Compost Blankets

The newest tool for low-impact development and post-construction runoff control

Although compost erosion control blankets have been widely specified and used for slope stabilization projects over the past decade and more recently have gained broader acceptance as a green best management practice, their latest application in low-impact development (LID) designs may be the best yet.

What is low-impact development? LID is a post-construction design and implementation goal that strives to have site post-development hydrologic flow and cycle patterns mimic the natural predevelopment patterns. While the overriding goal is to improve water quality, the means to that end incorporate landscape, engineering, and ecological design principles that utilize the functional services of natural systems in a manner that restores ecosystems and site aesthetics. Although LID design professionals often concentrate on site runoff volume and peak flow reduction, due to the link between runoff volume and stormwater pollutant loads and the destructive potential of elevated peak flows, the means to achieve these two objectives is where comprehensive LID design separates itself from conventional storm water management principles. These LID design criteria for post-development stormwater management include increasing surface absorption, increasing detention, increasing infiltration, increasing filtration, increasing surface evaporation, increasing plant transpiration, increasing raindrop interception, increasing surface roughness, decreasing slope steepness, and minimizing disturbance.

Growing compost socks on slope

Compost blankets are 100% recycled, bio-based, biodegradable, biologically stabile, high in organic matter and humus content, regionally manufactured, and locally available. As such, compost blankets are the management practice that is closest in nature to a forest duff layer. Compost blankets have been well documented on their performance for both establishing and sustaining vegetation over conventional vegetation establishment methods (Faucette, et al. 2006). In choosing plant materials that increase raindrop interception, ground cover evaporation, plant transpiration, and/or surface roughness—functions that mimic forest and pasture grass ecosystems—it is critical to have a growing media that will give these plant designs the best opportunity for survival, sustainability, and succession. Simply applying seed and fertilizer, hydroseed, or even a thin layer of straw does not mimic the natural system for vegetation establishment and gives current and future plant communities a lower chance for sustained ecosystem development. This becomes even more acute on cleared and graded landscapes where native vegetation and organic topsoils have been traded for bare, compacted, low-fertility subsoils.

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Perhaps just as important as sustaining plant communities is the functional ability of compost blankets to naturally manage rainfall and stormwater. As a ground cover, compost blankets naturally have a rough surface, which works to reduce peak flow rates, particularly compared to hydromulch and straw mulch applications (Faucette, et al. 2005 and Faucette, et al. 2007). This is principally due to the wide range of particle sizes typically used in standard specifications for compost blankets. Additionally, this same characteristic is responsible for raindrop interception at the soil surface, thereby reducing the energy of raindrop impact and splash erosion. Compost blankets have an enormous ability to absorb and detain rain water due to their high humus and organic matter content and porosity. Compost blankets are typically 50% organic matter (dry weight) and have been shown to absorb over 3 inches of rainfall in a 1-hour period (Faucette, et al. 2005 and Faucette, et al. 2007). This equates to over 11,000 cubic feet of rainwater per acre. Absorbing and detaining high volumes of rainfall at ground surface allows for higher rates of infiltration, surface evaporation, and plant available water, thereby mimicking the predevelopment hydrologic and water cycle pattern. Naturally, these high absorption rates (initial abstractions) and rough surface areas also delay the onset of runoff and peak flow conditions under high-intensity and duration storm events: studies have shown by as much as 40 minutes relative to hydromulch and nearly 15 minutes relative to straw mulch (Faucette, et al. 2005 and Faucette, et al. 2007).

Leveling effect, side view

So how does this help a designer who may want to include these in an LID plan to reduce runoff? From the body of published research (Demars, et al. 2000, Persyn, et al. 2004, Faucette, et al. 2005, Faucette, et al. 2007), it is now possible to develop average runoff coefficients for the rational formula, typically used to estimate peak flow rates to design stormwater conveyance systems, and runoff curve numbers (CN), typically used to predict runoff volumes, whether to assist in LID goals or to design stormwater containment and retention systems such as ponds and rain gardens. These design values are easy to determine if rainfall and runoff values are known under a variety of site conditions. Rational runoff coefficients for watershed surfaces are calculated as the ratio of runoff generated from the total rainfall, where 1 equates to 100% of the total rainfall is generated into runoff. Similarly, the Soil Conservation Service developed methods to determine runoff CNs for watershed surfaces if direct rainfall and runoff values or direct rainfall and initial abstraction values are known (SCS 1972). Published runoff coefficient values for natural pasture and forest systems range from 0.1 to 0.35 (paved surfaces are 0.95) (SCS, 1972), and runoff coefficients for compost blankets range from 0.1 to 0.32, with a median value of 0.28. Similarly, published runoff CNs for natural brush covered areas (75% covered) are 48, woodlands range from 55–66, and pastures range from 61–79 (SCS 1972), while vegetated compost blankets range from 47 to 61, with a median CN of 55. Although, compost blankets have been highlighted here as a natural fit for LID applications and design strategies, for many of the reasons described above compost will also likely prove to be a definitive tool in design and construction of engineered soils, bioretention and rain garden systems, infiltration trenches, constructed wetlands, streambank stabilization and restoration projects, and land and ecosystem restoration applications.

Author's Bio: Britt Faucette, Ph.D., CPESC, is an ecologist and Director of Research & Technical Services with Filtrexx International in Decatur, GA.