, which is higher than the potential benefits of £6,300k. Option 2Bii has a positive BCR of 1.5, with a raw partnership funding score of 9%, which is a 5% increase over option 2Bi. This increase comes from a significant reduction in the length of flood wall recommended the River Calder at East Ravensthorpe.
Option 2C has a low BCR value of 1.6. Option 2C does not result in any residential properties better protected, resulting in an extremely low partnership funding score. The benefits of £11,600k are associated with a reduction in flood depths rather than flood onsets. The overall scheme costs are high at £7,196k. As per Options 2Ai and 2Aii there is also the requirement for compensatory storage which would be required, to as not
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increase flood risk elsewhere. This is not currently costed in anticipation of a Do Nothing baseline being generated as part of future Do Nothing modelling.
Option 3 is economically viable with a BCR of 3.8. This is based on five residential properties and 65 non- residential properties being better protected against flood risk and overall scheme costs of £5,771k. Option 3 assumes that Options 2A and 2B are not required and that Ladywood Lakes can provide the full benefits required. There is a high degree of uncertainty with calculating the benefits of this option; the storage volumes and magnitude would need to be thoroughly tested using a hydraulic model in order to calculate actual benefits. Based on the findings of this study, Options 1 and 3 are the most cost beneficial; however, these options need to be assessed in conjunction with the other options to reduce flood risk across Mirfield and Ravensthorpe as a comprehensive flood risk management strategy.
Option 2C upstream of Ladywood Lakes reduces the fluvial flood risk in the immediate area but consequently could increase the potential flood risk downstream towards Option 1 in Central Mirfield. Option 1 proposes to reduce the flood risk to the properties along the canal; however this potentially increases the risk downstream along the River Calder. Ladywood Lakes provide a potential solution to mitigate these impacts as well as providing additional flood storage to reduce fluvial flood risk in the immediate area and downstream (Option 3).
The impact of implementing Options 1 and 2C without Option 3 would likely increase the flood risk further downstream in Ravensthorpe and potentially as far downstream as Dewsbury. These options would need to be assessed collectively to robustly understand the impacts, effectiveness and potential benefits.
Options 2A and 2B also reduce the fluvial flood risk to Ravensthorpe and can be implemented in place of or as well as Option 3. If Options 2A and 2B are implemented in place of Option 3, these options have the potential to exacerbate flood risk downstream in Dewsbury. Detailed modelling will be required to determine whether Option 3 can independently store the required volume of water to provide the necessary benefits locally and downstream. If the modelling suggests that there is not enough storage capacity available, Options 2A and 2B can be implemented along with Option 3 to help reduce the fluvial flood risk downstream.
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4. Conclusions and Recommendations
4.1 Conclusion
This study has assessed the technical, economic and environmental viability of undertaking flood risk management works in Mirfield and Ravensthorpe within the Metropolitan Borough of Kirklees. Six fluvial flood risk clusters have been defined and assessed as part of this study, namely West Mirfield, Central Mirfield, Lower Hopton, Ladywood Lakes, West Ravensthorpe and East Ravensthorpe.
The fluvial flood risk from the River Calder is considered high across Mirfield and Ravensthorpe with clusters of properties at risk and associated economic present value damages ranging from £4,000k in Ladywood Lakes to £18,000k in West Ravensthorpe. There is also a direct fluvial risk from the River Spen, which is a tributary of the River Calder, however this has been considered in the separate surface water study.
This study uses the Environment Agency’s 1D-2D hydraulic modelling and flood maps (2014) and utilises the defended case model runs as the present day baseline (do minimum) for the assessment of present day economic damages. The model shows that the Rivers Calder starts to overtop in different magnitude flood events within the clusters. The current standard of service (or onset of flooding) is listed below: West Mirfield – onset of flooding starts in the 10% AEP flood event Central Mirfield – onset of flooding starts in the 4% AEP flood event Ladywood Lakes – onset of flooding starts in the 10% AEP flood event West Ravensthorpe – onset of flooding starts in the 1% AEP flood event East Ravensthorpe – onset of flooding starts in the 10% AEP flood event
Given the topographical and spatial constraints, the individual options considered focus on reducing the fluvial flood risk utilising options such as flood walls along West Mirfield, West and East Ravensthorpe and replacing/raising the existing lock gate and the approaching pathway to the canal in Central Mirfield. Storage is also proposed utilising the existing Ladywood Lakes area and nearby land as an offline flood storage area.
Option 1 (raise existing sluice and approaching pathway to the canal) was identified as the most affordable solution with a BCR of 24.6 and a partnership funding score of 137%.
Options 2A, 2B and 2C consist of flood walls within West Ravensthorpe, East Ravensthorpe and West Mirfield. BCRs range between 0.6 and 1.7 suggesting that there is a weak economic argument to undertaking these options when considering the benefits of non-residential properties. This analysis also indicates that these options would generate a low partnership funding score meaning that it would generate a small percentage of funding from FDGiA. If they are to be considered, then the proposed flood walls predominately reduce economic damages to non-residential properties, but provide better protection to a small number of residential properties in West Ravensthorpe, East Ravensthorpe and West Mirfield respectively.
Option 3 (offline storage area) has been proposed as an affordable option to reduce flood risk with a BCR of 3.8. The benefits for this option are to three clusters identified in this report (Ladywood Lakes, West Ravensthorpe and East Ravensthorpe).
Based on the findings of this study, Options 1 and 3 are the most cost beneficial; however, these options need to be assessed in conjunction with the other options to reduce flood risk across Mirfield and Ravensthorpe as a comprehensive flood risk management strategy.
It should be noted that Option 1 presents a technical challenge particularly in relation to raising the existing lock gate by 0.5-1m and retaining the manual operation of the sluice due to its close proximity with the bridge. Furthermore the approaching pathway needs to be gradually raised so that once the lock gate is reached they are at the same elevation. The pathway and lock gate need to be tied in with each other and the side walls to ensure that flow cannot bypass the gate into the canal. Once the lock gate is reached the existing steps are kept in their current form and alignment to allow pedestrian access under the bridge.
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Option 3 utilises Ladywood Lakes and neighbouring land both of which were affected by the Boxing Day 2015 floods. The offline storage area covers Ladywood Lakes and a nearby field with a potential to hold up to 228,000m3 of attenuated flood water. The option presents technical challenges particularly as the ground and environmental conditions are unknown and to attain the additional 228,000m3, the water level in Ladywood Lakes would need to be maintained at a level approximately 1m lower than its current level. This would be supplemented by an embankment approximately 1m above exiting ground level with an inlet and outlet control structure to regulate basin operation during a 4% AEP flood event or greater.
4.1.1 Considerations of conclusions Whilst flood walls reduce the risk of fluvial flooding in the immediate area, there is the potential for the level of flood risk to be to be increased elsewhere as a consequence of defending the existing floodplain. The impact of defending certain risk clusters will need to be tested using the hydrodynamic model and may lead to the need for compensatory storage or other mitigation measures elsewhere in the catchment. The options focus on providing a 1% AEP SoP to properties in the floodplain. The majority of residential properties are located within the 1% AEP plus climate change flood extent or higher, and few would benefit from these options. Defra policy is to prioritise flood risk management expenditure on protecting residential properties and the partnership funding calculation reflects this. Consequently, most of the options considered return very low Partnership Funding scores which would mean that the majority of the cost of any FRM option would not be to be met by Central Government funding sources (FDGiA). This does not prohibit these options being progressed, but alternative sources of funding would need to be secured. It is uncommon practice to use 1% AEP plus climate change as an economic run, but the economic argument is likely to increase significantly with minimal cost increase for the designs and therefore it is recommended that the next stage whether a Do Nothing baseline is generated uses the 1% AEP plus climate change results.
4.2 Recommendations From the analysis undertaken, a number of options have been identified and therefore it is recommended that Kirklees Council consider the following: 1) It is recommended that the next phase of work would be an investigation of the Environment Agency’s modelled data to understand the river dynamics and conduct further analysis to determine whether the volume of stored water at Ladywood Lakes is adequate to deliver reductions in flood risk and to what SoP. 2) Funding should also be sought to carry out detailed hydraulic modelling to test the actual benefits of the measures proposed and generate an accurate Do Nothing model which will increase the benefits of the scheme, building on the Do Minimum, and ultimately, conservative benefits presented in this study. This modelling should also be used to assess potential impacts of options to the site and elsewhere. As part of this, option refinement will be required and consideration of incremental cost benefit analysis to define the most economically preferable SoP. IBCRs are only possible to create with a Do Nothing as the baseline. 3) As a result of detailed modelling, compensatory storage may be required to offset the increased flood risk elsewhere as a result of particular options such as flood walls. It is anticipated that some of the recommended schemes in this report, particularly the walls and storage in at Ravensthorpe could be undertaken in conjunction to both provide protection from flooding and meet the compensatory requirements. This is particularly important as Dewsbury is located downstream and has suffered from historical events of fluvial flooding. 4) All flood risk management activities should be undertaken alongside a structured programme of community engagement works to help raise awareness of flood risk and environmental issues.
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Appendix A. Economic Methodology
A.1 Introduction
This technical note sets out the economic assessment approaches used for the Mirfield and Ravensthorpe fluvial viability study, Kirklees.
The assessment of the economic impact of flooding is imperative to demonstrate a business case for flood works and in order to assess eligibility criteria for public funding. This technical note presents the economic assessment undertaken.
Economic assessment in its simplest form is a comparison between the costs of designing, building and maintaining a flood risk management scheme against the economic benefits that a scheme provides. Damages associated with property is categorised in a variety of ways which are described in Table A1:
Table A1. The costs of flooding to properties
Type of damage Description
Damage to the building structure / foundations or to the property fabric including electrics Direct damage and plasterboard.
Replacement and clean-up The cost of replacing damaged property possessions such as sofas or carpets and the cost costs of removing water after an event (i.e. making the property habitable again).
The emotional cost of flooding including loss of memorabilia, mental health issues and Intangible worry over future events.
This includes any temporary costs of accommodation and electricity consumption. For this Indirect damage study, increased traffic, damage to cars, business impacts on the community and utilities/services are not included.
The cost of providing emergency services during an event. Such services include: the fire Emergency services department, police, local authority, ambulance services, armed forces or the Environment Agency.
The two key variables in economic assessment are the frequency at which the onset of flooding occurs, and the depth of flooding.
A.2 A2. Calculation of Flood Damages
The calculation of flood damages is dependent on a number of factors including the type of property, average prices, flood depths and property threshold levels. The calculation of flood damages has been undertaken in accordance with Defra/Environment Agency guidance on appraising flood risk management schemes and using the Multi Coloured Manual (MCM) methodologies and datasets.
A.2.1.1 Property dataset
The Environment Agency’s National Receptor Dataset (NRD) was used as the basis for the property dataset. Property points with non-property Ordnance Survey Base Functions (e.g. allotment, electricity substation, bus shelters and post box) and upper floor properties were excluded.
A.2.1.2 Property Valuations
Property valuations are required to cap the damages at the value of the property.
Residential property values are based on national average property values derived from the Land Registry (http://www.landregistry.gov.uk/). Prices from January 2014 were uplifted using the Consumer Price Index (CPI)
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to November 2015 which was the latest available date. Average prices for Yorkshire and Humberside were used.
Table A2. Regional average residential property prices (January 2014 prices)
Property type Average valuation
Detached £ 202,524
Semi-detached £ 110,016
Terraced £ 77,699
Flat £ 109,166
Non-residential property valuations were calculated from rateable values per square metre, which were derived from the 2008 edition of the ‘Commercial and Industrial Floor space and Rateable Value Statistics’ (2005 Revaluation) provided by the Office for National Statistics. The rateable values were uplifted using the CPI. The rateable values were multiplied by the area of each property (as provided by the NRD) and using an equivalent yield factor the value of non-residential property was calculated through the formula:
100 ( ) ∗ Rateable Value EY Value
The Equivalent Yields (EY’s) were taken from the CBRE UK Prime Rent and Yield Monitor for the Yorkshire & Humberside region. The latest date published was Q2, 2015. The EY and rateable values are presented in Table A3. No code existed for ‘other bulk premises’ so the average of the four EY codes was used as a best estimate.
Table A3. Rateable values from the ONS ‘Commercial and Industrial Floor space and Rateable Value Statistics’ and the CBRE ‘UK Prime Rent and Yield Monitor’ (January 2014 prices)
OS Class Retail Office Warehouses Factories/ Industrial
MCM Code 200-299 300-399 400-499 >800
EY Code 6.57 8.09 5.5 7.43
England & Wales Average 133 89 35 27 Rateable Value (£/m2)
A.3 Threshold Levels
The property threshold level is the required flood depth for internal flooding to occur. Flood water can enter properties in a number of ways but predominantly through weak spots such as airbricks, doors or windows.
The threshold levels for were estimated from visual inspection and using industry standard. Given the high level nature of the economic process residential properties were given a threshold of 15mm (2 bricks) whilst non- residential properties were given 0mm (no bricks).
A.4 Extraction of Flood Depth
Each NRD point is spatially joined with its respective MasterMap Building Footprint in order to extract flood depths evenly across the property. The tool used for this was the QGIS-SAGA ‘Grid Statistics for Polygons’.
Fluvial depths were extracted from the River Calder defended scenario 2014 and River Spen 2009 models respectively. The maximum depths were used to assess fluvial flooding following MCM guidance.
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A.5 Return Periods used to assess damages
Return periods are expressed as Annual Exceedance Probability (AEP). The following depth grids were available and used to calculate ‘Do Nothing’ and ‘Do Minimum’ runs. 10% AEP (1 in 10 year) 4% AEP (1 in 25 year) 2% AEP (1 in 50 year) 1.33% AEP (1 in 75 year) 1% AEP (1 in 100 year). 0.1% AEP (1 in 1,000 year)
A.6 Discount Factor
A present value is defined as a future value expressed at today’s rate by discounting. Discounting is the process of establishing the rate of incline or decline respectively. Discount factors mirror the lifespan of capital works. The assessment period for fluvial flooding is 100 years. The PV discount factors were based on the UK Treasury Green Book for UK public infrastructure projects.
A.7 Conclusion
This Appendix has provided a description of the steps taken to produce the economic baseline for fluvial flood risk. These numbers will form the assessment of affordability for undertaking works.
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Appendix B. Options Appraisal
Likely to be positive Likely to be neutral or have no impact Legend: ~ ? Unknown, further analysis required Unlikely to be positive; potential to have a negative impact Technical Feasibility - Is it possible to put the measure in place and is it buildable? Technical Effectiveness - Will it work? Would it alleviate any of the flooding? Economic Viability - Is it likely to be affordable or would costs be far too high? Environmental Impacts - Would it have an impact on the environment? E.g. habitats or water quality Technical Technical Economic Environmental Applicable Continue to Option Considered Notes Feasibility Effectiveness Viability Impacts Sites SLO*? (Y/N)
Channel Containment Options
Effective only if erected at the correct time, therefore effective warning is required. Questions over feasibility due to time constraints and turnaround in Temporary defences ? ? All Sites N ~ the event of a flood. Unlikely to be safe for local public to erect, therefore only the EA/KC could install when required.
Embankments along Unfeasible in this area as there is not enough space immediately N/A N channel ? adjacent to the river channel for embankments. West Mirfield; Effective way of containing the water within the channel. Central Mirfield; Permanent localised Technically feasible, however space restrictions exist. West ? Y raised flood defences ~ Floodplain compensation would be required as a result of this Ravensthorpe; option. East Ravensthorpe Effective way of containing the water within the channel. Technically unfeasible in this area even though there are space Permanent stepped- restrictions. N/A N back flood walls Floodplain compensation would be required as a result of this option. Improved Channel Conveyance Options
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Technical Technical Economic Environmental Applicable Continue to Option Considered Notes Feasibility Effectiveness Viability Impacts Sites SLO*? (Y/N)
Unknown geomorphology issues mean that we cannot predict the built up rates of material removed. Removal of major silt Difficult to remove in places due to overhead restrictions, machine Central Mirfield N deposits from the river ? ? access and safety concerns. Ladywood Lakes Negative impacts on environment due to river bed disturbance, issues with regulations such as WFD.
Technically feasible but extent of effectiveness would require modelling. Bridge soffit raising ? ? N/A N ~ Any work would require bridge specific assessments which would then influence affordability.
Unlikely to be feasible due to space restrictions due to the urbanized nature of the areas. Channel widening N/A N Unlikely to be economically viable at this location as it would prove costly to relocate properties immediately adjacent to the channel.
This option is unlikely to reduce flood risk at Mirfield and could enhance flood risk elsewhere. The impact of this would require modelling. Movable Weir/ Limited knowledge of the costs of movable weirs lead to a high Removable Weir ? ? level affordability assessment being undertaken. The result of this N/A N lead to the option being ruled out on economic affordability grounds. Increase in velocity due to removing/lowering weir could make the River Calder unnavigable for canal boats
Proposed penstock or lock gate is an effective way of controlling Proposed Penstock or the water within the channel. Raising existing lock Proposed penstock or existing lock gate raised to base level of gate coupled with ? ~ overhead bridge to increase SoP. Central Mirfield Y temporary/permanent Additional flood barrier at same height as the structure to stop flow flood barrier down the footpath during a flood event. Measures can be temporary e.g. flood blocks or permanent e.g. flood door.
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Technical Technical Economic Environmental Applicable Continue to Option Considered Notes Feasibility Effectiveness Viability Impacts Sites SLO*? (Y/N)
This option is technically feasible but extent of effectiveness would require modelling
Technically unfeasible due to spatial restrictions immediately Channel bypass N/A N adjacent to the watercourse and very expensive option.
Technically unfeasible due to spatial restrictions adjacent to the River diversion watercourse N/A N Extremely costly option. Storage Options Upstream online The marshy area in West Mirfield opposite the River Calder and the West Mirfield storage (with canal from Batteyford Sporting Club has been identified, however (potential benefit N appropriate bunding) further investigation would be required. to all sites)
A high level survey has identified Ladywood Lakes and the marshy area in West Mirfield as options for offline storage (Ladywood Ladywood Lakes; Upstream offline Lakes). West Mirfield storage (with Precise costs are difficult to estimate at this stage. Y ? (potential benefit appropriate bunding) ~ Potential environmental issues during construction phase but to all sites) potential for longer term environmental and biodiversity benefits. Ladywood Lakes is not an environmental designation.
Technically unfeasible as no suitable locations available in close Underground storage proximity to the watercourse. N/A N tank Likely to have an impact on environment due to construction disturbance. Property Level Flood Protection Property Level Flood Protection (PLFP) Some properties may benefit from PFLP resilience measures where Resistance only ? depths are less than 1m but this is unlikely to be economically All Sites N effective up to 1m ~ viable. flood depth
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