penny coombes sarah wharton gary davies simon white river bee, desing flood alleviation feasibility
TRANSCRIPT
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