As the world of storm water management continues to evolve and municipalities face new requirements for the treatment and control of storm water runoff, storm water wet retention ponds are becoming a regular fixture for new and re-developments. Wet retention ponds are a great way to effectively meet pollutant reduction requirements while also controlling peak discharge release rates during storm events. The permanent pool of a wet retention pond helps settle out those nasty pollutants that are often carried in storm water runoff. The pollutants and sediment settle to the bottom of the wet pond where they often remain until they are removed.
Most municipalities and older developments own wet retention ponds that were built in the early 2000’s. It may be time to evaluate how much sediment has accumulated in those ponds and whether or not it is time to dredge them to remove the accumulated sediment.
What happens when too much sediment accumulates in my pond?
The accumulation of sediment often begins near the inlet to the pond. When sediment accumulates near an inlet, it may impede flow into the pond. In some cases, it may reduce the capacity of the inlet pipe, leading to flooding upstream.
When too much sediment accumulates, the quality and functionality of the pond begin to decrease. Depending on where the sediment is accumulated, it may reduce the storage capacity above the permanent pool of the pond. Additionally, when there is too much sediment on the bottom of the pond, storm water flow through the pond may scour the accumulated sediment and pollutants and carry it directly out the outlet structure. This likely means that your pond is no longer meeting the pollutant reduction requirements that it was originally designed to meet. Scouring and reduced functionality begins to happen when your pond has 3 feet or less of permanent pool depth left.
How do I know if there is too much sediment accumulated in my pond?
Sometimes you can visually see accumulated sediment above the water surface in or near an inlet structure. Another visual sign that there is too much sediment in your pond is when you begin to see cattails growing. Cattails tend to grow in about one to two feet of standing water and the permanent pool in a wet pond is designed to be deeper than that.
Most of the time, you’re not going to immediately know that you have an issue that you need to address and that is when a sediment survey comes in handy. The frequency of how often a sediment survey should be done depends on the size of the pond and the land use that is tributary to the wet pond. Areas with higher amounts of impervious surfaces or areas which are under construction generate higher levels of pollutants and sediment.
What is the best way to conduct a sediment survey?
In most storm water wet ponds, it is not practical to pump down the water, survey, and then refill the pond. As most ponds are 5 feet deep, it also may not be practical or safe to strap on a pair of chest waders and walk around the bottom of a pond to survey it.
One successful way to survey sediment in a pond is with the use of a small Jon Boat to ensure you can adequately cover all areas of the pond. The survey can be done in open water during warm weather or can be done in the winter when there is ice cover on the pond. This method will allow you to get accurate GPS survey elevations in a uniform grid throughout the pond. Another unique method may be using a fishing pole with weights and a bobber that you can adjust to cast into the pond and measure the depth of water throughout.
What can a sediment survey show me?
When conducting a sediment survey, crews lower a grade rod into the pond until they meet initial resistance which is the top of the sediment layer. They take an elevation reading and then push the rod further into the pond until they meet firm resistance, which represents the natural bottom of the pond, where they will take another elevation reading. If done correctly, a sediment survey can give you a visual representation of the depth of accumulated sediment across the bottom of your pond.
Bathymetric Map
With the information gathered during a survey, a bathymetric map can be generated creating a unique visualization of the depth of accumulated sediment at the bottom of your pond. These maps can be useful for determining if it is time to dredge your pond and for quantifying the amount of sediment that needs to be removed.
A sediment survey is not necessary to know that it is time to dredge your pond. Oftentimes, a visual assessment and inspection can be telling as to whether there is too much sediment accumulated in the pond or if the pond is still functioning as intended. A sediment survey can be beneficial in giving you more detail as you plan your dredging project.
How can Ruekert & Mielke, Inc. Assist Me?
R/M has experts that are experienced in evaluating storm water ponds and whether or not it is time to dredge accumulated sediment. Our survey staff have experience performing sediment surveys on a multitude of ponds. If your pond needs dredging, we can develop a cost-effective approach to your dredging project and assist with any necessary permitting, accumulated sediment end-use certifications, or facilitate sediment testing and disposal if needed.
About the Author
Peter Gesch, P.E.
Project Manager
Peter provides design and construction administration services on a wide array of projects, giving him a comprehensive knowledge of the industry. He understands the need for teamwork, communication, and leadership during the entire duration of a project. These skills allow him to manage projects and work with municipal officials and community members to exceed client expectations.
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