Pheasants Nest Detention Pond
Brookings, SD

The Pheasants Nest Detention Pond Project was identified in the City of Brookings Drainage Master Plan, and ranked as a top three priority drainage project by the City Council because of the potential benefit to downstream residents. The purpose of the detention pond is to regulate peak stormwater flows and reduce sediment loads entering the existing downstream open-channel drainage system. The plan included regrading the entire 10.5-acre lot which created room for a future dog park, construction of a two-phase storm water detention and sedimentation pond, and an outlet flow control structure. Plans also included installing native grasses, trees, wetland flora, and pedestrian walkways. Two rain gardens were also included to handle direct runoff from the adjacent street before entering the pond. Since there was an existing wetland onsite, a 404 wetland permit application was submitted to South Dakota’s Regulatory Office and the pond construction work was authorized under an Army Corps general permit. 

FEMA Map Revision: Unnamed Tributary to Skunk Creek
Hartford, SD

FEMA approved the Letter of Map Revision (LOMR) prepared by Banner for the City of Hartford for a 3.3-mile length of an unnamed tributary to Skunk Creek through the City and a small surrounding area in Minnehaha County. Hartford is developing at a rapid pace: to protect the public near this flooding source, the City elected to hire Banner to perform a detailed study. The City’s floodplain mapping was previously a Zone A approximate Flood Insurance Rate Map (FIRM) with no official Base Flood Elevations (BFEs) identified. The LOMR revised the 100-year (1% annual chance) boundary, added BFEs (Zone AE), and a 500-year (0.2% annual chance) boundary. This project was unique because of the accelerated timeline. The first FEMA map was submitted four months after the design contract was awarded. During this time, survey crews collected topographic data for the area and engineering staff conducted hydrologic and hydraulic models that were used to create the maps for the FEMA submittal. After the additional data requests were received from FEMA during their review process, a preliminary LOMR determination approval was made within six months. When the required 90-day appeal period concluded, the map became effective. This was a fast-track project especially considering the length of stream revised.

Twin Lakes Emergency Drawdown
Brookings County, SD

Twin Lakes and Lake Sinai have historically been a closed drainage basin. Brush Lake drains to Twin, and Twin drains to Lake Sinai when and if they meet their natural outlet elevations. In the summer of 2011, the lake water levels finally reached their natural outlet elevations and began to flow to a tributary to the Big Sioux River. A roadway embankment exists across the Lake Sinai natural outlet, which prompted the Brookings County Drainage Board to retain Banner to analyze options meeting county highway drainage standards. Banner recommended installing four culverts through the county road to relieve lake levels above the natural outlet elevation. Recommendations of culvert improvements for the downstream roadway crossings were also conducted and brought to the drainage board for their consideration. 

As Twin Lakes was discharging over its natural outlet, its water level was above the US Highway 81 centerline which caused safety and damage concerns for SDDOT. Additionally, the SDDOT retained Banner to help secure a Brookings County drainage permit for the emergency drawdown of Twin Lakes to a reasonable water level. The drainage permit included a hydraulics and hydrology study for the lake systems to analyze the impacts of a 90-day drawdown of Twin Lakes while taking into account the potential for several different scenarios of weather patterns. An EPA Storm Water Management Model (SWMM) watershed model was created to simulate scenarios and assist in recommending a reasonable drawdown rate. The downstream impacts from Twin Lakes flowing to Lake Sinai and from Lake Sinai downstream to the Big Sioux River were addressed in the study and presented at County Drainage Board hearings. Pumping and siphoning were temporary alternatives investigated for the drawdown. Ultimately, due to cost, SDDOT opted to install a more permanent solution which could be used in the future if a Permanent Drainage Permit with Brookings County and SD DENR is pursued in the future. The drawdown from Twin Lakes successfully occurred without adverse impacts downstream.