3rd Street Dam Improvement Project
3rd Street Dam Is a "Low Head Dam"
Low head dams are structures that generally span from one side of a riverbank to the other, partially blocking the waterway and creating a backup of water behind the dam. As the water reaches the wall it flows over the drop-off, which can be anywhere from six inches to twenty-five feet. Because the drop-off can be as low as six inches, the dangers of low head dams are often underestimated and the presence of a low head dam can be difficult to see when approaching from upstream.
The 3rd Street Dam is considered a low head dam that was built in 1936 for conservation purposes and to create a small reservoir for the City's water supply. Since then, the City has changed its water supply to ground wells and Mid-Dakota Rural Water, but the reservoir created is still utilized for recreational purposes. Although the upstream depth to the top of the dam is only a few feet, the downstream depth at the face of the 3rd Street Dam is a surprising 14-1/2 feet. This is deceiving when water is flowing over the low head dam and the dangers are often underestimated. Even a small drop over the dam can trap objects and claim lives.
Low Head Dams - "Drowning Machines"
Low head dams are extremely dangerous and have led to many fatalities of anglers, boaters, paddlers, tubers, swimmers, and would-be rescuers. Powerful recirculating hydraulics at these small dams can trap and drown unsuspecting river users, which is why low head dams have been dubbed the name "Drowning Machines". The 3rd Street Dam has claimed unfortunate lives over its controversial history.
"Drowning Machine" is the term used to describe the dangerous situation that is created when water flows over a low head dam. As the stream of water flows over the dam it drops into the water below creating a back-wash or a current. This recirculating current, which is also known as a "hydraulic", can take an object (including a person) to the bottom of the stream. The current then brings the object back to the dam. Once at the face of the dam the object is forced back to the bottom from the water pouring down from above. This recirculation can hold in an object, even a boat, indefinitely and can be impossible for even an expert swimmer to escape.
Why Low-Head Dams Are Perfect Drowning Machines
- Generally both sides of a dam consist of a vertical concrete abutment that can be difficult to scale if the victim manages to reach it.
- Fallen tree branches, tires, and other debris can be trapped in the hydraulic along with the victim, creating trauma hazards.
- In the months between fall and spring, the water temperature can be extremely cold which can significantly decrease the victim's survival time.
- Air bubbles mix into the water decreasing the buoyancy by one-third, which makes staying afloat even more difficult (even with a life jacket).
These factors, along with the force of the hydraulic, make low-head dams a very dangerous drowning machine.
3rd Street Dam Proposed Improvements
Stockwell Engineers, Inc. was hired by the City to complete a study on the 3rd Street Dam and the hazards associated with it. The goal of this study and proposed improvement project was to engineer a cost-effective, long-term solution to eliminate the safety hazard that the low head dam presents while limiting impacts to upstream and downstream segments of the James River.
To reduce ecological impacts and provide an economically feasible project, it was decided that the dam stay in place and undergo a retrofit to improve safety concerns surrounding the dam. A rock ramp, or rock arch rapids provides the structure that will eliminate the submerged hydraulic that currently exists on the downstream side of the dam. The rock ramp will lead up to the downstream side of the dam and break up recirculating currents. The final design of the project will include selecting the type and size of boulders used, setting the slope of the placement, determining the roughness coefficient for flood characteristics, and performing a shear stress analysis to ensure that the rocks do not move or wash away.
The use of angular "quarry" or crushed rocks is preferred because they interlock with each other and will not be washed downstream. This increases the cost because the rocks must be hauled into the area. The estimated construction cost of the project is $750,000 (excluding engineering and permitting). The James River Water Development District has realized the value in the project to address the safety hazard, and they have committed to funding construction costs in the amount of $150,000. The City is also pursuing funding options from other sources such as the Federal Emergency Management Agency (FEMA) and the South Dakota Game, Fish, and Parks.
Please take the time to watch some videos to see why low head dams are called "Drowning Machines". With the attention that the 3rd Street Dam receives from the public, especially during flooding events, the City of Huron is taking steps to eliminate the submerged hydraulic recirculating current below the dam and improve safety concerns. This follows an aggressive national movement underway to remove the dangers at low head dams, also known as "Drowning Machines".