Penny Coombes Sarah Wharton Gary Davies Simon White River Bee, Desing FLOOD ALLEVIATION FEASIBILITY

Location Cardiff Desing

Introduction Existing Situation –Hydrological Data –River Model –Damage Assessment Flood Alleviation Proposals –Off-line Storage –On-line options –Economic Appraisal

Hydrology Determine relationship between water level and flow Predict peak discharges at various return periods Provide inflow data during storm periods

Determine relationship Existing broad-crested weir Rating data

Determine relationship Weir equation

Maxima Data Maximum river level of each year for previous 25 years Use equation to calculate flows Use statistical analysis to calculate corresponding return periods

Relationship between peak flow and return period -catchment characteristics -maxima data Region curves using mean annual flood from: Extreme value distribution Synthetic hydrographs

Extreme Value Distribution Linear scale in the form of: Where u and a are statistical functions based on the maxima data y is a function of the return period

Region curves Maxima data Mean annual flood = 8.12 m 3 /s Relationship between mean annual flood and floods of various return periods

Synthetic hydrographs No runoff or rainfall data Use catchment characteristics to calculate a synthetic hydrograph Hydrograph-variation of flow with time

Estimated Maximum Flood Not the impossible flood Very small probability of being exceeded Time to peak reduced by third Snowmelt added but not ground-frost

10,000 Year Flood Based on region curves Estimated to be 10 times the mean annual flood

Cross-sectional data entered 5000m long reach modelled Data for 2 culverts entered Model calibrated using 1990 storm Creating the HEC-RAS model 5000m 3000m 1000m 0m Reach Plan Culvert 1 Culvert 2 Channel cross-section Culvert 1 (downstream) Culvert 2 (upstream)

Trial model

Calibrated model

Running the storm events

Net Present Value Interest Rate - 6% Time period - 60 years Annuity rate

Preliminary Damage Estimate Method 1 Averaging previous flood damages NPV = £5.17 million Method 2 Using Depth/Damage relationship from previous flood events NPV = £6.43 million

Depth/Damage Model Created as spreadsheet Estimates Direct, Tangible Damages Based on depth/damage graphs Calculates damage every 50m 2 sites of 100m x 2km –Residential –Business and Retail

Residential Layout 320 Terrace Houses (6.25m x 20m) 300 Semi- detached Houses (10m x 20m) 40 Detached Houses (15m x 20m)

Business/Retail Layout 4 Offices (25m x 100m) Clothing Store (50m x 160m) Electrical Store (20m x 250m) Household Store (20m x 400m) Supermarket (50m x 180m)

NPV - Direct Damages NPV = £2.91 million

Off-Site Storage Purpose - attenuate river flows Requirements Predominate Equation INFLOW - OUTFLOW = STORAGE Outlet Devices Energy Dissipation

Requirements Dam must not be overtopped by PMF/10,000 year flood Dam will stop all flooding at 50 year return period Water level behind reservoir must not reach 58.0 m AOD

Equation Terms INPUT - OUTPUT = STORAGE INPUT –Hydrograph data –Return periods: 2,5,10,25,50,100,200,500,10 000, PMF STORAGE –Contours –Areas –Volumes

OUTPUT Bankfull Discharge Device 1 –Radial Gate –5.2 m3/s –4 m x 0.235m Device 2 –Weir and spillway –For additional discharge –30m wide Radial ‘Tainter’ Gate ‘Ogee’ Spillway Inflow, Storage and Outflow from the Storage Structure

Off-site Storage Dam 100m wide Slope of 1 in m breadth Safety Fencing Up-stream Rip-rap protection Down-stream grass protection Plan of the storage structure

Energy Dissipation Create a hydraulic jump Convert super-critical flow to sub-critical flow Abrupt drop in level of stilling basin Dependant on Froude Number, water depth and step height Abrupt Drop Energy Dissipator

Existing water levels

Widening the culverts 2.5m 3m

Widening the culverts 3.5m 3m

Widening the culverts 3.5m

Widening the culverts 3.5m 6.75m

Widening the culverts Gabions

Without culverts widened

With fully widened culverts

Flood wall 1m No dam 1 in 200 year

Flood wall 1m No dam 1 in 500 year

Flood wall 1.2m No dam 1 in 500 year

Flood wall 1m With dam 1 in 500 year

Flood wall 0.5m With dam 1 in 200 year

Economic Appraisal 7 different alleviation schemes Total cost = Construction Costs + Residual Damages Do-nothing option = £2.91m Cost/benefit ratios calculated

Scheme A Widening the Culverts Damages prevented < 1 in 2 year flood Total Cost = £1.7m Cost/benefit = 0.77

Scheme B Widening the Culverts and 1m Flood Wall Damages prevented < 1 in 500 year flood Total Cost = £2.13m Cost/benefit = 0.96

Scheme C Widening the Culverts and 1.2m Flood Wall Damages prevented  1 in 500 year flood Total Cost = £2.42m Cost/benefit = 1.1

Scheme D Dam and Storage Reservoir Damages prevented  1 in 50 year flood Total Cost = £1.3m Cost/benefit = 0.59

Scheme E Dam and Widening the Culverts Damages prevented  1 in 50 year flood Total Cost = £1.76m Cost/benefit = 0.8

Scheme F Dam, Widening the Culverts and 1m Flood Wall Damages prevented  1 in 500 year flood Total Cost = £3m Cost/benefit = 1.37

Scheme G Dam, Widening the Culverts and 0.5m Flood Wall Damages prevented < 1 in 500 year flood Total Cost = £2.4m Cost/benefit = 1.1

Comparison of Schemes

Warning Systems 4 hour warning £2.55m Damages =12% reduction Combined with Scheme E: –Construction Cost £1.57m –Residual Damages £170,000 –Total Cost£1.74m Cost/benefit = 0.6

Conclusion Construction of Dam Widening of Culverts Establish 4 hour warning system Total Cost = £1.74m Cost effective Environmentally sound