Drainage - Erosion

Most of this Information is from Fairfax County website.

Method 1 Using Basic Erosion Prevention Techniques

Plant grass and shrubs. ...
Add mulch or rocks. ...
Use mulch matting to hold vegetation on slopes. ...
Put down fiber logs. ...
Build retaining walls. ...
Improve drainage. ...
Reduce watering if possible. ...
Avoid soil compaction.

Gutters and Downspouts: Wetness or Erosion Problems

Drainage problem near downspout. Wetness or erosion near your foundation walls and drip line can be very damaging. The source of these problems can include:

Gutters that need to be cleaned or are undersized
Downspouts and sump pump pipes
Poor grading around foundation walls

Poor grading around the foundation walls may exacerbate these problems. You may need to fill in depressionsthat have formed near the downspout. Land around your house should slope away from foundation walls. Ideally, a 5% grade should be maintained for a distance of ten feet around your home.

GUTTER INSPECTION AND MAINTENANCE

Drip line erosion

Inspect the outside of the gutters and downspouts for streaks or watermarks and look for erosion along the drip line. These are all indications that the gutters are overflowing when it rains and need to be cleaned.

Gutters should be cleaned free of debris at least once a year. This should be done more frequently if you have trees close to your house. Installing gutter guards can help reduce the frequency of gutter cleaning needed.

If the gutters appear to be clean, observe how they function during normal rainstorms. A spillover of runoff is an indication of undersized gutter system. If you see spillover, consider installing larger gutters.

PROTECT SOIL ALONG DRIP LINE AGAINST GUTTER OVERFLOW

If the gutters are spilling over but are not easily accessible or cost too much to replace, consider protecting the soil along the drip line. This can be done by spreading gravel or planting vegetation along the drip line.

Tips for protecting soil along the drip line:

A band of bluestone or pea gravel about 10 – 12 inches wide should provide adequate protection for the soil.
Before putting down the gravel, spread a sheet of landscape fabric over the area.
Install plastic edging (4 – 5 inches tall) along both sides of the area where the gravel will be added. The edging will keep the gravel in place.
Pour in a layer of gravel about 3 – 4 inches thick.
Ground covers or a row of thick leafy shrubs can also protect the soil from gutter spillage, complementing existing landscaping. Gravel and vegetative covers can also be used simultaneously.

SPLASH BLOCKS AND DOWNSPOUTS

Erosion at the end of the downspout.

When it rains hard, an average sized home can experience hundreds of gallons of runoff coming from the roof. If downspouts and sump pumps are not set up correctly, it can cause foundation problems, erosion or wet spots in the yard.

To start, make sure that the land around the house slopes away from foundation walls and fill in any depressions near the downspout. Ideally, a 5% grade should be maintained for a distance of ten feet around your home.

1. SPLASH BLOCK AND STONES: PROTECT THE PIPE OUTLET

Install a concrete or plastic splash block at the end of the pipe to direct flow away from the foundation wall and prevent erosion.

If erosion still occurs, place stones along the flow path at the end of the splashguard to protect the underlying soil. The stones should be fist-sized or smaller.

2. DOWNSPOUT EXTENSIONS: CHANGE THE DISCHARGE POINT

Downspout or sump pump extensions (non-perforated corrugated plastic pipes) can be attached to the downspouts to direct flow further from the house. The pipes can either be laid on the ground surface or buried underground.

Whenever you redirect stormwater, make sure it is discharged to a suitable area. Downspout extensions can be used in combination with swales, rain gardens or other structures that redirect or capture runoff, especially if a suitable area is not within a reasonable distance.

A splash block beneath gutters or downspouts prevents erosion Downspout extensions direct water away from the foundation of the house

CAPTURE AND STORE RUNOFF

Runoff can be captured and stored for use or allowed to soak into the soil. Rain barrels and cisterns are used for storage. Dry wells, soil amendment and rain gardens are used to infiltrate water into the soil.

These solutions have many environmental benefits: reducing runoff from your property, filtering runoff, watering your yard and recharging groundwater. They can also be combined with structures that redirect runoff. For instance, a grassy swale or downspout extension can be built to deliver runoff to a dry well or rain garden.

However, these practices may not be appropriate for every location.

Rain gardens or dry wells should not be located next to the foundation of the home to avoid water problems in the basement or foundation.
If your soil has a poor infiltration rate, an underdrain may need to be included in the design of a rain garden or dry well. Soil with high groundwater or very shallow depths to bedrock are also problematic. Find your soil type using Soil Survey maps. Before installing a rain garden or dry well, you should also do an infiltration test.

If more than 2500 square feet will be disturbed in constructing one of these practices, you will need a permit in Fairfax County. See: Site-Related Plans. In other jurisdictions, refer to local codes and ordinances.

1. RAIN BARREL OR CISTERN

A rain barrel captures water for later use. A rain barrel is a large container that attaches to the end of a downspout and stores water for later use, such as watering your garden or lawn. Rain barrels come in many shapes and sizes. A typical rain barrel holds about 50 gallons of water. Larger cisterns can hold hundreds or even thousands of gallons. Rain barrels are simple to install and inexpensive. Unless you have a cistern, this option generally does not provide enough storage capacity to greatly reduce the amount of runoff flowing to the lawn during large storms. Rather, rain barrels are most useful as an addition to other stormwater controls.

An overflow device allows excess water to escape when the barrel or cistern is full. Use corrugated plastic tubing to direct overflow away from the foundation of your home.
A downspout can be connected to one or more rain barrel(s). Connecting rain barrels increases the amount of runoff that can be captured.
For more information, see: Rain Barrel Workshops

2. RAIN GARDEN

Rain garden illustration A rain garden is a heavily planted landscaped depression situated in the path of runoff flow, with permeable soil. During storms, runoff will pool on the surface of the rain garden and soak into the soil, filtering the water and recharging groundwater.

The proper functioning of a rain garden depends on the permeabilityof the natural soil. In the absence of adequate permeability, an underdrain may be needed to allow filtered water to be discharged, which can add to the expense and time of construction.

In the rain garden, the natural soil is amended or replaced with a mixture of sand, topsoil and organic compost (or a pre-made rain garden mix). The depth of these amendments can vary depending on the effort and resources that you wish to commit, but eighteen inches should be considered a minimum. Underneath the amended soil, an optional gravel or sand bed stores stormwater so that it may soak into the underlying natural soil.

Rain gardens can be a good solution to large, widespread drainage problems in a backyard. They can come in many styles and sizes. Larger and deeper rain gardens will treat more runoff, smaller and shallower rain gardens will treat less, but all properly functioning rain gardens will reduce polluted stormwater and help to recharge groundwater.

More information and instructions for designing and building a backyard rain garden are available from NVSWCD's Rain Garden Design and Construction: A Guide for Homeowners or the Seven Myths about Rain Gardens article.

3. SOIL AMENDMENT OR COMPOST BLANKET

Often urban and suburban soil is dense, compacted and has low fertility. Adding organic matter or compost to the soil reduces soil density and improves soil structure. Structure refers to the clumping together of soil particles. Soil with good structure is more porous, promoting infiltration and reducing runoff. Soil amendment also improves the health and productivity of your soil by adding a long-term source of nutrients and organic matter. There are two general approaches.

Lower intensity method: Spreading a compost blanket Low intensity method:

Compost blanket. Put down a thin layer of compost and rake it in to your lawn, making sure that you do not bury your grass or other existing vegetation. If you are applying compost to a bare spot or un-vegetated section of your property, you may add a thicker layer.

What should you use for compost? You can purchase compost commercially, although you can also use leaf mulch. There are pick-up sites across the county for free leaf mulch. For an even simpler approach, you can recycle the organic matter you have on site by leaving grass clippings on the lawn (grass-cycling) and mowing your fall leaves into your lawn instead of bagging them. The clippings and mowed leaves will break down on their own and become compost in the soil. It may take a few seasons to see noticeable change in your soil.

Higher intensity method: Soil amendment High intensity method:

Soil Amendment. Think of a farmer plowing a field. Heavy-duty soil amending is basically the same thing. Instead of adding seeds, you will be mixing compost into your soil. This option is more costly and more intense (you’ll have to tear up your lawn!) but the results will be immediate and noticeable.

First, choose the area of your property that you wish to amend. Based on that area, you can calculate how much compost you’ll need. There is no set rule, but if you are going to use the more intense method, we recommend using at least four inches, if not more. You can either hire a landscaper or rent a rototiller or small backhoe. Till down 10 inches to two feet to mix in your compost. You can purchase compost commercially, or you can also get leaf mulch from the county for free. Be aware that the free leaf mulch from the county is not always of consistent quality, will not be fully decomposed and may contain weed seeds. Replant with your preferred vegetation immediately.

Terraces can often be backfilled with soil amendment to slow down runoff and also improve infiltration. For more information, see: How to Protect Eroding Land - Terraces

4. DRY WELL (INFILTRATION TRENCH)

Dry well illustration A dry well or infiltration trench is a gravel-filled underground storage area. Water captured by a dry well is stored in the void spaces between the gravel until it infiltrates slowly into the ground or flows out through an underdrain.

The sides of the dry well (not the bottom) are lined with landscape fabric.
Water can be directed to a dry well from a downspout or other pipe.
It may be used as a walkway or covered with topsoil and sod.
It is best to excavate until a permeable soil layer is found. A dry well is only a good option if a permeable layer of soil is found within a reasonable depth. If no such layer exists, a dry well might not be a good solution.

REDIRECT RUNOFF

Intercepting and redirecting runoff provides an opportunity to safely discharge the volume to a place beyond the problem area. This can be done using swales, French drains, catch basins or downspout and sump pump extensions. These methods of rerouting can be combined together with capture and storage practices (see below).

Whenever you are redirecting runoff, you must send it to a suitable outlet. Discharging runoff to an unsuitable area will just move the problems downhill. Be aware that redirecting runoff without soaking it into the soil can negatively impact neighboring properties.

1. SWALE

A swale is a wide, shallow ditch in which water can flow to a suitable outlet. The channel should be protected with grass or other vegetation. It also can be lined with appropriately sized stone to prevent erosion within the swale.

Stone is preferable in swales in which grass does not grow well due to shade or that are too steep or long for grass to prevent erosion. A swale lined with stone is sometimes called a dry creek bed. Stones used can be of varying sizes, with larger ones acting as stabilizers and smaller ones as fillers. Generally, the heavier the flows the larger the stones should be. Tightly woven landscape fabric should be placed below the stone lining.

A mowed grass swale Dry creek bed

2. FRENCH DRAIN

A French drain is an underground drainage device. It consists of a perforated pipe surrounded by gravel and lined with sturdy landscape fabric. A French drain conveys runoff underground to a suitable outlet. Downspout pipes and sump pump pipes can be connected to it, and a catch basin can be combined with it to help remove standing water. Deeper French drains can also be used to drain groundwater.

Tips for building a French drain. Pre-assembled French drain options (such as EZ-Drain) can save time and effort in installation.

Dig out a ditch where your French drain will go. A standard French drain ditch is about 1.5 feet deep and 10-12 inches wide, varying based on the size of the pipe chosen (usually 4-6 inches) and the desired depth.
Line the ditch on all sides with landscape fabric to prevent soil erosion.
Place a layer of gravel at the bottom of the ditch.
A perforated plastic pipe is laid on top of the gravel and surrounded by gravel on the sides and top.
If topsoil and sod will be used to cover the French drain, the landscape fabric should be pulled over the top of the gravel before adding the topsoil.
If the gravel will be left exposed at the ground surface, the landscape fabric does not need to be pulled over the top of the gravel, but the edges should be protected to prevent sediment from entering the French drain.

French drain: the perforated pipe is buried horizontally underground and surrounded by gravel French drain photo

3. CATCH BASIN

A catch basin is a collection box with a slotted drain at the top and a drainage outlet at the bottom. Surface runoff enters the inlet, passes through the collection box and exits through the outlet into a buried drainpipe. The catch basin should be placed at a low spot on the property so that water naturally runs to it (a grassy swale can be built to direct runoff to the basin). The buried drainpipe should discharge to a suitable outlet.

Catch basin illustration. A catch basin is a box underground into which water flows to a suitable outlet. Catch basin photo. A catch basin is a box underground into which water flows to a suitable outlet.

4. SPLASH BLOCKS AND DOWNSPOUT EXTENSIONS

Splash blocks and corrugated plastic pipe also can be used to direct roof runoff from downspouts or sump pumps away from foundation walls to a suitable area. See: Splash Blocks and Downspouts

GROUNDWATER

Groundwater is water below ground, filling up pore spaces in soil and joints in rocks. The water table is the depth to the surface of the saturation zone, the area of soil and rock that is saturated with groundwater.

Groundwater that is naturally close to the surface can cause wet conditions. High groundwater is generally found in low lying parts of the landscape, especially in floodplains next to perennial streams. Water seepage can also be found on slopes from natural springs. Groundwater is generally closest to the surface in colder months, but wetness can occur throughout the year. Click on image to enlarge.

GULLY AND GULLY EROSION

A gully is an incision or ditch created by the flow of running water. In the process of gully erosion, running water picks up soil or sediment in its path and moves it downstream, often depositing it into streams, lakes and rivers. Click on image to enlarge. Compare with Sheet Erosion.

INFILTRATION

Infiltration is the rate at which water passes through soil, generally measured in inches per hour. Soils with high clay content or compact soils have slower infiltration while loose or sandy soils have a higher infiltration.

It is important to conduct an infiltration test prior to installing a rain garden or dry well. If your soil has poor infiltration at the bottom of the rain garden, you will likely need an underdrain. You can use the following steps to assist you in conducting an infiltration test.

How to Conduct an Infiltration Test:

Dig a 12-inch (or deeper) hole at the proposed site of your rain garden or dry well and fill it completely with water.
Allow the water to sit for at least an hour so that the soil can saturate.
Refill the hole so it is full.
From this point on, observe to see if the water drains into the soil.
If the water drains within 48 hours, the site is suitable for a rain garden or dry well; however, it is highly recommended that the soil be amended with some compost and sand.
If the water does not drain in 48 hours, select a different location or amend or replace the soil and install an underdrain. It may be easier to choose another location.
You may choose to run the infiltration test more than once in the same hole or in different holes to ensure an accurate result.

LANDSCAPE FABRIC

Also called filter fabric or geotextile fabric. Landscape fabric is now typically made of synthetic (plastic) woven material. It is used to cover bare soil to prevent erosion. The tightness of the weave varies. In situations where water should not pass through the fabric, the weave should be tight. Looser weaves should be used when water is intended to pass through the fabric. Natural (non-synthetic) landscape material is a biodegradable option that can be used as an erosion control blanket until plant roots take hold.

MISS UTILITY

Miss Utility logo Miss Utility, also known as Virginia Utility Protection Service (VUPS), is a free service that provides markings for underground utilities, including power, sewer, gas, water and cable lines. Miss Utility can be contacted by dialing 811 (in Virginia) or 1-800-552-7001.

NATIVE PLANTS

A pinxterbloom azalea is a beautiful native addition to your landscape. A native plant is a plant found in the ecosystem in which it has evolved. It exists in equilibrium with other species in the same ecosystem. In the United States, plants found in the country prior to European colonization are considered native. People everywhere are discovering the beauty and benefits of native plants, from canopy trees to shrubs to meadow species and native groundcover. They help restore the ecosystem as more land is cleared for buildings or roads. They can require less maintenance than other plants. They also provide food and shelter for people and wildlife.

Non-native invasive plants can take over a landscape, choking all other vegetation. Invasive plants are aggressive non-native plants. These plants grow out of control when taken out of their native ecosystem. Kudzu or "the plant that ate the South" is an invasive that is commonly known. Invasive plants frequently escape from yards. Examples include English Ivy, Bamboo, Japanese Honeysuckle, and Garlic Mustard. For a full list of invasive plants that are problematic in Fairfax County, as well as how you can help, see Invasive Management Areas.

PERMEABILITY

The rate at which water passes through soil. Soils with high clay content or compact soils have slower permeability while loose or sandy soils have a higher permeability. See also: Infiltration.

PLAT

A map showing the boundaries and key features of a property. Similar to a plan or blueprint of a house or interior space, a plat of a lot shows the characteristics of the exterior space.

PROBE

Soil probe in a sinkhole. A soil probe is a long metal rod with a handle. It is particularly useful for determining the depth, size and extent of a sinkhole. If a probe is not available, a piece of rebar, shovel, fireplace poker or other implement can be used.

RESOURCE PROTECTION AREA

Perennial streams, coastline, wetlands, or other water bodies, and a buffer area (usually 100 feet) are protected as Resource Protection Areas, also known as Chesapeake Bay Preservation Areas. Soil disturbance, land clearing or development within an RPA is highly restricted and contingent upon permission from the County.

To determine if your property is in an RPA, use the Fairfax County Watershed Viewer. Enter your address to find your watershed, nearest stream and Resource Protection Areas. You can also contact the Conservation District(703-324-1460, TTY 711). District staff can help explain your options if your land is in an RPA.

RIPRAP

Riprap. Photo credit: US Army Corps of Engineers. Riprap consists of large rocks used to reduce water velocity at the outfall of pipes or to reduce erosion by flowing water on soil surfaces or in stream beds.

RUNOFF

Runoff is the flow of water over land that does not infiltrate into the soil or evaporate into the atmosphere. Runoff can come from rain, melting snow or even hoses or sprinklers. Problems with runoff are exacerbated by impervious surfaces that prevent infiltration such as roofs, roads and driveways. Click on image to enlarge.

SHEET EROSION

The removal of a uniform layer of soil from the surface by water runoff. Click on image to enlarge. Compare with Gully Erosion.

SOIL TEST

Applying too much fertilizer can kill your vegetation. A soil test is the best way to determine if your soil needs a pH or nutrient change. A soil test kit can be obtained from Fairfax County libraries or the Soil and Water Conservation District office. You can mail your soil sample to Virginia Tech and receive a detailed report for a small fee of $10.

Tips for reading your soil test report and applying fertilizer or lime:

A soil test can help you avoid fertilizer burn, when too much fertilizer kills grass or other vegetation.
Adequate quantities of nutrients may be present in the soil, but if the soil pH is too acidic, nutrients may not available for plant use.
Fertilizer recommendation is in pounds/acre (lbs./ac) and should be added once every year.
Lime recommendation (for pH) is in tons/acre and should be added only once in three years.
After a period of three years, a new soil test should be performed.

After correcting the pH or nutrient level, re-seed or replant the appropriate vegetation of your choice.

SPLASH BLOCK OR SPLASH GUARD

A plastic or cement plate placed at the end of a downspout to disperse and slightly slow down runoff and protect the underlying soil. Click on image to enlarge.

SUITABLE OUTLET OR AREA

A rain garden is one example of a suitable outlet. A suitable outlet is an area of land that is capable of receiving redirected runoff without causing erosion or sogginess. Examples include rain gardens, dry wells, existing swales or gently sloping vegetated land not immediately leading to neighboring properties. Ideally, runoff should be directed to an area where it can soak into the soil, filtering and cleaning the water. Downspout extensions carry water away from the foundation and discharge onto a gently sloped, grassy area in the lawn.

It is best to avoid connecting one impervious surface to another when discharging runoff. For example, from a water quality perspective, it is not preferable to discharge your roof runoff directly onto your driveway, flowing onto the street, into a storm drain and to the nearest stream, where the sheer volume of water flow can cause damage.

Conversely, an unsuitable or inadequate outlet is an area of land that is not capable of receiving redirected stormwater runoff without causing erosion, sogginess or other problems. Examples of inadequate outlets are areas with compacted soil that will cause stormwater to stagnate rather than soak into the ground, steep slopes that will produce rapid runoff and erosion, or areas next to foundation walls or other structures that could cause home flooding if used as a receiving area for runoff. Another example of an unsuitable discharge area is your property line. Don't solve your own issue by creating a problem for your neighbor.

SUMP PUMP

Sump pump diagram. Credit: Thrasher Basement. A small reservoir at a low point used to collect water. Many households have a sump designed into the foundation or slab to collect groundwater so it does not flow into the basement. A sump pump is a device that pumps that water through a pipe to the exterior of the house.

SWALE

Swale A wide, shallow channel designed to convey water safely. It has gently sloping sides. The swale can be lined with grass or stone to prevent erosion. Swales can trap sediment and pollutants if the water is flowing slowly.

TAMPER

Compacting with soil tamper. A tamper is a tool used to compact soil. It has a long handle with a flat metal plate at the end. To compact soil, the back of a shovel may also be used.

UNDERDRAIN

An underdrain refers to an underground drain pipe used with rain gardens or dry wells to allow excess stormwater to be carried away during heavy storms. An underdrain is often necessary in soil with very low permeability. Click on image to enlarge.

UNDERGROUND DRAINAGE SYSTEM

This is a less preferred option for handling widespread wet conditions. An underground drainage system is a series of perforated pipes buried underground to capture flow from high groundwater. It is similar to a French drain, but at a deeper level. The pipes are installed with a very slight grade. Groundwater enters the pipes and flows to a suitable outlet. This is sometimes referred to as drainage tiles, especially in agricultural settings.

WETLAND

Wetland A wetland that meets the Army Corps of Engineers definition is a jurisdictional wetland and is protected under the Clean Water Act. Any disturbance of a jurisdictional wetland in Virginia is prohibited without a permit from the Army Corps of Engineers Norfolk Office or the Virginia Department of Environmental Quality. Some wetlands in Fairfax County are within Resource Protection Areas protected under the Chesapeake Bay Preservation Ordinance.