Written by: Adam Treat, P.S.
Ponds and stormwater basins are a necessary part of the site development process and are typically designed to hold a specific volume of water. Soon after ponds or basins are built, they begin to fill up with sediment. This build up can be caused by organic sediments such as leaves, weeds, sand and silts. Underneath the water, this buildup can decrease the water volume capacity, decreasing the ability to retain and treat stormwater. Above the water, as the water depth decreases, there is a noticeable formation of pond scum, algae and weeds that emerge from the bottom of the pond.
This formation has the potential to attract mosquitoes and affect aquatic life in the area. These natural occurrences have adverse impacts on the water quality and stormwater requirements of the pond and it will not work effectively. The stormwater facility may eventually become nonfunctioning or even noncompliant with regulatory agencies. In an effort to restore the pond to be as close to its original condition as possible, the sediment must be removed. But how will you measure the volume of sediment to remove? One suggestion is a Bathymetric Survey.
Bathymetric Surveying is simply the measurement of the depth of a body of water. Environmental Design Group can measure these depths and map the bottom of your pond in its current condition. We can then calculate the volume of sediment by comparing the bathymetric mapping to the as-built drawings or proposed plans of the pond. Various surveying techniques and technology can be used to obtain the necessary measurements. Robotic total stations or GPS receivers can be used to measure a grid over the pond, including water depths. One traditional method of obtaining measurements involves wading through the pond, walking through the sediment and probing through to the original bottom of the pond. However, this method can cause great disturbance to plants and animals in the area and presents safety risks to the individual in the pond.
Where conditions will allow, Environmental Design Group uses less invasive technologies to mitigate these concerns. Sonar from a remotely controlled boat with a GPS receiver or a prism allows measurements to be taken at water level without disrupting the bottom of the pond. This method provides minimal disturbance to any plants and aquatic life. In some situations, sub-bottom profiler technology can be used to measure sediment depths through the muck to the original bottom of the pond. This method can take a higher number of measurements in a tighter grid pattern, on the bottom of the pond, in less time than traditional methods, yielding a more precise map of the existing pond bottom.
As your pond starts to fill with sediment and the sediment buildup begins to reduce the effectiveness of the pond, you may need a one-time volume calculation or regularly scheduled monitoring. Environmental Design Group can perform a bathymetric survey that will help you analyze the amount of work needed in an environmentally safe manner, with more data and less field time than traditional methods. You can contact me or Bob Warner, P.S. for further information on how we can help you with your pond sediment calculations!