Floodplain Management Strategies in an

Urbanized Central Texas

Topics Covered

• Storm Characteristics• Basic Runoff Principles• Detention Ponds (Regional vs. Onsite)• Channel Improvements• Considerations for Selection of

Improvements

The Hydrologic Cycle

LAKES & OCEANS

EVAPORATION

PRECIPITATION

TRANSPIRATION

GROUND WATER

INFILTRATION

LAKES & OCEANS

EVAPORATION

PRECIPITATION

TRANSPIRATION

GROUND WATER

INFILTRATION

Rainfall Measurements•Total Depth

•Intensity = Depth / Time

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

Runoff

Infiltration

TotalRainfall

Drainage

Area

Basic Runoff Principles•Depth = Precipitation – Infiltration

•Volume = Runoff Depth x Area

Return Frequency

Based on Probability

Frequency = 1 / Probability

1% Probability?

Frequency = 1 / .01

= 100 year event

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.)

Factors Affecting Runoff DepthsRainfall Intensity & Duration

Soil Type

Antecedent Moisture

Cover & Vegetation

Ponding (swamps)

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

ThunderstormsCommon

Day Long Drizzle Common

WARM FRONT

CoolCOLD

FRONT

Air Forced to Rise Abruptly

High IntensityShort Duration

Low IntensityLong Duration

Warm

Storm Variations

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

)

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

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

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

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.

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

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

Detention Ponds

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

RAIN EVENT

Detention Pond Cycle

POND FILLS AS METERED STRUCTURE DETAINS RUNOFF

RAIN EVENT ENDS

POND EMPTIES AS INCOMING RUNOFF

ENDS

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

Outflow

Pond Crests

StorageInflow

Time (hrs)

Q (c

fs)

Inflow / Outflow Hydrographs

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)

Impacts of Detention

Upstream Flooding

System Immediately Downstream

Regional Watershed

Maintenance Costs

Water Quality

Attracts Development

May not address increase in total runoff volume

RegionalOn-Site

Watershed Outfall Watershed Outfall

Detention Regional

Detention On-Site

Regional vs. On-Site Detention

Regional will have greater impact on downstream outlet

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

REGIONAL DETENTION POND IN BELL COUNTY

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

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?

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

Runoff Volume

Runoff

Infiltration

TotalRainfall

(Intergrated Area)

Drainage

Area

Time

Flow

Rat

eRetention Ponds

Channel Improvements

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

EXAMPLE OF THE BEST SOLUTION BEING NO DETENTION: THOMPSON ADDITION DRAINAGE CHANNEL, TEMPLE, TX

Considerations for Selection of Improvements

Who Will Provide Maintenance?

Owner?

City?

County?

State?

Who Will Inspect?

#1 Priority for maintenanceis a good inspection planwith adequate staffing.

Ponds must be inspectedon a scheduled basis.

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

FLOODPLAIN MANAGEMENTEXAMPLE 1:

EFFECTS OF NEW DEVELOPMENT ON DRAINAGE

PATTERNS

EXISTING DEVELOPMENT

EXISTING DEVELOPMENT

TRIBUTARY

MAJOR STREAM/ CREEK/RIVER

UNDEVELOPED

LAND

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

EXISTING DEVELOPMENT

EXISTING DEVELOPMENT

TRIBUTARY

MAJOR STREAM/ CREEK/RIVER

NEW DEVELOPMENT

NEW DEVELOPMENT

INCREASE IMPERVIOUS COVER

DECREASE TIME OF CONCENTRATION

INCREASE PEAK STORM FLOW

INCREASE CHANNELIZATION OF STORM FLOW

FLOODPLAIN MANAGEMENT

EXAMPLE 2:

CONSIDERATION OF DOWNSTREAM CONVEYANCES

VIEW “A” DETENTION POND No.1 (4.3 Acres)

VIEW ”B” DRAINAGE PATH THROUGH DOWNSTREAM PROPERTY

100 YEAR FLOOD PLAIN

VIEW “A”

VIEW “B”

TOPOGRAPHIC VIEW WITH 100 YEAR FLOOD PLAIN

EXISTING DEVELOPMENT

EXISTING DEVELOPMENT

DETENTION POND

NEW DEVELOPMENT

DELAYS PEAK FLOW FROM DOWNSTREAM DEVELOPMENT

CREATES POSSIBILITY OF COMBINING PEAK FLOWS FROM TWO WATERSHEDS

EXISTING DEVELOPMENT

EXISTING DEVELOPMENT

REGIONAL DETENTION POND

NEW DEVELOPMENT

DETAINS STORM FLOW FOR MULTIPLE DEVELOPMENTS

CREATES A SINGLE PEAK FLOW OUT OF POND

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

Questions?