Types of Stormwater Management Facilities, Howard County, MD

The Stormwater Management Department within the Howard County Bureau of Environmental Services is tasked with improving and managing the quality and quantity of water that originates in, falls onto, or passes through the county on its way to the Chesapeake Bay. One of the ways they do that is by monitoring and maintaining public and private stormwater management facilities.

In this article we cover:

• what a stormwater management facility is,
• the different types of stormwater management structures,
• features, benefits, and drawbacks of common stormwater BMPs, and
• how to decide which option is best for your property.

If you’re looking for information about stormwater management in Howard County in general, including your responsibilities as a property owner or manager, see our article on Stormwater Management in Howard County, MD.

We also have an article for you with  information about maintenance and repair of stormwater management facilities.

What is a stormwater management facility?

In basic terms, a stormwater management “facility” is something that facilitates the management of stormwater. You’ll also hear the term best management practices, or BMPs, in reference to structural, vegetative, or managerial practices used to treat, prevent, or reduce water pollution.

Generally speaking, a stormwater management facility is designed to do one or more of the following:

1. Control flooding
2. Control erosion
3. Control pollution
4. Recharge groundwater

The overall goal of these facilities is to protect public health and our environment.

Types of Stormwater Management Facilities

There are many types of stormwater management facilities that can help you achieve one or more of the goals above. In many cases, the structure isn’t noticeable to most people; it seems to be a normal part of the landscaping, natural environment, or surrounding infrastructure, or is hidden underground.

Many stormwater management facilities are ponds (either dry ponds/dry detention basins or wet retention basins). Other types include underground storage structures (usually located under parking lots in commercial areas), vegetated areas, and things like curbside inlets that remove oil and grit from stormwater runoff.

According to the Howard County Bureau of Environmental Services, there are currently over 9,900 active stormwater management facilities located throughout Howard County.

Aboveground vs Underground Stormwater Management Facilities

Structures used to control stormwater can be roughly divided into aboveground and underground types. We’ll start here with the more visible ones – aboveground structures.

Aboveground Stormwater Management Facilities

These types of structures can be further classified into three groups – wet basins, dry basins, and vegetative structures. While this isn’t an “official” classification, it’s a helpful way of looking at structural stormwater BMPs based on how they work.

Dry Basins

This type of stormwater basin is one of the most commonly applied stormwater BMPs. As the name implies, it normally has no water in it. If it hasn’t rained within the past 72 hours, we’d expect the surface of the basin bottom to be dry.

Dry basins work by holding stormwater runoff and discharging it slowly over a prolonged period of time, helping to control flooding. They also trap pollutants from runoff; while the water stays in the basin, particles and pollutants settle out of the water and sink to the bottom.

When the water reaches a certain level, it’s released from the basin (usually into a stream or storm sewer) through an outlet. Most basins also have an emergency spillway to take overflow water away from the basin during a major storm event.

As the basin dries out, pollutants stay at the soil surface while water soaks through the bottom of the basin into the subsurface layers.

Types of Dry Basin Stormwater BMPs

Common examples of dry basin stormwater management structures include:

• Infiltration basins – These are shallow, impounded areas designed to capture stormwater runoff and let it filter into the surrounding soil. The size and shape can vary from one large basin to multiple, smaller basins throughout a site. This system is effective for removing fine-grained pollutants, but larger sediments can clog infiltration basins.
• Infiltration trenches and berms – Similar to infiltration basins but shaped as wide channels or raised areas that capture water on the uphill side.
• Sand filters – Sand filters are large, sand-filled depressions in the ground (they sometimes look like a beach volleyball court or sand trap on a golf course). They’re designed to filter rainwater through the sand to remove pollutants and manage excess rainwater.
• Water quality inlets – Also known as oil/grease separators, these inlets remove sediments, oils, and grease from parking lots prior to discharge to the storm drain or infiltration basin.
• Dry wells – Aboveground dry wells (or micro-infiltration areas) are small trenches filled with stone. They’re designed to collect rainwater from paved surfaces (such as driveways, sidewalks, and small parking areas) and allow it to absorb into the surrounding soil.
• Bioretention systems – This term is often used interchangeably with rain gardens.  The one main difference is that bioretention areas have drainage underneath, while rain gardens depend on the soil for proper drainage.
• Bioswales – These are similar to bioretention areas in the way they’re designed with layers of vegetation, soil, and a perforated pipe within the bottom stone layer. Bioswales typically are located along a roadway and can be planted like gardens or covered in turfgrass.
• Porous/permeable pavement – These types of surfaces, such as interlocking tiles or bricks, porous asphalt, or permeable concrete, allow stormwater runoff to infiltrate the pavement and enter the soil. This removes fine-grained pollutants and helps with erosion control.

Is a dry basin right for you?

This is an excellent stormwater management facility in that it helps to manage flooding, as well as improving water quality by removing pollutants. However, it does require a fairly large area of level, uncompacted (preferably undisturbed), and relatively permeable soil. If the area is too small, the outlet is likely to clog during major rain events, rendering the entire structure ineffective. It’s also not recommended as the sole stormwater management option in areas where runoff is potentially highly contaminated or in “stormwater hot spots.”

Wet Basins / Wet-Type Stormwater Management Facilities

Just as the name suggests, in a wet-type stormwater management facility the basin is always filled with collected runoff.

Stormwater runoff transports solids and pollutants into the pond where heavy particles (sediment, pollutants) settle on the pond bottom. The clean runoff, or supernatant, is later discharged from an orifice located above the pond bottom. This type of facility can also be designed to accommodate additional water above the normal water surface level to provide temporary storage of stormwater runoff during heavy storms.

Types of Wet Basin Stormwater BMPs

Common wet-type stormwater management structures include:

• Wet ponds/retention basins – This permanent pond effectively controls peak stormwater rates and improves water quality by settling out sediment and pollutants. Wet ponds typically have additional capacity to temporarily store excess runoff. Benefits also include increased aesthetic value of the property and habitat for wildlife, but they provide only minimal flood protection.
• Constructed wetlands and shallow marshes – Also known as stormwater wetlands, these are shallow aquatic systems planted with wetland vegetation and built to mimic natural wetlands. Constructed wetlands are very effective at capturing stormwater and removing pollutants from runoff, mitigating peak flow rates, and reducing runoff volume. Beyond that, they provide aesthetic appeal and create a diverse wildlife habitat.
• Subsurface gravel wetlands

Is a wet-type stormwater management facility right for you?

Because there’s standing water present at all times, these types of structures can be a safety issue due to possible drownings and disease-carrying mosquitoes (although mosquitoes are usually not an issue in properly maintained wet basins). They require a substantial drainage area (usually 5 to 10 acres at a minimum) and relatively impermeable soils. You may also need one or more forebays to trap coarse sediment before it enters the basin.

Vegetative Type Stormwater Management Facilities

Vegetative facilities include a number of landscaping practices used to redirect, infiltrate, or slow down stormwater runoff.

• Vegetated/grassed swales or ditches – Also known as bioswales, these are channels that are densely planted with vegetation. Commonly seen in highway medians, these structures help reduce peak runoff downstream, facilitate infiltration, filter pollutants, and act as temporary water storage. They can also be used in small drainage areas with low runoff instead of underground storm sewers or concrete open channels.
• Vegetative filter strips or buffer strips – These are vegetated areas placed between sources of nonpoint source pollution (such as roads, highways, small parking lots, sidewalks, and other impervious surfaces) and a receiving body of water. They’re often planted with native grasses, shrubs, or trees (turfgrass can be used but it’s less effective in managing stormwater runoff). These strips help improve water quality by removing pollutants. Depending on the site, they may also help reduce runoff volume and can serve to recharge groundwater. They are best for gently sloping areas, where channelized flow isn’t likely.
• Green or vegetated roofs – We’re seeing more green roofs appearing on commercial buildings in urban areas of Howard County. These roofs are covered with specialized media and planted with a range of plants, from groundcovers to grasses and even flowering perennials. Many green roofs are built to moderate temperatures (both inside the building and in the surrounding environment to reduce the “heat island” effect). However, they also help to reduce the volume of stormwater runoff and improve water quality.

Underground Stormwater Management Facilities

• Underground filter systems – These facilities remove pollutants from stormwater runoff by passing the runoff through filter media (typically a mixture of sand and organic matter). These systems may be made by various manufacturers (such as Aqua-Filter^®, BayFilter^®, and StormFilter^®, among others) and contain special filter cartridges to remove pollutants. Filtration BMP’s typically treat catchment areas of 5 acres or less and are effective at treating total suspended solids (TSS), particulate phosphorus, metals, and organics.
• Underground storage – Underground detention and retention facilities store and then release stormwater runoff downstream at an allowable discharge rate. They reduce pollutant levels in the runoff by letting the sediment settle to the bottom of the system. These systems will build up sediment over time which will need to be removed periodically. Detention and infiltration BMP’s store stormwater runoff and infiltrate the stored runoff into the ground or release the runoff downstream at a required discharge rate. These facilities can provide flood control measures as well as reducing pollutants in stormwater runoff.
• Dry well (underground) – Sometimes called a seepage pit, a buried dry well is an underground pit filled with uniformly graded aggregate wrapped in geotextile, a prefabricated storage chamber, or a pipe segment. It 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’s often an overflow mechanism in the case of an intense storm event.

Stormwater Management Facility Resources

If you’re considering your stormwater management options, we recommend reading the Maryland Stormwater Design Manual.

These two volumes provide:

• an overview of how to size, design, select, and locate best management practices (BMPs) at a new development site to comply with state stormwater performance standards; and
• details on landscaping, BMP construction specifications, step-by-step BMP design examples, and design tools.

What type of stormwater management facility is best for you?

With so many options to choose from, it’s important to consider the key factors affecting the effectiveness of your system. Some things to consider include:

• site conditions (e.g. slope and soil types)
• existing and surrounding land uses
• available space
• priority stormwater management goals (e.g. water quality regulations)
• additional site development or redevelopment goals (e.g. recreational opportunities)
• maintenance schedule and anticipated costs

In Summary

If you’re interested in a stormwater management program from a DPW Bureau of Environmental Services approved contractor, give eos Outdoor Services a call at 410-648-9595. We provide proactive stormwater maintenance programs and repairs to both aboveground and underground stormwater management facilities. We’ll ensure all systems are fully functional and operating according to their design, and efficiently help you meet requirements for water quantity and quality.