By Tom Turchi
Marshfield City Engineer
With the colder temperatures approaching, I feel it is necessary to educate students and the general public of the potential dangers of stormwater ponds.
Originally, the city’s stormwater detention ponds were constructed to address flooding issues by storing peak stormwater flows in specially designed detention basins. This releases the stored stormwater at a controlled rate, and the water can be safely carried by downstream storm sewer systems. These types of stormwater ponds reduce the risk of flooding but do not provide stormwater treatment.
The city of Marshfield is classified as a Phase II community, and since 2008 the city has been required to obtain a MS4 permit for the discharge of stormwater. MS4 permit regulations now require municipal stormwater runoff be treated to specific water quality and quantity parameters typically referred to as Total Suspended Solids (TSS).
The city uses several methods to achieve these reductions in TSS such as detention ponds, bio-infiltration devices, and street cleaning. Stormwater treatment has been implemented with the ultimate goal of reducing polluted stormwater runoff, which is harmful to Wisconsin’s natural waters and detrimental to fishing habitats, swimming, drinking water, and other uses of our lakes and streams.
One of the most cost effective ways for the city to achieve its sediment reduction goals is to convert our current dry stormwater basins into wet basins. An example of a basin conversion from dry to wet is the Walnut basin located downstream from the Marshfield Clinic and St. Joseph’s Hospital campuses on the east side of Walnut Avenue.
Stormwater detention and sediment ponds incorporate numerous technical standards provided by the Wisconsin Department of Natural Resources. These include walkable 4-to-1 side slopes above the water level and a safety shelf with a flatter 10-to-1 slope at the water’s surface. The ordinary water depth in the city’s basins is 5 feet but can very quickly exceed depths in excess of 10 feet during typical rain fall events.
Fencing of stormwater ponds has been considered but is not economically feasible and could hamper access for emergency personnel in the event of an accident or emergency. Today there are more than 80 public and privately owned wet/dry stormwater basins within the city’s corporate limits. There are plans to convert several current dry basins to wet basins such as was done with the Walnut basin and the Peach Avenue basin located downstream of the fairgrounds east of Peach Avenue.
Education is a key to teaching our children of the potential hazards of these stormwater ponds. In addition to removing pollutants like TSS and reducing flooding, there are numerous hazards associated with these sediment basins.
Some such hazards are strong currents during storm events especially near inlet and outlet structures; soft, mucky bottoms of sediment; unsafe winter ice; and contaminants carried within the stormwater runoff such as heavy metals from brake shoes, oil, antifreeze, and other lubricants that drip from passing motor vehicles.
The greatest safety concern is the use of deicing solutions for road clearing operations during the winter months. The salt-based solutions used in deicing operations are necessary to keep our roadways safe for the traveling public but create runoff flows of melting snow and ice.
This constant runoff of snow and ice carry salt-based deicing solutions with them, which create an environment for extremely unsafe ice conditions throughout the winter months.
In closing, I encourage parents and school officials to continue educating our children regarding the purpose and potential hazards associated with stormwater ponds.