Alameda County Hydrology & Hydraulics Manual · Guidelines for hydrology and hydraulics •...
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Alameda County Hydrology & Hydraulics Manual Rohin Saleh, MS. PE. Supervising Civil Engineer, ACFCD
Transcript of Alameda County Hydrology & Hydraulics Manual · Guidelines for hydrology and hydraulics •...
Alameda County
Hydrology & Hydraulics ManualRohin Saleh, MS. PE. Supervising Civil Engineer, ACFCD
Alameda County Flood Control District
Established in 1949
Use the updated criteria for:
• Any project that drains to a District facility or is subject to District approval
• Modifying existing flood control infrastructure or designing new District facilities
• Drainage Studies and new developments within District watersheds
• ComprehensiveGuidelines for hydrology and hydraulics
• Regionally CalibratedUse of local and regional data
• AdaptableTo areas with little or no stream flow data
New H&H is:
From rainfall to runoff
• Answers basic challenge in hydrologic engineering
• Design discharges and FEMA floodplain maps are all based on runoff frequencies
Rainfall Event
Soil AMCInfiltration Rate
ImperviousArea
Other Factors
Runoff
Reliable Source for Design and Flood Plain Evaluations
Most available methodologies are based on hydrology text books or the “shopping card versions”
Alameda County methodology links rainfall statistics to runoff statistics
Unknown Level of Reliability
Know Level of Reliability based on measured data
Effects of commonly estimated Kinematic Wave ParameterPercent Difference from Measured Peaks
Why we need better estimates of Runoff Statistic :If you can design a drainage pipe with a velocity of 12 FPS you will need:
• 12” RCP for every 10 CFS
• 18” RCP for every 20 CFS
• 24” RCP for every 40 CFS
• 48” RCP for every 150 CFS
Why we need better estimates of Runoff Statistic :• Substantial impact to project cost
particularly if its used for improvement of existing facilities
• We need to distinguish between evaluation and improvement of an existing facility vs. adding a totally new drainage facility for the first time
Why we need better estimates of Runoff Statistic :• Flood Plain impacts (Unnecessary Flood
Insurance)
• Increase liability if you are evaluating an existing facility
• Increase risk by wasting public fund in a wrong location and ruining out of money where you truly need projects.
• Wrong runoff estimates can be obvious
Research & Development for Production of the H&H Manual
Rainfall Frequency forMAP “N” & Duration “X”
RainfallDepth
(inches)
1 100101 10010
Discharge(cfs)
Discharge “Q” at thePoint of Interest
Infiltration RateInfiltration
Rate(in/hr)
Initial Infiltration
Rate
UniformInfiltration
Rate
Time
Land Use
Unit Hydrograph Parameters
Q/Qp
U.H. for Steep CatchmentTime to Peak
(lag)
Time
U.H. for Average Catchment
U.H. for Flat Catchment
Design Storm
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hou
rly P
reci
pita
tion
(inch
es)
Elapsed Time (hours)
From Rainfall Statistic to Runoff Statistic• Infiltration:
Initial (AMC): StatisticalUniform: Physical
• Land Use & Effective Impervious Areas: Physical
• Hydrologic Soil Group: Physical
• Unit Hydrograph or Kinematic Wave: Physical
• Design Storm: Statistical
Active rain gauges & flow monitoring stations
60 Stream or flow stations
50 Recording rain gauge stations
150 Daily rainfall observers
1994: Initiation of calibratedhydrologymodels and methods
• Open channels• Closed conduits• Floodplains
Comprehensive studies of entire network
Floodplains
• Accurate representation of floodplain boundaries
• Impact to the community
High quality data from select small urban catchments in Alameda County and City of San Jose
Access to quality data sets
Detailed land usedata
• Building rooftops
• Driveways & sidewalks
• Roads
• Yards & greenways
Detailed land usedata
Calculation of Effective Impervious Area
• Significant impact on accuracy of runoff calculations
• Guidance on managing new low-impact development
Soils
• NRCS data
• Custom soil testing
• Reclassification of hydrologic soil groups
Time of concentrationand basin lag
Storm events
Production of rainfall spatial distributions and maps for hundreds of calibration events
Storm events calibration
Large and small storm events used for calibrations of various parameters
15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:0016Dec1914
Flo
w (
cfs)
0
2
4
6
8
10
12
14
16
18
20
CAL 01 RC OBSERVED FLOW CAL01 RC CALC FLOW
22:00 00:00 02:00 04:00 06:00 08:00 10:00 12:0014Dec1914 15Dec1914
Flo
w (
cfs)
0
1
2
3
4
5
6
7
8
CAL 01 RC OBSERVED FLOW CAL01 RC CALC FLOW
06:00 09:00 12:00 15:00 18:00 21:00 00:00 03:00 06:0011Dec1914 12Dec1914
Flo
w (
cfs)
0
5
10
15
20
25
30
35
40
45
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:0031Mar1914
Flo
w (
cfs)
0
2
4
6
8
10
12
14
16
18
20
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
00:00 03:00 06:00 09:00 12:00 15:0028Feb1914
Flo
w (
cfs)
0
5
10
15
20
25
30
35
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
14:00 16:00 18:00 20:00 22:00 00:00 02:0026Feb1914 27Feb1914
Flo
w (
cfs)
0
5
10
15
20
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
00:00 02:00 04:00 06:00 08:00 10:00 12:0006Feb1914
Flo
w (
cfs)
0
2
4
6
8
10
12
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30 17:0020Nov1913
Flo
w (
cfs)
0
2
4
6
8
10
12
14
CAL01 RC OBS FLOW CAL01 RC CALC FLOW
Data gathering & review
Calculated, calibrated, land use variables
Model results: validation
Review of model results for complex large watersheds with multiple stream gauges and long-term flow data
Model results: statistical comparison
Discharge frequency analysis
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Alameda Creek at Niles (11179000)
95% Low Bull 17B 95% Up Model
Model results: validation
Review of model results for complex large watersheds with multiple stream gauges and long-term flow data
Model results & validation
• Comparison of the model results with statistical gauge analysis (Log Pearson-Type III) where long period of record was available.
• Model result validation on consecutive gauges with long term data unaffected by man made operations
100
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Arroyo las Positas near Livermore (11176145)
95% Low Bull 17B 95% Up Model
100
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Arroyo Mocho near Livermore (11176000)
95% Low Bull 17B 95% Up Model
100
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Arroyo Valle below Lang Canyon near Livermore (11176400)
95% Low Bull 17B 95% Up Model
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Arroyo Hondo near San Jose (11173200)
95% Low Bull 17B 95% Up Model
100
1,000
10,000
100,000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Arroyo de la Laguna at Verona (11176900)
95% Low Bull 17B 95% Up Model
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Alameda Creek at Niles (11179000)
95% Low Bull 17B 95% Up Model
1000
10000
100000
1 10 100
Disc
harg
e (C
FS)
Return Period (YR)
Coyote Creek Upstream of Coyote Reservoir
95% Low Bull 17B 95% Up Model
Outcome with and without the calibrated model
• Improved floodplain accuracy
• Cost benefitsBEFORE AFTER
Main Components of H&H Manual
INSERT nice photo of Ala Co stream
Impervious percentage calibrated and more accurate
Clear definition of required levels of service and how to apply them
Refined hydraulic equations for junction losses
Refined synthetic hydrograph calculations
Detention pond criteria added
New mean annual precipitation map
New rainfall equations: intensity and depth
New runoff coefficientvalues per soil classification
6-hour and 24-hour design storm better defined
Tidal data based on latest FEMA studies
