Like all physical features of our communities, stormwater management facilities require ongoing maintenance to preserve their structural, functional, and aesthetic integrity. These features are designed to mitigate flooding hazards, as well as to remove sediment, nutrients, and other pollutants from stormwater runoff to protect downstream water resources. Their very function contributes to their impairment as these materials build up within the basins. Even when they are well maintained and functioning properly, stormwater management facilities will require periodic restoration activities that can be extremely expensive.
If you have a stormwater management facility in your community, it is important to ensure that copies of the design and/or as-built plans are available. These documents contain essential information regarding the original grading and depth contours of the facility, the configuration and elevation of the outlet structure and normal pool water level, and the location of inlet structures to the pond. The maintenance agreement for the facility should also be on file, which outlines the specific maintenance responsibilities of the community. If the homeowners association does not have copies of these documents, they can be obtained from the site engineer or developer, or from the city or county stormwater regulatory agency.
At a minimum, ongoing maintenance activities for stormwater management facilities should include periodic inspection and clearing of the inlet and outlet structures to maintain flow, control of woody growth on the embankment and near inlet and outlet structures, and the removal of vegetation and debris from areas where riprap has been installed for stabilization. Other activities may be required depending on the type of facility. Many localities have inspection programs for stormwater management facilities to ensure that they are in compliance with the maintenance requirements, but the frequency is often inadequate to address short-term deficiencies. Annual inspections are recommended, which can be performed by your lake management company in the years when they are not done by the locality.
Even with the most diligent maintenance program, major structural repairs and dredging of the facilities will ultimately be required and should be included in long-term budgeting considerations. Concrete pipes will begin to exhibit cracking and spalling and the joints may fail. Gate valves will become rusted and corroded. And sediment will continue to accumulate until supra-surface deltas appear and flow becomes restricted. The maintenance inspections will indicate when it is time to begin planning for structural rehabilitation. It is also recommended to have periodic bathymetric surveys completed to assess the extent of sedimentation in the pond. The results of these studies provide information on the current depths of the facility, as well as the magnitude and location of the unconsolidated sediment that has accumulated in the basin. By comparing the results of the bathymetric study with the as-built plans, an accurate estimate of the quantity of sediment to be removed during a dredging operation can be calculated. If the plans are not available, then the sediment quantity can be estimated using the unconsolidated sediment depth measurements.
There are two methods typically used to dredge stormwater management ponds. The first is to drain most of the water from the pond and to remove the sediment using traditional excavation equipment. The sediment is loaded into sealed dump trucks and hauled off-site for disposal. The other strategy is to leave the pond at full pool and to use barge-mounted suction equipment to pump the sediment into dewatering bags on the shore. The water slowly drains out of the bags back into the pond leaving the sediment behind and the filled bags can eventually be removed from the site when the material has dried out. The method selected for dredging is highly dependent on site-specific conditions such as the availability of access easements to the facility, the existence of open staging areas near the pond, the amount of material to be removed, regulatory permitting considerations and the budget for the project.
It is never too early to begin planning for the renovation of a stormwater management facility. A comprehensive inspection and maintenance program will help to prolong the eventuality of major rehabilitation projects and a bathymetric study can be completed at any time to provide current baseline information on the scope of sedimentation in the facility. As with any major structural repair and improvement project, proper budgeting and preparation are key to ensuring that the work can be completed in the most timely and cost-effective manner.
Contact the experts at 888-480-LAKE (5253) for all of your lake, pond and fisheries management needs.
Shannon Junior is an Aquatic Ecologist with SOLitude Lake Management. Since 1998, SOLitude Lake Management has been committed to providing full service lake and pond management services that improve water quality, preserve natural resources, and reduce our environmental footprint. Lake, pond and fisheries management services, consulting, and aquatic products are available nationwide. Learn more about SOLitude Lake Management and purchase products at www.solitudelakemanagement.com.