NEXRAD Radar for Hydrologic Modeling and Prediction in Houston

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NEXRAD Radar for Hydrologic Modeling and
Prediction in Houston

• Dr. Philip Bedient
• Rice University
• Houston, Texas

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Goals of Flood ALERT System

• Increase lead time for flood warning
• Provide accurate spatial and temporal rainfall
  estimates
• Provide a correlation between rainfall over
  watershed and flow rate in bayou
• Provide frequent updates via a user-friendly home
  page on the web
• Fast and easy-to-operate

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Brays Bayou Watershed, Houston, TX




















Harris Gully drains Rice/TMC Area
North
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Brays Bayou near the TMC
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Brays Bayou at Loop 610 in 1983
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Street Flooding in 1983
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Flood Prediction Rice/TMC Flood Alert System
(FAS)

• 1. Radar
• 2. Storm
• 3. Rainfall
• 4. Hyetograph
• 5. HEC-HMS
• 6. Hydrograph
• 7. ALERT

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Rice/TMC Flood Alert System www.floodalert.org
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Harris Gully drains the TMC
Low Flow Conditions During Tropical Storm
Frances
Peak Elevation 38.6 ft
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Brays BayouFederal Flood Control Project
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Brays BayouFederal Flood Control
ProjectEstimated Damage Reduction (when complete)
Meyerland
TOTAL DAMAGE REDUCTION 56.8 MILLION/YEAR
Rice/TMC/Zoo
DAMAGE ( MILLIONS)
TOTAL REMAINING DAMAGES 15.7 MILLION/YEAR
Astrodome
ECONOMIC REACH NUMBER
DAMAGES REDUCED
TOTAL REMAINING DAMAGES
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Greatest Needs

• Rice University and TMC flood-prone
• Need unique solutions for complex issue
• Combination storage and pipe diversions
• Expand NEXRAD to other watersheds
• Create customized hydrologic predictions
• Deliver on WEB for local communities

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Development of the FAS

• Three interrelated projects
• Calibration of NEXRAD to measured rainfalls and
  integration into HEC-1
• Creation of Flow Nomograph relating rainfall
  intensity and duration to flow in the bayou
• Creation of Links and Images over Brays Bayou,
  and animations for the web site
• System tested on Tropical Storm Frances

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Texas Medical Center Background

• Adjacent to Brays Bayou
• Major flooding in 1983 and 1976
• 42 member institutions including 14 hospitals, 2
  medical schools
• Approximately 25 million per day operating
  budget
• 4.5 million patient visits per year

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Brays Bayou T.S. Frances
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Backwater Effects

• Flooding in subwatershed influenced by backwater
  in Brays Bayou
• Integrated HEC-1 model for TMCs subwatershed
  with pipe flow model (StormCAD)
• Calibrated with three smaller storms
• Predicts overland flow based on water level in
  bayou and given rainfall

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Harris Gully Profile
Highest Nearby Ground Elevation
Tailwater 43.14 ft
Harris Gully Box Culverts
Flow 398 cfs
L 3.4 mi
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Brays Bayou Background

• Over 90 urbanized
• Responds quickly ( 6 hours) to small rainfall
  events
• Mild slopes ( 3 ft per mile)
• Overbanks near the Texas Medical Center (TMC)
  during a 5 to 10 year rainfall event
• Last major rainfall event 1983
• Close to flooding 1994, 1997, 1998

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Design Rainfalls for Houston (inches)

• Storm 6-Hour 24-Hour
• 5 year 4.8 6.8
• 10 year 5.7 8.2
• 25 year 6.7 9.5
• 100 year 8.5 12.5

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Brays Bayou Flood Response
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NEXRAD Rainfall Calibration Objectives

• Calibrate NEXRAD rainfall distribution to
  measured gauge rainfall.
• Integrate the Hydrologic Model (HEC-1) with
  NEXRAD rainfall for Brays Bayou and calibrate to
  measured historical flows.
• Develop a flood prediction method using variable
  rainfall input with the calibrated model.
• Test system with one or more historical storms

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NEXRAD Rainfall Estimates

• NEXRAD provides real-time data on a 16 km2 (6
  mi2) grid
• Equivalent to about 21 rain gages in Brays Bayou
  watershed
• Each estimate represents an average rainfall
  amount over the entire 16 km2 area
• NEXRAD rainfall estimates compare well with point
  rain gage measurements (r2 0.9)

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Converting Reflectivity to Useable Data

• Z-R Relationships
• Z 300R1.4 standard
• Z 250R1.2 Houston/Galveston area (1994)
• Calibration of Radar data
• Correlate radar to gauge data
• Adjust radar data accordingly

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RadarGage Correlation October 17, 1994
Sims Bayou
Total Rainfall Radar (in.)
Brays Bayou
Total Rainfall measured at the Gage (in.)
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RadarGage Correlation October 18,1994
Sims Bayou
Total Rainfall Radar (in.)
Brays Bayou
Total Rainfall measured at the Gage (in.)
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October, 1994 Calibration
Cumulative Rainfall (in.)
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October, 1994 Calibration
Cumulative Rainfall (in.)
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HEC-1 Model

• U.S. Army Corps of Engineers hydrologic model -
  recently updated to HEC-HMS
• Converts rainfall over a basin area into a runoff
  hydrograph at outlet
• Can be used for flood prediction and analysis
• Inputs include watershed size, length, slope,
  land cover, soils, rainfall intensity.

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HEC-1 Model Configuration
2
1
1
2
x
x
A
Route
3
4
3
y
Reach A
4
y
B
Route
5
5
Reach B
z
z
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HEC-1 Results October 1994 Storm
Flow (ft3/s)
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HEC-1 Results January 1998 Storm
Flow (ft3/s)
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HEC-1 Results April 1997 Storm

Flow (ft3/s)
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NEXRAD Calibration Results

• NEXRAD captured rainfall pattern in space and
  time over watershed
• The NEXRAD Data is useful for calibrating HEC-1
• HEC-1 generated hydrographs closely resemble
  measured hydrographs for historical storms
• NEXRAD is a powerful and accurate tool for
  real-time rainfall prediction

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Development of the FAS

• Three interrelated projects
• Calibration of NEXRAD to measured rainfalls and
  integration into HEC-1
• Creation of Flow Nomograph relating rainfall
  intensity and duration to flow in the bayou
• Creation of Links and Images over Brays Bayou,
  and animations for the web site
• System tested on Tropical Storm Frances

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Flow Nomograph

• Flow Nomograph based on 2000 HEC-1 runs for Brays
  Bayou
• Uses uniform rainfalls in space and time
• Calibrated by comparisons to past storms
• Rainfall intensity and durations plotted on
  nomograph give estimated peak flow for a storm
• Can be used in a real-time system

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Flow Nomograph
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Typical Flow Nomograph March 27-28, 2001
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Traffic Light Warning

• Top of every web page
• General level of concern
• Green-to-Yellow automatic
• Orange/Red require human authorization

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Typical FAS Information
Automated Real-Time Posting of Intensity/Duration
Points on Nomograph
Automated Real-Time Creation of
Cumulative Rainfall Plots
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Approaching Storm March 27-28, 2001
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Case Study Sept. 11, 1998

• Tropical Storm Frances
• Dropped 4-9 inches of rainfall over Brays
• Heavy rains in two watersheds to the north caused
  significant flooding

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Brays Bayou9/11/98 Midnight
Flow (cfs)
1600
Time
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Brays Bayou9/11/98 200 AM
Flow (cfs)
Time
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Brays Bayou 9/11/98 500 AMNote the intense
rainfall over White Oak
Flow (cfs)
Time
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Brays Bayou9/11/98 600 AM
Flow (cfs)
Time
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Brays Bayou9/11/98 1000 AM
Flow (cfs)
Time
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Sept. 11 Timeline

• Midnight - Heavy rainfall expected throughout the
  night.
• 500 AM - Flow at 20,000 cfs. Yellow Flood
  Caution.
• 600 AM - Flow at 24,000 cfs. Orange Flood
  Possible.
• 800 AM - Flow at 24,000 cfs after small peak.
  Radar indicates additional rainfall expected.
• 900 AM - NWS warns of severe line of
  thunderstorms in southern Harris County. Flow
  still near 24,000 cfs.
• 1000 AM - Brays Peaks. Radar indicates little
  expected rainfall in next few hours. Bayou begins
  to recede.
• 1030 AM - Rainfall over Brays Bayou ends.

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Case Study Results

• Flood ALERT System worked well
• Based on NEXRAD and bayou levels, did not
  recommend closing down TMC
• Without NEXRAD data, accurate flood decision
  would have been impossible
• System could have predicted heavy flooding in the
  bayous to the north

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Brays BayouFederal Flood Control ProjectChannel
Enlargements (Existing Mid Reach)
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Brays BayouFederal Flood Control ProjectChannel
Enlargements (Conceptual Design)
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Brays BayouFederal Flood Control ProjectBridge
Replacements

• Almeda
• South Braeswood (upstream of Holcombe)
• Greenbriar
• South Main
• Kirby
• Braeswood (upstream of SPRR)

• 75th Street
• Forest Hill
• Lawndale
• Telephone Rd.
• Scott Street
• Old Spanish Trail
• Ardmore
• State Highway 288

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Brays BayouFederal Flood Control ProjectWillow
Waterhole (bayou detention) Project
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Conclusions

• NEXRAD is a valuable tool for flood prediction
  for urban watersheds
• NEXRAD provides excellent rainfall data in space
  and time for modeling purposes
• Radar images allow for overall storm management
  as the flood progresses
• Real-time radar data takes flood prediction to a
  new level of accuracy