Managing and Removing Silt and Debris in Retention Ponds

Retention ponds need cleaning and plant trimming every few months, but they also need a full dredging for silt removal at least once every year. Oversized and less used ponds may be able to stretch out the dredging procedures to once every few years, but active and flood-prone ponds definitely need at least an annual cleaning. Allowing silt to build up reduces the pond’s total volume rapidly. This increases the chances that a heavy rainfall will overwhelm its holding capacity and flood the surrounding area. Keeping trash and debris off of the surface of the water and out of the muck at the bottom is also essential for a healthy, attractive, clog-free retention pond.

Assessing Sediment Depth

It’s important to measure the original topography of the pond with sonar or level rods immediately after it’s built to keep a record of the desired depth. If nothing else, record the depth of various parts of the pond during construction to establish baseline measurements. At least once every six months, use a staff gauge to see where the muck and silt begins. A staff gauge is essentially a large and waterproof measuring board that attaches in sections until you hit the bottom of the pond. Taking regular measurements at the same points you originally measured will tell you how quickly the pond fills with sediment. After a few years of seasonal measurements, you can likely build a standard cleaning pattern without the need for continual monitoring. If you know your pond tends to gain a few inches of silt each spring but doesn’t reach a critical point until the fall, you can plan around this pattern until there’s a major change in rainfall or water volume that would affect silt levels.

Preventing Silt Movement

Retention ponds are widely used for settling silt out of storm water. This is much easier to accomplish if the amount of dirt and sand that reaches the water is controlled from the beginning. Silt screens, around any raw and exposed soil, are essential for keeping retention ponds from filling up. Whenever construction or ground disturbances are necessary in the drainage area of the pond, silt screens must be installed and checked regularly. Even a single collapse due to overloading or flooding can dump hundreds of pounds of silt into the bottom of a pond. Methods like keeping the ground covered and establishing grass on bare soil as quickly as possible, will go hand in hand with the installation of temporary silt screens and fences.

Removing Accumulated Material

Traditionally, heavy equipment or vacuum units have been used to remove silt and muck from the bottom of a retention pond. However, it’s no longer the only option for increasing the depth and controlling organic material. Dredging is less than ideal because it’s hard to find places to safely dump tons of silt and it can disturb the natural ecosystem of the pond. Aeration systems that encourage the compaction and settling of silt and muck can stretch the period between full dredging jobs into multiple years. Hydro-raking is a less intrusive way of gently lifting out silt and muck from water as shallow as 1.5 feet. A backhoe or frontend loader is still used for the process, but it floats on the water’s surface rather than rolling around on the ground and crushing plant life. The rake attachment lifts out muck from the bottom of the pond while leaving plants and trees intact around the edges, unlike other dredging equipment.

Dealing with Trash and Debris

Retention ponds, in urban and rural areas alike, often end up holding solid trash that blows in from nearby structures or is dumped purposefully in the water. A pond full of old appliances, tires, and plastic bags, lowers surrounding property values and poses a health hazard to humans and animals alike. Schedule routine clean-up days for the pond and encourage neighbors to report any trash they see. If dumping is a problem, consider fencing or security cameras to discourage unauthorized access. If lightweight and small pieces of trash like wrappers are coming in with storm water and runoff, install trash traps on all water sources and clean them monthly to prevent clogs.

Leaves, Manure, and Other Organic Materials

Small amounts of leaves, animal manure, and other organic materials will enrich a planted pond’s ecosystem rather than overwhelming it. However, too much organic material releases high levels of nutrients in the water that encourage algae growth and can damage fish and plants. Manure, and its runoff, is particularly tricky because silt screens won’t stop nitrogen-rich water from reaching the pond. Earthworks, like ditches and berms, create a barrier to stop specific ground runoff from reaching the pond. Leaves are hard to keep out of a retention pond unless you cut down all the trees around the edges.  A lack of trees will lead warmer water that is more likely to experience an algae bloom, so it is best to leave them in place. Size the pond to handle the annual accumulation of muck from rotting leaves or float a net over the surface and pull it in weekly in the fall to dump the accumulated leaves.

Urban vs Rural Retention Ponds

Urban and rural retention ponds share a lot of characteristics, but they do face different challenges. Urban ponds are much more likely to develop high chemical pollutant loads from engine oil, pesticide residues, and other types of runoff. Rural ponds tend to develop problems from agricultural impacts instead. Despite being located closer to higher numbers of residents, urban ponds are less likely to attract visitors than secluded rural ponds. Rural retention ponds are often treated as a natural part of the landscape when they’re not, resulting in risks to wildlife and humans alike. Since rural ponds are more attractive as recreational features, it’s smart to design them for safe use since it can be hard to prevent access. Both types of ponds need proper security to prevent accidents and illegal dumping that threatens the health of the local ecosystem.

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