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

4. Location Cardiff Desing

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

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

8. Determine relationship Existing broad-crested weir Rating data

9. Determine relationship Weir equation

11. 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

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

13. 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

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

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

20. 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

21. 10,000 Year Flood Based on region curves Estimated to be 10 times the mean annual flood

24. 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)

25. Trial model

27. Calibrated model

28. Running the storm events

37. Net Present Value Interest Rate - 6% Time period - 60 years Annuity rate - 16.16

38. 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

39. 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

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

41. 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)

42. NPV - Direct Damages NPV = £2.91 million

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

45. 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

46. 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

47. 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

48. Off-site Storage Dam 100m wide Slope of 1 in 2.5 29m breadth Safety Fencing Up-stream Rip-rap protection Down-stream grass protection Plan of the storage structure

49. 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

51. Existing water levels

52. Widening the culverts 2.5m 3m

53. Widening the culverts 3.5m 3m

54. Widening the culverts 3.5m

55. Widening the culverts 3.5m 6.75m

56. Widening the culverts Gabions

57. Without culverts widened

58. With fully widened culverts

59. Flood wall 1m No dam 1 in 200 year

60. Flood wall 1m No dam 1 in 500 year

61. Flood wall 1.2m No dam 1 in 500 year

62. Flood wall 1m With dam 1 in 500 year

63. Flood wall 0.5m With dam 1 in 200 year

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

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

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

68. 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

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

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

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

72. 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

73. Comparison of Schemes

74. 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

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