Controlling erosion through a combination of mulching and vegetation establishment is a critical part of construction site erosion and sediment control practices. The most common approach has been to apply grass seed, fertilizer, and lime followed by straw and tackifier to prevent erosion and create a good environment for seed germination and grass growth. The other two types of materials commonly used for this purpose are erosion control blankets and hydromulch. This project compared runoff quantity and quality as well as grass establishment among five different grades of hydromulch and straw with and without polyacrylamide (PAM). The hydromulches included a wood-cellulose blend, 100% wood, bonded fiber matrix, stabilized fiber matrix, and flexible growth medium. The tests were conducted on plots established on five NCDOT project sites and included three fill and two cut slopes.
Soil erosion and sediment pollution can be major problems in and around construction sites due to land disturbing activities that leave areas of unprotected soil during active construction. Establishing vegetation to control erosion can be difficult due to poor soil, steep slopes, and no irrigation. Our study was conducted on five construction sites in North Carolina to evaluate different mulch treatments on steep slopes for erosion control and vegetative establishment. Site 1 was located in the Coastal Plain region near Kinston, Site 2 was located in the mountain region near West Jefferson, and the remaining three were in the Piedmont near Raleigh. Of the five, two were 2:1 cut slopes and three were 2:1 fill slopes. On all sites the area was divided into 20 plots that were either 3 m wide and 6 m long (site 2) or 3 m wide and 9 m long (sites 1, 3 and 4). After seeding a grass mixture, erosion control treatments were applied in a randomized complete block design and included: 3,000 kg ha-1 wheat straw+tackifier (straw) and 3,000 kg ha-1 wheat straw+ tackifier with 22.4 kg ha-1 of granular, linear, anionic polyacrylamide (straw+PAM) were applied at all four sites; 3,900 kg ha-1 flexible growth medium (FGM) and 3,900 kg ha-1 stabilized mulch matrix (SMM) were applied at three sites; 3,900 kg ha-1 bonded fiber matrix (BFM), 2,800 kg ha-1 wood fiber/cellulosic blend (WCB) and 3,360 kg ha-1 wood fiber mulch (WFM) were applied at two sites. Runoff volumes, turbidity levels, eroded sediment and nutrient concentration data were collected after natural rain events, and grass growth and cover was evaluated once it reached a height of 4-5’ (10-12 cm). At site 2, there were no differences between treatments most likely due to the combination of sandy soil texture (average 72% sand) and relatively light rainfall events that occurred there. At site 3, there was a trend of straw having higher runoff volume, turbidity, TSS and in general higher concentrations and amount of nutrient loss compared to all hydromulch treatments, and straw+PAM having higher losses than FGM. In contrast, at site 4 the straw treatment reduced runoff compared to FGM, WCB and WFM, while the only difference between treatments in reducing nutrient loss was between straw+PAM and WFM for total phosphorus (TP). The same trend of WFM and WCB ground covers producing higher turbidity, TSS and TSL compared to straw and straw+PAM was found on site 5. Addition of PAM did not have an effect on runoff volumes at any site, with turbidity reductions evident at two sites but only for individual storms and no significant reduction overall. Sites 2 and 4 had poor grass establishment with grass cover of <50%, while sites 3 and 5 had good grass establishment (67 and 85% cover, respectively). The reason for lower grass establishment under the hydromulches at some sites was explored further in a greenhouse study where our objective was to determine the effects of mulch type and rate on grass growth, resistance to penetration, and moisture holding capacity. The results suggest that straw may maintain sufficient soil moisture for plant growth for a longer period than hydromulch. Of the three grasses tested, tall fescue was inhibited by BFM but not FGM at recommend application rates or higher, while centipede and Bermuda grass had the same or higher growth as rates increased.
The results suggest that straw may maintain sufficient soil moisture for plant growth for a longer period than hydromulch. Of the three grasses tested, tall fescue was inhibited by BFM but not FGM at recommend application rates or higher, while centipede and Bermuda grass had the same or higher growth as rates increased.
