Comparison of HEC-RAS and MIKE11 Unsteady Flow Modeling for the Tillamook Valley

Abstract

Numerous flood events have historically devastated the Tillamook Valley in Tillamook County, Oregon. The City of Tillamook and both the commercial and agricultural activities in the Tillamook Valley are highly susceptible to flooding events, as the local topography is flat and broad before rising abruptly to the steep mountains of the Pacific Coast Range. As a result of the February 1996 flood, Congress authorized a request from Tillamook County to investigate water resource problems in the region. Under a contract with the U.S. Army Corps of Engineers, Portland District (District), WEST Consultants Inc. (WEST) developed one-dimensional unsteady flow hydraulic models of the system, originally applying the Danish Hydraulic Institute (DHI) MIKE 11 model and then later converting the model to the U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS). These two hydraulic computer simulation models are commonly utilized for unsteady flow application in the both the public and private sectors in the United States; however, it is in error to assume that the construction of these models should always be developed in the same manner and with the same modeling assumptions. WEST recognized the differences between these two models, as part of the study, and compared specific aspects between HEC-RAS and MIKE11 including the model equations and variables, approaches for model construction and development, and the effects of these variations on computation results as they relate to the Tillamook Valley. This paper discusses specifically the differences in computation of the channel roughness.