Drainage

Types of drainage solutions

A comprehensive drainage system will include surface and subsurface drain solutions. Surface drains remove the large amounts of water that fall in short periods of time and subsurface drains remove the excess water absorbed into the soil. The two systems work in conjunction to maintain the moisture in your soil at the proper level for protection of your landscaping and your home or building.

Grades

To protect structures, the most important grades on your property are those within 10 feet of your foundation or basement. This will prevent the water you just diverted away from the structure from soaking back through the soil toward your structure. Suitable grades vary depending on who you consult but a safe measurement is a 1 inch (or more) drop for every 1 foot out for the first 10 feet. This results in at least a 10 inch slope for the 10 feet closest to your foundation walls. The rest of your yard should contain a continuous slope downward to keep the water moving away from your foundation.

Surface Drains

Surface drainage can be defined as the controlled removal of water that collects on the land from rainfall, irrigation, snowmelt or hillside seeps. As gravity is the primary force driving this type of system, it involves shaping the land with a continuous fall in the ground level to provide a downhill passage for surface runoff at an appropriate rate of flow. For grass drainage channels, or swales, a minimum slope of 1% to 5% is desired. The contours of the land then direct the runoff to a suitable collection site, such as ditches, basins or storm sewers. At the low point of the ditch or interception point, area drains are installed which are connected to a main or submain and prevents the water from pooling in your yard. The underground pipes need a minimum slope of 1% or 1/8 inch per foot to keep water moving through them. If the ditch is long, several smaller drains should be spaced in a series, rather than one large drain in the middle, to help prevent erosion.

For driveways and other hardscapes, channel drains and exposed French drains are ideal. These linear trenches collect sheets of water that run off, as concrete and asphalt absorb none of the water as it falls. The open area of the channel/ exposed French drain is much greater than an area drain and is better suited to the greater volume of rain it will need to collect. Additionally, channel drains allow designers to modestly slope hardscapes, rather than requiring numerous, extreme slopes to direct runoff to area drains.

Subsurface Drains

While the benefits of subsurface drainage are hard to see because they occur within the soil, the difference will be noticeable in your plants, grass and soil. Subsurface drainage is the removal of gravitational water from the soil, which is accomplished by placing French drains underground to collect and remove water to a drainage outlet. Subsurface drains do not remove water necessary for plants, only excess water, which flows to the drains by gravity. Sub-Surface French drains consist excavating a sizable trench and lining it with a filter or geotextile fabric, which helps prevent soil particles from entering the French drain. The trench is then filled with clean rock/gravel and a proper sized perforated PVC pipe for the application is placed in the gravel.

Once the trench is filled with gravel, it will be covered with a layer a permeable filter fabric, installing a mixture of high quality topsoil/ sand and lastly installing new sod on top (assuming this French drain will be located in a grassy area). French drains function when water in the soil enters the gravel bed, flows into the perforated pipe and travels through connecting solid pipes to a discharge point. A general guideline for placing French drains is to use 4 – 6 inch perforated pipes, bury them 18 to 36 inches deep and space them 15 to 20 feet apart. In the trenches, pipes must maintain a .1% to a 1% slope. Soil construct, acreage and turf usage, however, may require variation from these guidelines and a professional can help you determine the best solution for your situation.

Discharge Outlets

Once water is collected in the pipes, it must be diverted to a suitable outlet to be released. This outlet can be a street gutter, a storm sewer or an onsite pond. Using a pop-up drainage emitter, water can be diverted to a water-safe area on your property away from your home or building. Pop-up drainage emitters are opened by the hydrostatic pressure of water flowing through the drain pipe, releasing water collected from gutters, downspouts, basins, grates, etc. If placed close to the street, the released water can flow over the curb and into the street without having to drill through the curb. The emitters then close as water flow diminishes, preventing debris and animals from entering the end of the pipe and clogging the system. Property owner or maintenance personnel need to make sure they perform routine maintenance on the pop-up emitters. This can be done by removing the pop-up to make sure there is no debris washed down from the roof gutters or surface drains that could potentially slow down the water flow in a heavy rainfall event.

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