In the following Q&A, Dr. Donald Gray, Professor of Civil and Environmental Engineering at the University of Michigan, responds to questions about the subject of erosion control, and in the process dispels some of the common misconceptions about the field while emphasizing significant preventive measures that should be used as a rule of thumb in any potentially erosion threatened area.

Q: What exactly is erosion and how does it differ from mass wasting?

A: Surficial erosion is the detachment and transport of individual soil particles as a result of drag or tractive forces exerted by flowing water (or air). Mass wasting, on the other hand, is the movement of relatively large, intact masses of soil and rock along well defined surface failures under the influence of gravity.

Q: What are the soil types or geological formations that are most susceptible to erosion?

A: Generally silts, silty sands, and fine sands are the most susceptible to erosion because these soil particles can be detached and transported with relative ease by flowing water.

Q: Are there ways that you can tell, by simple visual inspection when there might be a potential erosion problem?

A: Yes, a combination of erosion susceptible soils, steep slopes, and lack of protective vegetative cover presages a potential problem.

Q: What is the fundamental, underlying approach to erosion control?

A: All erosion control measures or techniques are based on either

(1) decreasing drag or tractive forces by practicing the 4 D's, viz.,

• Decrease (decrease velocity by reducing gradient)

• Detain (reduce amount of runoff by temporary storage)

• Divert (route flow away from critical areas)

• Dissipate (increase channel sinuosity and width, spread flow out, pass flow over baffles or roughened surface)

(2) increasing erosion resistance by:

• protecting/armoring the surface with a suitable ground cover

• stabilizing/reinforcing the soil with cement or fibers

Q: Are there fundamental erosion control principles and procedures that should be followed to minimize soil erosion losses regardless of what erosion control measures are employed?

A: Yes, definitely. Most of these principles relate to planning and execution of earth grading activities. They are listed briefly as follows:

• Fit the project to the existing topography, soils, and vegetation

• Minimize duration and extent of soil disturbance

• Retain natural vegetation whenever possible

• Protect disturbed areas with temporary vegetation and mulch.

• Divert runoff away from denuded slopes or critical areas

• Avoid or minimize concentration of flows

• Utilize channel linings or temporary structures in drainage channels.

• Retain sediment on site by using sediment basins, traps, and barriers.

• Install erosion control measures before grading and disturbance begins.

Q: How critical is efficient drainage in protecting potential erosion areas?

A: Drainage is critical, because erosion hazards will be greatly diminished if runoff can be intercepted, diverted, and/or slowed.

Q: What types of drainage systems work the best?

A: Surface drainage systems such as diversion dikes, ditch interceptors, cross road drains, slope drains, grass lined channels, etc. all work effectively to protect critical areas and limit erosion.

Q: What role does vegetation play is isolating erosion zones? What kinds of plants form the best matrix for erosion control?

A: Vegetation plays a key role and is probably one of the best ways of controlling and minimizing erosion. Grass and herbaceous plants that form a tight, dense cover at the ground surface work best.

Q: What kinds of synthetic erosion control products do you recommend?

A: I would recommend considering rolled erosion control products (RECPs) that are commonly manufactured from synthetic materials. These products include erosion control blankets (ECBs), turf reinforcement mats (TRMs), and geocellular containment systems (GCSs). Some of these products are manufactured from UV or biodegradable materials. RECPs can be thought of as a "soft armor" or biotechnical ground cover system that greatly improves the performance of vegetation on slopes and that increases the allowable tractive stress of vegetation in channels.

Q: Is erosion control a temporary fix? In other words, are landscape architects simply delaying the inevitable?

A: Erosion control is not a temporary fix. The most vulnerable or critical time for erosion is during or immediately after grading and earthmoving. The effectiveness of erosion control vegetation improves as it becomes well established at a construction site. Erosion control plantings and catch basins can also be incorporated into and enhance the landscape plan.

Q: Is there anything the Landscape Architect can do to prevent erosion after the soil has been graded?

A: The most important step to follow after grading is to protect bare and denuded soil surfaces by establishing a protective ground cover of grass and mulch. These can be applied most quickly and conveniently by hydro-seeding techniques. The conjunctive use of rolled erosion control products (RECPs) such as erosion control blankets and turf reinforcement mats helps as well as does soil bioengineering treatments such as live fascines.

Q: How has computer technology improved the ease and accessibility of designing for erosion control? What are some of the products currently available that you would recommend?

A: Computer technology has greatly improved the ease and accessibility of designing for erosion control by providing descriptive text, photographs, schematic drawings, and specifications for a wide variety of erosion control BMPs that can be readily downloaded and incorporated into an erosion control plan. Good examples, that I would recommend, include EROSION Draw 4.0 which describes conventional erosion control BMPs, and BIO Draw 1.0 which describes soil bioengineering and biotechnical techniques. Both these software packages, which are distributed by Salix Applied Earthcare, Redding, CA, were evaluated in the November 2000 issue of LASN.

Q: Do you think that the opportunity presented by erosion control projects has been largely unexploited by Landscape Architects?

A: I don't think so. My experience based as an instructor in erosion control short courses is that Landscape Architects are at the forefront of the field. In addition, landscape architects understand and work with vegetation and plant materials which are essential to controlling erosion.

Q: What are the opportunities available to Landscape Architects who would like to pursue a career in erosion control?

A: I suspect opportunities to pursue a career in erosion control are quite good. Stricter water quality standards and regulations limit the amount of erosion and sedimentation allowed at construction sites. The International Soil Erosion Control Association (IECA) has become very active during the past decade as a clearing house for soil erosion control technology and products; they also host many short courses and conferences on this topic. Landscape Architects can effectively combine or incorporate this knowledge about erosion control practices and products with their training in plant materials and site design to enhance demand for their services. LASN