Site Drainage Overview

 

Site drainage, a byproduct of the slopes and surface types found on the site, refers to the movement pattern of water across the surface (and subsurface) of a landscape, or a specific project site. Different types of land coverage (trees, dense undergrowth, pavement, etc.) will drain and infiltrate surface water at slower or faster rates, depending on the permeability of the surface and the pitch at which it is graded. For instance, a relatively flat parcel of sandy soil may absorb rainwater directly with minimal runoff, while a steeper-sloped paved area or roof will cause almost all water that lands on it to flow across the surface until it finds a basin or drain.

Site drainage patterns can be modified by performing grading adjustments, usually with the aid of a plan containing site contour and elevation data, called a site grading plan. This plan may also have elevation data at specific measured points, called spot grades, which can be helpful for ensuring the appropriate vertical relationships of site elements to each other.

The most fundamental tenet of site drainage sounds simple, but is easy to lose track of when wading through the minutiae of site grading plans: water flows downhill. In order to manage this overland flow and direct water intentionally to a destination, a number of site grading features are used:

Swales

A swale is a designed linear depression designed to direct water in a particular direction, usually to a catch basin, detention basin, or other surface drainage feature. For drainage swales where water may be moving in heavy volume or at a high speed, swales may be channelized, or reinforced with concrete or large aggregates to prevent erosion.

In some cases, water moving through a swale may need to be slowed down or its progress temporarily halted. Weirs or check dams may be installed for this purpose – in addition to their site engineering function for stormwater management, these can also be very attractive features if constructed with care in a fitting design vocabulary with the rest of the site. Monolithic granite weirs or stone wall check dams, often with a built in scupper to control water level and flow, are a common choice of site designers.

Detention Basins

A detention basin is a depression which is designed to receive a large volume of surface runoff from a site, but which may or may not sit dry at other times. Often fed by swales or other subsurface drainage outlets, these can be as large or small as the designer feels is needed or as the site allows. The goal of installing a detention basin is to reduce the volume of surface water capacity which needs to be handled by engineered (and more costly) drainage infrastructure.

Rain Gardens

Rain gardens are specially designed to absorb a large volume of rain water, and can tolerate being saturated for an extended duration, but are also designed as attractive site features when dry. A selection of plants which can tolerate a wide range of moisture conditions, and a particularly well-draining soil profile, are essential components of a rain garden. As with detention basins, their purpose is to infiltrate rainwater directly into the ground, or at least delay its concentration, at engineered drainage infrastructure.

Rain Barrels

Usually connected directly to roof downspouts, these store rainwater directly for future use on-site, most commonly for watering plants as needed. There are a wide range of prefabricated products available, but many people choose to design a custom installation to suit their particular requirements.

Catch Basins

Prefabricated sub-surface collection vessels, usually made of precast concrete, which serve to collect and redistribute surface water through below-grade piping connecting to storm drains or sumps/drywells. Water will typically drain directly into the basin by way of a drain grate at the surface, collecting at the bottom and filling up to a certain level before entering into the pipes (usually there is some clearance at the bottom to allow for sediment accumulation, so it does not get washed directly into storm drains).

 

In order to combat the effects of erosion on the site, especially if it is currently under construction and has lots of bare or unprotected soil, silt fences or hay bales are commonly installed at areas where surface water would drain off the site. These intercept particles, both large and small, which may have become suspended in runoff washing over the surface.

 

Where to Direct Surface Runoff?

Surface water should be directed away from buildings and site structures, at a sufficient pitch to avoid having water back up and cause flooding or other damage. A designer should also avoid altering drainage patterns on a site in such a way to direct new water onto neighboring property, as this may cause property damage and be grounds for litigation.

It is also a good idea to direct water away from surface features made of materials which may corrode, such as metal posts, bench mounts, flagpole bases, etc.

 

Soil Infiltration

Some surface water will infiltrate directly into the ground through the soil. Sandy soils or soils with a lower percentage of fine particles will absorb more water than heavy silt or clay soils.

 

Subsurface Drainage

In addition to surface water, subsurface drainage plays a critical role in managing both stormwater and groundwater behavior on the site.

Perforated pipe is commonly used to evacuate excess groundwater when a soil profile becomes saturated. Its site engineering applications are many, including in plant beds, lawn areas, and to alleviate hydrostatic pressure that can build up behind retaining walls or other grading features. The tiny perforations along the length of the pipe allow water to freely enter and travel with little resistance in the direction of the pipe, while filtering out gravel and other soil particles that might impede water flow.

Concrete is often used for larger diameter pipes connecting catch basins to storm drains. Owing to its relative ease of construction, strength, and low resistance to water flow, concrete is a common choice for civil engineers tasked with connecting to existing infrastructure. A culvert is used for its rigidity and strength when stormwater is piped under a driveway apron or roadway crossing with relatively little other supporting material.

A system of subsurface piping must either tie into catch basins and/or storm drains, or must daylight to again become surface water as a part of a stream or other drainage channel.

 

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