2013s6840 St Asaph Long Term Options and Economics ...€¦ · 2013s6840 St Asaph Long Term Options...

St Asaph Long Term Options and Economics Appraisal

Final Report

February 2014

Natural Resources Wales

Chester Road

BUCKLEY

CH7 3AJ

2013s6840 St Asaph Long Term Options and Economics Appraisal FINAL 13-02-14.doc

JBA Project Manager Chris Smith JBA Consulting Bank Quay House Sankey Street Warrington WA1 1NN

Revision History

Revision Ref / Date Issued Amendments Issued to

Draft v1 / 31/01/13 Richard Weston/Rob Green

Final / 19/02/13 Richard Weston/Rob Green

Contract This report describes work commissioned by Richard Weston, on behalf of Natural Resources Wales, by a letter dated 16th May 2013. Natural Resources Wales' representative for the contract was Richard Westion. Julia Hunt and Chris Smith of JBA Consulting carried out this work.

Prepared by .................................................. Julia Hunt BSc

Senior Analyst

Reviewed by ................................................. Chris Smith BSc PhD CEnv MCIWEM C.WEM MCMI

Principal Analyst

Purpose This document has been prepared as a final report for Natural Resources Wales. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

JBA Consulting has no liability regarding the use of this report except to Natural Resources Wales.


Copyright © Jeremy Benn Associates Limited 2014

Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 66g if 100% post-consumer recycled paper is used and 84g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex.

JBA is aiming to reduce its per capita carbon emissions.


1 Introduction ................................................................................................................. 1

1.1 Overview ....................................................................................................................... 1

1.2 Report Structure ........................................................................................................... 1

2 Long Term Options Appraisal ................................................................................... 2

2.1 Overview ....................................................................................................................... 2

2.2 Long Term Options Modelling ...................................................................................... 2

2.3 Upstream Storage Option ............................................................................................. 8

3 Economics Appraisal ................................................................................................. 11

3.1 Overview ....................................................................................................................... 11

3.2 Results .......................................................................................................................... 11

4 Conclusions ................................................................................................................ 14

List of Figures Figure 2-1 Options L1 and L2 1% AEP Results ........................................................................ 4

Figure 2-2 Options L3 and L4 1% AEP Results ........................................................................ 6

Figure 2-3 L5 1% AEP Results .................................................................................................. 7

Figure 2-4: Rating curve for 3.33%% AEP event at SA014 adjacent to Roe Parc (SA014) .................................................................................................................... 8

Figure 2-5: Design hydrographs for storage estimation ........................................................... 9

List of Tables Table 2-1 Long Term Options Modelled .................................................................................... 2

Table 2-2 Required storage volume .......................................................................................... 9

Table 3.1: Total Damages and top 20 contributing properties for no threshold scenario (all damages are £) .................................................................................................. 12

Table 3.2: Total Damages and top 20 contributing properties for 0.3m threshold scenario (all damages are £) .................................................................................... 13

2013s6840 St Asaph Long Term Options and Economics Appraisal FINAL 13-02-14.doc 1

1 Introduction

1.1 Overview

Following on from work carried out by JBA to update the flood mapping through St Asaph following the November 2012 flood event in the town, the calibrated model developed has been used to test a number of potential long term options to reduce flood risk to properties in St Asaph. The economic damages associated with flood risk in the town have also been briefly appraised. It should be noted that the consideration of long term flood risk mitigation options and economics in St Asaph is indicative at this stage to determine options that might work within the catchment. Following this work, options will be refined and tested further before any detailed scheme can be designed.

1.2 Report Structure

The long term options considered are discussed in Section 2 of this report, the economics appraisal is detailed in Section 3 and Section 4 provides the conclusions of this assessment.


2 Long Term Options Appraisal

2.1 Overview

Six long term options to alleviate flooding in St Asaph were considered. Five of the options were tested using the calibrated model of the River Elwy through St Asaph developed by JBA for the Flood Mapping Update

1; these are discussed in section 2.2 below. The sixth option is the use of

an upstream storage area to alleviate flooding through St Asaph; this has been assessed theoretically by considering the hydrology of the Elwy and is discussed in section 2.3.

All the six long term options are considered only a theoretical basis of whether they could be effective based on the application of the models and hydrology. No indication is given of whether these options are practical and buildable and no consideration is given of economic and environmental costs.

2.2 Long Term Options Modelling

Five potential long term options have been tested using the calibrated model developed by JBA as part of the Flood Map Update study. Three design events have been run through the model for each long term option considered; these were the 1.33%, 1% and 0.5% AEP events.

The five options that have been considered are listed in Table 2-1.

Table 2-1 Long Term Options Modelled

Option Code Option Description L1 Raise embankments

through St Asaph to downstream of Spring Gardens above flood levels

This option has been modelled by glass-walling the River Elwy through St Asaph to prevent flooding to the town. In reality, the embankments are unlikely to be raised as high as they have been in this scenario but this options shows the amount of water that would overtop downstream of St Asaph if water was contained within the river through the town.

L2 Raise embankments through St Asaph to downstream of Spring Gardens above flood levels and remove Spring Gardens Bridge.

This option is the same as option L1 in that the level of the embankments has been raised above flood levels but this scenario also includes the removal of Spring Gardens Bridge to determine the impact that this has on flood risk with the glass-walling through the town in place.

L3 Lower embankments downstream of Spring Gardens Bridge

This option involves lowering the embankments downstream of Spring Gardens Bridge to the ground level behind the embankment. The aim of this option is to increase conveyance through St Asaph by allowing more water to overtop onto the floodplain downstream of

1 St Asaph Flood Mapping Update, JBA Consulting, 2014


Spring Gardens Bridge and lower water levels within the river.

L4 Lower embankments downstream of Spring Gardens Bridge and remove Spring Gardens Bridge

This option is the same as L3 but additionally Spring Gardens Bridge has been removed to determine the impact of the structure on flooding in this scenario.

L5 Removal of trees from St Asaph Old Bridge to Elwy Crossing

The removal of the dense tree cover on the banks of the River Elwy from St Asaph Old Bridge to Elwy Crossing has been included as a long term option as removal of trees along this whole length could not be done in the short term.

The results of the long term options testing are discussed in the sections below with the main focus on the 1% AEP event results. GIS layers including flood outlines and depth grids have been provided with this report for all scenarios modelled.

2.2.1 L1 and L2 Options Testing

Glass-walling the Elwy through the urban area of St Asaph raises the water level in the channel significantly through the town as would be expected, both with and without Spring Gardens Bridge. The testing of this scenario without Spring Gardens Bridge (L2) shows that the bridge would have an impact on flood extent downstream of St Asaph. The extent of flooding without the bridge in place is actually increased downstream of St Asaph as the head loss across the structure does not exist and this allows more water to flow downstream increasing the amount of water overtopping slightly, leading to the larger flood outline as shown in Figure 2-1 for the 1% AEP event. Despite the glass-walling used in both scenarios L1 and scenario L2, the flood extent predicted by the model downstream of the glass-walled area is reduced for both options compared to the 1% AEP design event. This is because overtopping at Roe Parc is prevented in the L1 and L2 scenarios and this reduces the amount of water on the floodplain on the left bank of the River Elwy, significantly acting to reduce flood extent in this area.

For the 0.5% AEP event, the flood outlines produced for the L1 and L2 options are similar in extent with the L2 scenario producing a slightly larger flood extent downstream of St Asaph. Significant flooding occurs through St Asaph even with the glass walling in place during the 0.5% AEP event as water overtops the banks of the Elwy upstream of the defences through the town and flows across the floodplain parallel to the river. There is no flooding predicted to properties in St Asaph in the glass-walled scenarios (L1 and L2) for the 1.33% AEP event. As in the 1% AEP event, the L2 flood extent is larger downstream of St Asaph than the L1 flood extent.


Figure 2-1 Options L1 and L2 1% AEP Results

© Crown Copyright. All rights reserved. Natural Resources Wales, 100024198 2014


2.2.2 L3 and L4 Options Testing

Lowering the embankments downstream of Spring Gardens Bridge to increase conveyance in the channel upstream does not, on its own, reduce flood risk significantly through St Asaph. The L3 scenario where the embankments downstream of Spring Gardens Bridge have been removed but the bridge is still in place predicted similar flooding upstream of the A55 to the 1% AEP design event. Downstream of the A55, the L3 option predicts a slightly reduced flood extent at Roe Parc and the sewage treatment works is not predicted to flood by the model as it is in the 1% AEP design event.

The L4 scenario that involves lowering the embankments downstream of Spring Gardens Bridge and removing Spring Gardens Bridge from the model predicts less flooding than the L3 scenario. The predicted flood extent is reduced to the west of The Roe, upstream of the A55 for the L4 scenario and flooding to Roe Parc is significantly reduced. Removing the bridge from the model means that there is no head loss at the location of the bridge and conveyance in the channel is further increased, leading to lower water levels in the Roe Parc area. The L3 and L4 predicted flood extents for the 1% AEP event are shown in Figure 2-2.

For the 1.33% AEP event, the L3 and L4 scenario predicted flood extents are very similar and both show significantly reduced flooding to Roe Parc compared to the 1.33% AEP design event. The sewage works downstream of the A55 is not predicted to flood in the L3 and L4 scenarios for the 1.33% AEP event, whereas it is within the 1.33% AEP design event flood outline. For the 0.5% AEP event, the L3 scenario does not predict flooding to the sewage treatment works downstream of the A55 but otherwise the flood extent predicted is very close to that predicted for the 0.5% AEP design event. The predicted flood extent for the 0.5% AEP event for the L4 scenario is slightly reduced compared to the design event. Flooding is not predicted to the sewage treatment works in the L4 scenario and the extent of flooding in Roe Parc is reduced compared to the design event. Upstream of the A55, the flood extent for the L4 scenario in the 0.5% AEP event is similar to the design event.


Figure 2-2 Options L3 and L4 1% AEP Results



2.2.3 L5 Option Testing

Option L5 involves removing the tree cover on the Elwy banks throughout St Asaph from St Asaph Old Bridge to Elwy Crossing. The biggest impact is seen through the centre of St Asaph upstream of the A55 where flood risk is significantly reduced compared to the 1% AEP design event as shown in Figure 2-3. Downstream of the A55, removing the bank tree cover causes only a minimal reduction in flood extent and this is mainly towards the downstream boundary of the model away from St Asaph itself. Flood depths downstream of the A55 are reduced by up to approximately 100mm in this scenario.

Figure 2-3 L5 1% AEP Results



The 1.33% AEP design event predicts flooding through St Asaph on the right bank of the Elwy between St Asaph Old Bridge and the A55. In the L5 scenario, this area is not predicted to be flooded, similarly flood risk is reduced upstream of St Asaph Old Bridge during the 1.33% AEP event in the L5 scenario particularly on the left bank of the River Elwy. Flood risk predicted for the 0.5% AEP event in the L5 scenario is significantly reduced compared to the 0.5% AEP design Event. The extensive flooding predicted on the left bank of the Elwy upstream of the A55 during the 0.5% AEP design event is reduced to a much smaller area in the L5 scenario. The flood extent at Roe Parc is also reduced for the 0.5% AEP event L5 scenario compared to the design event.

2.3 Upstream Storage Option

2.3.1 Overview

One of the options under consideration to alleviate flooding in St Asaph is to store flood water in the valley upstream of St Asaph. This has the potential to allow the excess flow above that which can be passed within existing defences heights to be skimmed off the hydrograph peak. A key step in determining whether storage is a viable flood alleviation option is to determine the approximate volume of storage that would be required. It has been agreed with NRW that this assessment will focus on the 0.5% AEP flood event (and November 2012 event) and assume the St Asaph flood defence embankments retain their current crest levels. The analysis is laid out in a series of steps below.

2.3.2 Step 1: Estimate pass forward flow

The peak of the 3.33% event overtops the defences with a peak flow of 171m3/s and peak level of 13.2mAOD at Roe Parc. A flow of 150m

3/s gives a level of 12.8 mAOD which is below

surveyed bank crests. The lowest bank crest surveyed is over 12.9 mAOD but more typical low levels are 13.1 mAOD. Therefore 150m

3/s is a reasonable value to use as a pass forward flow

for this assessment. A range of values either side of this are also assessed for comparison.

Figure 2-4: Rating curve for 3.33%% AEP event at SA014 adjacent to Roe Parc (SA014)

2.3.3 Step 2: Design hydrographs

For this initial estimate of storage, hydrographs for the 0.5% AEP event as used in the hydraulic modelling and the modelled November 2012 event are used as the design hydrographs for storage estimation. The hydrographs are taken at the upstream extent of the St Asaph hydraulic model. These hydrographs are plotted below in Figure 2-5 with the peaks set at the same time.


Figure 2-5: Design hydrographs for storage estimation

2.3.4 Step 3: Calculate storage volumes

The flow hydrographs are analysed to determine the flow that must be removed at each 5 minute timestep (i.e. the flow above the pass forward flow) and that is then converted to a volume that needs to be stored. The total of these volumes for 5 minute intervals is the total required to be stored.

The results from this analysis are shown below in Table 2-2. For the likely pass forward flow of 150m

3/s (assuming existing embankments) the storage estimate is in excess of 2,300,000 m

3.

To give this some context, to store this volume of water would require a 1km2 storage area at

2.3m deep. It would seem unlikely that this volume of storage could be found in the valley upstream of St Asaph although this has not been investigated. The extremely large storage volumes are driven by the fact that the flows are very high and the event duration is relatively long.

Table 2-2 Required storage volume

Pass forward flow (m

3/s)

200 175 150 125

Storage volume required (m

3)

0.5% AEP design event 863,134 1,516,876 2,320,989 3,272,100

November 2012 event 746,178 1,470,041 2,387,650 3,498,224

2.3.5 Step 4: Magnitude check on results

The event hydrographs are 20 to 24 hours in duration. For the 0.5% AEP design event, the difference between the peak flow and the pass forward flow (~260m

3/s to ~150m

3/s) is

approximately 110m3/s which extends over approximately 10 hours of the event duration (-4 hrs

to +6hrs on Figure 2-5). The average flow to store is therefore 50-60m3/s over 10 hours. This

equates to an approximate magnitude of water to store of 1,980,000m3

(based on 55*10*3600).


Although, this is a very approximate calculation, this does suggest that the 2.3 million cubic

metres of storage calculated in Table 2-2 is of the appropriate magnitude.

2.3.6 Step 5: Consider uncertainties

There are 2 key factors that mean the above estimates give a storage volume that is smaller than that would be needed if a more detailed assessment were undertaken:

1. The estimation is based on the design event used in the hydraulic modelling. It is also possible to have a 0.5% AEP event with a longer duration but a lower peak flow. It is possible that this would require a greater storage volume. This has not been calculated in this preliminary assessment.

2. The calculation assumes perfect control of flows at the required pass forward flow. In reality the control mechanism would not operate this efficiently and in order to limit flow to the pass forward flow it would likely start storing water before that flow is reached. This would therefore require a greater storage volume. Again this has not been calculated in this preliminary assessment.

2.3.7 Outcomes

The headline storage volume to come out of this assessment is 2.3million cubic metres and this is likely to be an underestimate. This would require significant infrastructure and land area to accommodate and is probably unlikely to be achievable (although that has not been specifically assessed). To give this some context the recently constructed flood storage basin on the River Douglas in Wigan has a storage volume of 370,000m

3.

Based on this preliminary assessment, the following recommendations are made:

1. Consideration in more detail is required to determine whether this volume of storage is realistically achievable.

2. Storage could form part of a suite of measures to reduce flood risk to St Asaph. For example, improvements to the defences through the town to allow a higher pass forward flow could reduce the required storage volume to a more manageable amount.

3. A smaller design event could be considered for storage. It should be noted however that climate change is expected to increase flood event magnitude and will reduce the design event that can be mitigated through storage.

Realistically flood storage, although not ruled out entirely, seems unlikely to be a practical option to alleviate large flood events in St Asaph.


3 Economics Appraisal

3.1 Overview

JBA’s flood risk analysis tool FRISM has been used to assess economic damages associated with the flooding at St Asaph. This analysis has been used only on the baseline situation modelled in the flood mapping report (i.e. model as calibrated to 2012 flood event) and the mean depth at a property (as defined within FRISM).

The modelled flood event grids have been interrogated against NRD property points to determine flood depth at each property for each event. Multi-coloured manual methods have then been used to convert that into a damage associated with each property for each event. These have then been annualised to give an average annual damage (AAD) figure and converted into present value (pV) damages over 100 years.

Values have been calculated for both using a 0.3m property threshold and for using no property threshold. Results of both are presented to illustrate the uncertainties that occur from estimating flood depths within properties.

Capping has not been required for residential properties. Only one pV for a residential property is greater than £100,000 and in most cases it is much less than this. These are generally substantially lower than the typical property values in St Asaph.

3.2 Results

The top 20 properties and total damages figures are given in Table 3.1 for the no threshold scenario and Table 3.2 for the 0.3m threshold scenario, showing damages relating to each event considered and AAD and pV.

Average annual damages are estimated at ~£300,000 and total pV damages are estimated to be around £8.8 million for the no property threshold scenario. With a 0.3m property threshold uniformly applied the figures reduce to ~£120,000 AAD and £3.6 million pV damages. Just looking at the difference the threshold level makes shows that these figures are fairly uncertain. Some of the key uncertainties include:

• Potential variations in depth of flooding across a property and threshold levels.

• Return period of flooding is uncertain - some short term changes to alleviate flooding have already been put in place.

• Some of the larger damages are associated with properties that are not well defined within NRD and MCM codes.

• Some uncertainty over lower threshold of flooding (10% AEP assumed) although this has been tested and the damages have been shown to be relatively insensitive to this.

Some of the uncertainty associated with the above factors could be reduced with further investigation such as:

• Property threshold levels could be surveyed to improve the flood depth estimates

• Commercial properties could be checked and assigned a more appropriate MCM class if possible.

Significant uncertainty will remain within any calculations of damages even with these refinements.


Table 3.1: Total Damages and top 20 contributing properties for no threshold scenario (all damages are £)

OBJECTID_1 mcmcode os_class Floorarea Q30_Damage Q75_Damage Q100_Damage Q200_Damage Q1000_Damage Annualise_Damage pV

1572 999 2379 0 0 31849.04 763537.1 1875467 9195 274103

866 213 SHOPPING 654 0 113907.3 294759.8 524322.3 863192.4 7506 223757

1598 999 308 6803.631 293258.8 303284.1 306916.8 309279.6 7288 217279

1297 999 1255 0 0 101947.9 522622.6 1056626 5946 177264

1597 999 233 6697.968 218481.1 226855.5 229635.1 231450.7 5512 164316

1669 511 HOTEL 1439 0 0 0 399365.3 956960.7 4668 139153

985 1 DWELLING 208 0 128987.5 168211.9 197382.5 220915.2 3756 111987

1395 1 DWELLING 85 34866.2 55333.71 56403.89 59118.92 67468.93 2859 85253

1946 1 DWELLING 103 12710.62 92474.04 95552.91 96774.09 97522.84 2755 82151

1742 1 DWELLING 105 36630.63 42944.34 45381.4 51092.95 64466.01 2700 80509

908 1 DWELLING 211 0 105791.3 124021.6 115141.5 127242.8 2650 79021

859 21 GENERAL COMMERCIAL 367 0 20151.15 113954.1 195763 340649.7 2612 77887

893 1 DWELLING 208 0 69584.95 117147.7 130614 148770.6 2334 69576

1813 1 DWELLING 83 29751.42 41461.79 43411.57 46132.53 54314.38 2324 69289

1721 1 DWELLING 103 26464.45 40748.74 44406.75 50773.03 64006.48 2227 66408

1941 660 HOSPITAL 673 0 40067.23 47866.72 142431.1 294980.8 2192 65366

1512 214 RETAIL WAREHOUSE 272 0 0 47029.91 216381.5 329101.5 2156 64299

1394 1 DWELLING 95 21859.55 46172.91 47596.06 50005.26 61312.23 2093 62398

1692 1 DWELLING 89 22568.94 39791.82 44335.37 51467.58 66881.24 2059 61384

1794 1 DWELLING 99 20048.1 43304.56 45792.56 51853.83 62413.57 1985 59183

Number of properties contributing 57 179 379 534 824

Total damage 810037 4208143 8685079 16912220 33246908 296230 8830621


Table 3.2: Total Damages and top 20 contributing properties for 0.3m threshold scenario (all damages are £)

OBJECTID_1 mcmcode os_class Floorarea Q30_Damage Q75_Damage Q100_Damage Q200_Damage Q1000_Damage Annualise_Damage pV

1598 999 308 0 272917.6 285195.6 289922.1 292996.2 6555 195434

1572 999 2379 0 0 0 224800.7 1603353 5821 173543

1597 999 233 0 202693 212146.4 216012.3 218374.5 4875 145349

1297 999 1255 0 0 0 233404.2 929910.6 3840 114471

985 1 DWELLING 208 0 110171.6 153906.4 187728.6 199795.7 3370 100482

866 213 SHOPPING 654 0 0 0 158019.4 552169.6 2367 70577

1946 1 DWELLING 103 1377.76 84253.75 89818.59 91592.72 92452.72 2106 62792

1669 511 HOTEL 1439 0 0 0 80978.07 554289.3 2027 60432

1512 214 RETAIL WAREHOUSE 272 0 0 0 157124.5 293364.5 1587 47312

886 1 DWELLING 169 0 3108.32 92522.9 134230.7 145907.7 1463 43627

893 1 DWELLING 208 0 2782.27 81795.58 113483.4 135895.2 1291 38503

1925 513 CAMPING 0 0 52034.27 56847.4 57774.13 58381.08 1279 38128

908 1 DWELLING 211 0 14727.93 97112.74 66434.17 104445.2 1188 35436

1491 21 GENERAL COMMERCIAL 272 0 0 0 77372.73 267732.8 1151 34322



859 21 GENERAL COMMERCIAL 367 0 0 6563.89 81350.85 252247 1150 34286

1526 211 HIRE SHOP 272 0 0 0 74111.23 265466.4 1129 33682

896 1 DWELLING 113 0 1511.52 75318.86 91794 103699.1 1055 31468

986 1 DWELLING 120 0 1605.15 74622.35 90796.87 105751 1055 31464

Number of properties contributing 48 132 284 440 758

Total damage 62754 1075207 3163968 6722553 20345808 119731 3569203


4 Conclusions This report provides an indicative assessment of potential flood risk mitigation options in St Asaph and the associated baseline economics. This document is only intended as a starting point at looking into options for St Asaph and further work is required before a detailed scheme can be devised. The testing of options within the model shows that small changes in water levels along the Elwy can have a marked effect on predicted flood extent within St Asaph due to the presence of the embankments throughout the town.

It should be noted that the hydrology for the catchment is subject to change as a rating review at Pont Y Gwyddel gauge upstream of St Asaph on the River Elwy is planned following the findings of the post November 2012 event analysis undertaken by JBA as part of the St Asaph Flood Mapping Update

2. The rating at the gauge may be adjusted for high flows and this could have

an impact on the design event flows calculated along the Elwy through St Asaph. This may mean that the reduced flood risk to St Asaph predicted for the long term options in Section 2 of the report may not be entirely realised should the design flows change following a rating review at Pont Y Gwyddel.

The high level initial economic appraisal that has been undertaken highlights the uncertainty associated with calculating damages for flood events. A robust baseline damages assessment will be required as part of any future detailed economic appraisal undertaken for St Asaph.

2 St Asaph Flood Mapping Update, JBA Consulting, 2014.

Offices at Coleshill

Doncaster

Edinburgh

Haywards Heath

Limerick

Newcastle upon Tyne

Newport

Saltaire

Skipton

Tadcaster

Thirsk

Wallingford

Warrington

Registered Office South Barn

Broughton Hall

SKIPTON

North Yorkshire

BD23 3AE

t:+44(0)1756 799919 e:[email protected]

Jeremy Benn Associates Ltd Registered in England

3246693

Visit our website

www.jbaconsulting.com

2013s6840 St Asaph Long Term Options and Economics ...€¦ · 2013s6840 St Asaph Long Term Options...

Documents

Transcript of 2013s6840 St Asaph Long Term Options and Economics ...€¦ · 2013s6840 St Asaph Long Term Options...