Clear Creek General Reevaluation Report
Hydrologic Analysis of Without-Project Conditions
Executive Summary  June 2003
General—The hydrologic analysis of without-project conditions for the Clear Creek
General Reevaluation Report (GRR) has been finalized. The study is documented in a
comprehensive report that is available for public release (see distribution below). The
analysis included the development and calibration of a rainfall-runoff model for the Clear
Creek watershed and a hydraulic model for the mainstream of Clear Creek. Dannenbaum
Engineering Corporation and the Galveston District Corps of Engineers conducted the
analysis. Halff Associates performed the independent technical review.
Significance—The models and flood frequency results will provide the hydrologic basis
for economic flood damage calculations. These damage calculations will be used to
judge the economic feasibility of all structural and non-structural flood risk management
measures considered in the Clear Creek GRR.
New GIS Based Models—The rainfall-runoff model (HEC-1) and hydraulic model
(HEC-RAS) were created using newly acquired data. The HEC-1 model covers the entire
watershed of Clear Creek. The HEC-RAS model includes the mainstream of Clear Creek
from the Fort Bend County line to the outlet at Galveston Bay. Both models benefit from
modern mapping and surveying technologies. The models were verified with available
data in the form of stream gage records, historical flood hydrographs, and observed high
water marks.
Model Simulations and Results—Simulations with the new models were conducted for
eight hypothetical flood frequency events to generate flow and stage frequency, the main
products of the analysis. The flow frequency results compared favorably with
independent flow frequency estimates from statistical analysis and also regression
equation methods. The entire computational sequence in the analysis included over 380
passes of the final rainfall-runoff and hydraulic models. Sample tables and figures from
the report are attached to this summary to illustrate the modeling results.
Follow-up Work—Economic modeling studies that will quantify flood damages and
provide counts of inundated structures by floodplain for without-project conditions are
currently underway. This effort is nearing completion. Additional hydrologic modeling
is underway for the first series of flood risk management measures and is approximately
50 percent complete.
Distribution of Report—Paper copies of the report can be found at public libraries
in the study area. A limited number of CD-ROM copies are available on a first come first serve basis
by email request to gerald.m.dunaway@usace.army.mil.
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