02-01-04 | News

Drainage Principles
and Erosion Control

By Jeffrey B. Tyler, NDS, Inc.,
product manager, drainage and landscape

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.

Adequate drainage is more than a necessary evil; it is a major influence on proper water management and plant health, and is a first step toward effective erosion control.

Are you doing site development? There’s plenty of information for a variety of land uses available to guide your development or landscape efforts. However, drainage is a subject often overlooked or downplayed. For example, capillary moisture is essential to the maintenance of good soil characteristics and healthy turf. Gravitational moisture, and in many cases surface runoff, constitute excess water that is detrimental to the 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.

Water from rainfall or irrigation that does not infiltrate the soil becomes 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.

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.

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.

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.

Sidewalks are a common obstruction. Drains or piping must cross under walks or other pedestrian areas to reach the street or stormdrain.

Benefits of Drainage

Surface Drainage

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

Excess rain or irrigation water will naturally flow to areas of lower elevation. The excess water may remain ponded, causing poor aesthetic conditions and destroying turf or damaging buildings, homes and hardscapes. Surface drainage devices can prevent these undesirable conditions and can often be located so they will not interfere with the planned use of the site.

Subsurface Drainage

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:

1. Soil firmness and structural capability maintained
2. 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).
3. Helps the soil warm earlier in the spring.
4. Provides increased aeration in the root zone; air is necessary in the root zone for healthy growth.
5. Deepens the root zone in drained soil, compared to undrained soil, especially during the spring and summer seasons.
6. 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.
7. 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.

Common Drainage Applications

Gravity is the primary vehicle for carrying away stormwater runoff. Here are a few general guidelines for conveying runoff water to a discharge point. There must be a continuous minimum fall in the ground level to assure adequate drainage for:

• grassy swales, a minimum slope of one to five percent;
• pipe with a smooth interior, a minimum slope of one-eighth inch per foot or one percent.

The minimum slope needed for drainage systems involving these common applications must be determined:

swales
dry wells
subsurface foundation
retaining wall
landscape surface
driveways
parking areas
planters
slope retention
road surfaces
road bed erosion protection
landfills
golf course greens, tees, fairways, sand traps
cart and truck washdown areas
swimming pool decks
walkways, paths
children’s play areas
baseball, football, soccer fields
tennis courts
spring interception or collection

Leading Indicators of Drainage Problems

Erosion is a big problem in drainage. Slopes carrying runoff water must be carefully calculated to ensure continuous flow, yet not steep enough to erode. Slow moving water will create a bog, while water moving too fast will cause erosion, form gullies and weaken foundations. A properly designed drainage system should eliminate both extremes. However, if not well designed, the following indicators of drainage problems may be present:

Indicators of Drainage Problems

• client observations;
• your personal observations;
• standing water 24 hours after a hard rain or irrigation;
• odors from stagnate water;
• concentrations of yellowing or patches of turf that has minimal original turf;
• yellowing plant life;
• areas of turf that are thin despite plenty of sun and no obvious disease;
• trees and landscape material that are dying for no obvious reason;
• areas that are constantly being treated for fungus problems;
• water debris that has accumulated in affected areas (leaves, pine straw, trash, etc.);v
• water stains on fences, buildings and hardscapes;
• mud or silt deposits on walkways, porches, or flat hardscapes;
• structural damage related to moisture or excess water problems;
• water intrusion into the home through doors sills, basements, garages, etc.

Surface Obstructions

Sidewalks are a common obstruction to deal with. Large amounts of water should not cross a sidewalk to reach the street or the stormdrain. Use drains or install piping to cross under walks or other pedestrian areas to prevent hazards. Other leading types of drainage problems caused by surface obstructions include:

Surface Conditions that Cause Drainage Problems

• Soil level and topography: If the property is not graded correctly it will not drain properly.
• Tree root systems may cause impediments to surface and subsurface water flow.
• Shrubbery root systems and certain types of ground cover will block or impede surface water flow.
• Landscape timber, bricks, etc., will cause water flow obstructions and will inhibit or create directional water paths on the surface.
• Plastic edging, and a wide variety of other types of edging, may affect surface drainage problems.
• Flat work or hardscapes such as sidewalks, driveways, patios,etc., may have been installed too high and either inhibit, block,or retain water.
• Poor soil conditions that are indigenous to the region or brought in for construction purposes.
• Swimming pools or buried structures that severely limit the soils holding capacity.
• Root barriers may inhibit and block water movement.
• Broken or poorly adjusted irrigation systems.
• Adjoining properties that may have all of the above potential problems that are routed onto your client’s property.

Drainage problems can be classified into three categories. Until you determine the base problem, you cannot determine the root cause of the drainage problem.

Three Categories of Drainage Problems:

• Nuisance: Water standing for extended periods of time. This contributes to mosquito infestation (e.g., West Nile virus), will be harmful to the turf and shrubs, and limits the recreational use of the affected area.
• Potential Damage: Water damage to outbuildings, such as storage sheds, fences, flooded patios, etc.
• Damage: This category will cover all instances where there is water intrusion into the home or outbuildings and/or plant damage.

Design

Keep it simple! Over-design of a stormwater drainage system is expensive. However, some basic features, like cleanouts, should be installed for added convenience or to comply with local codes. Engineers and architects typically divide drainage into surface and subsurface.

Surface Drainage

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.

Subsurface Drainage

Subsurface drainage has previously been defined as the removal of gravitational water from the soil. The source of subsurface water is percolation (water moving vertically and laterally underground) that is generally attributable to precipitation or irrigation. The lack of volume and velocity at which water flows through a subsurface perforated drain pipe allows sediment to settle and potentially clog (or reduce flow in) the pipeline, limiting the life span of the system. When soil becomes saturated, water movement is reduced and water may be unable to flow to the subsurface drain. These shortcomings underscore the need to install surface drains in conjunction with subsurface drains to minimize the ground water volume introduced into a subsurface drain.

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 excess water. The design is simply a continuation of what size the catch basin or channel drain system needs to be, what size and type the conducting pipe system needs to be, and what format the disposal system should take.

Combining Surface and Subsurface Drain Systems

Surface water should not be connected directly to a subsurface drainage system unless the system is designed to handle the combined flow. For example, the excessive volume and velocity of water from a surface drain system tied directly to a french drain may leach out of the perforated pipe defeating its function as a ground water collection device. It is possible to join nonperforated 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 may be separate systems, one to collect and convey surface water, one to collect and convey subsurface water.

Discharge Outlet Design

Once the stormwater is collected and conveyed by the drain pipe, it must be discharged to a safe location. The outflow rate potential must at least be equal to the expected inflow rate. The final step in design of subsurface or surface drainage systems concerns the disposal of collected water in compliance with local codes and stormwater regulations.

Discharge Collected Water

Several options are available to discharge water. You can discharge on site, into a pond or dry well for example, or discharge off site into the street gutter or directly into the storm sewer in compliance with local codes and stormwater regulations. You can combine different options for the best solution. The best solution is often the simplest solution. Two of the most popular include the use of pop-up drainage emitters and drywells.

Pop-Up Drainage Emitter

Pop-up drainage emitters allow water to be diverted and released to water-safe areas away from structures, erosion-prone landscapes and poor drainage areas. Water captured by grates, catch basins, channel drains, downspouts and roof gutter systems flow through the drainage pipe and away from structural foundations to safe or useful areas; this system terminates with a pop-up drainage emitter.

For example, water can be routed from a low area next to a foundation to a water safe area such as a street curb, or the center lawn area, with a sloped grade which will ensure flow of the water from the emitter to a safe area. Install 10 feet of perforated pipe prior to the pop-up emitter. This will allow any standing water remaining in the pipe to leach into the soil.

To control water flow from a roof gutter downspout, it is recommended to install a 9 or 12 inch catch basin below the downspout elbow; pipe the catch basin to a far-off discharge point at least 10 feet away and end the discharge line with a pop-up drainage emitter. Connecting the downspout elbow directly to a discharge pipe is usually not recommended since the flow cannot be visually monitored. A downspout clogged with leaves can backup, split in freezing weather and contribute to ice damming or significant roof damage. In areas not prone to freezing, a clogged downspout will cause rain gutters to overflow, spilling water to the foundation. Proper drainage mandates carrying this runoff away from the building.

Pop-up drainage emitters are opened by the hydrostatic pressure of water flowing through the drain pipe. As flow diminishes the emitter closes again. For ideal performance, the top of the emitter should be installed at a lower elevation than the invert of the pipe inlet (downspout connection, connection to a basin, etc.), to ensure proper flow. Maintain a minimum slope in the drain pipe leading to the pop-up drainage emitter; a weep hole in the pop-up drainage emitter elbow drains any standing water in the pipe. Since the emitter is closed during dry weather, debris and rodents cannot enter the drain pipes.

Pop-up drainage emitters should also be used to terminate an overflow discharge line coming off of a drywell. This allows any excess runoff water, beyond the capacity of the drywell, to discharge into the surrounding soil (located at least 10 feet down slope from the nearest structure). Gravel backfill will increase a drywell’s capacity, however in poorer soils (e.g., clay) larger or multiple dry wells may be required.

Whether for downspouts or eliminating puddling (below) the bottom of the drywell should never be installed less then two feet above the peak water table level, otherwise, the drywell will simply become a reservoir for ground water. The top of the drywell should be recessed below grade at least eight inches, usually deeper in freeze zones to prevent back-up.