SURFACE DRAINAGE

Surface drainage begins with shaping and smoothing the land into a watershed that directs runoff to ditches, catch basins, storm sewers or French drains. They also must try to prevent erosion by managing surface water so it will not reach damaging proportions.  Surface drainage alone is generally inadequate to assure soil moisture levels best suited for low-cut, high-traffic turf.  Poor soil moisture conditions affect the condition of the plant material and restrict the use of the site. However, don't reject the idea that reshaping will be necessary to deliver a long-term solution to a drainage problem.

Any surface under 2 percent (50:1) slope is considered flat. However, water will move across a slope of 0.5 percent (200:1) if the surface were perfectly smooth. This slope can be less for sandy soils.   What appears flat can be sloped enough to move surface water to a collection point or ditch. Vegetation growing on the slope creates resistance for water moving across it. This is why turf grass is an excellent means of erosion control.
The required slope for surface drainage is greater than that of a ditch, French drain or subsurface drain.  Provide a drop of one to two feet per 200 feet of length. Water will move at a lesser slope, but you also need to provide enough slope so the velocity of the water in the pipe will move solids mixed with the water.

Catch basins and other collection devices should be located to prevent their blockage by leaves, eroding soil, or future pavement. The size of the basin should be matched to the proper size of pipe. Don't skimp on pipe or fittings.   Like irrigation, only a certain number of catch basins can drain into a single pipe. Obviously, the storm sewer or other receptacle  for the water from the drain pipe must also be adequate. 

SUBSURFACE DRAINAGE

Subsurface drains depend upon infiltration and percolation of water through the soil. Soils vary in these characteristics. Water begins to enter the perforations in buried drain pipe when the soil next to the pipe becomes saturated.   Once the soil is saturated, the water accepted by the pipe is limited by the size and number of openings. The size of the laterals and drainage mains can also restrict flow once they reach capacity.

Heavy clay soils will require closer and shallower installation than sandy or loamy soils. These details will also be influenced by the typical rainfall of the locality and the "worst case scenarios" for the location.

Sand is not a solution by itself. Sand is very limited in water and nutrient holding abilities. It has very limited chemical buffering if you make a mistake with any fertilizer, herbicide or fungicide.  Properly selected organic amendments, such as peat, can give you the insurance you need without sacrificing drainage. Agricultural specifications will not suffice for urban landscapes.

Perforated drain pipe buried 3-5 feet deep and 40 feet apart will be insufficient in areas with heavy soils and frequent rainfall. Turf applications run in the range of 18 inches to 3 feet deep and 15-20 feet apart.  Where leaching out salts from rootzones is necessary, closer spacing will be needed for the drain pipe. A well-designed drainage system can remove salts from the primary root zone when used in conjunction with a flushing irrigation cycle.

Vertical drainage structures are an alternative to buried perforated pipe. They can function as both a collector of surface and subsurface drainage. These prefabricated drainage structures are usually installed in narrow trenches.  The top of the structures should be below the depth reached by aerifier tines. For surface drainage, the drain should not be covered with soil. Instead, cover it with sand to the surface.

French drains are trenches filled with pea gravel sloped to carry water away from an area. They are an option to concrete drainage culverts. The gravel must extend to the surface and could present a problem for mowing equipment.

Channel drains might be a better solution than French drains in some areas. These are essentially buried gutters covered by long and narrow grates. Like any drainage structure, they are sloped and lead to a storm sewer or drainage sump. The grates can be removed to clean the gutter beneath the surface.