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Development and Application of Unsteady Flood Models Using Geographic Information Systems

By Daniel Baldwin Snead and David Maidment

ABSTRACT

This document presents the application of two unsteady flow hydraulic models used for flood routing and visualization: the MIKE 11 model from the Danish Hydraulic Institute (DHI) and the HEC River Analysis System model, better known as HEC RAS, from U.S. Army Corps of EngineerＴ Hydrologic Engineering Center. In this study, both hydraulic models use rainfall-runoff data in time series format from an existing HEC Hydrologic Modeling System (HEC HMS) model. The approach for both models leads to the spatial integration of unsteady flow simulations into a geographic information system (GIS) for flood visualization and animation. The study area applied to both models is the Mill Creek Watershed located in Cincinnati, Ohio. The Mill Creek watershed area is approximately 165 square miles consisting of 28 main stream miles. The study area used for the hydraulic models, referred to as the Primary Damage Center, is approximately 5.3 square miles in area consisting of 3.97 stream miles. The results found from this project support an on-going flood analysis study conducted by the Louisville District, U.S. Army Corps of Engineers. The primary source for the data used in the project was the Louisville District.

The studyＴ focus was on 1) the development of an accurate and workable digital terrain model of the study area; 2) the development of a MIKE 11 model based on surveyed, stream cross-section data; 3) the development of a HEC RAS model based on stream cross-section data extracted from the terrain model; and 4) the creation of flood animations from the two hydraulic model simulations. The results of this study provide information on the two unsteady flow hydraulic model methods as well as what advantages they have over steady flow hydraulic models.

The MIKE 11 modelＴ stream geometry was based on surveyed data, which did not extent over the full width of the inundated flood plain. The HEC RAS modelＴ stream geometry was extracted from the digital terrain model, which ensured that the flood plainＴ extent was fully accounted for. The results were faster flood wave attenuation, higher maximum water surface elevation, and shorter flood duration for the MIKE 11 model simulation as compared to the HEC RAS simulation. The results of the HEC RAS unsteady flow model were also compared to the HEC RAS steady flow model based on steady flow peak runoff discharge values. The unsteady flow hydraulic modelＴ maximum water surface elevation was less than the steady flow hydraulic modelＴ water surface elevation because the steady flow hydraulic model assumes peak runoff occurs simultaneously in the individual drainage basins within the watershed, while the unsteady flow model more closely mimics the movement of the flood wave through the drainage area.

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