Floodplain Management Strategies in an Urbanized Central...
Transcript of Floodplain Management Strategies in an Urbanized Central...
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Floodplain Management Strategies in an
Urbanized Central Texas
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Topics Covered
• Storm Characteristics• Basic Runoff Principles• Detention Ponds (Regional vs. Onsite)• Channel Improvements• Considerations for Selection of
Improvements
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The Hydrologic Cycle
LAKES & OCEANS
EVAPORATION
PRECIPITATION
TRANSPIRATION
GROUND WATER
INFILTRATION
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LAKES & OCEANS
EVAPORATION
PRECIPITATION
TRANSPIRATION
GROUND WATER
INFILTRATION
Rainfall Measurements•Total Depth
•Intensity = Depth / Time
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0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.5 1.0 1.5 2.0 2.5
Rai
n D
epth
(in)
Time (hrs)
FillsOverTime
Rain GaugeTotal Depth
1.0 inches
2.7 inches0.2 hrs
0.9 hrs
Average intensity for a
given time slice can be
computed using the
incremental depth
for that time slice.
Gauged Rainfall
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Runoff
Infiltration
TotalRainfall
Drainage
Area
Basic Runoff Principles•Depth = Precipitation – Infiltration
•Volume = Runoff Depth x Area
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Return Frequency
Based on Probability
Frequency = 1 / Probability
1% Probability?
Frequency = 1 / .01
= 100 year event
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RATIONAL METHOD vs. SCS METHODRational Method generally used on smaller watersheds (<200 acres).
SCS Method generally used on larger watersheds.
Rational Method: Q = CiA
C = Runoff Coefficient
i = Rainfall Intensity (in / hr)
A = Drainage Area (acres)
Q = Peak Runoff Rate (cfs)
SCS Method: TR-20 , TR-55
Computer Programs
Runoff Curve Number (CN)
Rainfall Distribution Type
Drainage Area (sq. mi.)
Time of Concentration (hr.)
24-hr Rainfall (in.)
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Factors Affecting Runoff DepthsRainfall Intensity & Duration
Soil Type
Antecedent Moisture
Cover & Vegetation
Ponding (swamps)
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Runoff Hydrograph
Peak Flow
Rising Limb
Time of Flood
Falling Limb
Time to Peak
Volume(Integrated Area)
Vol. / Time
Volume
Time
Base Flow Time
Flow
Rat
e
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ThunderstormsCommon
Day Long Drizzle Common
WARM FRONT
CoolCOLD
FRONT
Air Forced to Rise Abruptly
High IntensityShort Duration
Low IntensityLong Duration
Warm
Storm Variations
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Hydrograph Variations
"Thunderstorm"Duration = 2.5 hrs
20
40
60
80
100
02 4 6 8 10 120
P = 3.1"
"Low Intensity Storm"Duration = 9.0 hrs
P = 3.1"
Hyd. Vol.9.6 acre-ft
(SCS UH Method, Area=150 acres, CN = 70, Tc = 1.0 HRS)
Hydrographs Computedfor Undeveloped Site
Time (hrs)
Flow
(cfs
)
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SCS Distribution Types(Geographic Locations)
II
II
I
I A
IIIIII
III
III
TYPE I TYPE IA TYPE II TYPE III
Bell County, TX Zone II
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0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 3 6 9 12 15 18 21 24
Time (hrs)
Rai
n Fr
actio
n
SCS Synthetic Distributions(Fixed 24-Hour Duration)
I A
III
I
II
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0.0
500.00
1000.0
7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0
Time (hrs)
Flow
(cfs
)SCS Synthetic Distributions
Effects on Hydrograph Shape
I AIIII
II
Computed Using TR-55
Tabular Hydrograph MethodCN = 75
Area = 0.05 sq. mi.
P = 5.00 inches
Tc = 0.05 hours
Tt = 0.00 hours
Equal Volume for All Three Hydrographs
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DECREASES VOLUMEDOWNSTREAM
Tc
t
QRUNOFF
LESS ROUGH
t
QRUNOFF
MORE ROUGH
t
QRUNOFF
LITTLE STORAGE
t
QRUNOFF
MORE STORAGE
t
QRUNOFF
GENTLER
t
Q
RUNOFF
STEEPER
Slope, Roughness & PondingA. Relationship of slope to peak discharge.
B. Relationship of hydraulic roughness to runoff.
C. Relationship of wetland storage to runoff.
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0
100
11 12 13 14 15
Time (hrs)
Flow
(cfs
)
20
40
60
80
Effects of CN
CN = 85
CN = 75
CN = 65
CN = 55
Computed Using SCS UH MethodP = 5.00 inches
Area = 0.05 sq. mi.
Tc = 0.05 hours
Type II 24-Hour Rainfall
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120
0
100
11 12 13 14 15
Time (hrs)
Flow
(cfs
)
20
40
60
80
Effects of Tc
Tc = 0.10 Hours
Tc = 0.50 Hours
Tc = 0.75 Hours
Tc = 1.0 Hours
Computed Using SCS UH MethodP = 3.8 inches
CN = 75
Area = 50 acres
Type II 24-Hour Rainfall
Volumes of all four hydrographs are equal
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Detention Ponds
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YIELD
= DETENTION POND
ACCESS ROADS / ON RAMPS
= TRIBUTARIESINTERSTATE HIGHWAY
= MAIN CHANNEL
THE DIFFERENCE IS THAT IN DRAINAGE, RARELY CAN WATER FIND AN EXIT RAMP!
A Highway Analogy
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RAIN EVENT
Detention Pond Cycle
POND FILLS AS METERED STRUCTURE DETAINS RUNOFF
RAIN EVENT ENDS
POND EMPTIES AS INCOMING RUNOFF
ENDS
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Basic Routing EquationVolume = [Inflow – Outflow] x Time
If Inflow > Outflow, then…-Volume is increasing
-Water surface is rising
If Inflow < Outflow, then…-Volume is decreasing
-Water surface is falling
If Inflow = Outflow, then…-Volume is constant
-Water surface is constant
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Outflow
Pond Crests
StorageInflow
Time (hrs)
Q (c
fs)
Inflow / Outflow Hydrographs
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AB
A + BA + B
Pond Outflow
(With Detention)(No Detention)
Area B FullyDeveloped
Detention Pond
A+BOutfall
AB
0.0
250.0
11.0 12.0 13.0 14.0 15.0
Time (hrs)
Flow
(cfs
)Adding Hydrographs
(Detention Effects Downstream)
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Impacts of Detention
Upstream Flooding
System Immediately Downstream
Regional Watershed
Maintenance Costs
Water Quality
Attracts Development
May not address increase in total runoff volume
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RegionalOn-Site
Watershed Outfall Watershed Outfall
Detention Regional
Detention On-Site
Regional vs. On-Site Detention
Regional will have greater impact on downstream outlet
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Regional Detention
Positives-May Attract Development
-Better for regional flow rate control
-Less maintenance sites
-Can be incorporated into golf courses & parks
Negatives-Politics associated with pond location
-Politics of funding
-May have reduced on-site quality impacts
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REGIONAL DETENTION POND IN BELL COUNTY
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On-Site DetentionPositives-Controls flooding (Immediately downstream)
-Traps sediment from site
-Fewer Politics
Negatives-Sometimes not effective for regional flood control
-More maintenance sites
-More sites will require space for detention
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When Designing A Detention Pond:1. Evaluate existing conditions:
Drainage Basin BoundariesLand UseTopography
2. Evaluate developed conditions
3. Determine required storage capacity of pond based on condition that release rate can not exceed peak flow for undeveloped condition
4. Identify potential locations for pond
5. Evaluate down steam conveyance features:Is there adequate capacity for storm flow generated by current conditions?Are there online ponds or dams downstream that could be affected?
Can roads and/or buildings be flooded by the concentration of storm flow?
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Compute Inflow Hydrograph(s)
Estimate Pond Storage
Develop Grading PlanCompute Design Volumes
Select Trial Outlet Structure,Compute Rating Curve
Route Inflow Through Pond
ResultsAcceptable?
Structural DesignDetails of Outlet
Outlet RatingCurve Meet
Target Values?
No
Yes
No
Yes
Steps for Detention Design
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Runoff Volume
Runoff
Infiltration
TotalRainfall
(Intergrated Area)
Drainage
Area
Time
Flow
Rat
eRetention Ponds
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Channel Improvements
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Channel Improvements
• Cleaning and / or widening a channel to contain the 1% annual chance storm.
• Clearly defines the floodplain.• Can potentially reclaim property from
floodplain.• Does not hold back peak flows.• Best suited for lower third of watershed
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EXAMPLE OF THE BEST SOLUTION BEING NO DETENTION: THOMPSON ADDITION DRAINAGE CHANNEL, TEMPLE, TX
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Considerations for Selection of Improvements
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Who Will Provide Maintenance?
Owner?
City?
County?
State?
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Who Will Inspect?
#1 Priority for maintenanceis a good inspection planwith adequate staffing.
Ponds must be inspectedon a scheduled basis.
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Good Design PracticesDesign for easy maintenance (and easy access)
Provide pond bottom drainage
Stabilize trickle channels
Allow for sediment buildup
Improve water quality
Account for impacts immediately upstream & downstream
Account for regional impacts
Design structures that have low (or dissipated) outlet velocities
Guard against overtopping
Check design with multiple frequency storms
Check design with multiple storm durations / distributions
Assess impacts of increased runoff volume
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FLOODPLAIN MANAGEMENTEXAMPLE 1:
EFFECTS OF NEW DEVELOPMENT ON DRAINAGE
PATTERNS
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
TRIBUTARY
MAJOR STREAM/ CREEK/RIVER
UNDEVELOPED
LAND
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
TRIBUTARY
RECEIVING STREAM/ CREEK/RIVER
UNDEVELOPED
LANDSTORM FLOW SPREADS OUT OVER PERVIOUS, UNDEVELOPED LAND
RESULTS IN HIGHER TIME OF CONCENTRATION AND LOWER PEAK FLOW
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
TRIBUTARY
MAJOR STREAM/ CREEK/RIVER
NEW DEVELOPMENT
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NEW DEVELOPMENT
INCREASE IMPERVIOUS COVER
DECREASE TIME OF CONCENTRATION
INCREASE PEAK STORM FLOW
INCREASE CHANNELIZATION OF STORM FLOW
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FLOODPLAIN MANAGEMENT
EXAMPLE 2:
CONSIDERATION OF DOWNSTREAM CONVEYANCES
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VIEW “A” DETENTION POND No.1 (4.3 Acres)
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VIEW ”B” DRAINAGE PATH THROUGH DOWNSTREAM PROPERTY
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100 YEAR FLOOD PLAIN
VIEW “A”
VIEW “B”
TOPOGRAPHIC VIEW WITH 100 YEAR FLOOD PLAIN
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
DETENTION POND
NEW DEVELOPMENT
DELAYS PEAK FLOW FROM DOWNSTREAM DEVELOPMENT
CREATES POSSIBILITY OF COMBINING PEAK FLOWS FROM TWO WATERSHEDS
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
REGIONAL DETENTION POND
NEW DEVELOPMENT
DETAINS STORM FLOW FOR MULTIPLE DEVELOPMENTS
CREATES A SINGLE PEAK FLOW OUT OF POND
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EXISTING DEVELOPMENT
EXISTING DEVELOPMENT
TRIBUTARY
MAJOR STREAM/ CREEK/RIVER
NEW DEVELOPMENT
EXISTING DETENTION
POND
DELAYS AND REDUCES PEAK FLOWS
DIRECT DISCHARGE WITHOUT DETENTION ALLOWS PEAK FLOW TO PASS BEFORE PEAK FROM UPSTREAM ARRIVES
DOWNSTREAM CAPACITY MUST BE EVALUATED
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Questions?