Bozeman Creek Floodplain Mapping

AESI provided review and refined modeling services for the completed Bozeman Creek and Tributaries project. The original modeling effort, completed by others, aimed to update floodplain mapping for the Bozeman area using updated elevation data and the Army Corps of Engineer’s hydraulic modeling software HEC-RAS.

In the first phase of the project, AESI provided select review and evaluation of the Bozeman Creek and Tributaries Floodplain Mapping Project hydraulic model and floodplain mapping developed by the study’s contractor. In addition to general model review, possible flood mitigation measures were evaluated to lower base flood elevations in areas susceptible to flooding. Specifically, AESI evaluated the effect on the floodplain of increasing the flow split from Bozeman Creek to the Mill Ditch Diversion including a rough order of magnitude cost estimate to improve a roughly 9,250 foot section of the ditch to handle an increased flood flow. AESI also evaluated the addition of an inset floodplain on Bozeman Creek at Creekside Park, as well as changes to the model to more accurately reflect the flooding characteristics at the park.

The second phase of the project included refining the Bozeman Creek and Tributaries Floodplain Mapping Project hydraulic model through a Letter of Map Revision (LOMR). The LOMR remodeled Bozeman Creek from upstream of the downtown area to its confluence with the East Gallatin River. The new model addressed the city’s concerns regarding the computation of split flows and the floodway extents. To refine the model, AESI completed a detailed 2D model in HEC-RAS 5.0.3 to quantify split flows and then created a 1D model based on the 2D results. The updated 1D model also included the Bond Street Bridge as surveyed by AESI during the first phase of the project. The bridge was constructed after the initial floodplain model was developed by FEMA. Ultimately, several miles of Bozeman Creek floodplain and floodway were successfully updated and will be used by the community to regulate development in the future.