Ward County Inundation Mapping and Hydraulic Modeling, Ward County, North Dakota

Location:  Ward County, North Dakota Client:  Ward County, North Dakota The historic 2011 flood on the Mouse and Des Lacs Rivers caused significant damage to homes, businesses, and infrastructure.  The lead up to the flooding began with significant rainfall in the fall of 2010, followed by a heavy snowpack.  The snowmelt runoff was compounded by record rainfall in the spring of 2011.  The result was record runoff and flooding that resulted in significant damage to homes, businesses and infrastructure in the Mouse and Souris River Valley.Prior to the spring snowmelt, Ward County contracted with Houston Engineering, Inc. (HEI) to develop flood inundation maps for key areas along the Mouse and Des Lacs Rivers.  The inundation maps were developed using LIDAR data that was acquired in 2010 and the effective Flood Insurance Study hydraulic profiles and models.  HEI completed this analysis on a compressed schedule and provided hydraulic profiles and inundation maps that were displayed by Ward County on their web site to assist with planning efforts for the 2011 flood.  Additionally, HEI provided technical assistance in the form of hydraulic modeling and mapping support to Ward County, the North Dakota State Water Commission (NDSWC), and the U.S. Army Corps of Engineers (USACE) during the flood.The project began with the creation of digital elevation models using the available LIDAR data to use for the mapping effort.  The hydraulic models (HEC-2) and profiles from the effective FIS were reviewed and intermediate profiles were developed for mapping.  The profiles were tied to key National Weather Service reporting stations, including the Mouse River gages at Sawyer, Logan, Minot Broadway Bridge, and Minot 3.5w; and the Des Lacs River gage at Foxholm.  The profiles were mapped using the LIDAR-based DEMs and Arc GIS software.  PDF mapbooks for a range of flood stages at each gage were created and provided to Ward County for display on their web site and dissemination to the public. Once the flood event began, HEI was contracted to provide additional hydraulic modeling and technical assistance during the flood event.  This included working with the NDSWC and USACE in developing and calibrating hydraulic models for the Mouse and Des Lacs Rivers.  The effective FIS HEC-2 hydraulic models were converted to HEC-RAS and calibrated based on the measured discharges and high water marks during the flood event.  Hydraulic profiles were then generated based on the discharges forecasted by the National Weather Service using the HEC-RAS model.  The hydraulic modeling also included the analysis of impacts of emergency flood protection measures and potential structure removals.  The resultant hydraulic profiles were mapped using the LIDAR-based DEMs and ArcGIS software.  The flood inundation boundaries and depth grids were key tools in evacuation planning and flood protection efforts during the flood event.

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