Hard spring rains in Georgia that come before vegetation is growing again often cause serious erosion problems in our rivers and banks. Strong grass and woody vegetation are nature’s primary defense against erosion, yet plant life doesn’t always hold and can create safety issues when we have increased water flow. A stronger defense can be the placement of granite riprap, although this type of application is often perceived as unsightly in area parks and neighborhoods that border tributaries. Granite riprap can also minimize the ecological benefits provided by natural vegetation. Aquascape Environmental has provided many safe and scenic solutions for stream restoration and habitat enhancement projects, such as the featured project at East Cobb Park.
East Cobb Park includes 13 acres of woodlands and open spaces with amenities including multiple playgrounds, a walking loop, several pavilions, and nature trails—some of which traverse along the adjacent Sewell Mill Creek. Given those amenities and the park’s location central to many neighborhoods, it is one of Cobb County’s most highly utilized parks. Sewell Mill Creek offers visitors an opportunity to play and enjoy the ambience of a creek-side setting. The portion of the creek within the park is a rather robust waterway, with a large drainage area of 12.57 square miles. The county installed a main sewer line trunk along the creek to take advantage of the central gradient position that the creek provides. This is common practice for urban tributaries.
Due to a large storm event that occurred within the watershed, a 250-foot portion of the creek’s bank was severely eroded, which threatened to compromise the integrity of the adjacent sewer line. Based on the conditions created by the erosion, Cobb County implemented an emergency action plan to stabilize the impacted section through placement of large granite riprap along the creek bank. While this action did provide a strong element of stability to the bank, the rock placement left a very prominent “scar” on the creek, in stark contrast to the otherwise natural landscape along the creek within the park.
Faced with concerns expressed by park visitors about the visual impact of the rock placement along the creek, the county decided to explore options for adding a vegetative component. While there are some tried-and-true soil-bioengineering stream restoration techniques, such as live staking, the county wanted to avoid the placement of woody vegetation within the sewer easement, because this type of process could create issues with the maintenance of the sewer line. Aquascape Environmental was contacted by the county to assist in the development and implementation of a plan using an alternative method to live staking.
In reviewing the conditions and constraints of the site with the Cobb County Stormwater and Parks Department, Aquascape Environmental advised that the best and simplest option available was to utilize a technique called “buried riprap.” The objective of this technique is to establish a soil matrix within the open void sections in the rock to allow for the establishment of native herbaceous vegetation. This method accelerates a process that would otherwise occur naturally from soil migration into the open voids because of repetitive overbank flooding during large storm events. Aquascape Environmental utilized a specialized contractor to assist in the application of 40 cubic yards of a soil compost material via a specialized blower truck. The material was applied along the face of the bank into the existing riprap. This operation had to be completed in a careful manner to reduce the amount of spillage of the compost material into the creek. Upon completion of the compost application, the compost was seeded with a mixture of native grasses and non-grass flowering plants called forbs, which were adapted for the riparian conditions along the bank.
While this project seemed to be a relatively easy option to establish the desired vegetation along the bank, the success of it could have been jeopardized not only by drought conditions but also by any large storm events that would mobilize the compost off the riprap. It took over a year for the project to develop, but there was sufficient soil placement created within the rock voids, so that vegetation was established along the rock face and the visual scar to East Cobb Park was minimized: “A Lemon turned into Lemonade.” Cobb County has been very pleased with the results of this project and is considering similar projects in the future.
Buried Riprrap – Soil Compost Material Application
