1 INTRODUCTION 1.1 Study Area Wyke Beck is a left bank tributary of the River Aire situated in the east of Leeds, from its source in Roundhay Park it runs in a generally southerly direction to its confluence with the River Aire, as shown on Figure 1.1. The beck is approximately 9km long from Waterloo Lake to its confluence with the River Aire (10.5km long including the tributaries which feed into the Waterloo and Upper Lakes) and has a catchment area of approximately 28km2. The majority of the catchment is heavily urbanised, with some open areas of parks, playing fields and a small amount of rural fields. Properties were flooded by the Wyke Beck in August 2004, May 2005 and June 2007, and there was also a ‘near miss’ event in January 2008. However there is not an extensive history of property flooding from the beck. The majority of the properties which flooded in these recent events are located in the Dunhill Estate. The frequency and extent of these flood events has led the Environment Agency to look into developing a flood scheme for the Wyke Beck. 1.2 Flood Investigation Study Overview and Objectives This Flood Investigation Study follows on from the Stage 1 Study carried out by Royal Haskoning in January 2008. The Stage 1 Study highlighted the need for a thorough review of the existing hydraulic model and an understanding of the causes of the flooding from the Wyke Beck before any options could be investigated further. This study focuses on reviewing the hydraulic model and updating it so that it accurately represents the flooding which was observed during the three recent flood events. The objectives of this study are to; • Identify the factors influencing the recent flooding; • Review and update the existing hydraulic model from the Phase 2 Floodplain Mapping Study carried out by JBA Consulting to gain confidence in its accuracy; • Establish an understanding of the problem at Wyke Beck; and • Carry out an economic assessment for the entire catchment. It is anticipated that this study will form the basis of a Pre-Feasibility Study for Wyke Beck; with the development and analysis of options for a scheme required to complete the Pre-Feasibility Study. This report has been laid out in a Pre-Feasibility format in order to make the transformation into a full Pre-Feasibility Study easier and quicker. 1.3 Structure of Report The report is set out in 5 sections as follows; 1. Introduction: This section puts the study into context and gives the background and physical characteristics of the catchment; 9T1287/R/303348/PBor Final Report - 1 - 2PndP June 2008 2. Definition of the Problem: the results of the historic review are discussed and the different aspects of the problem at Wyke Beck are identified and their impacts discussed, with the key problems summed up; 3. Review of the Existing Hydraulic Model: The results of the review are catalogued and the further investigations into the urban drainage that were carried out are detailed. The changes and updates to the hydraulic model are detailed and the impact of these changes discussed; 4. Economic Assessment: the details of the economic assessment carried out for the entire Wyke Beck catchment are listed and the results of the assessment discussed and the amount of money potentially available for a scheme analysed; 5. Conclusions: the conclusions from this Investigation Study are laid out; 6. Recommendations: the recommendations for further work suggested. 1.4 Catchment and Watercourse Description The upstream limit of Wyke Beck is the southern end of Waterloo Lake in Roundhay Park. Waterloo Lake is fed by Great Heads Beck and an unnamed tributary (known as ‘Fox Wood’ Beck in this study) and is also linked by a series of waterfalls to Upper Lake in Roundhay Park. The Upper Lake is fed by another unnamed tributary (known as ‘Ram Wood Beck’ in this study). Wyke Beck is fed by the outflow from Waterloo Lake and then runs in a southerly direction to its confluence with the River Aire. The catchment is predominantly urbanised being situated in the eastern suburbs of Leeds, however the beck cuts a narrow rural path with marginal land of woodlands, parklands and playing fields, see Figure 1.1. The catchment is generally low lying with a reasonably gentle gradient of 12m/km, being approximately 150mAOD in the uppermost catchment and approximately 20mAOD at the confluence with the River Aire. The catchment boundary shown in Figure 1.1 has been derived by adjusting the FEH catchment boundary to include all the extents of the sub-catchments which were determined during the drainage assessment (see Section 3.1.2). There are a variety of structures on the Wyke Beck, including weirs, footbridges, road bridges and culverts. The main structures included in the hydraulic model are listed in Table 1.1 below. 9T1287/R/303348/PBor Final Report - 2 - 2PndP June 2008 Table 1.1 Structures on the Wyke Beck included in the hydraulic model Chainage Structure Type (confluence = 0m) Waterloo Lake Outfall Weir Round nosed weir 9055 Wetherby Road Culvert 8840 Easterly Road Culvert 7973 Grange Park Crescent Footbridge Bridge 7596 Foxwood Footbridge Culvert 7354 Brooklands Footbridge Bridge 6804 South Parkway Approach Culvert 6473 Foundry Lane Culvert 6240 Pembroke Grange Footbridge Culvert 6141 Denbigh Heights Footbridge Bridge 5492 A64 Culvert 4990 Railway Embankment Culvert 4830 Dunhill Rise Culvert 4617 Ancient Footbridge Bridge 4512 Selby Road (A63) Culvert 4401 Cartmell Drive Bridge 4285 Neville Road Culvert 3842 Coronation Parade Footbridge Bridge 3241 Halton Moor Road Culvert 2976 Pontefract Lane Culvert 2327 M1 Culvert 936 Knowsthorpe Lane Bridge 687 Skelton Grange Culvert Culvert 212 9T1287/R/303348/PBor Final Report - 3 - 2PndP June 2008 2.2.6 Waterloo Lake Waterloo Lake is the source of Wyke Beck. In 1991 Leeds City Council modified the outfall from Waterloo Lake due to concerns over seepage and the stability of the southern embankment from the old outfall. The old outfall discharged over the south facing embankment, and comprised a 6.5m wide flat crested concrete overflow weir with a secondary overflow weir 2.55m wide for high flow events in a similar location to the new weir. There was also a 450mm diameter pipe (79.01mAOD invert level) approximately 3.2m below the weir; this was a draw down pipe used for draining the lake for maintenance and repair. The new discharge weir is a 22m long spill weir and discharges from the south-eastern corner of the lake. The new weir was designed to be longer than the old weir in order to accommodate the peak maximum flow; the old geometry of the weir was under capacity. The new weir (82.42mAOD) is approximately 190mm higher than the original weir level (82.23mAOD). As both the old and new outfall weirs would discharge under normal conditions the level of Waterloo Lake has not significantly changed. The modifications to the outfall of Waterloo Lake have not significantly altered the amount of flood storage which the lake provides. Following the modifications to the outfall weir the lake was refilled in stages. However in the 1998 minor subsidence along the crest of the embankment necessitated the lake to be drawn down again while repairs were carried out. A clay liner was applied to the inside face of the middle third of the embankment and a layer of rock revetment with a bitumen coating was constructed. The lake was then restored to its normal level. In order to ascertain whether any changes at Waterloo Lake would have an effect as far downstream as the Dunhill Estate a sensitivity run of the hydraulic model was carried out. The sensitivity run eliminated the upstream part of the catchment from Wetherby Road culvert upstream, including Waterloo Lake, Upper Lake and the four inflows into the lakes. A nominal flow (2m3/s constant flow-time boundary hydrograph) was introduced at the new upstream extent to replace the upstream portion of the catchment in order for the model to function correctly. The results of the sensitivity test show that Waterloo Lake does not have a significant impact on water levels as far downstream as the Dunhill Estate. The inflows into the Upper and Waterloo Lakes only provide approximately 13% of the total inflows into Wyke Beck. Comparative hydrographs were extracted at chainage 4617m (at Dunhill Rise Road Bridge), Figure 2.6, and 7870m (at Easterly Road), Figure 2.7 below. The hydrographs at Dunhill Rise show that the flows from the catchment upstream of Wetherby Road only accounts for an additional 0.8m3/s on the peak of flow at the 100 year return period and only 0.1m3/s at the 5 year return period. However further upstream at Easterly Road the contribution from the flows from Waterloo Lake are more significant, accounting for an additional 4.5m3/s on the peak of flow at the 100 year return period and 1.35m3/s at the 5 year return period. 9T1287/R/303348/PBor Final Report - 10 - 2PndP June 2008 Figure 2.6 Comparative hydrographs at Dunhill Rise Road Bridge (chainage 4617m) for the 1 in 100 year and 1 in 5 year events showing the effect of removing the flow from the catchment upstream of Wetherby Road, including Waterloo Lake. 9T1287/R/303348/PBor Final Report - 11 - 2PndP June 2008 Figure 2.7 Comparative hydrographs at Easterly Road (chainage 7870m) for the 1 in 100 year and 1 in 5 year events showing the effect of removing the flow from the catchment upstream of Wetherby Road, including Waterloo Lake. 9T1287/R/303348/PBor Final Report - 12 - 2PndP June 2008 3 EXISTING MODEL REVIEW 3.1 Hydrological Input Parameters The methodology and assumptions of the hydrological input parameters for the Floodplain Mapping Phase 2 Study ISIS model have been reviewed.