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Exterior Drainage Principles and Erosion Control, Part 2

By Jeffrey B. Tyler, NDS

Part 1 of this article


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).

These sturdy plastic drywells (e.g., Flo-Well(TM) 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.

Whether for downspouts or eliminating puddling (bottom) 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.

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

The optimal design is to connect grates directly to a catch or Spee-D(TM) basin, which captures sediment in the sump area of the basin before the sediment can enter and clog the drainage pipes. In contrast, connecting intake grates directly to pipe can promote sediment blockage.

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).

Design drainage lines with catch basins or cleanouts at appropriate intervals to allow access to clogged pipelines, permitting use of a high pressure water device or a plumbing snake to clean the pipeline.

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(TM) 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.

Water flowing from gutter spouts emptying near foundations may remain ponded. The simple solution is catch basins to connect down spouts into a drainage system or into a drywell system (provided the drywell is at least 10 feet from the building).

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.

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, reducing construction damage to existing landscape and plants.

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.

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.

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

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October 20, 2019, 5:58 pm PDT

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