Presented by

George Doubleday1

•What is The Woodlands

•Purpose of this Research

•Build and Calibrate VfloTM model for The Woodlands

•Compare storms with different development scenarios

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Master planned community north of Houston, TX Idealized by George Mitchell

Minimize impact of urbanization on hydrology

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Founded in 1974

Over 97,000 residents

today

160 miles hike and bike

trails

6,000 acres of green

space

Source: www.thewoodlands.com

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“LID enhances our ability to protect surface and ground water quality, maintain the integrity of aquatic living resources and ecosystems, and preserve the physical integrity of receiving streams.” Department of Natural Resources – Maryland

Utilize stormwater control practices throughout an urban watershed

Aim to preserve the pre-development hydrology while allowing for beneficial human use

“LID is a comprehensive technology-based approach to managing urban stormwater” DNR - Maryland

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Use hydrologic model, VfloTM, to demonstrate The Woodlands as a premier example for stormwater management

Compare stormwater outflows from different development scenarios Undeveloped 2006 Development Highly Urbanized

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GIS Pre-processing

Input data into VfloTM

Calibrate to observed data

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Fully distributed physics-based hydrologic model Variability of runoff controlling factors Soil properties, land use/cover, elevation data

Infiltration calculated by Green and Ampt Equation

Runoff is routed from grid cell to grid cell Finite Elements method Kinematic Wave Analogy

Rainfall input can be rain gage data or radar rainfall

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Developed by Vieux, Inc.

Land Use

Soils

DEM

VfloTM

Model 9

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Overland

Direction

Channel

Flow Characteristic

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Infiltration Excess

Calculated at Each Cell Rainfall Rate

Infiltration Rate

Runon From Upslope

Whooo!!

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Created storage-elevation relationships with GIS

Rating curves developed with spillway equation(3/2)H 2gCb

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2=Q

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Ordered NEXRAD-Radar Rainfall data April 2009 event and Hurricane Ike Calibrated model at Sawdust Rd. (USGS

gage 08068450)

Calibration adjustments: channel cell and overland cell roughness▪ Mostly effected timing and peak flows

Initial saturation Hydraulic conductivity

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4% volume difference

3% peak flow difference

2.3 hr time to peak difference

8,000 cfs peak

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16% volume difference

37% peak flow difference

0 hr time to peak difference

3500 cfs peak

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Even with no losses the peak flow is under-predicted

Conclude that the Radar Rainfall does not accurately represent entire event

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Five historical storm events

1979 USEPA study on The Woodlands

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1983 USGS LandSat image to verify pre-development land use type

Manning’s roughness value for forest and woody wetland vegetation Initially set n = 0.094

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Undeveloped model matches observed historical flows

Channel n = 0.032Overland n = 0.066

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2006 development causes about 100-200 cfs increase in peak flow

Some shift in timing

Design of The Woodlands effectively mitigates the impact of development on hydrology

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Existing Cypress Creek VfloTM model used to average roughness and impervious values for Houston highly urbanized areas n = 0.024 Imperviousness = 27%

Added roughness of 0.015 for all channel cells to represent a hypothetical concrete channel network

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Replace with this…

Channel n = 0.015 Overland n = 0.024

Drastic change in timing and peak flow

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Peak Flow Comparisons (cfs)

 Undeveloped

2006 Development

(% diff.)

Intense Development

(% diff.)

10/28/1974 375 573 (53%) 1543 (312%)

11/10/1974 825 952 (15%) 3301 (300%)

11/24/1974 755 807 (7%) 2102 (178%)

12/10/1974 500 491 (2%)  -

 Hurricane Ike 6112 8119 (33%) 17017 (178%)

Avg. Diff.  - 22% 242%

Time to Peak Comparisons (hrs)

 Undeveloped

2006 Development

(∆ hrs)

Intense Development

(∆ hrs)

10/28/1974 20 14.67 (5) 7.5 (12.5)

11/10/1974 15 5.0 (10) 2.8 (12)

11/24/1974 26 13.3 (13) 5.5 (21)

12/10/1974 27.3 5.33 (22)  -

 Hurricane Ike 16.8 5.66 (11) 1.5 (15)

Avg. Diff.  - 12.5 hrs 15.1 hrs

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Can VfloTM accurately predict stormwater runoff from small sub-areas within the watershed? Select small sub-areas within the

calibrated model of The Woodlands ▪ 10 – 200 acres in size

Compare to peak flows calculated from the Rational Method

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