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.
Project Goal #1
Alleviate flooding of US Highway 81 by recommending a controlled outlet rate and design
- Culvert design
- Erosion protection
- Watershed Modeling with EPA SWMM and Precipitation Analysis
- Hydraulic Analysis
- Drainage Study and Presentation
- Testimony at South Dakota Water Management Board Hearing
- Drainage Easement Exhibits