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 datah Q m m3/s

0.108 0.2210.109 0.2140.188 0.490.268 0.8710.354 1.3850.357 1.5130.586 3.7210.597 3.64

Determine relationship

• Weir equation

5.1705.1 hCbCQ dv5.1hkCQ f

5.1hC

Qk

f

5.1165.7 hCQ f

0

0.5

1

1.5

2

2.5

3

3.5

4

0.1 0.2 0.3 0.4 0.5 0.6

h (m)

Q (m3/s)

Q given Q formula

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:

ayuQ

Where u and a are statistical functions based on the maxima data

y is a function of the return period

yQ 646.1166.7

4

5

6

7

8

9

10

11

12

13

14

-2 -1 1 2 3 4

y

Q

(m3 / s)

Q maxima data

Q = u + ay

Region curves

• Maxima data

Mean annual flood = 8.12 m3/s

Relationship between mean annual flood and floods of various return periods

7

12

17

22

27

32

0 2 4 6

y=-ln(-ln(1-1/Tr))

Q (m3 /s)

EVD: Q=u+ay

Maxima dataQbar/Q(T)

CharacteristicsQbar/Q(T)

7

9

11

13

15

17

19

21

23

0 1 2 3 4 5 6 7

y

Q (m3 / s)

Synthetic hydrographs

• No runoff or rainfall data

• Use catchment characteristics to calculate a synthetic hydrograph

• Hydrograph-variation of flow with time

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Time (hours)

Discharge

(m3/s)

2 5 10 25 50 100 200 500 1000

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

050

100150200250300

0 5 10 15 20 25 30 35 40

Time (hours)

Q

(m3/s)

synthetic hydrograph EMF adjusted EMF 10,000 year flood

• Cross-sectional data entered

• 5000m long reach modelled

• Data for 2 culverts entered

• Model calibrated using 1990 storm

Creating the HEC-RAS modelDesing

Bee

5000m

3000m

1000m

0m

Reach Plan

Culvert 1

Culvert 2

-120 -100 -80 -60 -40 -20 0 2045

46

47

48

49

50

51

design project Plan 41 Flow: Dam 1 in 500 year eventupstream end of main reach

Station (m)

Elev

atio

n (m

)

Legend

Ground

Bank Sta

Channel cross-section

-120 -100 -80 -60 -40 -20 0 2042

43

44

45

46

47

48RS=3.010 Upstream (Culvert)

Station (m)

Elev

ation

(m)

Legend

Ground

Ineff

Bank Sta

-120 -100 -80 -60 -40 -20 0 2039

40

41

42

43

44

45RS=1.010 Upstream (Culvert)

Station (m)

Elev

atio

n (m

)

Legend

Ground

Ineff

Bank Sta

Culvert 1 (downstream)

Culvert 2 (upstream)

Trial model

47.08

42.78

44.20

41.34

41.91

44.72

45.65

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 1.000 2.000 3.000 4.000 5.000 6.000

Chainage (km)

Ele

vatio

n (m

AO

D)

Series1Recorded level

Trial model

47.08

45.65

44.72

41.91

41.34

44.20

42.78

45.35

41.33

47.30

45.85

44.19

43.02

42.06

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 1.000 2.000 3.000 4.000 5.000 6.000

Chainage (km)

Ele

va

tio

n (

mA

OD

)

Recorded profile HEC-RAS predicted profile

Calibrated model

47.08

45.65

44.72

42.78

44.20

41.34

41.91

47.19

45.71

44.82

44.22

42.91

41.36

41.89

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 1.000 2.000 3.000 4.000 5.000 6.000

Chainage (km)

Ele

va

tio

n (

mA

OD

)

Recorded profile HEC-RAS predicted profile

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr 1 in 100yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr 1 in 100yr1in 200yr Bed level

Culvert 1

Culvert 2

Running the storm events

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr1 in 100yr 1in 200yr Bed level 1 in 500 yr

Culvert 1

Culvert 2

Net Present Value

• Interest Rate - 6%

• Time period - 60 years

• Annuity rate - 16.16

Preliminary Damage EstimateMethod 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

£0

£500,000

£1,000,000

£1,500,000

£2,000,000

£2,500,000

£3,000,000

£3,500,000

0 2 4 6 8 10

Y=-ln(-ln(1-1/Tr))

NP

V

NPV = £2.91 million

Off-Site Storage

• Purpose - attenuate river flows

• Requirements

• Predominate EquationINFLOW - 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

00.5

11.5

22.5

33.5

44.5

55.5

66.5

77.5

88.5

99.510

10.511

11.512

12.513

13.514

14.515

15.5

0 10 20 30 40 50 60 70

Time(hrs)

Inflow

Outflow total

MAXIMUM ALLOWABLE DISCHARGE:

5 . 2 m3/S

Inflow, Storage and Outflow from the Storage Structure

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

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

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr1 in 100yr 1in 200yr Bed level 1 in 500 yr

Culvert 1

Culvert 2

Widening the culverts

2.5m

3m

Widening the culverts

3.5m

3m

Widening the culverts

3.5m

3.5m

Widening the culverts

3.5m

6.75m

Widening the culverts

Gabions

Without culverts widened

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr1 in 100yr 1in 200yr Bed level 1 in 500 yr

Culvert 1

Culvert 2

40.00

41.00

42.00

43.00

44.00

45.00

46.00

47.00

48.00

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000

Chainage (km)

Wa

ter

su

rfa

ce

lev

el (

mA

OD

)

1 in 2yr 1 in 5yr 1 in 10yr 1 in 25yr 1 in 50yr

1 in 100yr 1in 200yr Bed level 1 in 500 y

Culvert 1

Culvert 2

With fully widened culverts

Flood wall

1m

No dam1 in 200 year

Flood wall

1m

No dam1 in 500 year

Flood wall

1.2m

No dam1 in 500 year

Flood wall

1m

With dam1 in 500 year

Flood wall

0.5m

With dam1 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

£0

£500,000

£1,000,000

£1,500,000

£2,000,000

£2,500,000

£3,000,000

£3,500,000

- A B C D E F G

To

tal

Co

st

ResidualDamages

ConstructionCosts

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