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Using Calibrated Hydrology, Unsteady Hydraulics, and Stream Geomorphology to Better Define Flooding and Erosion Risks Along Mary's Creek

Presented at the 2012 Texas Floodplain Management Association Fall Conference

Presenters:

Hector Olmos, P.E., CFM - Freese and Nichols, Inc.           Katie Hogan, E.I.T - Freese and Nichols, Inc.      

Ranjan Muttiah, Ph.D., P.E., CFM - City of Fort Worth        David Gattis, AICP, CFM, ICMA-CM - City of Benbrook

Using calibrated hydrology, unsteady hydraulics, and stream geomorphology to better define flooding and erosion risks along Marys Creek

Introduction

Freese and Nichols, Inc. was hired by the cities of Fort Worth and Benbrook to perform the Lower Marys Creek open channel study. The study included using a calibrated hydrologic model, unsteady HEC‐RAS, and stream geomorphology to better define the floodplain and erosion concerns along Marys Creek. This project benefits the Cities of Fort Worth as well as Benbrook by creating a more accurate floodplain and removing several structures from the floodplain.

Hydrology

With a 55.2 square mile drainage area, Marys Creek is the largest contributor to the Clear Fork of the Trinity River downstream of Benbrook Dam. The 100‐year discharge listed in the FEMA FIS from a USACE study completed in the early 1980s is almost twice as much of the 100‐year discharge on the Clear Fork downstream of the confluence with Marys Creek. This suggests that either the flows on Marys Creek are too high, or that the flows on the Clear Fork are too low. Freese and Nichols, Inc. created a calibrated hydrologic model to revise discharges on Marys Creek. Close coordination with the Cities of Fort Worth, Benbrook, and the U.S. Army Corps of Engineers (USACE) was necessary.

Hydraulics

The effective FEMA hydraulic model for Marys Creek was developed by the U.S. Army Corps of Engineers (USACE) Fort Worth District using HEC‐2 in the early 1980s. Given the special circumstances at the confluence of Marys Creek with the Clear Fork Trinity River, including diversion structures, flow spills between the two streams, and a pond, an unsteady‐flow model is more appropriate than a steady‐flow model. A new HEC‐RAS unsteady‐flow model was created to better represent the area and more accurately route the flows from the hydrologic model. Approximately 9.2 miles of Marys Creek was modeled using unsteady HEC‐RAS. The resulting hydraulic models and inundation maps provide a more accurate representation of the floodplains and floodways of Marys Creek and the Clear Fork Trinity River within the study area.

Stream Assessment

Fluvial geomorphology standard methodologies were used to complete a stream condition assessment for over 7,800 ft of Mays Creek. The stream assessment identified areas of high shear forces, vertical walls, and areas with potentially severe erosion. Locations along Marys Creek have vertical walls up to 40 feet and banks that are encroaching upon homes. The stream assessment helps the City determine which areas of the creek need improvements first to prevent future structural damage.

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