Background

In July 2021, the City of Sioux Falls adopted a new floodplain ordinance to reduce flood risks across the city and improve its flood resilience. Through this, the City began regulating its floodplain based on interim Base Level Engineering (BLE) hydrologic and hydraulic modeling and mapping, even though the mapping is not expected to become effective until late 2024 or 2025. This proactive floodplain management approach allows the City to use the best available mapping to identify flood risk areas and to preserve the floodplain by preventing development from occurring in areas that will soon become mapped.

Through the floodplain ordinance update, developers seeking to build in future floodplain areas will need to ensure that the site does not adversely affect the base flood elevations (BFEs) for adjacent properties. Additionally, the developer must provide compensatory storage that offsets the volume of fill placed in the floodplain.

The Project

HEI contracted with the City to manage the overall floodplain models so that proposed developments meet the floodplain ordinance requirements. This includes verifying that a proposed development results in “no rise” to the BFE, and that compensatory floodplain storage is provided resulting in a net balance of cut and fill in the floodplain.

HEI is currently managing the floodplain models as floodplain permit applications are submitted to the City. For complex development sites, HEI has also independently contracted with developers to conduct the necessary hydraulic analyses using the interim 2D BLE models so that the proposed developments meet the floodplain requirements for permitting. HEI is also conducting hydraulic analyses for City-led street design projects to be sure they meet the floodplain development requirements.

When the floodplain maps become effective, they will show the recently developed areas in the floodplain. Since the developments were constructed to the new standards with raised elevations, the City will submit a Letter of Map Revision that will show the developments out of the floodplain.