Ice Jam Discharge Frequency and Its Impact on Flood Insurance Elevations

The Heart River drains approximately 3,360 square miles in southwest North Dakota, flowing through the city of Mandan, ND, before entering the Missouri River near Bismark, ND. Flood protection and irrigation projects have been constructed over the years in the Heart River basin, including the Heart Butte Dam constructed in 1949 by the U.S. Bureau of Reclamation approximately 100 miles upstream of Mandan. While this altered the hydrology of the basin slightly by providing storage, irrigation opportunities, and increasing evaporation, the Lower Heart River can cause both open water and ice jam flooding in the city of Mandan. Flood protection levees have been constructed over time in Mandan starting in 1959 with improvements added in 1963 and in 1989. An effort to provide increased flood protection and to re-certify the levee system in Mandan to Federal Emergency Management Agency (FEMA) protection levels is being undertaken by the Lower Heart Water Resource District. The effort included a reanalysis of the historic discharge and stage records of the USGS gage at the upstream edge of the city. An analysis of past ice jamming events, river and floodplain geometry, and ice jam mechanics found that there is a peak discharge for which an ice jam can remain in place and that this discharge varies depending on river location and local geometry. The analysis indicated that the 1% annual exceedance probability ice jam discharge is 42,000 cfs, but the peak discharge for which an ice jam could remain in place varied between 15,000 and 25,000 cfs depending on river location. The paper describes the analysis techniques used to develop the combined stage frequency values for the levee recertification.