IECA Member’s Insight on Perimeter Berms

by Steve Gucciardi, CPESC City of Charlotte

Over the past decade, the North Carolina Piedmont region has experienced a tremendous amount of growth and development. Charlotte is a rapidly expanding city nestled in the lower portion of the piedmont bordered to the west by the Catawba River. With topographical challenges and clays that are notorious for their colloidal particulate compositions, erosion control becomes a difficult task. During construction, protection of our waterways, wetlands, public lands and private property rests with the City of Charlotte’s Erosion Control Team.

In an effort to help the development community in Charlotte comply with provisions of NPDES Phase 2 and increased regulatory requirements at the state and local levels, Charlotte Erosion Control Team members have been experimenting with sediment basin configurations. In addition to the increase in regulations, developers are maximizing developable square footage on their parcels. This results in construction sites that don’t leave much room for erosion control measures, in particular sediment basins. In addition to the construction site constraints, development in Charlotte has been pushed into areas that have historically been passed over by developers because of extreme topography, proximity to wetlands, flood plains and streams. With increased regulations, tight site conditions and environmentally sensitive areas outside the denuded limits, the standard rectangular type sediment basin fails to provide adequate protection. These conditions prompted the erosion control coordinators to seek a change in sediment basin configuration.

A sediment basin configuration that has been experimented with over the past few years in Charlotte is the perimeter berm. The perimeter berm basin provides protection along the periphery of a job site with weirs strategically placed and sized for drainage areas. This type of system is becoming a standard practice in Charlotte in lieu of the standard rectangular basin. Once the perimeter system has been constructed, the berm is then stabilized with either seed and straw or mulch from clearing activities. This system is also used in conjunction with silt fence.

This approach evolved from standard rectangular basins, to requirements that sediment basin berms be tied into existing topography. This process eventually led to tying the basins together and creating a series of basins attached by continuous berms.

The resulting protection has proven to be exceptionally effective for several reasons. Generally, this type of system provides more storage capacity than is necessary. However, it should be pointed out that in the design and engineering review process storage capacities could be more difficult to determine than the conventional sediment basin. Computation of storage volume involves measuring irregular contour lines, which may be difficult to read if the site plan contours are on a small scale. One of the most significant benefits to this type of sediment basin is the fact that the perimeter of the project is usually protected for the duration of the project. With the standard basin, too much reliance is placed on diversion ditches. If the diversion ditches are not maintained, flow sometimes makes its way around the sediment basin. The end result is off site sedimentation.

With the scarcity of land leading to the absolute maximization of developable square-footage, it has become common for developers and or contractors to request that sediment basins be removed prior to site stabilization. In many cases, infrastructure and building pads could not be completed due to the fact that sediment basins were in the way. With the perimeter basin system, protection is able to remain in place throughout the construction process. In short, the developer/contractor is not constantly making requests to erosion control inspectors that basins be removed.

Overall, the Charlotte Erosion Control Staff and the development community have had great success with perimeter berms. In an effort to better meet the turbidity requirements of NPDES, experiments with baffles and polymers are currently underway. Erosion control is not a static field. The Charlotte Erosion Control program is constantly searching for new and innovative approaches to better handle rapid development in environmentally sensitive areas.