Erosion

By Kenton Shepard 

Erosion is the loss of soil through the actions of a fluid. In this case, we’re concerned with water.

Erosion can take two forms:

• Mechanical erosion, in which soil particles loosened by the impact of raindrops or flowing water are carried away by running water.
• Chemical erosion, in which chemicals in water dissolve minerals in the ground or even in the atmosphere.

Whatever its source, erosion forces can have damaging effects both at the homesite and after they leave. Offsite erosion can result from poor onsite practices. Erosion can cause problems such as…

• Aesthetics- development of ruts, gullies and soil loss
• Environmental problems- sedimentation and pollution of waterways and the loss of wildlife habitat.
• Structural problems- foundation undermining and/or hillside instability.

ONSITE EROSION

The process of erosion begins with water droplets dislodging soil particles. Small valleys called rills are created as this particle-carrying water flows downhill and if left unchecked, rills will eventually develop into gullies as erosion progresses. Steep slopes or soil types which are easily eroded will speed the process.

Unchecked erosion can cause the loss of topsoil, degrade driveways, undermine foundations and cause hillsides to slide.

Limiting grading to within 20 feet of the building footprint minimizes damage to plants growing on the lot whose root systems help hold soil in place.

Source

A few methods for helping stop on-site erosion

OFFSITE EROSION

Unprotected areas lose soil to erosion which can eventually find it way onto neighboring properties or into streams, rivers, lakes and the ocean. Soil particles (sediment) cause a number of problems in bodies of water. Fast moving water can carry sediment long distances, but as water slows, sediment begins to settle to the bottom.

Stream, river and lake beds are eco-systems which support a variety of life both in and out of the water. As sediment falls to the bottom, it can build up layers which smother plant life and damage or destroy the local food chain. Spawning grounds for fish can be destroyed. In this way, large areas can become empty of aquatic life.

The effect can carry over onto land, since the land-based food chain upon which many animals depend is tied to the water-based food chain.

If soil washed into waterways contains agricultural chemicals such as fertilizers or pesticides, in addition to downstream wildlife habitat, human drinking water sources may be affected.

Hillside Erosion

The following commonly-used methods for controlling hillside erosion are often more effective if they’re used in some combination.

1. Establishing cover crops
2. Mulching
3. Erosion prevention mats
4. Structures

The most effective method is to combine established cover crops with one of the other three methods.

1. Cover Crops

Roots are effective at holding soil in place and plant leaves help reduce the impact of rain droplets, so planting fast-growing ground cover plants are a good way to prevent soil erosion. Erosion mats or mulch may need to be left in place over seed beds until plants are mature enough to be effective.

2. Mulching

Mulch is an inert material such as straw, wood chips or shredded wood. Spreading mulch over a surface serves two purposes. First, by softening the impact of raindrops it helps prevent soil particles from being dislodged. Secondly, mulch helps seeds to germinate by retaining moisture.

3. Erosion Control Blankets (ECB’s) also called Erosion Mats.

Source: Kenton Shepard

An erosion mat protects a slope damaged by highway construction

Erosion Mats are placed to protect seeded areas on slopes of up to 5 degrees. They consist of a layer of biodegradable material sandwiched between two layers of lightweight netting. Over time, the biodegradable material will be absorbed into the soil and the netting will eventually decay and disappear among the bedding plants.

4. Structural Controls

Soil erosion may be controlled on steep slopes in a couple of ways…

• Retaining walls- must be constructed in such a way that they support not only the weight of the soil they contain, but also the weight of the water in the soil, which may actually outweigh the soil itself.Retaining walls may be built from various materials. All should be constructed with a method for releasing water from behind the wall and with footings or tie-backs into the hillside which allow them to resist gravity and lateral pressure.

Source: Kenton Shepard

A inspection common find… failure due to inadequate design and construction.

• Terraces- often used in conjunction with planting, prevent erosion by shortening the long slope into a series of shorter, more level steps. Terracing slows runoff and allows it to soak into the soil.

Source

Terracing, cover crops and retaining walls in Machu Picchu, Peru. Built about 1450

Perimeter Erosion

The purpose of protecting the site perimeter is to prevent soil on one building site from washing onto a neighboring site. Straw bales and silt fences are typical systems. Both work by forming a barrier which slows runoff, forcing it to drop suspended silt, and both provide an 18-24 inch wide silt basin on the upstream side of the barrier. Silt may need to be removed periodically, ensuring that water runs through the barrier rather than over it.

Both methods are more effective if existing ground cover vegetation is left in place for a minimum of 10-20 feet along the perimeter to be protected.

Silt fences

Silt fences are barrier systems which prevent erosion by blocking soil-carrying surface runoff.

Strawbale Silt fence

Because they deteriorate, straw bales are appropriate for short-term projects only. They’re often used to protect the site until seeded areas develop root systems capable of preventing erosion.

Bales should be in contact with the surface of the ground along their length to prevent water from running beneath them. Bales are typically held in place with 3 stakes per bale.

Fabric Silt Fences

Silt fences are more common and preferable to straw bales for situations in which long-term effectiveness is required. A special fabric is used which will allow water to pass through. The base of the fabric should be buried in a 6-inch deep trench to prevent runoff from undermining the barrier, leaving approximately 18 inches above the surface.

Source: Kenton Shepard

A silt fence protects adjacent property

Fabric is supported either by stakes or may be attached to an existing fence, in which case the fabric bottom must still be buried.

Channel Erosion   

Water flowing through lower areas of a site may be heavy enough to form channels. Severe erosion can result if these areas are unprotected, especially if they contain disturbed soil. Paving is the best long-term solution for controlling channel, although they may also be lined with rip-rap.

Source: Kenton Shepard

      A channel protected by a bag

Hillside/channel erosion controls

Source: Kenton Shepard

Erosion mat above a sandbag retaining wall above a channel lined with rip-rap