Development of HEC-HMS and HEC-RAS models for urban

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					Hydrologic Models for Urban Floodplain Mapping
   and Damage Reduction in Brownsville, TX

   Philip Bedient
  Rice University
  CEE Department
  Houston, Texas

                    March 17, 2006
Background of Floodplain Studies
 Floodplain studies:
   Provide Water surface profiles and floodplain maps
   for land development and impact statements caused
   by urbanization
   Include the analysis of historic floods for model
   Feature the computation of the water surface profile
   for at least the 1% annual chance (100-yr) flood
   Usually do not include solutions for the flood problem
New Technologies for Floodplain Analysis

   LIDAR and Radar provide accurate data sets for
   topography and storm events
   GIS Linked with new hydrologic models allows
   rapid ability to map results
   The floodplains created can be overlain on
   aerial, land use, and public utilities maps to show
   potential impact
The models developed will be within a GIS framework for easy data
  manipulation and integration.
  Use a lumped parameter hydrological model, HEC-HMS, and a
  hydraulic river analysis system, HEC-RAS, to develop a
  floodplain based on existing land use and land classification,
  LIDAR, and both NEXRAD and design storm rainfall data.
  Use the same hydrologic and hydraulic models to predict the
  impact of urbanization on the North Main Drain and Cameron
  County Drainage District 1 Watersheds
  Develop and model feasible flood mitigation options to reduce
  the effects of flooding on North Main Drain and Cameron County
  Drainage District 1 watersheds.
Overview of Brownsville, TX
                      Brownsville is a medium-
                      sized city in the south of
                      Texas, located just north of
                      the Rio Grande.
                      It is characterized by very flat
                      slopes and clay-rich soils
                      which make the area very
                      susceptible to flooding
                      According to the 2000 US
                      Census, Brownsville’s
                      population has grown from
                      99,000 to 140,000 since
Overview of Brownsville, TX
 Brownsville has 4 watersheds
   2 main drainage ditches:
       Cameron County Drainage District 1 Ditch – 23 mi2
       North Main Drain – 10 mi2
   2 resaca networks:
       Resaca de la Guerra – 5 mi2
       Town Resaca – 6 mi2
LIDAR and Watersheds
Why Accurate Floodplain Predictions
are Important for Brownsville
                                                              Major events within the last
         Storm Rainfall (inches)                              40 years have caused
DATE     TOTAL            DAILY    REMARKS                    extensive flooding, a couple
Sep-67    15.4             12.1    Hurricane Beulah           being larger than the 100-yr
Aug-80    6.9              5.5     Hurricane Allen            rainfall total of 11.7 inches
Sep-84    15.2             7.9                                within 24 hrs.
Sep-88    5.4              4.7     Hurricane Gilbert
                                                              Causes of flooding:
Oct-96    10.6             10.6    Tropical Storm Josephine      Slope and soil type
                                                                 Topographic anomalies
                                                                 Rapid urbanization
                                                                 Undersized drainage channels
Overall Methodology
Methodology - LIDAR
                      LIDAR is Light Detection and
                      Ranging and uses laser light
                      reflection, GPS, and INS
                      Flight Specs:
                         Flying Speeds: 200 – 250 km/h
                         Flying Height: 300 – 1000 m
                         Scan Angles: 20 – 30 deg
                         Pulse Rates: 2000 – 50,000
Disadvantages of LIDAR
                   Accuracy depends on the
                   accuracy of the GPS and INS
                   Processing the raw data is not
                        The digital terrain model is a
                         regular surface which does not
                         present remarkable discontinuities
                        The height of the points are
                         independent of far points, but are
                         correlated to points in the same
Rainfall Data
NEXRAD            TP-40 Design Storms
                Rainfall    2-      10-    100-
                Duration   Year    Year    Year

                1 hour        2      3.2     4.6

                2 hours    2.68     4.08     5.9

                3 hours      2.9    4.48    6.53

                6 hours      3.3    5.42    8.25

                12 hours     3.9    6.48      10

                24 hours     4.6   7.475   11.75
HEC-HMS Transforms

 Model created by HEC-GeoHMS
 extension in ArcView
 Simulates the runoff-precipitation
 response of a watershed
 Basin translation and attenuation
 represented by Clark Unit
 Hydrograph (TC&R)
 Flood wave attenuation in an open
 channel is modeled via the
 Modified Puls method (Storage –
 Discharge relationship depended
 on momentum and continuity

     Cross Sections   WSE Profiles
HEC RAS Input Window

Multiple Cross Sections

HEC RAS Profile Plots
3-D Floodplain
Output Table
Extracting Cross Sections

Geo-RAS Extension
Elevations extracted from
LIDAR by cross section
Cross sections, streamlines,
flowpaths, are exported from
ArcView into HEC-RAS
HEC-RAS and HEC-HMS Iterations
                          Storage/Outflow calculated based on
                          inputted flows and channel geometry

                                   Rating Curve inputted into
                                   HEC-HMS Modified Puls

Rating Curves developed based on
Calculated storage/outflow
Floodplain Delineation Based on Land Use

                        Used current and future Land
                        Use and Land Classification
                        for impervious values, %
                        developed, and roughness
                        Used the 2, 5, 10, 25, 50,
                        and 100-yr design storms for
                        Calibrated the models with
                        the May 7, 2004 rainfall data
                        from NEXRAD
Floodplain Analysis - Houses
Used the May 7, 2004 storm
for calibration:
   8.41 inches in 34 hours
   Modeled between a 2- and 5-yr
   Observed high water marks:
       Paredes Ln Rd: 20.53 ft
       Old Port Isabel: 16.93 ft
   Modeled high water marks:
       Paredels Ln Rd: 20 ft (3%)
       Old Port Isabel: 16.75 ft (1%)
Flood Mitigation Options
 Detention/Retention Ponds
    With controlling inlet and outlet structure
 Channel Modifications/Improvements
    Channel Maintenance
    Channel Widening and Lining
 Hydraulic Structure Improvements
    Improving Culverts
    Elevating Bridges
 Development Controls
Option A: Construction of 12 detention ponds
    and the improvement of a hydraulic structure

Option B: Option A plus a channel improvement
    from Paredes Ln Road to FM 802

Option C: Option B plus an extend channel
    improvement to the end of the watershed
    and the elevation of two bridges
Existing Conditions
Existing Conditions
Option A 100-yr
Existing Conditions
Option B 100-yr
Existing Conditions
Option C 100-yr
Flood Mitigation Results
100-yr Existing Dev WSE alternative comparison for CCDD1



                                                                Water Surface Elevation (ft)
                                                           20                                  Existing
                                                                                               Option A
                                                                                               Option B
                                                           15                                  Option C


    60    50    40          30           20   10       0

                 Distance from Outlet (ft)
Full Development
Conditions 100-yr
Full Dev. Conditions
Option A 100-yr
Full Dev. Conditions
Option B 100-yr
Full Dev. Conditions
Option C 100-yr
Flood Mitigation Results
100-yr Full Dev WSE alternative comparison for CCDD1




                                                              Water Surface Elevation (ft)

                                                         22                                  Existing
                                                                                             Option A
                                                                                             Option B
                                                         18                                  Option C




   60    50   40          30           20   10       0

               Distance from Outlet (ft)
Conclusions - Objectives
 Creating a flood study within a GIS framework allows for
 easy manipulation of the data and models.
 Using a lumped parameter hydrological model (HEC-
 HMS) in conjunction with a river analysis system (HEC-
 RAS) can accurately predict floodplains
 The use of a high resolution DEM (LIDAR) provides
 accurate floodplain prediction with little calibration.
 The models allowed easy flood analysis to determine
 feasible mitigation options for the area
Conclusions – Flood Analysis
 The topography, soil, and rapid development of
 Brownsville makes the area susceptible to flooding for
 even small storm events.
 Option C for the CCDD1 ditch provides protection from
 the 100-yr storm for existing and future development
 Stream and rain gages are needed for better model
 Restrict the amount of allowable discharge into the
 drainage systems from future developments

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