By Jeffrey B. Tyler, NDS, Inc.
A channel grate is used to capture water where it runs off hardscape surfaces, as seen with a sidewalk here. Longer grates can keep water from flowing over and off patios, parking lots or athletic courts.
Drainage is a subject that is intertwined with irrigation but is often overlooked or downplayed. For example, capillary moisture is essential to the maintenance of good soil and healthy turf. Gravitational moisture, and in many cases surface runoff, constitute excess water that is detrimental to turf and other plant life. Excess water retards plant growth, so gravitational water must be removed from the soil if healthy turf and plant life conditions are to exist. Surface runoff must be removed from all areas so that erosion will not occur and water will not be retained in surface depressions.
Now, let’s focus on aspects of exterior drainage, from the identification of problems to the design and installation of solutions. To start, some background information is important and is intended as a guideline for exterior drainage. The landscape architect or engineer should always be consulted for the critical areas of drainage design.
It’s important to carefully plan so the top of the drainage grate sits at the ground level. A common mistake is leaving the grate too high for water to easily flow into the system.
Whether adjacent to a building, or at a low point in the middle of an athletic field, turfgrass doesn’t like standing water. Dead zones marked by mud (even during dry spells) are a sign that drainage in an area may be insufficient.
Surface Water Sources
Water from rainfall or irrigation that does not infiltrate the soil appears as surface water. Surface water runoff is a major concern in urbanized areas where development results in a high percentage of impervious surfaces such as roofs, driveways and streets. Surface water may flow to adjacent areas (runoff) and contribute to soil saturation in another zone. Some surface water may be retained on the ground surface in depressions which, if soil permeability is extremely low, will puddle or pond.
This square catch basin and other surface drainage devices can prevent ponding and damage to turf, hardscapes and structures. These drainage devices can often be located so as not to interfere with the site plan.
Subsurface Water Sources
Most subsurface water results from surface infiltration, although water can enter the subsoil from aquifers or adjacent areas. Another potential contributor to excess soil wetness is a perched water table that generally forms above an impermeable soil layer.
Benefits that occur due to surface drainage systems are:
- erosion control
- controlled removal of surface water
- healthy soil and plant life
- prevents structural damage to buildings
Surface drainage begins with shaping and smoothing the land into a watershed that directs runoff to ditches, catch basins, storm sewers or other drainage systems. Without proper surface drainage, subsurface drainage efforts are considerably more difficult. Surface drainage has been defined as the controlled removal of surface runoff resulting from precipitation, irrigation, spring thaws, or hillside seeps. In most cases, turf will not survive or hardscapes and buildings may be damaged.
Stormwater runoff must never be deliberately directed from one property onto another property. Although it is acceptable for water which flows naturally from one property to another to continue, never increase this flow artificially through grading and piping.
A view of the same driveway area after the installation of a channel grate and its accompanying drainage line. The system maximizes safety and useable outdoor space while protecting building foundations (and subsurface floors and basements) from water damage.
A view of a hardscape in a low position (adjacent to a building) without an effective drainage system.
Removal of gravitational water from the soil profile provides many benefits. These benefits are often inconspicuous because they occur within the soil and the root zone.
The benefits of subsurface drainage:
- Soil firmness and structural capacity
- Controls timeliness of maintenance operations. Continued removal of excessive soil water during the recreation season permits extended, more intensive use, resulting in increased revenue (e.g., golf courses after a summer rain).
- Helps the soil warm earlier in the spring.
- Provides increased aeration in the root zone; air is necessary in the root zone for healthy growth.
- Deepens the root zone in drained soil, compared to undrained soil, especially during the spring and summer seasons.
- Increases the supply of available plant nutrients. Many plant nutrients must change in their chemical form during the period between when they are applied to the soil and when they become available to the plants; air exchange in the soil promotes this process.
- Decreases the damage due to freezing. Frost heaving can raise and buckle concrete slabs, sidewalks, and hardscapes. Drained soils have less water to freeze, thus frost heaving is less of a problem.