Soil Ecology and Restoration Group
Erosion is much less costly to prevent than it is to repair. Even a small erosion gully can involve many cubic meters of soil that must be collected from the bottom of the slope and replaced at a cost of hundreds, or more likely, thousands of dollars. Erosion can limit plant establishment from seeds and destroy irrigation systems and installed landscaping. Costs are also transferred to others directly through increased flooding, sedimentation and subsequent flooding damage. Sediment also causes serious damage to aquatic and riparian ecosystems. In 1998 we reported on research to find materials that can provide erosion control, enhance native seed germination, and control weeds without harm to wildlife. These were almost all natural materials without the more common plastic or plastic mesh reinforced materials which have proved to be harmful in many cases by trapping wildlife (especially snakes and lizards but also birds) and looking ugly as they break down.
These tests showed that the combination of natural erosion control materials at Palos Verde site in March 1999 after a 0.9 inch rain from 2 m x 5 m plots was only 0.5 pounds per plot for the Curlex, Encs2, Jute, and Cocoa mulch. This was about 1/4 the erosion on the control plots with plants and 1/6 the plots with coir erosion fences.
To achieve success with biodegradable erosion control methods the materials should be selected to fit the slope steepness, soil type, weed control method, and anticipated foot traffic. The most effective materials for erosion control appeared to be the coir and Encs2 mats. Jute netting does well for its relatively low cost, but it is not as stable or easy to install. The Encs2 coir/straw mat material provided very effective erosion control and good seed germination and plant survival. These mats are made of completely natural, biodegradable materials, and can be generally recommended for areas where little or no foot traffic is expected. They should also be used whenever lizard or snake populations are present.
The Curlex mats were also effective but include plastic reinforced net. The Curlex netting had the best plant survival of all treatments and the best germination among the mats. These results may reflect the greater thickness of the Curlex and better light penetration than with Encs2. However, the green plastic photodegradable netting that holds the material together made it difficult to walk on while planting and in other areas created an unsightly mess. Although problems with bird and reptile entrapment using plastic materials are not uncommon, none were observed at these sites, perhaps because the top layer of plastic netting was removed from some test areas (effectiveness or durability was not noticeably reduced) and reptile populations are low. These Curlex mats could be reformulated with a jute, hemp, or coir net, but until that is done they should only be used on sites without lizard and snake populations.
The coir fiber erosion control fences have worked well in some cases, but spacing should be closer than manufacturers recommend and it must be carefully installed. The net should be stapled to the stakes used to hold the net in place, back-filled and carefully compacted. Long stakes should be used on soft sandy slopes. Combining cocoa mulch with these fences cut erosion in half and some form of mulch should generally be used with these erosion fences.
Mulch alone was more effective than erosion than erosion control fences at one site. The cocoa mulch appeared to improve native seed germination and has the advantage of not having any weed seeds which may survive composting operations. This material was heavy enough to keep the seeds from blowing away and retained moisture, but readily allowed the seedlings to grow. The new seedlings were noticeably clustered in patches of mulch.
Pitting and mulch showed very variable results depending on the soil conditions and slope. At the Tank 76 south site pitting and mulch controlled erosion better than jute netting. At the FCTCP site pitting appeared to disrupt the soil and created more erosion than the control.
Mulch alone or pitting and mulch can work well on shallow slopes or small pockets of disturbed soil where sheet flow is not expected. Dense planting alone is suitable on some sites. This might be more effective with grasses than with shrubs. The roots of container plants also help stabilize slopes.
Straw flake check dams are inexpensive and recommended for areas which are narrow and may have substantial water flow, such as old dirt roads. These dams are inexpensive and easy to install. A self-propelled trencher would increase the speed of installation. A 7.5-10 cm wide slot 15-25 cm deep is cut across the slope and then flakes of a straw bale 5-10 cm thick are placed in the trench. The soil is then back-filled and compacted around the vertical straw fence. Weed free straw or rice straw, which persists longer thanks to high silica content, is recommended to reduce the risk of introduction of exotic species.
These natural fiber erosion control methods should all be more widely used. These are more costly than hydroseeding but more effective than conventional hydroseeding practices. At one Point Loma site an adjacent site was hydroseeded by a different contractor, this became an eroded, weed infested slope which will require costly repair work.
In an effort to improve hydroseeding effectiveness we compared several application methods on an erosive sandy slope. Treatments include: conventional hydroseeding with fiber, hydroseeding followed with compost, hydroseeding followed with blown straw, punched straw followed with hydroseed, and a dual application of seeds first, well churned into the soil, followed by a second fiber application. The goal was to control erosion, encourage germination of native seeds and improve survival of container plants.
The dual application, seeds first then fiber was most effective. Plant density was double conventional hydroseeding application and the plant cover was highest. We suspect this increase results from the churning application of seeds and water to the soil that ensures good seed/soil contact. It may also benefit from deeper wetting of the soil, giving the seedlings more moisture to grow. This double application is more expensive, but added substantial benefits in plant establishment. For less erosive slopes this application technique should be more cost effective than natural mulches and container planting.
This was followed by hydroseeding followed by compost 10%+ better than conventional methods, which had the second highest cover. The punched straw followed by hydroseeding, showed a 25% increase in density but much reduced cover. Hydroseeding followed by blown straw had reduced density and cover.
For less erosive slopes a hydroseeding application with just seeds churned into the soil in a first coat, followed by a second layer with fiber appears to be a realistic and cost effective treatment. Low points should be treated with straw flake, coir, or straw wattles or check dams. Conventional hydroseeding is rarely effective in dry areas unless the timing is fortunate enough to catch seasonal rains or if irrigation systems are installed.