Keyword Site Search

Exterior Drainage Principles

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.

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 landscape architect or engineer determine the minimum slope needed for drainage systems involving these common applications:

  • 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:

  • 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.);
  • 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.

Drainage Problems Caused by 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:

  • 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

Drainage Problems Caused by Subsurface Obstructions

Common subsurface obstructions can contribute to these drainage problems:

  1. poor soil conditions that are indigenous to the region or brought in for construction purposes;
  2. swimming pools or buried structures that severely limit the soils holding capacity;
  3. root barriers may inhibitand andblock water movement;
  4. broken or poorly adjusted irrigation systems;
  5. 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.

  1. 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.
  2. Potential Damage: Water damage to outbuildings, such as storage sheds, fences, flooded patios, etc.
  3. Damage: This category will cover all instances where there is water intrusion into the home or outbuildings and/or plant damage.


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.

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

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.

Dry Wells

Early drywells, consisting of a pit filled with gravel, were very labor intensive to construct and required considerable excavation and the strain of hauling gravel. Fortunately, modern options favor sturdy plastic drywells. They offer the convenience of using simple hand tools to install, disturb only four square feet of sod, require the removal of only 10 cubic feet of soil to install, and do not need to be filled with gravel (gravel fill only diminishes water storage capacity).

Drywell Applications

For Gutters & Downspouts
To Eliminate Puddles

These sturdy plastic drywells (e.g., Flo-Well?) can be easily stacked or connected side-by-side to increase capacity. However, these are not to be confused with perforated plastic drums or buried 55 gallon barrels; such retrofitted industrial containers are not recommended as substitutes for sturdy plastic drywells. Larger sized drywells are available in concrete; however these have a drawback of requiring heavy duty equipment for installation.

Unlike other drainage systems, drywells have no need for piping systems to transport stormwater to a far-off discharge point; this minimizes the need for considerable trenching to locate and connect to existing drainage lines (thus minimizing construction damage to existing landscape and plants). For this reason, sturdy plastic drywells are a favorite retrofit solution for golf courses needing to evacuate water puddling in low lying areas; this allows for golf play to resume quickly following a summer rain.

Recently, there has been a dramatic increase in the use of drywells as a best management practice (BMP) for stormwater management in direct response to the National Pollutant Discharge Elimination System (NPDES) Phase 2 regulations. This is particularly helpful for controlling silt and sediment runoff from new construction sites of one to five acres. However, some areas do not permit the use of drywells out of concern for the possible impact on local groundwater quality.1 Thus, a reminder to architects and specifiers that drywells should be used in accordance with local codes and stormwater regulations.

Note that proper drywell design requires knowledge of many factors, including the estimated discharge volume over time and the soil percolation rate. For example, sandy soils drain quickly compared to heavy clay soil; poor soil percolation slows the discharge process. Fortunately, online drainage calculators can simplify these calculations; many architects use the drainage calculator found at

Designing a back-up drainage plan with a safety valve to handle possible peak season overflow is smart design; a discharge pipe leading away from the drywell, ending with a pop-up emitter, is a popular and economical solution. In this case, the drywell and the overflow discharge should be located at least 10 feet away from any building.

The bottom of the drywell should never be installed less then two feet above the peak water table level (usually highest in the spring), otherwise, the drywell will simply become a reservoir for ground water. Finally, the top of the dry well should be well recessed below grade at least eight inches or to a depth in compliance with local codes (usually deeper in freeze zones to prevent back-up caused by freezing). Local code may also mandate that the top of the drywell be below the level of any nearby underground utilities (i.e., usually at least three feet below grade).

Other Options for Discharging Stormwater

BMPs exist for many other common options to discharge collected stormwater, including:

Wet Ponds/Retention or wet ponds are basins which contain a permanent pool of water.

Dry Ponds/Detention Facilities

Detention facilities, or dry basins are used as a means of controlling peak discharge rates through the temporary storage of storm runoff.

Culvert Outfall

Discharges collected water into swales, drainage ditches, creeks, and ponds. However, always observe local stormwater regulations and avoid the following when designing discharge outlets:

Design for Troubleshooting Surface Drains

Drainage system design can directly contribute to the ease of troubleshooting maintenance problems after installation. For example, regular maintenance requires removal of debris like leaves, grasses, and mulch. Many problems are associated with debris entering the pipe and reducing flow. To minimize this risk, do not connect intake grates directly to pipe (a common residential practice to "save" the cost of a catch basin). Saving the cost of a $25 catch basin in the design phase will likely contribute to a greater expense in maintenance costs after installation.

A better design is to connect grates directly to a catch basin or to a Spee-D? basin. This recommended installation captures sediment in the sump area of the basin before the sediment can enter and clog the drainage pipes. However, catch basin sumps need periodic cleaning. Sediment and heavy debris can collect in the sump over long periods of time. If left unchecked, the sediment can build-up to a level where it restricts the outlet flow from the basin to the discharge pipe.

Clogged Pipelines

Design drainage lines with clean outs at appropriate intervals to service the pipeline. For example, if all of the surface inlets and basin sumps are clear, then the pipeline may be clogged. Access can be gained through one of the clean outs to use a high pressure water device or plumbing snake to clean the pipe line. Note: plumbing snakes are not recommended for corrugated pipe (since corrugated pipe does not have a smooth interior), and should not be run through a backwater valve installed in a run of pipe with a smooth interior; damage is likely to occur.


Design discharge outlets to allow for ease of access for regular maintenance at least once a year. The discharge outlet is where the drainage line comes to the surface to discharge water; however, a clog at the outlet can compromise the entire drainage system; as a result, the outlet is perhaps the weakest link in the drainage system. Better design can be achieved by understanding the following common causes of failure and the importance of regular preventative maintenance:

Clogged outlet: Outlets must be kept clear of weeds and debris that may cause the discharged water to pond and back up into the discharge pipe.

Dead rodents or animal nests: Rodents and other small animals often crawl into the outlet to build nests if the outlet is an open pipe. Install a rodent prevention device; often a simple round grate with openings less than 0.25 inches wide will work just fine.

Broken or crushed pipe: The outlet may be broken or crushed by heavy equipment. Use an outlet pipe of sufficient strength or reinforce the open pipe end by installing a grate.

Erosion of soil from the outlet: Water flowing from the outlet may cause erosion and cause the pipe to move out of alignment. To prevent this, use erosion control fabric and large size rock or rip rap to stabilize the end of the outlet pipe.

Common Problems and Solutions

Not planning for an adequate drainage system as an integral part of a project can cost you referrals and your reputation. In other words, erosion of a landscaped area can also destroy the aesthetic value that your client expects, and that reflects on you and your business. Avoiding common drainage problems can help to control potential damage; damage that can be as minor as a yellow lawn spot or as extensive as damage to a building's foundation.

Using a "problem-solution" format to explain the importance of an adequate drainage system to your client is usually a good place to start. Photos to illustrate this point are very effective tools. For example, foundation damage that can occur from water accumulation is easily communicated to property owners by using problem-solution photos. Several common drainage problems to be aware of and solutions offered to deal with them are listed below.

Common Drainage Problems and Solutions

Down spouts from gutters empty near foundations: Use catch basins to connect down spouts into a drainage system.

Ground water around foundations: Install surface and subsurface drainage.
Stormwater puddles in low spots in the yard: Install surface drains.
Contoured landscape design causes water runoff to pond: Install surface drains.

Driveway slopes towards garage: Install channel drain in front of garage door.
Hardscape slopes toward the building: Install channel drain around the building perimeter.

Retaining walls: Weep holds, french drains and surface drains can be installed as needed.

raised planters: weep holes, french drains and surface drains can be installed as needed.

Flat turf area that is usually soggy: install a french drain.

New Federal Regulations

Since any runoff collected by a drainage system ultimately has to be discharged, exterior drainage principles cannot be adequately discussed without some mention of new federal regulations. In 2003, new federal stormwater runoff regulations went into effect impacting all construction activity disturbing between one to five acres. Known as the Phase II rule, it is an extension of the National Pollutant Discharge Elimination System (NPDES) stormwater program implemented by the Environmental Protection Agency. Simply put, the new stormwater regulations are about capturing and treating nonpoint source pollution and are designed to eliminate the discharge of pollutants into America's waters.

The impact of the NPDES Phase II rule is far-reaching and includes any method of conveying surface water, including streets, gutters, ditches, swales, or any other manmade structure that alters and/or directs wet-weather flows. These regulations impact:

construction site runoff
postconstruction site runoff

Thus most stormwater drainage from one to five acre sites is covered and needs a NPDES permit backed by comprehensive stormwater management plans utilizing BMPs to handle the runoff. Numerous landscape architects, engineers, developers and municipalities are impacted by this for the first time.

BMPs and Site Development

Controlling excessive amounts of silt and pollutants in the runoff is of primary concern during site development. NPDES permits for stormwater discharge, for example for construction sites, require the implementation of BMPs at that site. Landscaping a newly graded area or a large landscape renovation project are impacted by these requirements too.

Although the range of possible BMPs can be exhaustive, two general categories are particularly applicable to landscape design:

runoff control
erosion control

Understanding drainage solutions within these BMP categories and how they can be worked into your site development is necessary for a successful project. Be familiar with a few of the simple Best Management Practices that can be worked into your site development. Local permitting authorities and municipal public works departments are a good source of information for BMP solutions; civil engineers are a good resource to use for more complicated projects.

Part 2 will run in the December issue.

About the Author

Known for his creative, innovative, and entrepreneurial style, Jeffrey Tyler has extensive marketing and business development experience in the drainage, landscape, and plumbing industries working for NDS, Black & Decker, and IdeationPro.

Drainage Design Simplified

Checklist for Drainage Design

When designing a system, consider the following points and work from the discharge point toward the highest elevation.

  1. Analyze the job site topography:
    a) Check the off-site drainage pattern. Where is water coming onto the site? Where is water leaving the site?
    b) Check the on-site topography for surface runoff, puddling and percolation.
  2. Determine the on-site runoff pattern; high points, ridges, valleys, streams, and swales. Where is the water going? (See aerial view diagram.)
  3. Overlay the grading plan and indicate watershed areas; calculate square footage (acreage), points of concentration, low points, etc.
    a) Check the means of discharge (to comply with local codes and NPDES stormwater regulations).
  4. On-site (pond, creek, retention basin, dry wells*)
  5. Off-site (street, storm drains)
  6. Natural drainage system (swales)
  7. Existing drainage system (drain pipe)
  8. Proposed drainage system
  9. Analyze the other site conditions.
    a) Land use and physical obstructions: walks, drives, parking, patios, landscape edging, fencing, grassed area, landscaped area, tree roots.
    b) Soil type, to determine water absorption.
    c) Vegetative cover, to determine the amount of slope possible without erosion.
  10. Analyze areas for probable location of drainage devices.
  11. Identify what type and size drains are required; be sure to account for anticipated peak flow volumes. 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 dry well or discharge point in the most direct and simple manner possible.

Pop-up drainage emitters allow water to be diverted and released to water-safe areas.

About NDS

NDS is a recognized leader in the drainage and landscape industries. Over 25 years ago, NDS began producing plastic drainage and landscape products. Today, NDS offers a wide range of drainage products, landscape products, fittings, valve and meter boxes, equipment pads, and drip and micro irrigation products. NDS is headquartered in Woodland Hills, CA. NDS offers a complete line of products from six regional warehouses in the U.S. and one in Puerto Rico.

1 U.S. Environmental Protection Agency, Fact Sheet: Stormwater Discharges Regulated as Class V Wells, June 2003.

Related Stories

November 19, 2019, 11:12 pm PDT

Website problems, report a bug.
Copyright © 2019 Landscape Communications Inc.
Privacy Policy