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 calibration 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

Objectives 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 1990

Overview of Brownsville, TX Brownsville has 4 watersheds 2 main drainage ditches: Cameron County Drainage District 1 Ditch – 23 mi 2 North Main Drain – 10 mi 2 2 resaca networks: Resaca de la Guerra – 5 mi 2 Town Resaca – 6 mi 2

LIDAR and Watersheds

Why Accurate Floodplain Predictions are Important for Brownsville Major events within the last 40 years have caused extensive flooding, a couple being larger than the 100-yr rainfall total of 11.7 inches within 24 hrs. Causes of flooding: Slope and soil type Topographic anomalies Rapid urbanization Undersized drainage channels Storm Rainfall (inches) DATETOTALDAILYREMARKS Sep Hurricane Beulah Aug Hurricane Allen Sep Sep Hurricane Gilbert Oct Tropical Storm Josephine

Overall Methodology 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 pulses/sec

Disadvantages of LIDAR Disadvantages: Accuracy depends on the accuracy of the GPS and INS Processing the raw data is not exact 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 surroundings

USGS 30m DEM

LIDAR DEM

Rainfall Data Rainfall Duration 2- Year 10- Year 100- Year 1 hour hours hours hours hours hours NEXRAD TP-40 Design Storms

HEC-HMS

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 equations)

HEC-RAS Cross SectionsWSE Profiles

HECRAS Bridge CS

HEC RAS Input Window

Multiple Cross Sections River Bridge

HEC RAS Profile Plots

3-D Floodplain

Output Table

Extracting Cross Sections Geo-RAS Extension Elevations extracted from LIDAR by cross section shapefiles 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 Curves developed based on Calculated storage/outflow Rating Curve inputted into HEC-HMS Modified Puls

Floodplain Delineation Based on Land Use Used current and future Land Use and Land Classification for impervious values, % developed, and roughness coefficients Used the 2, 5, 10, 25, 50, and 100-yr design storms for Brownsville Calibrated the models with the May 7, 2004 rainfall data from NEXRAD

Floodplain Analysis - Houses

Calibration Used the May 7, 2004 storm for calibration: 8.41 inches in 34 hours Modeled between a 2- and 5-yr storm Observed high water marks: Paredes Ln Rd: ft Old Port Isabel: ft Modeled high water marks: Paredels Ln Rd: 20 ft (3%) Old Port Isabel: ft (1%)

Flood Mitigation Options Detention/Retention Ponds With controlling inlet and outlet structure Diversions Channel Modifications/Improvements Channel Maintenance Channel Widening and Lining Hydraulic Structure Improvements Improving Culverts Elevating Bridges Pumping Buyouts 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 100-yr

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

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

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 validation. Restrict the amount of allowable discharge into the drainage systems from future developments