kirkstall forge flood defence background papers

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Weetwood Commercial Estates GroupElm House Farm, Saighton Lane, Central HouseSaighton, Chester, CH3 6EN Beckwith KnowleTel. 01244 330111 Otley RoadFax. 01244 332111 Harrogatewww.weetwood.net HG3 1WZ

KIRKSTALL FORGE FLOOD RELIEF CHANNELMITIGATION MEASURES

OCTOBER 2007


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Weetwood i 761R01/AG30 October 2007

SIGNATURE SHEET

Report Title : Kirkstall Forge Flood Relief Channel MitigationMeasures

Client : CEG

Report Status : Final

Date of Issue : 30 October 2007

Prepared by

:

A. Morriss BSc (Hons), MSc

Checked by

:

C. D. Whitlow BSc, PhD

Approved by

A. F. Grime BEng MBA CEng MICE FCIWEM


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CONTENTS

Page

Signature SheetContentsList of Tables, Figures & Appendices

iiiiv

1 INTRODUCTION................................................................ 1

1.1 BACKGROUND..........................................................................................11.2 OBJECTIVES AND DELIVERABLES ...............................................................11.3 APPROACH TAKEN IN THIS STUDY..............................................................21.4 PURPOSE OF THIS REPORT ........................................................................2

2 FLOOD RELIEF CHANNEL DESIGN REFINEMENT ..................... 3

2.1 EXISTING FRC DESIGN..............................................................................32.2 EXISTING FRC DESIGN: COMMENTS ...........................................................32.3 FRC SUGGESTED REFINEMENTS .................................................................4

3 HYDRAULIC MODELLING .................................................... 6

3.1 HYDRAULIC MODELLING REQUIREMENTS ....................................................6

3.2

HYDROLOGY ............................................................................................6

3.3 HYDRAULIC MODEL REFINEMENTS..............................................................63.4 HYDRAULIC MODELLING SIMULATIONS.......................................................73.5 HYDRAULIC MODELLING: RESULTS & DISCUSSION ......................................83.5.1 Main River Aire: Rein Road Weir to Kirkstall Bridge ....................................83.5.2 Flood Relief Channel...............................................................................83.5.3 Sensitivity Tests ..................................................................................11

4 GEOMORPHOLOGICAL ASSESSMENT.................................. 13

4.1 GEOMORPHOLOGICAL ASSESSMENT REQUIREMENTS..................................13

4.2 GEOMORPHOLOGICAL ASSESSMENT METHODS..........................................134.2.1. Calculating Sediment Transport Rates .......................................................144.2.2. Sediment Transport Limitations and Assumptions .......................................154.2.3. Scenario 1: Without FRC..........................................................................164.2.4. Scenario 2: With FRC ..............................................................................164.3. GEOMORPHOLOGICAL ASSESSMENT RESULTS..............................................174.3.1. Sediment Transport Rates .......................................................................174.3.2. Sediment Transport Rates: Results Summary.............................................234.4. GEOMORPHOLOGICAL ASSESSMENT DISCUSSION ........................................234.4.1. Impacts of the FRC on the Existing Channel Geomorphology........................244.4.2. Geomorphological Consequences: Summary ..............................................27

4.4.3. Additional Impacts of the Proposed FRC on the Existing Channel ..................274.5. FRC DESIGN: GEOMORPHOLOGICAL CONSIDERATIONS.................................29


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4.5.1. Sediment Transfer ..................................................................................294.5.2. Bed Configuration...................................................................................304.5.3. Bank Stability.........................................................................................314.6. MITIGATION MEASURES.............................................................................324.6.1. Reducing Bed-Scour................................................................................32

4.6.2. Reducing Meander Loop Aggradation and Possible Bank Erosion ...................324.7. GEOMORPHOLOGICAL ASSESSMENT CONCLUSIONS......................................33

5 SUMMARY & CONCLUSIONS.............................................. 35

6 BIBLIOGRAPHY............................................................... 37


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LIST OF TABLES

Table 2.1 FRC hard engineering design drawings as available for this study...............3High flow design hydrological estimates were drawn from the existing iSIS model forthe 100-year and 100-year plus allowance for climate change (100-year CC) events.The QMED estimate was drawn from the HiFlows-UK website whilst the two low flowdesign estimates for Q95 and Q50 were read from the flow duration curve held on theNational River Flow Archive (NFRA). Design estimates as used are summarized inTable 3.1.Table 3.1. Design hydrological estimates and sources as input into the iSISmodel...............................................................................................................6 Table 3.2. Model scenarios as run during this study...............................................7Table 3.3. Simulated results for the 100-year and 100-year CC events comparing thepre- (outline design) and post-refinement alterations to the FRC. ..........................11

Table 3.4. Results of roughness sensitivity tests. Sensitivity results shown againstrefined FRC simulation outputs for 100-year CC design event. ...............................11Table 4.1. Modelled flows for sediment transport calculations. ..............................15Table 4.2. Sediment sizes as used for sediment transport calculations...................15Table 4.3. Channel cross sections modelled for scenario 1 (without FRC). ..............16Table 4.4. Channel cross sections modelled for scenario 2 (with FRC).................... 16

LIST OF FIGURES

Figure 2.1. Proposed plan view of FRC: possible pool-riffle sequence as illustrated isapproximate only...............................................................................................5Figure 2.2. Proposed refinements to the hard engineering cross section as supplied.Levels and measurements indicative only. Note that TRHB has been drawn to the leftof the cross section. ...........................................................................................5Figure 3.1. FRC iSIS cross section showing revised Mannings n estimates (0.033 inlow flow channel and 0.045 on margins) to reflect the introduction of marginalvegetation to screen the hard engineering structures. ............................................7Figure 3.2. Results of 100-year CC design event in long section showing outline FRCcross section (pre-refinement) and FRC post-refinement from Rein Road Weir to

Kirkstall Bridge. .................................................................................................9

Figure 3.3. Results of 100-year CC design event in long section showing outline FRCcross section (pre-refinement) and FRC post-refinement from Rein Road Weir to thelower railway bridge. ..........................................................................................9Figure 3.4. Results of 100-year design event in long section showing baseline (noFRC), outline FRC cross section pre-refinement and FRC post-refinement from ReinRoad Weir to Kirkstall Bridge. ............................................................................10Figure 3.5. Results of 100-year design event in long section showing baseline (noFRC), outline FRC cross section pre-refinement and FRC post-refinement from ReinRoad Weir to the lower railway bridge. ...............................................................10Figure 3.6. Results of 100-year CC design event in long section showing refined FRCagainst sensitivity test results. ..........................................................................12Figure 4.1. The Meyer-Peter and Muller (1948) sediment transport function...........14Figure 4.2. Sediment transport sand fraction Q95. ...........................................17


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Figure 4.3. Sediment transport sand fraction Q50. ...........................................18Figure 4.4. Sediment transport fine gravel fraction Q50. ...................................18Figure 4.5. Sediment transport sand fraction QMED..........................................19Figure 4.6. Sediment transport fine gravel fraction QMED..................................19Figure 4.7. Sediment transport coarse gravel fraction QMED..............................20

Figure 4.8. Sediment transport cobble fraction QMED. ......................................20Figure 4.9. Sediment transport sand fraction 100-year CC................................. 21Figure 4.10. Sediment transport fine gravel fraction 100-year CC.......................21Figure 4.11. Sediment transport coarse gravel fraction 100-year CC................. 22Figure 4.12. Sediment transport cobble fraction 100-year CC. ...........................22Figure 4.13. Areas of potential increased erosion and deposition. Based on Drawing360 Required Bank Heights, 05/10/2006, Weetwood. ........................................25

LIST OF APPENDICES

Appendix A: 761 070808 Meeting Note

Appendix B: Hydraulic Modelling Results


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1 INTRODUCTION

1.1 BACKGROUND

In the present situation, the existing flood risk to the proposed redevelopmentat Kirkstall Forge is unacceptable to the Environment Agency largely due to the

potential blockage to the passage of the River Aire beneath a pair of railwaybridges. Flood mitigation has been proposed which involves the introduction ofan entirely new, engineered flood relief channel (FRC).

The construction of the FRC is intended to reduce the flood risk associated withblockage of the two bridges. Previous modelling work has confirmed thepotential of a FRC to lower flood risk to acceptable levels and has allowed basiccross section area and geometry to be outlined: the new channel will be aminimum of 25m wide (based on a Mannings n roughness coefficient estimateof 0.033).

Weetwood Environmental Engineering (Weetwood) has been supplied with aproposed hard engineering design of gross cross section geometry based onthese design specifications. This cross section requires refinement in order to:

Ensure that fish habitat is accommodated within the new reach

Ensure that a decrease in the quality or availability of fish habitat presentin existing reaches is avoided

Ensure the flood relief function of the proposed FRC is maintained ingiving due consideration to the above

Accommodate ecological and aesthetic enhancements where appropriateusing established river enhancement and rehabilitation techniques.

The introduction of a new reach to the existing channels at Kirkstall Forgerepresents a profound change to the channel configuration. As such, a broad-scale, qualitative geomorphological investigation is required in association withflood risk modelling in order to examine the probable gross morphologicalimpacts of the construction of the FRC. This includes a consideration of theprobable geomorphological impacts of the new channel on the existing meanderand implications for fish habitat.

1.2 OBJECTIVES AND DELIVERABLES

Throughout this study, the main considerations taken into account whenexamining the proposed FRC are (in descending order of priority):

1. Mitigation of flood risk at the Kirkstall Forge site

2. Potential changes to flood conveyance in local reaches if thegeomorphological regime is significantly altered by the introduction of thebypass channel

3. Potential changes to bank and bed morphology due to the FRC (as 2)above)


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4. Potential changes to fish habitat both in local reaches and furtherdownstream

5. Further potential ecological implications and aesthetics of the FRC.

In addressing the outstanding concerns of the Environment Agency regardingflood risk and associated issues at the Kirkstall Forge site, the main objectivesof this study are:

1. FRC design refinement

2. Broad-scale assessment of likely adjustments of local geomorphology

3. Identification of potential impacts on fish habitat and possible mitigationstrategies.

It should be noted that the geomorphological content of this report is semi-quantitative only i.e. results quoted are intended to be indicative of probablelarge-scale change. In conducting this analysis, a number of assumptions havebeen made which are detailed in the body of the text below. Observations andrecommendations made regarding fish habitat also rest on a number of broad-scale, qualitative assumptions regarding habitat requirements.

1.3 APPROACH TAKEN IN THIS STUDY

In addressing the considerations outlined above, the approach taken has beento:

Outline a refined cross section design using established riverenhancement and rehabilitation techniques

Re-run the iSIS model to ensure that the hydraulic performance of theFRC is largely unaffected

Carry out a broad-scale geomorphological assessment of the implicationsof the introduction of the FRC incorporating the design refinements

Refine the design recommendations as necessary in keeping with thenatural geomorphological processes expected to operate within thereaches local to the site.

1.4 PURPOSE OF THIS REPORT

This report is designed to primarily address the outstanding concerns of theEnvironment Agency as stated in a meeting held with Commercial EstatesGroup (CEG) on 8thAugust 2007 summarized in 761 070808 Meeting Note(Appendix A).


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2 FLOOD RELIEF CHANNEL DESIGN REFINEMENT

2.1 EXISTING FRC DESIGN

Weetwood were supplied with several preliminary drawings incorporatingseveral proposals for hard engineering geometry consisting of the following

documents as listed in Table 2.1.

Table 2.1FRC hard engineering design drawings as available for this study.

Document Title File Name & Type Author Date

DV-SK-011 Rev P2A024328-DV-SK-011 - P2 -FLOOD RELIEF CHANNEL.pdf

RD White YoungGreen Consulting

20.06.2007

Kirkstall Forge BypassChannel.dwg No.675_SK03 Rev.C10.09.07

Bypass Channel 675_SK03

Rev. C copy.jpg

Randle Siddeley

Associates10.09.2007

Kirkstall Forge -Bypass-ChannelSection A-A.dwg No.675_SK04 Rev.D10.09.07

Bypass Channel cross section675_SK04 RevD copy.jpg

Randle SiddeleyAssociates

10.09.2007

This information formed the basis of the cross section and planform geometryrefinements as detailed below.

2.2 EXISTING FRC DESIGN: COMMENTS

As specified in 761 070808 Meeting Note, part of the remit of this study wasto soften the appearance of the proposed hard engineering structures withparticular reference made to the rip-rap battering below the railway line (TRHBof the FRC).

In examining the potential for aesthetic softening of the appearance of theseinstallations, the following observation was made regarding the configuration ofthe bank protection.

Section 1 as shown in White Young Green drawing DV-SK-011 Rev P2 showsrip-rap overlying a 0.17m thick Reno mattress. It is unclear what benefit thismattress will offer when positioned beneath rip-rap: it is possible that setting amattress at this location (i.e. beneath the rip-rap) will actually be counter-productive. It is recommended that further justification for this section of thehard-engineering design is sought since the mattress may act as a plane ofshear to the overlying material. (Draft soft-engineering refinement drawings inthis report are presented with no Reno mattress below the rip-rap).

The following soft-engineering design refinements proceed with the

assumption that the general cross section geometry of the proposed hardengineered structures within the reach will remain largely unchanged.


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2.3 FRC SUGGESTED REFINEMENTS

The gross cross section design as dictated by the hard engineering structuresrequired refinement mainly to:

1) Provide additional potential fish habitat by the creation of a more variedflow pattern and potential refugia and lying up areas

2) Provide aesthetic softening of the appearance of the hard engineeredstructures as proposed.

In order to address 1) and 2) above, the approach taken was to introduce apair of marginal coir net berms to create a sinuous, low flow channel runningalong the middle of the FRC. It is proposed to tie this material into the hardengineering structures and to plant emergent and marginal aquatic vegetationdesigned to partially screen the vertical revetment on the TLHB and the base ofthe rip-rap on the TRHB.

It is understood that, due to the risk of blockage associated with vegetation andlarge woody debris within local reaches, planting trees along these marginswould be unacceptable to the Environment Agency. However, given the verticalface of the proposed revetment, more effective screening of the TLHB may bepossible using trees and shrubs tolerant of periodic flooding. Costs in terms ofa decrease in conveyance and increase in flood levels are likely to be slight ifonly one bank is vegetated in this manner. However, it may be necessary toset the berm on this bank at a slightly higher level to ensure that any treesplanted here would survive

Without using trees to soften the rip-rap, an alternative method would be to

green this feature by blinding the batters with gravel and soil duringinstallation and seeding with grass and herbs of local provenance. This mayallow the surface of the rip-rap and the face of the TRHB to appear morenatural and established soon after seeding with an appropriate mixture.

Further structural refinement is proposed by incorporating the engineering of asequence of pools and riffles designed to introduce some low flow variabilityinto the new reach. Running the riffles diagonally across the FRC will, inconjunction with the marginal mattresses, force flow from side to side of thechannel with passage over each feature. Associated pools and zones of scourcould be added at alternate banks in order to provide a wider, more naturalvariety of habitat for fish within the reach. A possible configuration of thisarrangement is presented in Figure 2.1 and Figure 2.2.

Further flow diversity could be ensured by lowering the invert level of one sideof the riffle slightly to force the flow to alternating sides of the FRC duringperiods of low discharge if required. Larger material may also be incorporatedwithin the surface of the riffle i.e. in the boulder fractions in order toincrease small-scale bedform and flow diversity. Information regardingappropriate minimum sizing of material can also be found within thegeomorphological assessment section below.


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Figure 2.1. Proposed plan view of FRC: possible pool-riffle sequence as illustrated isapproximate only.

Figure 2.2. Proposed refinements to the hard engineering cross section as supplied.Levels and measurements indicative only. Note that TRHB has been drawn to the left

of the cross section.


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3 HYDRAULIC MODELLING

3.1 HYDRAULIC MODELLING REQUIREMENTS

As specified in 761 070808 Meeting Note, the Environment Agency requiredthat the existing iSIS model was altered to incorporate any refinements made

to the proposed outline FRC design in order to ensure that the hydraulicperformance of the FRC as input into previous model runs (i.e. incorporatingthe outline FRC cross section geometry) was substantially replicated.

In addition to an assessment of the refined FRC to convey design flooddischarges, low flow hydrological estimates were required in order to examinethe behaviour of the local reaches at low design flows. Both low and high flowdesign estimates were necessary components of the broad-scalegeomorphological analysis in order to allow assessment of the potential forsediment transport and deposition within the reaches of interest.

3.2 HYDROLOGYHigh flow design hydrological estimates were drawn from the existing iSISmodel for the 100-year and 100-year plus allowance for climate change (100-year CC) events. The QMED estimate was drawn from the HiFlows-UK websitewhilst the two low flow design estimates for Q95and Q50 were read from the flowduration curve held on the National River Flow Archive (NFRA). Designestimates as used are summarized in Table 3.1.

Table 3.1. Design hydrological estimates and sources as input into the iSIS model.

Flow Exceedence

Probability / ReturnPeriod

Discharge (m3

/s) Source Comment

Q95 3.367 NFRA As quoted on website

Q50 9 NFRAApproximate (readfrom flow duration

curve)QMED 138.8 HiFlows-UK As quoted on website100yr 300.63 Atkins Scaled to Newlay Weir

100yr CC 360.756 Atkins Scaled to Newlay Weir

3.3 HYDRAULIC MODEL REFINEMENTS

In order to accommodate the proposed refinements to the FRC, iSIS crosssections representing the new channel were altered as outlined in Figure 3.1 toreflect the introduction of marginal berms. Proposed planting of vegetationdesigned to soften the appearance of the hard engineering structures wasincorporated by raising roughness estimates to 0.045 for all points within thecross section bordering the new, sinuous low flow channel. A roughnessestimate of 0.033 was preserved for the channel bed between the proposedmarginal berms (representative of the gravel/ cobble bed expected to beestablished within this reach).


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Figure 3.1. FRC iSIS cross section showing revised Mannings n estimates (0.033 inlow flow channel and 0.045 on margins) to reflect the introduction of marginal

vegetation to screen the hard engineering structures.

3.4 HYDRAULIC MODELLING SIMULATIONS

Simulations for both the 100-year and the 100-year CC event both included anassumption of 23% blockage at the upper railway bridge (to account for thepotential build up of trash material during these high return periods known to

add to flood risk to the site in the existing situation). Model runs for lowerreturn periods (i.e. in this study, QMED and below) assumed no blockage at thislocation.

Table 3.2. Model scenarios as run during this study.

Model Return Period Comment Purpose

Q95 -/-Q50 -/-

QMED -/-100-year -/-

Baseline (No FRC)

100-year CC -/-

Baseline Scenarios forGeomorphological

Analysis

Q95 -/-Q50 -/-

QMED -/-100-year -/-

100-year CC -/-

GeomorphologicalAnalysis and Flood Risk

100-year CCS1: Roughness of FRC

margins raised to 0.055

Flood Risk SensitivityIncreased Growth of

Vegetation

100- year CCS2: Roughness of leftbank margin raised to

0.07

Flood Risk Sensitivity:Addition of Trees to Left

Bank

FRC Included

100-year CCS3: Roughness of entireFRC bed raised to 0.045

Flood Risk Sensitivity


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Baseline scenarios were also re-modelled (i.e. without the inclusion of the FRC)for use in the geomorphological assessment. Runs as executed during thisstudy are listed in Table 3.2.

3.5 HYDRAULIC MODELLING: RESULTS & DISCUSSION

3.5.1 Main River Aire: Rein Road Weir to Kirkstall Bridge

In order to examine the effects of incorporating the above changes to theoutline FRC design on flood risk, the 100-year CC results from earlier modelruns carried out to estimate the required broad-scale geometry of the bypasschannel were compared with the results of simulations of the same designevent including the above refinements. These results are presented in longsection in Figure 3.2 against earlier results incorporating the basic outline FRCcross section design. Figure 3.3 shows the same results in long sectionrunning from Rein Road Weir to the lower railway bridge. Figure 3.4 and

Figure 3.5 show the results as simulated for the 100-year design event.

Comparing results from the outline design FRC to the refined FRC scenario,reveals a slight rise in flood levels upstream from the upper railway bridge ismodelled in both events. This is just visible in the long sections but isnegligible throughout the reach below Rein Road Weir. The maximum changein flood level was simulated at the upstream face of the upper railway bridge(node 02671700176C) an increase of 36mm in the 100-year CC event andof 35mm in the 100-year event. Neither of the simulations indicated asignificant increase in overall flood risk to the development site in the mainriver reaches stretching from Rein Road Weir to Kirkstall Bridge.

Results from all runs described in this section are included in Appendix B.

3.5.2 Flood Relief Channel

Simulated results for the 100-year and 100-year CC events comparing thepre- and post-refinement alterations to the FRC are presented in Table 3.3.As with the results for the main channel, a comparison of pre- and post-refinement flood levels shows a negligible increase in simulated levelsthroughout the FRC. In both the 100-year and 100-year CC events the largestincrease in simulated flood level is found at the entrance to the FRC: 35mmfor the 100-year and 36mm for the 100-year CC event.

The modelled increase at the FRC entrance in the 100-year CC event may beof some note in that this level (38.736mAOD) is 36mm higher than theexisting minimum flood level to the car park (as shown in White Young Greendrawing DV-SK-011 Revision P2). However, the estimated flood levels arewell below the minimum required bank top heights in the agreed planningconditions.


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100yr CC Design Event: Long Section

30

32

34

36

38

40

42

44

46

48

0 500 1000 1500 2000 2500

Chainage From Rein Road Weir (m)

Elevation/SG

(mAOD)

Left bank Right bank Bed 100yr CC Outline FRC Design 100yr CC Refined FRC

UPSTREAM RAILWAY

BRIDGEDOWNSTREAM

RAILWAY BRIDGE

Figure 3.2. Results of 100-year CC design event in long section showing outline FRCcross section (pre-refinement) and FRC post-refinement from Rein Road Weir to

Kirkstall Bridge.

100yr CC Design Event: Long Section

30

32

34

36

38

40

42

44

46

48

0 100 200 300 400 500 600 700 800 900 1000


El

evation/SG

(mAOD)

Left bank Right bank Bed 100yr CC Outline FRC Design 100yr CC Refined FRC

UPSTREAM RAILWAY

BRIDGE

DOWNSTREAM

RAILWAY BRIDGE

Figure 3.3. Results of 100-year CC design event in long section showing outline FRCcross section (pre-refinement) and FRC post-refinement from Rein Road Weir to the

lower railway bridge.


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100yr Design Event: Long Section

30

32

34

36

38

40

42

44

46

48

0 500 1000 1500 2000 2500


Elevation/SG

(mAOD)

Left bank Right bank Bed 100yr Outline FRC Design 100yr Refined FRC

UPSTREAM RAILWAY

BRIDGEDOWNSTREAM

RAILWAY BRIDGE

Figure 3.4. Results of 100-year design event in long section showing baseline (noFRC), outline FRC cross section pre-refinement and FRC post-refinement from Rein

Road Weir to Kirkstall Bridge.

100yr Design Event: Long Section

30

32

34

36

38

40

42

44

46

48

0 100 200 300 400 500 600 700 800 900 1000


Elevation/SG

(mAOD)

Left bank Right bank Bed 100yr Outline FRC Design 100yr Refined FRC

UPSTREAM RAILWAY

BRIDGEDOWNSTREAM

RAILWAY BRIDGE

Figure 3.5. Results of 100-year design event in long section showing baseline (no

FRC), outline FRC cross section pre-refinement and FRC post-refinement from ReinRoad Weir to the lower railway bridge.


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Table 3.3. Simulated results for the 100-year and 100-year CC events comparing thepre- (outline design) and post-refinement alterations to the FRC.

Node

100yr

OutlineFRC

Design

(mAOD)

100yrRefined

FRC(mAOD)

Difference(m)

100yr CC

OutlineFRC

Design

(mAOD)

100yr

CCRefined

FRC

(mAOD)

Difference(m)

BY_130 38.368 38.403 0.035 38.7 38.736 0.036

BY_065 38.256 38.262 0.006 38.57 38.577 0.007

BY_000 38.13 38.13 0 38.423 38.423 0

3.5.3 Sensitivity Tests

A short series of sensitivity tests to varying roughness estimates within the

bypass channel were run using the 100-year CC design estimate in order toassess impact on flood risk. These included:

S1: Raising the roughness of the FRC margins to 0.055 to represent increasedvegetation

S2: Raising the roughness of the FRC TLHB to 0.07 to represent tree growth

S3: Raising the roughness of the entire FRC to 0.045

These results are presented against the results of runs using the refined FRC

(Mannings n of 0.033 and 0.045) in long section in Figure 3.6. Resultswithin the FRC are presented in Table 3.4. None of the sensitivity testscaused a significant increase in flood risk to the development site. This is tobe expected since any decrease in conveyance in the FRC is likely to result inmore flow through the existing meander.

Table 3.4. Results of roughness sensitivity tests. Sensitivity results shown againstrefined FRC simulation outputs for 100-year CC design event.

Node

100yrCC

RefinedFRC

S1:Channel0.033

Banks0.055

Differencefrom

100yr CC

RefinedFRC

S2:Banks0.0450.07

Differencefrom

100yr CC

RefinedFRC

S3:EntireFRC

0.045

Differencefrom

100yr CC

RefinedFRC

BY_130 38.736 38.775 0.039 38.738 0.002 38.818 0.082

BY_065 38.577 38.599 0.022 38.578 0.001 38.622 0.045

BY_000 38.423 38.423 0 38.423 0 38.423 0


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100yr CC Design Event Sensitivity: Long Section

30

32

34

36

38

40

42

44

46

48

0 500 1000 1500 2000 2500


Elevation/SG

(mAOD)

Left bank Right bank Bed 100yr CC Refined FRC

100yr CC FRC TLHB 0.07 100yr CC FRC Margins 0.055 100yr CC FRC 0.045

UPSTREAM RAILWAY

BRIDGEDOWNSTREAM

RAILWAY BRIDGE

Figure 3.6. Results of 100-year CC design event in long section showing refined FRCagainst sensitivity test results.


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4 GEOMORPHOLOGICAL ASSESSMENT

4.1 GEOMORPHOLOGICAL ASSESSMENT REQUIREMENTS

The broad aims of the geomorphological assessment were to:

Assess the potential impact of the proposed Flood Relief Channel (FRC)on the geomorphological processes operating within the existing channelprimarily via sediment entrainment/ transport modelling

Address any additional consequences of the FRC as a result of themodified flows and geomorphological processes operating within the river(e.g. fisheries habitats, integrity of existing infrastructure and flood risk)

Assess the geomorphological processes operating within the FRC andprovide advice to the final design of the new channel

Provide advice for the implementation of any mitigation measuresrequired (e.g. erosion control).

4.2 GEOMORPHOLOGICAL ASSESSMENT METHODS

To assess the impact of the FRC on the geomorphological processes operatingwithin the existing channel, it is important to understand how changes to flowhydraulics will impact on patterns of erosion and deposition within localreaches. Any alteration to the rates of erosion and deposition may have morewidespread consequences for fisheries habitats, flood risk and/ or channelinfrastructure.

For this study, a sediment transport function was used to calculate rates ofpotential sediment transport through a section of river and, thereby, infer anylikely changes to the morphological characteristics (e.g. bed scour, deposition).Although sediment transport functions are a useful tool, they generally requirecalibrating with field data. For this study, no such data was available:consequently the application of the model results is limited and must only beviewed as indicative of relative directions of change rather than absolutemagnitudes of change.

Sediment transport models can be used to give indications of the following:

Potential volume or mass of material transported through a reach.

Downstream fluctuations in the rate of sediment transport

Likely changes of sediment dynamics through time, or changes inresponse to natural or man-made perturbations to the river or catchment

Energy available for bed scour, in response to changes in flow dischargesand hydraulics

Patterns of sediment deposition and potential channel aggradation

The potential for different sediment sizes to be scoured, entrained andtransported at various discharges.


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2/3

3 )1()1(8

)1(

=

gDsgDsgDs

q cs

Sediment transport modelling coupled with the existing Fluvio report (Fluvio,2006) and additional design plans and iSIS model results were used to gain anunderstanding of the dynamics of potential morphological change within theRiver Aire and to indicate likely patterns of change which may otherwise beunknown.

4.2.1. Calculating Sediment Transport Rates

Sediment transport rates were calculated for a number of cross sections in thevicinity of the proposed FRC. Results from the hydraulic modelling undertakenas part of this study were used to provide the key hydraulic parameters. Foreach cross section, the Peter-Meyer and Muller transport function (see Figure4.1) was used to calculate potential sediment transport rates for a range offlows and a range of sediment sizes.

The Meyer-Peter and Muller sediment transport equation (Meyer-Peter andMuller, 1948) and as reviewed in Wilcock (2001).

(1)

Where is Shield`s shear stress, c is critical shear stress, qs is sedimenttransport rate, D is sediment size, s is sediment density, g is acceleration ofgravity and is fluid density.

(2)

Where S is slope and h is hydraulic depth (approximated by flow depth in widechannels).

At a given river cross section and for a given grain size, flow depth, flow widthand water surface slope a sediment transport rate (as a volume or mass perunit of time) can be calculated.

Figure 4.1. The Meyer-Peter and Muller (1948) sediment transport function

Models were run for two scenarios:

Scenario 1: without the designed FRC (current conditions), and

Scenario 2: with the FRC refined.

The flows used to drive the models are shown in Table 4.1 below. These flowsare chosen as they provide a full range of flows from very low to very highand can be used to observe likely sediment dynamics over a full range of flowconditions. In natural channels, QMED is approximate to bankfull dischargeand is indicative of the most geomorphologically effective flows within manyrivers (Knighton, 1998).

ghS =


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Table 4.1. Modelled flows for sediment transport calculations.

Flow Exceedence

Probability / ReturnPeriod

Discharge Comment

Q95 3.367 m3/s -/-

Q50 9 m3

/s -/-QMED 138.8 m3/s

Recurrence interval ~2.33years

100yr CC* 360.756 m3/sClimate Change scenario(100yr flow plus 20%)

* Note: the sediment transport calculations use a 100yr CC flow incorporating an assumption of a23% blockage at the upper railway bridge.

Sediment sizes used within the models are based on typical size classthresholds cited in fluvial geomorphology literature (Knighton, 1998), seeTable 4.2 below. The sediment size used in the transport models is theminimum size within the size classes. Fluvio (2006) indicate that the gravel

size fraction (10-40mm) is dominant (40%-50%) along with sand material,although a range of sediment size up to the cobble fraction is found within thechannel at Kirkstall Forge.

Table 4.2. Sediment sizes as used for sediment transport calculations.

Sediment SizeClasses

Size in Metres(Lower Size Limit)

Boulder 0.256

Cobble 0.064

Gravel (Coarse) 0.016Gravel (Fine) 0.002

Sand 0.00006

The results of the iSIS hydraulic model for the range of flows listed abovewere extracted and imported into an Excel spreadsheet. A rearrangedManning`s formula was used to derive a water surface slope (based onvelocity and stage outputs from iSIS) which was then used along with flowdepth, flow width and sediment size to derive a sediment transport rate for aparticular sediment size fraction.

In this study, the sediment transport rates are expressed as a volume ofsediment moved over a period of time (m3/s). The volume (m3) can beconverted into a mass (kg) by multiplying the volume by 2650 (dry weight ofsediment in kg per m3).

4.2.2. Sediment Transport Limitations and Assumptions

The fluvial geomorphology literature lists many assumptions and limitations inthe use and interpretation of sediment transport functions. Some keyassumptions and limitations are:

Sediment transport rates assume a readily available supply of sedimentfor entrainment within the flow.


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Sediment transport rates are based on averaged cross section hydraulicparameters (velocity, slope and area).

The transport function assumes a steady and uniform flow. It does nottake into account turbulence or velocity fluctuations.

Additional sediment parameters, such as packing and sorting are notaccounted for.

As outlined earlier, sediment transport data have not been collected for thisstudy and therefore the model results have not been calibrated: the figuresobtained are therefore used for indicative purposes only. The relative valuesbeing more appropriate to this study than the absolute values.

4.2.3. Scenario 1: Without FRC

Sediment transport rates are calculated for cross sections within the vicinity ofthe proposed FRC. Cross sections used are shown in Table 4.3 below.

Table 4.3. Channel cross sections modelled for scenario 1 (without FRC).

Channel CrossSection

Location

1700200 Located 24 m upstream of railway bridge1700176C Upstream railway bridge

1700066Located within the existing channel meander (between thetwo rail bridges)

1607120DLocated immediately downstream of the proposed FRCoutlet

4.2.4. Scenario 2: With FRC

Sediment transport rates are calculated for cross sections within the vicinity ofthe proposed FRC. Cross sections used are shown in Table 4.4 below.

Table 4.4. Channel cross sections modelled for scenario 2 (with FRC).

Channel CrossSection

Location

1700200 Located 24 m upstream of railway bridge

1700176C Upstream railway bridge1700066 Located within the existing channel meander (between the

two rail bridges)1607120D Located immediately downstream of the proposed FRC

outletBY-130 Located within the upstream end of the proposed FRCBY-065 Located within the mid-section of the proposed FRCBY-000 Located within the downstream end of the proposed FRC

It should be noted that a limited number of cross sections within the Kirkstall Forge reacheswere modelled to provide sediment transport estimates.


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4.3. GEOMORPHOLOGICAL ASSESSMENT RESULTS

4.3.1. Sediment Transport Rates

The results of the two model scenarios are plotted for comparison (see Figure4.2 to Figure 4.12 below). Each plot shows the sediment transport rate(m3/s) for each cross section. Separate plots are shown for a single discharge

(Q95, Q50, QMED and 100yr CC) and an individual sediment size class. Whereresults indicate that material is not transported through a reach, the resultsare not plotted (e.g. boulders are not transported at Q95flows for Scenario 1and 2).

The three FRC cross sections (Scenario 2 with FRC, blue line) are plotted toappear downstream of the channel meander cross sections. This is for easeof result presentation only: they must be viewed and interpreted accordingly.

Sediment Transport (Sand Fraction): Q95

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010

0.00012

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section

Sedim

entTransport(m3/s)

Sand With FRC Sand Without FRC

Figure 4.2. Sediment transport sand fraction Q95.


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Sediment Transport (Sand Fraction): Q50

0.00000

0.00020

0.00040

0.00060

0.00080

0.00100

0.00120

0.00140

0.00160

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section

SedimentTransport(m3/s)


Figure 4.3. Sediment transport sand fraction Q50.

Sediment Transport (Fine Gravel Fraction): Q50

0.00000

0.00010

0.00020

0.00030

0.00040

0.00050

0.00060

0.00070

0.00080

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section


Fine Gravels With FRC Fine Gravels Without FRC

Figure 4.4. Sediment transport fine gravel fraction Q50.


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Sediment Transport (Sand Fraction): QMED

0.00000

0.02000

0.04000

0.06000

0.08000

0.10000

0.12000

0.14000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section



Figure 4.5. Sediment transport sand fraction QMED.

Sediment Transport (Fine Gravel Fraction): QMED

0.00000

0.02000

0.04000

0.06000

0.08000

0.10000

0.12000

0.14000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section



Figure 4.6. Sediment transport fine gravel fraction QMED


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Sediment Transport (Coarse Gravel Fraction): QMED

0.00000

0.01000

0.02000

0.03000

0.04000

0.05000

0.06000

0.07000

0.08000

0.09000

0.10000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section


Coarse Gravels With FRC Coarse Gravels Without FRC

Figure 4.7. Sediment transport coarse gravel fraction QMED.

Sediment Transport (Cobble Fraction): QMED

0.00000

0.00200

0.00400

0.00600

0.00800

0.01000

0.01200

0.01400

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section


Cobbles With FRC Cobbles Without FRC

Figure 4.8. Sediment transport cobble fraction QMED.


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Sediment Transport (Sand Fraction): 100yr CC

0.00000

0.05000

0.10000

0.15000

0.20000

0.25000

0.30000

0.35000

0.40000

0.45000

0.50000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section



Figure 4.9. Sediment transport sand fraction 100-year CC.

Sediment Transport (Fine Gravel Fraction): 100yr CC

0.00000

0.05000

0.10000

0.15000

0.20000

0.25000

0.30000

0.35000

0.40000

0.45000

0.50000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section



Figure 4.10. Sediment transport fine gravel fraction 100-year CC.


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Sediment Transport (Coarse Gravel Fraction): 100yr CC

0.00000

0.05000

0.10000

0.15000

0.20000

0.25000

0.30000

0.35000

0.40000

0.45000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section


Coarse Gravels With FRC Coarse Gravels Without FRC

Figure 4.11. Sediment transport coarse gravel fraction 100-year CC.

Sediment Transport (Cobble Fraction): 100yr CC

0.00000

0.05000

0.10000

0.15000

0.20000

0.25000

1700200 1700176C 1700066 1607120D BY 130 BY 065 BY 000

iSIS Cross Section

Sedime

ntTransport(m3/s)

Cobbles With FRC Cobbles Without FRC

Figure 4.12. Sediment transport cobble fraction 100-year CC.


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4.3.2. Sediment Transport Rates: Results Summary

Scenario 1: Without FRC

Boulders are not transported at any of the modelled flows within any ofthe modelled cross sections.

At Q95flows, sand is the only size fraction potentially transported.

At Q50flows, sand and fine gravel are the only size fractions potentiallytransported.

At QMED and 100yr CC flows, sands to cobbles are potentiallytransported, although only very low rates for cobbles at 100yr CC flowsupstream of the proposed FRC (cross section 1700200).

The greatest potential transport rates, for a given sediment size, are

located at cross section 1700200.

Potential sediment transport rates are greater within the existing channelmeander compared to potential rates at the locations of the entrance andexit to the proposed FRC.

Scenario 2: With FRC Refined

Boulders are not transported at any of the modelled flows within any ofthe modelled cross sections.

Sand is potentially transported at all flows, although at only very lowrates through the existing meander.

Within the existing channel meander (cross section 1700066) rates ofpotential sediment transport, for all size fractions are greatly reduced,compared to the present scenario without the FRC. This is evident for allflows (except 100yr CC cobbles).

Potential transport rates upstream of the FRC (cross section 1700200)are significantly increased (over 400%) as a consequence of the FRC.This additional stream power is likely to be translated into bed scour, inthe absence of sediment available for transport from upstream sources.This increase is modelled for all sediments below the boulder sizefraction, but becomes more pronounced with increased flows.

Potential transport rates are consistently higher within the FRC comparedto those in the existing meander, for a given sediment size and flow.

4.4. GEOMORPHOLOGICAL ASSESSMENT DISCUSSION

The results of the sediment transport modelling presented in this report,coupled with the information obtained from Fluvio (2006) have been used tomake some broad (semi-quantitative) observations about the likely impact ofthe proposed FRC to the geomorphological processes operating within the RiverAire at Kirkstall Forge.


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In addition, the iSIS model results, the sediment transport modelling and anunderstanding of fluvial processes has been used to make some keyrecommendations about the geomorphology related design aspects of the FRC.

4.4.1. Impacts of the FRC on the Existing Channel Geomorphology

4.4.1.1. Erosion and Deposition Pre-FRC

The River Aire at Kirkstall forge is a laterally constrained channel, but with along profile characterised by pool and riffle sequences: a consequence ofchanges in bed elevation to accommodate temporal and spatial fluctuationsin flow velocities and sediment transport. Sediment within the channel isdominated by gravels (10-40mm) and sands, but also includes cobbles.Fluvio (2006) suggest that coarse material is only entrained and transportedvery infrequently during high flows whereas the sand fraction is highlymobile. The absence of exposed gravel bars within the reach and protectedchannel banks indicates the river is sediment supply limited.

The results of the sediment transport modelling are broadly in line with theseobservations. Sand material is shown to be mobile through all flows aboveQ95, whilst coarse-gravels and cobbles are mobilised at flows of QMED andabove.

Results suggest that the highest rates of potential sediment transport(increased flow velocities) occur upstream of the proposed FRC (cross section1700200) and remain high (albeit at a lower rate) within the existing channelmeander (cross section 1700066). However, flow velocities and thereforepotential sediment transport rates, are significantly lower at cross sections

1700176C and 1607120D and deposition here is more likely, particularly forsand size material at lower flows where sediment transport rates are zero.The main reason for the reduction in flow competence here is the widening ofthe channel within the meander loop (~50m) near the railway bridgescompared to further upstream (~25m) near the footbridges. This propensityfor deposition is indicated in the field by the raised bed elevations (riffles)within the meander loop - particularly on the inside of the meander bend.

4.4.1.2. Erosion and Deposition Within the Existing Channel (Post-FRC)

There are two main impacts of the FRC on sediment dynamics within theexisting channel. The first is a significant increase in channel velocity and

therefore potential sediment transport approximately 24m upstream of theupper railway bridge (cross section 1700200). The second is a markedreduction in velocity and potential sediment transport rates within theexisting channel meander (1700066), in between the two railway bridges(see Figure 4.13). The likely impacts of these changes are discussed in moredetail below.


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Figure 4.13. Areas of potential increased erosion and deposition. Based on Drawing360 Required Bank Heights, 05/10/2006, Weetwood.

4.4.1.3. Cross Section 1700200

Model results suggest that the FRC will cause a significant increase in flow

velocities and therefore potential sediment transport rates at cross section1700200: most pronounced at flows of QMED and above. This cross sectionis located upstream of the new footbridge and at the location of a riffle orhump (see Fluvio, 2006). Although not modelled for sediment transportrates, the cross sections further upstream (upstream to 1700350) show asimilar increase in flow velocity and therefore, by extension, there ispotential for increased sediment transport rates at these locations as well.

The increased potential transport rates are most pronounced at flows ofQMED and above and likely to be a result of the greater flow conveyancethrough the FRC at these flows.

The model results suggest there may be significant effects on the stability ofthe channel at and immediately upstream of cross section 1700200. Thesediment transport rates calculated can only provide an indication ofpotential sediment transport: they are based on the assumption of anunlimited supply of sediment available for transportation. As the River Aireat Kirkstall Forge appears to be a sediment supply limited channel, then theexcess stream power is likely to be transferred into bed and or bank scourwithin this section of river. The likely consequences of this excess streampower are threefold:

Bed scour is likely to be initiated at the location of increased streampower and erode upstream through a process of nick point migration

Area of possible increasedsediment deposition and channelbank erosion

Area of possible increased

erosion around the `hump` and

possibly further upstream

AREA OF POSSIBLE INCREASED

SEDIMENT DEPOSITION AND

CHANNEL BANK EROSION

AREA OF POSSIBLE INCREASED

EROSION AROUND THE HUMP &

POSSIBLY FURTHER UPSTREAM


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The engineered channel banks may become undermined as thechannel bed level is lowered

Eroded material will be transferred downstream during the next andsubsequent flows of sufficient competence and become depositedfurther downstream.

The Fluvio report (Fluvio, 2006) suggests that skimming of the hump (theraised bed level at cross section 1700200 and upstream) would only result ina very short-term lowering of the bed elevation. Under current flow andsediment transport conditions the feature would reform within several yearsor a decade. However, erosion of the hump resulting from significantchange to the flow hydraulics (a consequence of the construction of the FRC)would be likely to cause a permanent change in the bed elevation at thislocation and further upstream.

4.4.1.4. Cross Section 1700066

Model results indicate a significant reduction in potential sediment transportrates within the existing channel meander (in between the two railwaybridges), once the FRC is in operation. This modelled reduction in streamcompetence is a direct consequence of the reduction in flow through theexisting channel meander. At Q95, Q50and QMED discharges approximatelyhalf of the flow within the river is diverted through the FRC. This is increasedto ~63% at 100yr CC flows (with 23% railway bridge blockage).

Material that would normally be transported through this reach at Q95 andQ50 flows (largely sand), is more likely to be deposited within the channelmeander (as indicated by the cross section 1700066 model results). Even at

QMED flows, gravel material may be deposited whereas previously it wouldhave been more likely to be transported through the reach. When theeffects of the increased likelihood of bed scour upstream of the FRC areconsidered (i.e. increased availability of material for potential deposition asoutlined above) the effects of the reduced stream power within the existingmeander are compounded.

Although sand material may be deposited within the meander section duringlow to moderate flows, model results indicate that this material would beremoved at QMED and above. However, more coarse material (gravels) mayonly be re-mobilised and transported from this reach during the very highestdischarges. Cobble material (if eroded and transferred to this reach from thehump as outlined above) may reside within the meander permanently.Material deposited within the reach may form a more prominent gravel baron the inside of the meander loop, adding material to the existing barfeature. The river here may respond to the formation of a gravel bar featurein one of two ways.


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Firstly, the river channel may become narrower and more incised (forcedtowards the outside of the meander bend), with a concomitant increase inflow velocity in order that continuity of flow is maintained. Secondly, andpotentially less desirable, is a tendency for the river channel to migratelaterally, causing erosion of the channel banks on the outside of the meanderloop. The likelihood and extent of the latter depends to a large extent on the

composition and strength of the material of the channel banks: soft material(alluvial material) will be more prone to erosion than more resistancematerial (bedrock). Information about the composition of the channelmaterial at this location is not available. However, Fluvio (2006) indicatethat the meander loop has undergone a degree of migration over the last150 years indicating that the banks are prone to erosion. It should be notedthat meander evolution at this location may eventually threaten the stabilityof the steep slope at the top of which sits the Leeds and Liverpool Canal.

4.4.2. Geomorphological Consequences: Summary

The broad-scale effects of the FRC on the existing channel morphology in the

River Aire at Kirkstall Forge are summarized below.

Potential erosion of the hump upstream of the FRC as streamvelocities are increased during moderate to high flows causing apermanent lowering of bed elevations.

Material eroded from the `hump` is likely to be deposited within theexisting channel meander. Fine material (sand to gravel) is likely to bere-mobilised and transported out of this reach at moderate to highflows, whereas coarse material may reside within the meander for muchlonger periods of time (10s to 100s of years).

A tendency towards sediment deposition (short-term) within theexisting channel meander as flow velocities are reduced: a consequenceof water being diverted through the FRC. Potential erosion of thechannel banks on the outside of the meander loop and possible threatto the stability of the steep slope at the top of which sits the Leeds toLiverpool Canal.

4.4.3. Additional Impacts of the Proposed FRC on the Existing Channel

The main probable geomorphological consequences of the FRC are outlinedabove. The effects of these shifts in sediment dynamics may have wider

reaching impacts within the River Aire at Kirkstall forge. These effects areoutlined below.


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4.4.3.1. Impact on Local Infrastructure

Increased flow velocities upstream of the FRC inlet are likely to promote bedscour within the channel between the two footbridges. The channel banks inthis location are protected with block-stone revetments (see photograph onpage 9 of the Fluvio report). There is a potential that these revetments maybecome undercut as the channel bed erodes, although the depths of the

revetment footings are unknown. Furthermore, it is not clear how farupstream the zone of potential erosion will migrate: the River Aire has arelatively low gradient bed and this distance could, therefore, be considerable(10s to several 100s of metres). This is further dependent on thecomposition of the bed material below the surface of the exposed alluvium:sub-surface bedrock will tend to resist erosion more so than alluvial material.

The railway bridge piers are located away from this zone of potential bedscour and are therefore unlikely to be affected directly from bed scourfurther upstream. However, the railway bridge at the downstream end of themeander loop may be compromised if lateral channel migration (see above)

is significant. Detailed information, including pier location and design wasnot available for inclusion within this study and, therefore, any assessment ofpotential impact is, at present, only speculative.

4.4.3.2. Impact on Fish Habitats

Baseline fisheries surveys conducted in 2004 (Hydrosurveys, 2004) andreviewed by Fluvio (2006) indicate relatively low fish populations within theRiver Aire at Kirkstall Forge. This reflects the relatively poor habitats for fishin general. Fish habitats within the meander loop between the two railwaybridges are potentially more favourable to fish (Fluvio, 2006) due to thehigher flow velocities and shallower water.

Disturbance of this particular reach due to increased deposition of finematerial is likely to periodically degrade the fish habitat in this location.However, as mentioned earlier, accumulation of fine material (sand and finegravel) is likely to be temporary, as material is likely to be removed duringhigher flows. The more significant, long-term changes are likely to be anincrease in the accumulation of coarse gravels and cobbles and changes tothe flow conditions within this reach and a narrower channel with highervelocities or a wider and shallower channel configuration - depending on theresponse of the channel to flow/sediment modifications in this reach.

Erosion of the hump is indicated in this study, and will therefore remove ormodify this fish habitat within the river. Fluvio suggest this particularlocation is not an anomalous feature within the Aire at Kirkstall and thereforealternative fish habitats are available.


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Impacts on fish habitats downstream of the proposed FRC are likely to beshort-term only. One consequence of the potential bed scour at the humpwill be mobilisation of bed sediments and re-deposition of this materialfurther downstream. It has already been indicated that fine material fromthe bed scour may be deposited within the existing channel meander.During subsequent floods, this material is likely to mobilised further,

entrained within the flow and then re-deposited in downstream sections.However, the flow regime downstream will not be affected and any finematerial that is deposited (as a consequence of bed scour) is likely to beremoved during subsequent moderate to high flows. Furthermore, thevolume of fine material potentially entrained from upstream bed scour isconsiderable, but finite and, therefore, may not pose a long-term issue.

Although the fish habitats are likely to be affected to varying degrees, theadditional habitat afforded by the FRC may counter any disturbance tohabitats within the existing channel. A key objective of the FRC is to providea suitable habitat for fish populations, notwithstanding the mainconsideration for flood risk mitigation.

4.4.3.3. Impact on Flooding

The impact on flood risk caused by the direct morphological alterations of theexisting channel, are likely to be insignificant. The creation of the FRC willfar outweigh any changes to the capacity of the channel, either at theupstream end of the reach (bed scour) or within the meander loop (increasedaggradation). In particular, the potentially reduced capacity of the channelat the meander loop may cause some degree of flow impedance at highflows. This is not likely to cause any great effect to overall flood risk if theFRC is operating as planned (i.e. conveying over 50% of the total flow).

Any degradation of the channel bank revetments, in response to thepotential for accelerated bed scour upstream of the FRC (as outlined above)will have obvious implications for any associated infrastructure, including apotential increase to local flood risk.

4.5. FRC DESIGN: GEOMORPHOLOGICAL CONSIDERATIONS

4.5.1. Sediment Transfer

The results of the sediment transport models were used to outline sediment

transfer and associated issues within the channel.

For the majority of sediment size classes and flows, material that enters theFRC from upstream sources is likely to be transferred through effectively.This material will then be incorporated into the flow within the natural channeland moved downstream as per current conditions.

However, the models indicate that for the cobble size fraction at QMED,material entering the FRC may be deposited within it. This does not mean theFRC will completely fill with cobbles and restrict flow completely: however, itdoes suggest that the flow within the FRC may have to modify itself (changes

to width, depth and velocity) in order to accommodate the increased volumeof sediments from upstream sources.


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The current FRC designs provide the scope for hydraulic variability (i.e. theincorporation of pools and riffles), and these will promote a degree ofturbulence within the channel and help promote localised fluctuations invelocity leading to more effective sediment entrainment. This engineered bedtopography should provide a broad template for the hydraulic variability toensure an effective transfer of sediment through the new channel. However,

there are likely to be additional natural bed forms that form and move withinthe channel in response to the fluctuating influx of flow and material.

4.5.2. Bed Configuration

Pools and riffles are common topographical bed features within coarse gravel /cobble bed rivers and form as a function of stream energy and bed materialsize. They are found in both straight and meandering rivers. A huge volumeof literature exists describing their characteristics and theories of theirevolution. However they tend to have several common characteristics, whichinclude:

A more or less regular spacing of successive pools or riffles at adistance of 5 7 times the channel width.

Riffles tend to have a coarser bed material than adjacent pools

High flow sediment transfer is greater through pools than riffles and lowflow storage at riffles is greater than at pools. This provides theconditions for the coarse material on riffles and the maintenance of poolriffle sequences

Successive sequences of pools and riffles often have their high points

(on riffles) and low points (in pools) alternating from one side of thebank to the other.

This list of key, but by no means exhaustive, riffle-pool characteristics isimportant when considering the design of artificial pool-riffle sequences.

The initial broad FRC geometry indicates a channel length of 130m and aminimum or low/moderate flow width of 15m. Two pool-riffle sequences aretherefore an appropriate number for this width, as indicated in the RandleSiddeley design plans. Furthermore, this spacing is between a multiple of 1and 0.5 of the spacing of the riffle-pool sequences in the existing channel, butgiven the reduced (multiple ~ 0.5) flows in the FRC and narrower channel(multiple ~ 0.5), riffle height (above mean bed level) should also reflect thenatural and FRC dimensions and flow/sediment characteristics. As anestimate, an appropriate riffle height would be approximately 0.25 to 0.35 mabove mean bed level.


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Material used to construct the pool-riffle sequence must also reflect thecharacteristics of flow nature of sediment within the channel. As indicated bythe sediment transport models, the cobble size fraction of bed load ispotentially entrained within the FRC however, only at 100yr CC flows. Thissuggests that a conservative estimate of material used to `seed` the riffles,must be somewhat larger than cobbles (i.e. an intermediate axis greater than

64mm). As flows vary in velocity (and therefore entrainment capacity) arounda modelled mean value, an appropriate minimum size for riffle seeding wouldbe in the order of 100mm. Occasional larger material may also be used, toprovide localised flow variability, to act as zones of deposition and for fishhabitat purposes.

Additional flow variability may also be provided by alternating the location ofsuccessive riffles from one side of the channel to the other (see above). Notonly will this provide valuable flow diversity (undercurrents, secondarycirculation) important as fish habitats, it will provide the channel with agreater scope for internal adjustments (i.e. hydraulic and bed formvariations). This is important as the new channel attempts to attain a formand flow that is at equilibrium with the prevalent flow and sediment regimes.

Once the gross riffle and pool sequences are constructed, flows within thechannel will modify the dimensions of these features to a certain extent.However, as Fluvio (2006) and this study have indicated, local reaches of theRiver Aire appear to be sediment supply limited. As such any attainment ofequilibrium within the man-made pool riffle sequence (i.e. material sorting,armouring, subtle morphological adjustments) make take several years orflood cycles to mature.

4.5.3. Bank Stability

Stable banks are important for any channel design. However, a compromiseis often necessary between channel bank stability and aesthetics in order toavoid over-engineering (often visually unappealing and environmentallyunacceptable). A thorough examination of channel bank stability within theFRC is outside the scope of this assessment. Nevertheless, design plans andcross sections used in the flow models indicate the use of coarse rip-rapmaterial for the channel banks associated with various propositions forgreening materials and features.

Model outputs indicate velocities within the FRC in the region of 3 m/s (bulkcross section velocity) at 100yr CC flows. Although this figure is not fullyrepresentative of velocities adjacent to the channel banks - and flow velocityis by no means the only force acting on channel bank stability (e.g. additionalgravitational forces) - material no smaller than boulders (i.e. 256mm) wouldbe an initial, very general estimate for appropriate sizing of rip-rap material.In addition, any changes in slope through the cross section of the channel (i.e.change in slope between bank and bed) should reflect as far as possible thoseoccurring in natural, stable channels: abrupt changes in slope should beavoided when possible.

It should be noted that more detailed geo-technical assessments arenecessary for a full understanding of the design criteria for any channel bank

protection measures.


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4.6. MITIGATION MEASURES

This study has identified two main areas of concern with respect togeomorphological integrity of the River Aire at Kirkstall Forge. Theseobservations are based on hydraulic modelling and sediment transportfunctions, the latter of which has not been calibrated by any site-specificsediment transport data.

The significant velocity increase upstream of the proposed FRC inlet,leading to potential bed scour and possible destabilising existing channelbank revetments.

The accumulation of sediment within the existing channel meander andpossible erosion of the channel banks towards the outside of themeander loop.

There are a number of options that may be employed to reduce the impact ofthese potential issues and may be viewed as a starting point for further

investigation.

4.6.1. Reducing Bed-Scour

The cause of the possible bed scour upstream of the FRC inlet (upstream ofcross section 1700200) may be a throttle effect through the FRC at moderateto high flows. Moderation of this impact could be achieved through treatingthe cause or halting the effects by:

Reducing the flow competence through the FRC at high flows therebyslowing flow velocities through the channel (increasing roughness) andthus causing a backing-up effect further upstream, leading to reduced

flow competence (and channel velocities) at the point of possible bedscour. Although this is an effective means of reducing or even haltingthe bed scour potential, it should be noted that this method is not aneffective means of delivering the required flood protection benefits ofthe FRC.

Preventing upstream migration of the zone of bed scour by theconstruction of a series of bed-check structures. These preventupstream migration of nick points by preventing bed erosion, byproviding stable, erosion-resistant barriers within the bed of thechannel. They may range in design from a series of lateral boulders

(often only in small streams) to concrete sills (often required in largerchannels, such as the found in the River Aire). The design of thesestructures is guided by the long profile gradient, flow magnitude andbed material/ depth. However, they can be a successful means bywhich to control the upstream migration of bed scour.

4.6.2. Reducing Meander Loop Aggradation and Possible Bank Erosion

The accumulation of coarse material within the existing meander loop andpossible channel erosion is a likely consequence of the initiation of bed scourupstream of the proposed FRC. A number of possibilities are available toreduce or prevent these impacts. Further work is likely to be required to

assess the likely effectiveness of these various options.


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Removal of the upstream bed scour problem (see above) may preventcoarse material from being entrained and further deposited in themeander loop. However, even by halting or reducing the bed scourissue, sediment transferred through the reach at the highest flows, willstill be deposited within the meander loop causing a build-up of sedimentdue to the reduced flows with the operation of the FRC.

Increasing the flows through the existing channel meander may increasethe competence of flows to remove sediment deposited within themeander loop. This option, however, may reduce the effectiveness of theFRC for reducing flood risk as less flow would pass through in high flows.Furthermore, the entrance to the new channel would require engineeringin such a way that during high flows, only a pre-determined volume ofwater is able to enter.

One potential indirect impact of increased accumulation in the meanderloop is for the river to erode the banks towards the outside of themeander loop (hydraulic modification in response to the increased bedlevel). Fluvio (2006) indicate that this section of river has undergonesome lateral shift during at least the last 150 years, indicating that it isprone to erosion. Some form of bank protection is likely to be required iferosion on the outside of the meander loop is to be halted.

4.7. GEOMORPHOLOGICAL ASSESSMENT CONCLUSIONS

The main conclusions drawn from this study are listed below.

Sediment transport models indicate that there are likely to be significantlocal changes to the sediment dynamics within the existing channel as a

consequence of the proposed FRC.

Increased channel velocities upstream of the FRC and upstream of thetopographic high point or hump are likely to initiate bed scour in thislocation. The zone of bed scour may transfer upstream as the riverattempts to attain a new base level potentially causing undermining ofthe existing channel bank revetments. Measures to mitigate the effectsof bed scour may include the construction of bed check weirs and/orlowering conveyance rates through the FRC.

As a consequence of the FRC, flows through the existing meander loopwill be reduced, resulting in a greater propensity for sediment deposition(particularly by material eroded from the zone of bed scour upstream).Fine material is likely to be re-mobilised during high flows whereas largermaterial may remain within the channel in the form of a bar feature. Theriver may respond to this by eroding into the channel banks on theoutside of the meander loop. Bank protection may be required(depending on the extent of this possible erosion).

Channel morphology downstream of the FRC is unlikely to be modified toany great extent, if at all, although fine material may be depositedtemporarily.


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Fish habitats are likely to be modified by increased deposition within theexisting channel meander loop and possible erosion of the bed upstreamof the FRC. However, additional habitats within the existing channel andwithin the new FRC will offer favourable conditions for fish.

Changes to the sediment dynamics and channel morphology, resulting

from the operation of the FRC, are not likely to have any significantimpact on the effectiveness of the FRC as a control of flood risk.

Modelled sediment transport rates through the FRC indicate that materialwill be transferred effectively through the new channel, with no majormorphological changes expected. However, some morphologicalreadjustment of the designed bed features (pool and riffle sequences) isto be expected as the channel readjusts to flow and sediment inflows.

The broad-scale bed features and overall design of the FRC are generallyappropriate for the flow characteristics and sediment dynamics within theRiver Aire. However, detailed design features, such as the nature of bankmaterial and cross section dimensions may require further assessmentfor a full understanding of its long-term operational effectiveness andstability.


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5 SUMMARY & CONCLUSIONSThis study has investigated the potential for refinements to be made to the hardengineering proposals for the FRC at Kirkstall Forge. A draft refined design basedon an outline FRC cross section geometry has been produced which aims to bothsoften the appearance of the hard engineering and create potential fish habitats

within the new reach. Proposed features within the new channel broadly include:

marginal berms within the FRC designed to encourage a sinuous flow pathand provide a platform for marginal vegetation

a sequence of pools and riffles aimed at providing a diverse flow patternwithin the reach

blinding with gravel and soil of the rip-rap battering in order to allowvegetation to be readily established and provide a more naturalappearance.

The design of these features is broadly in-line with the geomorphological analysis.Material with an intermediate axis >64mm could be used to provide a relativelystable platform from which to initiate or seed a semi-natural pool-riffle sequence.It is currently proposed to use material of 100mm to 300mm to form the basis ofthe features introduced to the new FRC. Subsequent development and maturationof these bedforms should be expected over several flood events in response toanticipated sediment delivery from the main channel and scour both within themain channel and the FRC. However, initial use of material of the above size isexpected to resist transport by all but the most extreme discharges therebyensuring that the main riffle and habitat features as installed are not destroyed.

The design of the features within the FRC has been deliberately broad-scale.Over-engineering the in-stream features is to be avoided and natural sorting andarmouring of the large-scale features as proposed should be expected as a naturalresponse of the river to the significant geomorphological change in local reachesprompted by the introduction of an entirely new channel.

Section 1 as shown in White Young Green drawing DV-SK-011 Rev P2 shows rip-rap overlying a 0.17m thick Reno mattress. It is unclear what benefit thismattress will offer when positioned beneath rip-rap: it is possible that setting amattress at this location (i.e. beneath the rip-rap) will actually be counter-

productive. It is recommended that further justification for this section of thehard-engineering design is sought since the mattress may act as a plane of shearto the overlying material.

iSIS simulations incorporating the changes to the overall FRC design have shownthat there is a negligible increase in flood levels due the introduction of themarginal berms and vegetation to soften the engineered channels appearance.However, subsequent broad-scale geomorphological modelling using results fromthe hydraulic model suggests that there may be several significant changes to thelocal geomorphological regime which may need to be addressed prior to finalisingand implementing any designs. Most notably, these include:


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Possibility of the formation of a nick point in the main channel due toincreased velocities and scour upstream of the FRC with the potential toundermine the existing revetment structures if left unchecked

Possibility of erosion of the outer bend of the meander between the tworailway bridges due to development of the point bar from aggradation of

sediment transported into the reach from upstream scour. Meanderevolution at this location may eventually threaten the stability of the steepslope at the top of which sits the Leeds and Liverpool Canal.

Mitigation options for both these scenarios are outlined above including bed checkstructures and additional bank and revetment protection. However, further moredetailed investigation is required in order to reduce the current uncertaintyconcerning the potential magnitude of these impacts. Hydraulic model runsexamining these mitigation scenarios have not been attempted in this study.

The modelling and geomorphological assessment have also revealed that there issome potential for an impact on fish habitat within the existing meander.However, this is expected to be short term only and is mitigated by theintroduction of new habitat features within the FRC. Deposition of fine materialduring periods of low flow is possible due to the decrease in velocities within themeander reach between the two bridges a direct consequence of theintroduction of the FRC. However, this material is expected to move through thereach during higher flows on an annual basis. It should be noted that this cycle ofdeposition and transport is a natural process and, whilst a change from theexisting regime within the meander, is likely to be consistent with patterns oferosion and aggradation in the River Aire locally.

Finally, it should be reiterated that the approach taken in this study has beennecessarily broad-scale and semi-quantitative. Results as quoted within the bodyof this report rest on a number of assumptions which would, ideally, be subject tofield verification and confirmation in order to reduce the levels of uncertaintysurrounding the qualitative judgements as presented.


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6 BIBLIOGRAPHY

Fluvio, 2006.Geomorphological investigation of the River Aire at Kirkstall Forge,Leeds. Report Prepared for Weetwood Environmental Engineering.

Hydrosurveys, 2004.Baseline fisheries survey of the River Aire, Kirkstall, Leeds.Unpublished Report, Hydrosurveys, Kenilworth.

Knighton, D., 1998. Fluvial Forms and Processes: a new perspective. Arnold,London

Meyer-Peter E, Muller R., 1948. Formulation for bed load transport.Proceedings International Association for Hydraulic Research, 2nd Congress,Stockholm; 3964.

Randle Siddeley Associates., 2007. Kirkstall Forge Bypass Channel. Drawing

number 675 SK03 REV C.

Wilcock, P. R., 2001. Towards a practical method for estimating sediment-transport rates in gravel-bed rivers. Earth Surface Processes and Landforms. 26,1395-1408. J Wiley.


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Weetwood 761R01/AG30 October 2007

APPENDICES

APPENDIX A: 761 070808 Meeting Note

MEETING WITH ENVIRONMENT AGENCY STAFF AT PHOENIX HOUSE, LEEDS ONWEDNESDAY 8 AUGUST 2007 AT 2PM

PURPOSE OF MEETING:

To review the current flood risk mitigation measures and surface water drainage

proposals in light of the approved planning conditions and secure agreement inprinciple regarding consents required by the Environment Agency

ATTENDEES:

Representing CEG - Charles Johnson, Gareth Chambers, Granville Davies, AndrewGrimeRepresenting the Environment Agency Gillian Turner & Robert Sanderson(Development Control), Ian Thynne (Planning), Erica Adamson (Biodiversity), NeilTrudgill and Pat OBrien (Fisheries), Karen Wooster (Water Resources)

NOTE OF MEETING:

1. PROPOSED FLOOD RELIEF CHANNELDrawing numbers 675_SK03 Rev B. 18.07.07, 675_SK04 Rev C. 06.08.07 and DV-SK-011 Rev P2 formed the basis of the discussions although it was noted that theadditional rip rap shown on the last drawing was not yet fully included on thelandscape sketches. It was agreed in principle that the proposals tabled wouldallow Clause 28 of the planning conditions to be discharged subject to thefollowing.

1.1 rerunning the model to demonstrate that the propos

kirkstall forge flood defence background papers

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