Without an action plan to address this issue, any decertification of the levees would put significant areas of the city in the floodplain.
Lower Heart River Hydraulic Analysis
Lower Heart River Water Resource District
The Lower Heart River Water Resource District (LHRWRD) needed to complete a hydraulic study on the Lower Heart River to evaluate alternatives that would preserve the certification of the flood levees through the Federal Emergency Management Agency. The levees provide flood protection to the City of Mandan. Recent reevaluation of the watershed hydrology indicates that the flow for the 1 percent chance flood (100-year event) is higher than the flow under the current certification criteria and compromises the 3-foot freeboard requirement for certification. Without an action plan, any decertification of the levees would put significant areas of the city in the floodplain.
Bolton & Menk obtained a current hydraulic HEC-RAS model from FEMA and evaluated various channel conveyance area modifications that would preserve the levee freeboard under the higher one-percent flood discharge condition. A series of recommendations of conveyance area modifications to preserve the 3-foot levee freeboard requirement along with cost estimates were developed and incorporated into a final report.
With the report in hand, the LHRWRD and associated regulatory agencies are now aware of the modifications required to preserve the levee certification and are pursuing funding for the improvements.