Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: WEDNESDAY, February 17, 1993 TAG: 9302170135
SECTION: CURRENT PAGE: NRV-12 EDITION: NEW RIVER VALLEY
SOURCE:
DATELINE: LENGTH: Medium
EROSION CAN STEAL A PRECIOUS ASSET: THE GROUND YOU WALK ON
Soil erosion is a major consequence of storm runoff on unprotected areas. Sediment constitutes the largest volume of contaminant carried by runoff. The annual erosion of bare soil can amount to 100 to 200 tons per acre.Small projects such as tilling or installing pavement can create problems. Most of the phosphate and pesticides entering streams and ponds are attached to soil particles. Therefore, controlling erosion will make a significant contribution to the control of water pollution.
The erosion process is initiated when the impact of falling raindrops or irrigation water detaches soil particles. When there is too much water to soak the soil, it fills surface depressions and begins to flow. With sufficient velocity, this shallow surface runoff, called sheet erosion, carries away the detached soil particles.
If the degree of slope increases, the spread of the water movement increases and a more severe form of erosion can occur. Rill and gully erosion are caused by concentrated flows of water. Rills (small, narrow channels) converge to form gullies (deep, wide channels). They usually contain water only during or immediately after rain.
To recognize erosion problems look for these signs:
Tree roots, small stones or rocks becoming exposed.
Small rills or gullies beginning to show.
Build-up of silt in certain low areas.
Soil splashed on windows and outside walls.
The widening or deepening of stream channels.
To control sheet and rill erosion, plant a ground cover or use erosion-control mats over the soil. These mats are made of biodegradeable or permanent materials, such as jute, wood excelsior, coconut fiber or synthetic fibers.
Ground covers include any plant material that covers the ground surface so the soil cannot be seen from above and rain does not strike directly upon it. Turfgrass is one important type of ground cover, but many other low-growing plants are used this way, including herbaceous perennial plants and low shrubs. Besides controlling erosion on slopes, ground covers fulfill other important functions:
Conserve soil moisture and reduce soil temperatures during periods of extreme heat.
Use nutrients in the soil for plant growth, which otherwise could be lost to erosion and leaching.
Reduce lawn maintenance and fill narrow, odd-shaped areas where mowing and edging might be difficult.
Obstruct foot traffic without impeding view.
Produce interesting patterns with variation in height, texture and color.
Significant maintenance is necessary for the first one to three years until the ground cover becomes established. Use fertilizer to encourage vigorous growth for good cover, irrigate in times of drought and control disease and pests.
Newly cut banks and any slopes greater than 12 percent are best treated with ground-cover plantings other than sod to reduce maintenance. Around buildings, ground covers are superior to paving or structural controls for reducing heat, glare, noise and dust.
Careful planning is necessary to prevent gully erosion. Determine where the runoff should be channelled and take steps to protect the channel bottom and sides from erosion. For slow-moving flows, use ground covers or mats. Channels from fast flows may require heavy stone lining, paved lining or piping.
If erosion-control measures cannot be implemented immediately to eliminate bare soil, then sediment-control measures are needed so sediment does not spread and cause damage.
Sediment is almost impossible to clean from stream beds and can smother grass and stain pavements. To trap sediment, filter runoff through a silt fence or a check dam (a filter made of layers of crushed stone). Another way to trap it is to divert runoff to a pit or basin to slow the water and allow the soil particles to settle to the bottom.
Stones and sand readily settle out of storm water; however, fine soil particles, such as clay and silt, settle out only after the runoff has been contained for a long time.
Be careful not to direct runoff into ponds or streams. Professional assistance may be necessary to design effective control measures.
Joe Hunnings is the Virginia Cooperative Extension Service agent for agriculture in Christiansburg. If you have questions call the Montgomery County extension office at 382-5790.