Types of Stormwater Management Facilities

There are two types of dry wells:

• A surface dry well (micro-infiltration area), is a small trench filled with stone that collects rainwater from paved surfaces such as driveways and allows it to absorb into the surrounding soil. An infiltration area receives rainwater from surface runoff, not underground pipes like a buried dry well.  Infiltration areas are common on residential lots, where they are typically located next to driveways and at least 20 feet from a building.

• A  buried dry well is a small underground pit filled with stone that collects rainwater from roof gutters and allows it to absorb into the surrounding soil. Underground piping connects the dry well to the roof downspout.  There may be three or more dry wells on one lot and since most are buried and covered with grass, people can recognize their location by the observation well cap that is typically at least 20 feet from the house. 

A grass drainage swale is an open channel that collects water from hard surfaces and allows it to percolate into the ground, reducing the amount of runoff leaving the road or property. The grass covering the side slopes and swale bottom provides a filtration surface for the water and helps to reduce the flow velocity.

• Swales are commonly found along roads, parking lots, or between properties of residential lots.

• In steeper areas, some swales have stone or concrete ‘check dams’ across the width to help slow the flow rate, promote infiltration, and prevent erosion.

• During large storms, swales can direct extra runoff to other stormwater facilities or the storm drain system. 

• Some grass swales are bioswales and have special soils to increase infiltration.

Types of Green Roofs

The two main types of green roofs are  extensive or intensive green roofs.

• Intensive roofs are thicker, heavier, and are designed to support trees and larger shrubs

• Extensive green roofs are typically lighter and thinner, and are designed to support plants like sedum.

Common  non-structural drainage practices include directing rain water (stormwater) from your roof gutters and driveways to nearby vegetated areas where it can soak into the ground. Examples include:

• Rooftop Disconnection: Directing gutter downspouts to grass or planted areas

• Non-Rooftop Disconnection: Driveways or parking lots are sloped so that rainwater flows onto grass or planted areas 

• Sheetflow to Conservation Area: Directing rainwater to natural planted areas

A pond is a type of stormwater management facility designed to collect rainwater and pollutants and prevent downstream flooding.

• Wet ponds always have a pool of water.

• Dry ponds only have water after rain.

Ponds have an embankment (called a dam) to hold back water that is entering the pond. Most dams are earthen (not concrete) and are grass covered. All stormwater ponds also have a control structure that releases water at a much slower rate than the stormwater entering the pond. While water remains in the pond, pollutants have time to settle at the bottom.

Most ponds located throughout Montgomery County are stormwater management ponds, even ones that are community amenities. Ponds can have many different designs, including ponds with concrete channels, dry ponds with sand filters on the pond bottom, and ponds that provided wetland habitat around the edges. Learn more about ponds.

Permeable pavement is built with materials that let water pass through. Replacement of traditional concrete or asphalt driveways with permeable pavement allows rainwater to naturally filter through the ground and reduces stormwater runoff.  Permeable pavement provides a strong, solid surface that can be installed on driveways, walkways, and patios and can also serve as attractive landscaping features, raising property values.

There are three main forms of permeable pavement. In addition to these forms, listed below, there are grass pavement systems and gravel-based pavement systems. Others could be submitted for consideration on a case-by-case basis.

• Permeable interlocking concrete pavement (PICP)

• Pervious concrete (PC)

• Porous asphalt

What is the Difference Between a Rain Barrel and Cistern?

• Rain barrels come in a variety of sizes but typically as a 55-gallon container that collects roof runoff. Rain barrels can be added to any building with gutters and downspouts. All rain barrels require an overflow port. Rain barrels are usually around the size of a trash can, but cisterns can be much larger, ranging from the size of a washing machine to a car, to collect and store more water.

• Cisterns are sealed tanks that can be located above ground, partially buried, or below ground. Cisterns are larger than rain barrels and they can collect water from several downspouts from one building’s roof or from multiple roofs if they are large enough. Large cisterns may require a permit, so please check with the  County’s Department of Permitting Services. 

Rain gardens are saucer shaped gardens that water flows into that are planted with grasses, flowers, shrubs, and sometimes small trees.  The soils and basin fills with water for a short amount of time before soaking back into the surrounding soil. 

A sand filter is a type of stormwater management facility designed to filter rainwater through sand to remove pollutants. Filters are typically a depression in the ground filled with sand that helps to manage polluted or excess rainwater. To the untrained eye, it may look like a sand box or volleyball court. You can find sand filters in residential neighborhoods and around commercial businesses. For information on underground sand filters, click here.

Proprietary underground filtering facilities remove pollutants from stormwater runoff by allowing it to flow through a filtration system. These systems are made by various manufacturers and have special filter cartridges to remove pollutants. These facilities are located underground and include devices such as Aqua-Filter ^®, Bay-Filter ^®, and Storm-Filter ^®, among others. You can visit the manufacturers’ websites for more information on your device. 

Underground filtering facilities collect and filter stormwater runoff from your site. Flow splitters are often used to send a certain quantity of untreated water, known as the “first flush,” to an underground filtering facility. Stormwater flows into the facility through an underground inlet pipe and passes through filtration cartridges that remove pollutants. The filtered stormwater is then discharged downstream through an outlet pipe. This process removes pollutants and returns clean water back to the local stream or into the storm drain system. 

A flow splitter is a stormwater structure that divides stormwater runoff between multiple stormwater facilities or between a stormwater facility and an offsite storm drain system. Flow splitters are located in residential neighborhoods and around commercial businesses.

The flow splitter sends a portion of the stormwater to facilities such as wet or dry ponds, constructed wetlands, infiltration trenches, or hydrodynamic separators for treatment. The portion of untreated water that is sent to a facility for treatment is referred to as the “first flush.” Excess stormwater from large storms passes through the flow splitter and continues to the storm drain system.

Hydrodynamic separators remove oil, grease, trash, and sediment from stormwater runoff. These underground structures include oil and grit separators and proprietary hydrodynamic separators, such as Baysaver ^®, Aqua-Swirl ^®, and Stormceptor ^®. Please visit the manufacturers’ websites for more information about these devices. Underground hydrodynamic separators are commonly located under parking lots at commercial sites or multi-family residential sites (condominium, apartments, etc.). 

During a storm, rainwater collects pollutants as it flows across impervious surfaces, such as rooftops, sidewalks, and roads. Flow splitters are often used to send a certain quantity of untreated water, known as the “first flush,” to a hydrodynamic separator. The oil and grit separator captures and treats stormwater by separating oil, grease, trash, and sediment from the captured stormwater through three chambers. The clean water is then returned to the local stream or to the storm drain system.  Proprietary systems follow similar processes to remove oil, grease, trash, and sediment from stormwater. The designs of  these systems vary.

An underground sand filter is a type of stormwater management facility designed to filter rainwater through sand to remove pollutants. Sand filters can be located on the ground surface or underground. Please see the section for surface sand filters if your facility is above ground. You can find sand filters in residential neighborhoods and around commercial businesses.

During a storm, rainwater collects pollutants as it flows across hard surfaces, such as rooftops, sidewalks, and roads. Flow splitters are often used to send a certain quantity of untreated water, known as the “first flush,” to an underground sand filter.  An underground sand filter is typically contained within a concrete shell with three chambers. In the first chamber, heavy grit, debris, and oil are removed from the stormwater. The second chamber contains a sand filter with layers of gravel, sand, and a filter fabric (filter materials). Additional pollutants are removed as the stormwater passes through the filter media. A pipe beneath the filter collects and directs the stormwater into the third chamber, which releases the clean water back to the local stream or into the storm drain system.

Underground storage structures are underground pipes or vaults that are typically used to store stormwater. Underground structures and vaults 
are used for stormwater storage where there is not enough space for an aboveground facility. The water is then slowly released to local streams, which reduces downstream flooding and erosion. Underground storage structures are commonly located under parking lots at commercial sites or multi-family residential sites (condominium, apartments, etc.).

The underground structure receives stormwater runoff from the surrounding area through stormwater inlets and slowly releases the stormwater through an outlet pipe to a stream or pond. Flow splitters are often used to send a certain quantity of untreated water, known as the “first flush,” to an underground storage structure. The stored water is released gradually to allow time for pollutants to settle out of the water. This slower release rate also helps prevent flooding and erosion downstream. Underground storage structures may have portions of perforated pipe to allow water to infiltrate back into the surrounding soil.