Vico Property Group Proposed Retail Development, Dalmarnock Road Flood Risk Assessment May 2002

Halcrow Group Limited Vico Property Group Proposed Retail Development, Dalmarnock Road Flood Risk Assessment May 2002

Halcrow Group Limited

Halcrow Group Limited The Octagon 35 Baird Street Glasgow G4 0EE Tel +44 (0)141 552 2000 Fax +44 (0)141 552 2525 www.halcrow.com

Halcrow Group Limited has prepared this report in accordance with the instructions of Vico Property Group for their sole and specific use. Any other persons who use any information contained herein do so at their own risk.

© Halcrow Group Limited 2002 Proposed Retail Development, Dalmarnock Road Flood Risk Assessment

Contents Amendment Record This report has been issued and amended as follows:

Issue Revision Description Date Signed

1 0 Initial issue 31-5-02 AMcG

2 0 Final Issue 11-6-02 Contents

1 Introduction 1 1.1 Context 1 1.2 Scope of the study 1 1.3 Site description and proposed development 1 1.4 Previous Studies 2 1.5 History of Flooding 2 1.6 Methodology 2

2 Hydrological assessment 4 2.1 General 4 2.2 Description of the catchment 5 2.3 Flood flow assessment methodology 5 2.4 Choice of donor catchment 6 2.5 Annual peak flow analysis for the River Clyde 7 2.6 Comparison with previous studies 8 2.7 December 1994 Flood Event 11 2.8 Climate change 11

3 Flood Alleviation Measures 14 3.1 Degree of Protection 14 3.1.1 Existing Protection 14 3.2 Methods of Flood Alleviation 15 3.2.1 General 15 3.2.2 Raising Existing Ground Levels 16 3.2.3 Flood Embankments or Walls 16 3.2.4 Raising Ground Levels and Constructing Flood Embankments or Walls 16 3.3 Predicted Flood Level 17 3.3.1 Flow/ Level Relationship 17 3.3.2 Discussion of Results 17 3.4 Freeboard 18 3.5 Options for Flood Alleviation 19 3.5.1 Option 1 – Flood Defence Wall/ Embankment Around Perimeter of Site 19 3.5.2 Option 2 – Partially Infill Site and Construct Buildings at a Higher Level 19 3.5.3 Option 3 – Partially Infill Site and Construct Flood Defence Wall/ Embankment 20 3.5.4 Option 4 – Infill Site to Design Protection Level 20 3.5.5 Option 5 – Construct Flood Defence Wall/ Embankment and Raise Existing Defences 20 3.5.6 Option 6 – Construct Flood Defence Wall/ Embankment to the Same Level as the Existing Defences 21 3.6 Impact Upstream and Downstream 21 3.7 Secondary Flooding 22

4 Conclusions and Recommendations 24

5 References 26

Figures

Figure 1: Site location Figure 2: Plan of Catchment Areas Figure 3: Annual peak discharge statistics - River Clyde at Daldowie Figure 4: Annual peak discharge statistics - River Clyde at Dalmarnock Figure 5: Flow Level Relationship at Dalmarnock

Tables

Table 1: Maximum flood discharges for a range of return periods Table 2: Comparison of Maximum flood discharges for a range of return periods Table 3: Maximum predicted flood levels for a range of return periods Table 4: Comparison of predicted flood levels for a range of return periods

Appendices

Appendix A: Supporting drawings Appendix B: Hydrology 1 Introduction

1.1 Context This study investigates the potential flood risk to the proposed retail development by the Vico Property Group at Dalmarnock Road, Glasgow, adjacent to the River Clyde. The site is located just downstream of Dalmarnock Road Bridge at national Grid Reference NS 261 677 as shown on Figure 1.

1.2 Scope of the study The scope of the study is outlined below:

Assess river flows for a variety of return periods up to the 1 in 200 year event
Determine the flood risk of the proposed development site considering the possible likely impacts of climate change upon river flows
Make recommendations for accommodating/mitigating the flood risk

1.3 Site description and proposed development The proposed development is located in the east of Glasgow on the south bank of the River Clyde. The site is just downstream of Dalmarnock Road Bridge located on part of the former Dalmarnock Industrial Estate. The total site area is approximately 11.2 ha. Existing ground elevation ranges between approximately 4.2m AOD to 7.0m AOD (at the river bank). The land is partly developed with a number of industrial units having recently been demolished. A line of trees runs along the river bank between the site and the river.

The site is part of the River Clyde floodplain and was inundated during the largest recent flood event of December 1994 with reported water depths in excess of 2 metres.

The proposed area of buildings is approximately 26,500m2 out of the 11.2 ha total site area. The development comprises a mixture of retail units, industrial units, flats and food outlets with associated parking area as detailed on the following drawings contained in Appendix A:

UG/DALM/FS01 – Proposed Layout

Doc No 1 Rev: 0 Date: May 2002 1 1.4 Previous Studies In 1997, Babtie carried out a Flood Risk Assessment1 for South Lanarkshire Council for various sites along the River Clyde including Dalmarnock Industrial Estate.

The study provided an assessment of flood levels at Dalmarnock for a variety of flood return periods based on a hydraulic modelling exercise. In addition, it made recommendations for the construction of a flood defence wall or embankment along the south bank of the River Clyde along Downiebrae Road (upstream of the site) and for a short section adjacent to the site to protect the area around Dalmarnock Industrial Estate.

1.5 History of Flooding The industrial estate in the immediate vicinity of Dalmarnock Bridge was significantly inundated during a flood event in December 1994 to water depths of up to 2 metres in places. The study under taken by Babtie in 1997 confirmed the cause of flooding was due to the River Clyde overtopping its banks over a low stretch of riverbank along Downiebrae Road and also, to some extent due to surcharging of the local sewerage system.

1.6 Methodology The location of the development adjacent to the River Clyde necessitates a thorough assessment to determine flood risk at the site. It is also necessary to include the possible impacts of future climate change, which could influence the site through increased flood magnitudes.

Factors affecting the flood levels at the site include:

Magnitude of river flood flows in the River Clyde
Hydraulic constraints on the river channel e.g. the presence of bridges on the River Clyde.

In order to obtain a thorough understanding of the interaction of the various parameters and form the basis of a robust flood risk assessment, the present study is based on:

1 South Lanarkshire Flood Study, Babtie Group, September 1997

Doc No 1 Rev: 0 Date: May 2002 2 1. A hydrological assessment of river peak flows at the site for a range of return periods, using the methodology of the Flood Estimation Handbook (FEH) (Institute of Hydrology, 1999) and the most up-to-date recorded flow data provided by Scottish Environment Protection Agency (SEPA).

2. A review of the hydraulic modelling results and recommendations made to South Lanarkshire Council in the 1997 study by Babtie taking account of the updated hydrological assessment.

Doc No 1 Rev: 0 Date: May 2002 3 2 Hydrological assessment

2.1 General Flood risk involves both the statistical probability of an event occurring and the scale of the potential consequences. The degree of risk is calculated and expressed in terms of the expected frequency of a flood for a given magnitude e.g. the 10 year, 50 year or 100 year flood. The risk is expressed in terms of these ‘return periods’. This means that there is a 10%, 2% and 1% chance respectively of such an event happening in any given year. Over a longer period of time, the probability of occurrence is considerably greater. For example:

For the 100 year return period:

there is a 1% chance of it occurring in any year, but
a 26% chance of at least one such flood in a 30 year period, and
a 51% chance of at least one such flood in 70 years, the minimum lifespan of many developments.

For the 200 year return period:

there is a 0.5% chance of it occurring in any year, but
a 14% chance of at least one such flood in a 30 year period, and
a 30% chance of at least one such flood in 70 years.

A point of concern in the flood derivation and further analysis is that it assumes that the climate (the long term weather patterns including statistically stable patterns of flood and drought) is static. Such an assumption is now thought to be insufficient as changes to the climate patterns are believed to be occurring as a result of:

(a) Long term climate change as a result of global warming. Climate models have predicted increases in the long term average precipitation over the north west of Britain and this is supported by analysis of measured natural phenomena. This will increase the size of floods experienced but the magnitude of this is at present unclear.

Doc No 1 Rev: 0 Date: May 2002 4 (b) Cyclical changes in precipitation patterns over the west of Scotland with cycles of the order of 10 to 20 years.

The effects of climate change have only recently started to be quantified, with the nature and magnitude of these changes being disputed. Thus it is extremely difficult to incorporate these effects into the statistics of a flood study. However it is necessary to acknowledge their existence and include provision for such uncertainties arising from these sources within the design. Incorporating a conservative estimate of increase in discharge (as a result of increased rainfall) is necessary to take into account the above. Also using a conservatively high flood return period i.e. 1 in 200 year and adopting a suitable freeboard within the design of flood defences to account for future changes aids the provision for the associated uncertainties.

2.2 Description of the catchment The catchment of the River Clyde at Dalmarnock Industrial Estate extends to 1969km2 and is a large catchment developed on a mixed geology, Ordovician (in the South) to Carboniferous with drift cover below the headwaters. Hill pasture is the major land use, some mixed farming and urbanisation in the lower valley. The average annual rainfall within the catchment is 1123mm. The closest gauging station to the site operated by SEPA within the catchment area is located, approximately 20km upstream of the site (Daldowie gauging station, catchment area: 1902km2, period of observation: 1978-2001). Figure 2 shows the catchment areas at both the Daldowie gauging station and at the downstream extent of Dalmarnock Industrial Estate.

2.3 Flood flow assessment methodology An assessment of peak flow statistics at the downstream end of the site has been derived from the annual maxima flows measured at SEPA Daldowie gauging station, by using the methodology of the Flood Estimation Handbook (FEH, Institute of Hydrology, 1999).

The FEH methodology is based on the derivation of an index flow, denoted QMED (1 in 2 year return period) and a growth curve, the product of which is a flood frequency curve, which allows the flood peak for a given return period to be determined. The FEH is based upon an extended rainfall and river flow data set in the UK, which is accessible via a CD ROM accompanying the handbook and associated software (WINFAP-FEH).

Doc No 1 Rev: 0 Date: May 2002 5 QMED can be calculated from recorded data when available, or in the absence of

flood peak data as at ungauged catchments, QMED is estimated from the catchment descriptors based on digital data (FEH CD-ROM).

At ungauged sites, an adjustment to QMED based on catchment descriptors is also applied using a nearby “donor” gauged catchment by using the following equation:

QMEDs,adj = QMEDs,cds (QMEDg,obs/QMEDg,cds)

Where:

QMED median annual discharge (m3/s) adj adjusted s subject site g gauged site cds catchment descriptors obs observed data

Two methods have been considered to derive the growth curves:

The pooling-group analysis: the recommended method from the FEH is the pooling-group analysis (as this is employed when estimating to return periods greater than half the record length i.e. where T > 1/2N, where N denotes the number of years of annual maximum flow data). This approach produces a ‘pooled growth curve’ based upon the hydrologically similar sites within the UK to that of the subject site.

Single-site analysis: at gauged sites, the single-site analysis consists of fitting a distribution to the recorded annual maximum peak flows for the period of record and extrapolating the distribution curve to derive the peak flow the desired return period.

2.4 Choice of donor catchment The River Clyde at Daldowie was adopted as a donor catchment following a search for local gauged catchments and consultation with SEPA. The River Clyde at Daldowie is an ideal ‘donor’ catchment as it is located on the same river as the site under analysis and it therefore has similar catchment characteristics (see ‘catchment descriptors’ in Appendix B and Figure 2).

Doc No 1 Rev: 0 Date: May 2002 6 2.5 Annual peak flow analysis for the River Clyde The FEH pooling-group method (with application of a recommended distribution) was applied to the River Clyde at Daldowie gauging station and compared with the actual annual peak flow data, to ascertain the growth factors for design events up to the 200 year return period. The pooling-group estimates were considerably less than recorded events at high return periods and thus a single-site analysis was undertaken on the data set (Figure 3). The single-site approach (using the general logistic distribution), although normally recommended in the FEH only for use with return period predictions < 1/2N, provided more compatible estimates with that of the recorded data. As the record length is only 24 years, estimating large return periods from single-site analysis would not normally be recommended. However, in recent discussions (e.g. British Hydrological Society. The Flood Estimation Handbook - Initial Experiences. 8th March 2002) it has been acknowledged that in certain situations use of the single-site approach may have an advantage over that of the pooling-group method, especially with the current data uncertainties associated with the FEH dataset.

Following recent discussions with SEPA regarding the most appropriate method to use in such cases, the single-site analysis (with application of the 3-parameter General Logistic distribution with L-median technique) has been adopted which gives a much closer fit to the middle and upper ranges of the observed annual maxima data and an overall more conservative estimate of design flows (Figure 3).

The growth factors derived for the River Clyde at Daldowie (single-site analysis) were adopted for the River Clyde at Dalmarnock. The maximum flood discharges derived for various return periods are shown in Table 1, Figure 3 and Figure 4. In addition Appendix B details the catchment descriptors and the data transfer method used within the hydrological analysis.

Peak flood discharge (m3/s) Return Period River Clyde at Daldowie River Clyde at Dalmarnock

2 years 460 470

5 years 602 615

10 years 721 738

Doc No 1 Rev: 0 Date: May 2002 7 Peak flood discharge (m3/s) Return Period River Clyde at Daldowie River Clyde at Dalmarnock

25 years 915 936

50 years 1102 1127

100 years 1334 1364

200 years 1625 1661

Table 1: Maximum flood discharges for a range of return periods

Daldowie is the most relevant gauging station for estimating flows at Dalmarnock Industrial Estate. However, the flow rating for Daldowie gauging station is under review by SEPA and there is currently a low level of confidence attached to the flow data and therefore the assessment of peak flow for the River Clyde at Dalmarnock is only indicative.

2.6 Comparison with previous studies Comparison of the flood discharges estimated at Daldowie (Table 1) with the flood discharges used by Babtie in 1997 (obtained from report entitled: The River Clyde Flood Study, 1996) shows that differences in magnitudes exist, particularly at high return periods, see Table 2 and Figure 3. The flood discharges used by Babtie were based upon the Flood Studies Report (FSR) (NERC, 1975), which was superseded by the FEH in 1999.

Subsequently, differences exist in the magnitudes of flows estimated via the FSR and the FEH methods and several explanations exist for the differences, these relate mainly to the differences in methodologies, data sets and user application.

River Clyde at Daldowie Peak flood discharge (m3/s) Return Period Halcrow (2002) Babtie (1997)

2 years 460 -

5 years 602 580

10 years 721 720

Doc No 1 Rev: 0 Date: May 2002 8 25 years 915 840

50 years 1102 980

100 years 1334 1100

200 years 1625 1324

Table 2: Comparison of Maximum flood discharges for a range of return periods

The principal differences of the FEH are:

• The median annual flood QMED (the 2-year flood), rather than the mean annual flood QBAR (2.33-year flood), is used as the index variable. QMED is the middle ranking value and thus puts less emphasis on the high values in the data set and compensates for the extreme values not to be over valued, whereas the FSR QBAR reflects the high flow values in the data set.

• Pooling of flood peak data for growth curve derivation is flexible rather than fixed, and is tailored to the subject site – via standard criteria and the users discretion; stations (from anywhere in the UK) are selected to form part of the pooling-group according to catchment similarity rather than according to their Hydrometric Area (as in the FSR) i.e. not by geographical region, but by similar hydrological characteristics.

• Catchment similarity is initially judged in terms of size (AREA), baseflow (BFIHOST) and rainfall (SAAR); then by the user.

• The data at all stations within the pooling-group can be updated to reflect the longest record possible; whereas the FSR regional analysis was based upon curves constructed on the basis of past data only.

• The Generalised Logistic (GL) distribution rather than the Generalised Extreme Value (GEV) distribution is the default recommendation to describe annual maximum floods in the UK.

• Single-site analysis is applicable to estimate to return periods <1/2N (where N is the number of years of data) as opposed to the 2N rule in the FSR.

Doc No 1 Rev: 0 Date: May 2002 9 • In the absence of flood peak data, QMED is estimated from catchment descriptors based on digital data rather than derived manually from maps.

Thus the difference in index flood and the generation of the growth curves in each method lead to different estimates of design flood magnitudes for respective return periods.

Other aspects that could explain the difference in results are also detailed hereafter:

Data quality Data quality and availability is highly important in the FEH. Although data sets for each river are contained within the software (WINFAP-FEH), the data is only that as input up to the point when the FEH was produced, approximately 1993/1994. Also, often the data on the FEH software differs with that of the respective hydrometric authority. Therefore to have a consistent data set the flows used within the FEH analysis were obtained from SEPA (flow record: 1978-2001). Daldowie is the most relevant gauging station for estimating flows at Dalmarnock Industrial Estate. However, the flow rating for Daldowie gauging station is under review and there is currently a low level of confidence attached to the flow data.

Length of data set The Babtie flows will have utilised flow records up until 1995 (as report produced in 1996). Whereas, the data set used in the FEH analysis uses data from 1978- 2001, therefore the range of data is different. The fact that QMED is now used instead of QBAR also produces different results (when multiplied by the growth factors).

Application of the FSR methodology Babtie obtained the flows tabulated in Table 2 above from a report entitled: River Clyde Flood Study (1996). The FSR methodologies adopted were: • Extreme value distribution approach (single-site analysis)

• Mean annual flood/flood growth factor approach (regional analysis)

The first method is applicable to predicting to return periods up to twice the record length. The second method is suggested for predicting to return periods greater than twice the gauge record. The final flows produced were based on a combination of the two methods above i.e. a combination of single-site analysis and regional analysis:

Doc No 1 Rev: 0 Date: May 2002 10 “The results of the analysis indicated that both methods individually would not fully describe the trend of the flood frequency relationship and therefore a judgement needs to be made taking into account all information available. The values…are the results of this judgement and represent the “recommended values”” (Babtie report, 1997).

It is unclear how the actual flows were decided upon, as no explanation exists for the final “recommended” flows. Attempts to obtain the previous flood flows by applying the FSR methods as detailed above do not produce the same values as those tabulated in Table 2.

2.7 December 1994 Flood Event The peak flow of the December 1994 event was measured at 1107 cumecs by SEPA at their gauging station at Daldowie, some 20km upstream of the proposed development site. Based on the hydrological assessment detailed in Section 2.5, this equates approximately to a 1 in 50 year return period event.

2.8 Climate change The main sources of information about the effects of climate change upon flooding and the implications on levels of protection offered by flood prevention schemes stem from the UK Climate Impacts Programme (UKCIP), set up by the Department of Environment, Transport and the Regions (DETR) in 1997. The UKCIP is responsible for the integrated approach of climate change information from all levels of studies on the topic within the UK and hence aids the provision of information and advice regarding impact assessment and risk appraisal. A consequence of the UKCIP is the development of climate change scenarios for the next 100 years in the UK undertaken by the Climate Research Unit (CRU) at the University of East Anglia.

The latest published information about climate change in Scotland in particular (Werritty et al.; Hulme et al.; Price and McInally; Kerr et al.) has been reviewed, which in turn utilises the findings of Hulme and Jenkins (1998), The UK Climate Impacts Programme - Technical Report 1. This report provides one of the most authoritative published works related to projected UK climate change. Its findings utilise the Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) and the Hadley Centre HadCM2 model climate scenario experiments. Werrity et al. estimate an average (annual) increase in precipitation between 10% - 21% by 2080 for the Scotland, with a mean increase of 16%.

Doc No 1 Rev: 0 Date: May 2002 11 In April 2002 the UKCIP02 climate change scenarios for the UK (Hulme et al. 2002) were published replacing the UKCIP98 climate change scenarios. These scenarios represent an important advancement in the understanding of the nature of future climate change in the UK.

The scenarios present four different descriptions on how the world may develop in the decades to come, being based on for different emission scenarios from the IPCC, which are: Low Emissions, Medium-Low Emissions, Medium-High Emissions and High Emissions.

The UK Climate Impacts Programme climate scenarios suggest that the UK will become warmer in summer and autumn, with the average annual temperature in the UK rising between 2°C2 and 3.5°C3. High summer temperatures will become more frequent and very cold winters more rare. Winters will become wetter and summers drier than at present. Heavy winter precipitation (rain and snow) will become more frequent by 2080, for example daily winter precipitation intensities that are experienced once every 2 years on average may become between 5% and 20% heavier. There will be a decrease in snowfall by throughout the UK, on average Scotland may experience a decrease between 60% - 90% by 2080.4 Winter precipitation is estimated to increase by between 15% - 20% by 20805 in the Scotland.

The scenarios suggest a decrease of gales overall, though there may be an increase in the frequency of very severe gales. The scenarios do not account for a ‘cooling’ situation occurring - as for this to happen the North Atlantic thermohaline circulation would have to shutdown and the probability of this is said to be low. In addition the scenarios also neglect the collapse of the West Antarctic ice sheet. These results however are based upon grid based General Circulation Models (GCM), which represent Scotland by two cells only. Therefore the strong west- east precipitation gradient and the influence of the mountainous landscape cannot be properly taken into account.

2 Low Emissions scenario 3 High Emissions scenario 4 For the High Emissions scenario 5 Based upon the Medium-High Emissions scenario

Doc No 1 Rev: 0 Date: May 2002 12 However, the current precipitation predictions fall within the range of natural variability and due to the uncertainty associated with climate predictions and the absence of a clear impact assessment methodology, it is proposed to adopt the following hypotheses:

Adoption of an increase of 16% to event rainfall depth equating to approximately a 16% increase in peak flows to carry out a sensitivity analysis in increased river flows, due to the estimated range of increases in precipitation.

Adoption of a suitable freeboard, assessed by undertaking sensitivity analyses.

The above hypotheses utilise the precautionary principle advocated by NPPG7.

Doc No 1 Rev: 0 Date: May 2002 13 3 Flood Alleviation Measures

3.1 Degree of Protection There is no general guidance on the appropriate degree of protection for new developments in Scotland. NPPG7 makes it clear that flood risk has to be considered alongside the scale and type of development proposal, and the consequences of flooding. Until recently the generally accepted minimum standard of protection for a development of this nature was the 1 in 100 year event. The Scottish Executive accept design against the 100 year return period event plus an allowance for climate change as acceptable for statutory flood prevention schemes under the Flood Prevention (Scotland) Act 1961. Notwithstanding this, some local authorities now require higher standards of protection for new developments in accordance with guidelines published by the Association of British Insurers (1 in 200 years).

3.1.1 Existing Protection There is an existing flood defence embankment east of Dalmarnock Road adjacent to the River Clyde from Dalmarnock Road to the access road to the industrial estate in the Cunningar Loop as shown on Figure 1. This embankment was constructed by South Lanarkshire Council following recommendations made in the 1997 Babtie Study. Although the embankment prevents direct overtopping along Downiebrae Road, it does not offer any protection to the existing properties in the area due to low ground levels downstream of Dalmarnock Bridge where flood water is able to overtop and cause inundation to properties in Dalmarnock Road and Downiebrae Road. Babtie also recommended that a short section of wall be constructed downstream of Dalmarnock Bridge. This section of wall has not been constructed and is essential to provide protection to the Dalmarnock area.

The top of the embankment was constructed to a level of 8.2m OD which represented the 1 in 200 year predicted flood level3 or the 1 in 100 year flood level3 plus 800mm freeboard at the time of construction.

3 South Lanarkshire Flood Study, Babtie Group, September 1997

Doc No 1 Rev: 0 Date: May 2002 14 The embankment was formed by constructing a sheet pile wall along Downiebrae Road with the top of the piles being cut off or driven to 7.7m OD. Type 1 Material was then placed either side of the piles and topsoil placed to form a crest level of 8.2m OD4. South Lanarkshire Council have confirmed the embankment is designed to be effective until the crest level is breached at 8.2m OD.

Based on the revised hydrological assessment, the existing embankment offers protection to approximately the predicted 1 in 100 year flood level with no freeboard or the 1 in 50 year event plus 600mm freeboard.

3.2 Methods of Flood Alleviation

3.2.1 General As a principle, it is preferable not to build on floodplains where the development may be exposed to flood risk. However, it is generally recognised that this is sometimes necessary for other reasons such as the redevelopment of existing sites such as Dalmanock. There are three basic alternatives for flood protection of new development sites. These are:

i) raise the ground levels above predicted flood levels; ii) protect the sites by means of flood embankments or walls; iii) partially raise ground levels and construct smaller embankments or walls.

In each case the protection is provided against a particular level of risk. A design flood event is chosen based on an assessed exceedance probability, which is often expressed as a return period. A freeboard allowance is then applied to cater for potential inaccuracies in the analysis, survey data, super-elevation at bends and wind and wave action etc. The exceedance probability and freeboard are chosen based on a range of factors including the consequences of failure and the uncertainties in the assessment and analysis. In most cases economics dictate the flood protection is provided to a level such that there remains a degree of risk that the defences will be overtopped.

4 Extract of Construction Drawing Received from South Lanarkshire Council

Doc No 1 Rev: 0 Date: May 2002 15 3.2.2 Raising Existing Ground Levels If the ground levels of the proposed development at Dalmarnock were raised above the predicted flood levels, this would provide an assessed degree of security against flooding for any development built on that ground. If the flood level exceeded the raised development platforms, it is likely this would occur relatively slowly and to shallow depths. This method would not provide protection to existing adjacent developments, nor guarantee access during flood events because of the level of existing roads. This option would require the importation of large volumes of upfill material which has obvious cost implications.

3.2.3 Flood Embankments or Walls Another option would be to protect the development by flood embankments or walls which would provide an assessed degree of security against flooding for both new and existing developments. There are additional hazards associated with flood banks and walls which do not apply in the case of raised development platforms. If the flood bank or wall were to fail when flood water in the River Clyde was above the protected ground level, flooding might occur to significant depths. Depending on the mode of failure, the flooding may develop very rapidly. In the case of embankments and walls the three likely failure modes are overtopping, internal erosion and under seepage.

Further problems arise with the option of flood banks or walls with regard to drainage. A means of dealing with rain falling on the protected area, and surface water draining to it from higher ground during periods when the adjacent river level is above the protected ground level. The effect of elevated river levels on the ground water regime within the site must also be considered. Where the underlying ground below the flood banks is permeable, the possibility of flow into the protected area and upwelling of ground water may exist.

3.2.4 Raising Ground Levels and Constructing Flood Embankments or Walls A further alternative would be to partially raise the ground levels, to say the predicted flood level, and construct a wall or embankment to provide the required freeboard or construct the floor levels at a higher to provide the required freeboard.

Where it is necessary to provide flood protection to new development sites it is considered preferable to use raised platforms rather than flood banks if other circumstances are equal. However, it is possible that in the case of the

Doc No 1 Rev: 0 Date: May 2002 16 redevelopment of the Dalmarnock area that economics and the interface with existing roads and property may dictate otherwise

3.3 Predicted Flood Level

3.3.1 Flow/ Level Relationship A flow/ level relationship for the site at Dalmarnock has been established using the data obtained from the study by Babtie in 1997. The flows calculated in Section 2.5 have been superimposed to allow a prediction of flood level for various flood events. Figure 5 contains a graphical plot of the flow/ level relationship derived for Dalmarnock.

Using the above relationship the predicted flood levels are as noted in Table 3 below:

Return Period Predicted Peak Predicted Flood (Years) Flow (m3/s) Level (m OD) 2 470 5.40 5 615 5.88 10 738 6.28 25 936 6.94 50 1127 7.57 100 1364 8.35 200 1661 9.33 Table 3: Maximum predicted flood levels for a range of return periods

3.3.2 Discussion of Results The predicted flood levels for the 1 in 2 and 1 in 200 year events have been obtained by extrapolating the flow/level relationship outwith the data used to construct the relationship. For this reason these predicted levels should be treated with some caution.

For comparison purposes, Table 4 below shows the water level prediction obtained in Section 3.3.1 and those derived by Babtie in their 1997 study:

Doc No 1 Rev: 0 Date: May 2002 17 Return Period Predicted Flood Babtie Predicted (Years) Level (m OD) Flood Level (m OD) 2 5.40 5 5.88 5.80 10 6.28 6.20 25 6.94 6.70 50 7.57 6.90 100 8.35 7.40 200 9.33 8.20 Table 4: Comparison of predicted flood levels for a range of return periods

As can be seen, the largest difference occurs for floods with return periods of 1 in 50 years or higher. The main reason can be attributed to the difference in predicted peak flow for the high return period events. The flood frequency curve derived by Babtie is quite ‘flat’ compared to that derived using the FEH methodology as shown in Figure 3. Therefore the difference in flow for a particular return period increases as the return period increases.

3.4 Freeboard In order to cater for wind and wave action, climate change and potential inaccuracies in the analysis it is necessary to incorporate a suitable free board above the predicted flood level.

Based on the information available at this stage we recommend that an allowance of 1.0 metre is a suitable freeboard to be adopted in conjunction with the predicted 1 in 100 year flood level (excluding climate change). Therefore, all development will need to be constructed at a level of 8.35 + 1.0 = 9.35m OD or protected by the construction of walls or embankments to this level.

The protection level of 9.35m OD is equivalent to:

Predicted 1 in 100 year flood level + 1.0m freeboard
Predicted 1 in 100 year flood level + climate change + 0.3m freeboard
Predicted 1 in 200 year flood level

Doc No 1 Rev: 0 Date: May 2002 18 3.5 Options for Flood Alleviation This section details the various options available to provide protection against direct inundation from the River Clyde. Drawings of the relevant options are contained in Appendix A.

3.5.1 Option 1 – Flood Defence Wall/ Embankment Around Perimeter of Site This option would require the construction of a wall or embankment around the entire boundary of the site enclosing the new development as shown on drawing UG/DALM/FS02. The crest level of the proposed flood defences would be 9.35m OD. Given the existing ground levels the height of the wall is likely to vary between 1.0 - 3.75m on average.

A flood gate would also be required at each point where the access road crosses the flood defence wall or embankment. The gate would require to be closed in the event of a flood to isolate and protect the development.

The new surface water and foul drainage systems from the site should incorporate measures to either store water/sewage temporarily on site or incorporate a pumped system to ensure the site if free from secondary flooding during times where gravity drainage may not be possible. In addition it will be necessary to ensure that the existing drainage systems do not provide a path for water to enter the development from the areas outwith the development that will continue to flood.

This Option would not offer any additional protection to any existing properties in the surrounding area.

3.5.2 Option 2 – Partially Infill Site and Construct Buildings at a Higher Level This option would require the partial raising of ground levels within the site boundaries. Achieving the required level of protection could be accomplished by raising the site to the predicted 1 in 100 year flood level of 8.35m OD and constructing the floor levels of the buildings at 9.35m OD to provide the required freeboard. Alternatively parts of the car parking areas could be maintained at a lower level and allowed to flood for lower return period events with only the buildings raised to 9.35m OD and protecting against the design flood event.

The surface water and foul drainage systems would also require special attention as noted in Option 1.

Doc No 1 Rev: 0 Date: May 2002 19 This Option would not offer any additional protection to any existing properties in the surrounding area.

3.5.3 Option 3 – Partially Infill Site and Construct Flood Defence Wall/ Embankment This Option is very similar to Option 2, although in this case a flood wall or embankment would need to be constructed around the perimeter of the site to 9.35m OD to provide the required free board, with the ground levels raised to 8.35m OD. This would allow the floor levels of buildings to be constructed at a level between 8.35m OD and 9.35 m OD.

A flood gate is also likely to be required as noted in Option 1. The surface water and foul drainage systems would also require special attention as noted in Option 1.

This Option would not offer any additional protection to any existing properties in the surrounding area.

3.5.4 Option 4 – Infill Site to Design Protection Level This Option would require the entire site to be raised to 9.35m OD. This option is likely to negate the need for any additional surface water storage or pumped drainage systems, as gravity drainage should be feasible under most flood conditions up to the predicted 1 in 100 year flood level.

This Option would not offer any additional protection to any existing properties in the surrounding area.

3.5.5 Option 5 – Construct Flood Defence Wall/ Embankment and Raise Existing Defences This Option would require the construction of a new flood defence wall or embankment along the northern boundary of the site adjacent to the River Clyde between Dalmarnock Bridge and higher ground to the west with a crest level of 9.35m OD. Babtie also recommended a wall in this area to protect the site and surrounding area. However, due to the limited topographical survey information available at this stage we are unable to confirm if the ground level to the west of the site is high enough for this option to be practical.

In order to provide the required level of protection, the existing defence along Downiebrae Road would also have to be raised to 9.35m OD. In addition a flood gate would need to be constructed across Dalmarnock Road to tie the new defences in to the raised defences. This gate would remain open for the majority of

Doc No 1 Rev: 0 Date: May 2002 20 the time until the flood levels threatened to overtop the footpath of Dalmarnock Bridge at which time it would be closed to protect the site and surrounding areas.

This option would not only offer protection to the proposed development, but would also ensure the existing properties in the area were protected against a 1 in 100 year return period event.

The possibility of secondary flooding from existing drainage systems is high for this option and is discussed more fully in Section 3.7.

3.5.6 Option 6 – Construct Flood Defence Wall/ Embankment to the Same Level as the Existing Defences This option would require the construction of a flood defence wall or embankment along the north side of the proposed development (see Drawing UG/DALM/FS03) although the crest level would be lower at 8.2m OD. This would offer protection approximately to the 1 in 100 year predicted flood level with no freeboard.

This option would offer protection to a lower standard than that normally accepted and would not be recommended without a detailed examination of the potential damages and costs when compared to the cost of providing the minimum recommended standard of protection.

The possibility of secondary flooding from existing drainage systems is high for this option and is discussed more fully in Section 3.7.

It is possible that this option is the only economical and environmentally acceptable solution to progress with the redevelopment of the site Dalmarnock as Options 1 - 5 will have significant cost implications as well as the potential to be visually intrusive. In order to reduce the visual impact of the proposed defences it may be possible to use a demountable form of defence along Dalmarnock Road where the visual impact is likely to be greatest.

3.6 Impact Upstream and Downstream The development of the site at Dalmarnock will have an insignificant impact on flood levels upstream or downstream of the site as protecting the site will only remove a small amount of flood storage but will not alter flow conveyance.

Doc No 1 Rev: 0 Date: May 2002 21 3.7 Secondary Flooding In addition to direct inundation we have also considered the possibility of indirect inundation due to backing up and surcharging of the existing sewerage system in the area.

We believe there are a number of existing surface water and foul drainage systems in the Dalmarnock Road/Downiebrae Road area with several outfalls and overflows into the River Clyde although we are still awaiting a response from Scottish Water to confirm this.

In the 1997 study by Babtie, cover levels were noted to vary between 4.6 and 7.6m OD. Therefore it is highly likely that the sewerage system would become surcharged during high flood levels and cause flooding through manholes due to backing up from water levels in the River Clyde via the outfalls. The installation of well maintained flap valves on the outfalls would prevent backing up through the pipes. We understand that Scottish Water may have installed flap valves on these outfalls, but this has still to be confirmed by Scottish Water. The installation of flap valves would prevent the backing up through the pipes. However it would not prevent surcharging of the sewerage system and eventual flooding through the manholes should the volume of runoff created by a storm event over the catchment be greater than the storage capacity of the sewerage system.

In Option 1, the risk of secondary flooding can be substantially reduced by ensuring there is no hydraulic connectivity between the new development and the existing drainage systems. This would require the new drainage systems serving the development to have sufficient storage or be pumped in the event of high river levels preventing gravity drainage.

In Option 2, the risk of secondary flooding from the existing drainage systems would also be substantially reduced by the raising of the site level. Any surcharging of the existing systems is likely to affect existing properties located at a lower level. Special attention to the new drainage systems would also be required to prevent flooding due to the storm runoff exceeding the storage capacity of the new drainage system.

In Option 3, the risk of secondary flooding would be very similar to Option 2. However, as the buildings would be constructed at a lower level the risk would be slightly higher.

Doc No 1 Rev: 0 Date: May 2002 22 In Option 4, the risk of secondary flooding would be virtually negated providing sufficient storage and/or pumping arrangements were incorporated into the new site drainage in the event of gravity drainage not being possible.

In Options 5 and 6, the risk of secondary flooding is highest out of the various options for protecting the site. It may be possible to alleviate the impacts of any secondary flooding by ensuring none of the new buildings are located in low areas and ensuring all ground levels fall away from the building.

Doc No 1 Rev: 0 Date: May 2002 23 4 Conclusions and Recommendations

The following conclusions and recommendations can be drawn from this study:

The largest recorded flood on record at Daldowie Gauging Station (which is upstream of the development site) occurred in December 1994. The peak flow was in the region of 1107 cumecs and at the time had a reported return period of about 1 in 75 years6. The proposed development site was inundated to reported depths in excess of 2 metres;

With an extended data set available and adopting the new FEH hydrological assessment methodology, the return period associated with the 1994 flood event at the site is approximately 1 in 50 years.

In order to protect the areas vulnerable to flooding it will be necessary to raise existing ground levels above the design flood level, or construct a flood embankment or wall around the perimeter of the development. Alternatively, a combination of ground raising and the construction of an embankment or wall could be considered;

It is recommended that the site is protected against the 1 in 100 year return period flood event plus 1.0m freeboard, corresponding to a level of 9.35 m OD. This level of protection is also equivalent to the predicted 1 in 200 year flood level;

The site could be at risk from secondary flooding via surcharged outfalls to the watercourses. It will be necessary to carry out a detailed investigation of the existing drainage system during the detailed design stage of the project.

There is no predicted significant impact on flood risk at upstream or downstream locations from the proposed development.

6 South Lanarkshire Flood Study, Babtie Group, September 1997

Doc No 1 Rev: 0 Date: May 2002 24
The flood water levels predicted in this report are based on a previous hydraulic modelling exercise carried out by Babtie (1997). There is limited confidence in extrapolating these results beyond the predicted 1 in 100 year flood level (8.35 m OD). Therefore further hydraulic modelling of the River Clyde would be required if protection against the 1 in 200 year event plus climate change is required.

Doc No 1 Rev: 0 Date: May 2002 25 5 References

Hulme, M. Jenkins, G. (1998). Climate change scenarios for the UK: scientific report. UKCIP Technical Report No. 1. Climate Research Unit, Norwich.

Hulme, M. Crossley, J. Lu, X. (2001). An exploration of regional climate change scenarios for Scotland. Report on behalf of the Scottish Executive Central Research Unit. HMSO Edinburgh.

Hulme, M. Jenkins, G. J. Lu, X. Turnpenny, J.R. Mitchell, T. D. Jones, R. G. Lowe, J. Murphy, J. M. Hassell, D. Boorman, P. McDonald, R. and Hill, S. (2002). Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. 120pp.

IH. 1999. Flood Estimation Handbook - Volumes 1-5. Institute of Hydrology, Wallingford.

Kerr, A. Allen, S. Shackley, S. Milne, R. (1999). Climate change: Scottish implications scoping study. Scottish Executive Central Research Unit, Edinburgh.

NERC. (1975). Flood Studies Report Volumes 1-5. Natural Environment Research Council, London.

Price, D. McInally, G. (2001). Climate change: review of levels of protection offered by flood prevention schemes. Scottish Executive Central Research Unit report, 63pp.

The Scottish Office. (1995). National Planning Policy Guideline. NPPG7: Planning and Flooding.

Werritty, A. Black, A. Duck, R. Finlinson, B. Thurston, N. Shackley, S. Crichton, D. (2002). Climate Change: Flooding Occurrences Review. Scottish Executive Central Research Unit.

Doc No 1 Rev: 0 Date: May 2002 26 Figures Figure 1 : Site Location Plan

Existing Flood Defence

Site Location

Based upon the Ordnance Survey Map with the permission of the controller of HMSO. Crown Copyright Reserved

Halcrow Group Limited 35 Baird Street, Glasgow, G4 0EE

Licence No. AL 100017424 River Clyde @ Dalmarnock Industrial Estate

River Clyde @ Daldowie (84013)

Figure 2: Catchment areas (FEH, 1999) Figure 3

Annual peak discharge statistics - River Clyde at Daldowie

1600 Annual Maximum Discharge: 1978-2001 FEH single-site analysis FEH pooling-group analysis 1400 Babtie report (1997)

1994 1200 /s)

3 1000

800 Discharge (m 600

400 Return period (years)

2 5 10 25 50 100 200 200

0 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Logistic Reduced Variate [y = ln(T-1)] Figure 4

Annual peak discharge statistics - River Clyde at Dalmarnock Industrial Estate

1800 FEH 1600

1400

1200 /s) 3 1000

800 Discharge (m

600

400 Return period (years)

2 5 10 25 50 100 200 200

0 -1 0 1 2 3 4 5 6

Logistic Reduced Variate [y = ln(T-1)] Figure 5 Flow/ Level Relationship (Levels Based on Babtie Study - September 1997, Flows Based on FEH Methodology) 12

10

8 y = 0.0033x + 3.8493

6

4

2

0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Flow (cumecs) Appendix A – Supporting Drawings Appendix B – Hydrology FEH analysis: River Clyde

Catchment description Catchment description River Clyde @ Daldowie River Clyde @ Dalmarnock (Grid reference: NGR 267000 661800) (Grid reference: NGR 261700 662700)

DTM AREA 1901.6 DTM AREA 1968.58 BFIHOST 0.412 BFIHOST 0.413 DPLBAR 60.73 DPLBAR 68.66 DPSBAR 92.4 DPSBAR 90.6 FARL 0.962 FARL 0.962 PROPWET 0.59 PROPWET 0.59 SAAR 1128 SAAR 1123 SPRHOST 41.9 SPRHOST 41.6 URBEXT1990 0.022 URBEXT1990 0.028 ALTBAR 265 ALTBAR 258 ASPBAR 356 ASPBAR 354 ASPVAR 0.06 ASPVAR 0.06 LDP 128.56 LDP 138.48 RMED-ID 35.8 RMED-ID 35.7 RMED-2D 49.7 RMED-2D 49.4 RMED-1H 9.2 RMED-1H 9.1 SAAR4170 1135 SAAR4170 1129 URBCONC 0.742 URBCONC 0.755 URBLOC 0.316 URBLOC 0.354 Data Transfer

QMEDs,adj = QMEDs,cds (QMEDg,obs/QMEDg,cds)

Bonchester QMEDs,adj

QMEDs,cds = 383.22 m3/s QMEDg,cds = 374.83 m3/s 3 QMEDg,obs = 459.55 m /s 3 QMEDs,adj = 469.836 m /s

Fittings for growth curve and associated design event discharges

3 3 River Clyde Daldowie QMED = 459.550 m /s River Clyde Dalmarnock QMEDs,adj =469.836 m /s Single Site Analysis Single Site Analysis

T growth factor Q (m3/s) T growth factor Q (m3/s) 2 1.000 459.550 2 1.000 469.836 5 1.310 602.011 5 1.310 615.485 10 1.570 721.494 10 1.570 737.643 25 1.992 915.424 25 1.992 935.913 50 2.398 1102.001 50 2.398 1126.667 100 2.903 1334.074 100 2.903 1363.934 200 3.535 1624.509 200 3.535 1660.870 500 4.624 2124.959 500 4.624 2172.522