Drainage – A Necessity for Healthy Soil
Whether its new construction or an existing drainage issue, Poly Drip has the products and the know-how you need to get the job done.
A well designed drainage system will eliminate troublesome wet areas from your landscape, greatly improve plant and turfgrass health, and help to reduce breeding areas for mosquitos.
Sources of Water Background:
Surface Water Sources
Water from a rainfall or irrigation event 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 be free to flow to adjacent areas (runoff) and contribute to soil saturation in another zone. Some surface water is retained on the ground surface in depressions which, if soil permeability is extremely low, will pond.
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 of Drainage in Relation to Healthy Soil and Plant Life
Benefits that occur due to the controlled removal of surface water by surface drainage systems are:
- Erosion control.
- Removal of surface water.
Excess rain or irrigation water will naturally flow to areas of lower elevation. The excess water may remain ponded, causing poor aesthetic conditions as well as destroying turf or damaging buildings, homes and hardscapes. Surface drainage structures can prevent these undesirable conditions and can often be located so they will not interfere with the planned use of the site.
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 include:
- Maintain soil’s structural capability and improving trafficability.
- Timeliness of maintenance operations. Continued removal of excessive soil water during the recreation season permits extended, more intensive use, resulting in increased revenue.
- 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. Figure I -1 shows the progression of root development during the spring and summer seasons under conditions of drained and undrained land.
- Increases the supply of available plant food by increasing the presence of air in the root zone. Many plant nutrients must change in their chemical form during the period between when they are applied to tile soil until they become available to the plants.
- Decreases the damage due to freezing. Frost heaving can raise and buckle concrete slabs, sidewalks, and hardscapes. Drained soils have less water to freeze and frost heaving is less of a problem.
Topography and Grades
The length and steepness of slopes influence the amount and rate of storm water runoff. As the extent and gradient of slope increase, the amount, rate, and velocity of runoff increase, thereby increasing the potential for erosion. On the other hand, soils that are flat, or with relatively no relief, do not drain well, but they seldom provide any threat in terms of erosion.
Slopes’ influence on surface runoff and subsurface ground water. The greater the slope the more surface runoff and less soil absorption. Conversely, the lesser the slope the more soil absorption and less surface runoff are experienced.
There are three basic forms of drainage control on a steep slope: above ground drainage diversion above the cut slope area; sub-surface drainage within the slope; and pipe drainage from within the steep slope, primarily for draining active seepage such as springs.
- By far the most beneficial and effective of these drainage techniques is the entrapment of surface water above the steep slope and the transportation of it to a safe area. Entrapment of up slope waters can be accomplished through a number of different drainage methodologies, including berming, use of concrete-lined or grass-lined ditches. Entrapment water can thereafter be transported to a point away from the slope and safely released.
- A secondary, and overall less effective method of entrapping and diverting drainage water on the slope is the use of sub-surface interception ditches parallel to the contours of the slope. Improvements have been made, however, through the use of geo-textile fabrics and gravel envelope drains to make sure that if these systems have to be relied upon that they have a more predictable life.
- Lastly, rigid perforated pipe can be installed in seepage areas on a slope, especially in any area that perpetually weeps water, and can be drained out beyond the toe of the slope to keep water from degrading the hillside. These rigid pipes are buried into the slope perpendicular to its face.
Many times all three of these methodologies are utilized on one project.
Comprehensive Drainage System
A complete drainage system incorporates both surface and subsurface drains. Surface drains to remove heavy volumes of rainfall that fall in short spans of time and subsurface drains to remove water which percolates into the soil. Soil has a natural ability to absorb just so much water. At the point the soil becomes 100% saturated with water, it cannot absorb anymore. With no place to go, additional rainfall accumulates on the surface resulting in flooding and erosion. This is another reason it is critical to incorporate surface drains into any drainage plan.
Drainage Design Simplified
Drainage in its most simplified form is a process of collecting, conducting, and disposing of drainage water. The design process is simply a continuation of what size the catchment system needs to be, what size and type the conducting system needs to be, and what format the disposal system should take.
Checklist for Drainage Design
- Check off-site drainage pattern. Where is water coming onto site? Where is water leaving the site? Water flows perpendicular to contour lines.
- Check on-site topography for surface runoff and percolation.
- Determine runoff pattern; high points, ridges, valleys, streams, and swales. Where is the water going?
- Overlay the grading plan and indicate watershed areas; calculate square footage (acreage), points of concentration, low points, etc.
- Check means of disposal (also check local codes).
- On-site (pond, creek, retention basin)
- Off-site (street, storm drains)
- Natural drainage system (swales)
- Existing drainage system (drain pipe)
- Proposed drainage system
Analyze other site conditions.
- Land use and physical obstructions such as walks, drives, parking, patios, landscape edging, fencing, grassed area, landscaped area, tree roots, etc.
- Soil type determines the amount of water that can be absorbed by the soil.
- Vegetative cover will determine the amount of slope possible without erosion.
Analyze areas for probable location of drainage structures.
Identify what type and size drains are required.
Design the system using a combination of surface and subsurface drain systems and underground pipes. Design pipe layout to convey water from the drains to the discharge point in the most direct and simple manner possible.
Combining Surface and Subsurface Drain Systems
Surface water should not be connected directly to a subsurface drainage system, unless it is designed for it. The excessive volume and velocity of water from a surface drain system tied directly to a french drain may actually leach out of the perforated pipe defeating its function as a ground water collection device. It is possible to join non-perforated pipe conveying water from surface drainage systems and subsurface drainage systems when the junction is at an elevation lower than any perforated pipe. The most cost-efficient system in terms of lifecycle costs may include completely separate systems, one to collect and convey surface water and one to collect and convey subsurface water.
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