Since the 1970s, these hidden features of the developed landscape have been the workhorses of stormwater management in the state of Pennsylvania. Most of the time, they look like nothing but grass-lined holes in the ground. But when it rains, they fill up with rainwater and allow it to discharge slowly over a longer period of time than it would otherwise. This prevents the increased runoff from developed sites from causing spikes in the water level of rivers, streams, and drainageways that would result in erosion and flooding. Detention basins have become the default stormwater management feature for two simple reasons: They’re cheap, and they work.
Well, they sort of work. Detention basins do a good job of controlling the peak rate of runoff, but they don’t do much to control the runoff volume or the quality of the water that they discharge. All of those gallons and gallons of water that didn’t sink into a newly paved parking lot would still make their way to area waterways, but they’d just do so at a somewhat more leisurely pace. Even worse, they would still contain all of the contaminants that they had picked up along their journey.
This “classical” model of stormwater management was convenient for a design engineer like me. It was a well-studied and time-tested approach. The calculations involved were straightforward, and surprises were few and far between. It was convenient for municipalities, commercial property managers, and homeowners associations that would be responsible for the stormwater management systems because required maintenance was minimal. But it was an incomplete solution. Waterways continued to see an increase in pollutant concentrations, and summertime base flows continued to fall due to decreased groundwater recharge.
Which is why, in 2007, the Pennsylvania Department of Environmental Protection (DEP) enacted new regulations that would turn the business of stormwater management in the state of Pennsylvania on its ear. These new regulations focused on areas like stormwater infiltration, thermal pollution, and water quality that had previously been unaddressed. The ideas weren’t new-regulations in other states had been working on the same principles for years, and some municipalities right here in the Keystone State had enacted their own requirements to address the same issues. But the new regulations from the DEP marked the first statewide effort to shift from a “rate only” control criterion to something more comprehensive.
Don’t worry about the humble detention basin; it won’t be going away any time soon. But instead of functioning as the only stormwater management facility on any given development site, the detention basin will now be only a part of a suite of facilities, or best management practices (BMPs), geared at addressing both the quantity and quality of stormwater that finds its way back into our streams and groundwater supplies.
Some of these facilities will continue to be earthworks constructed by contractors onsite, and some will be commercially available products. But no matter what type of facilities, or combination thereof, are selected for a particular site, none of these facilities will be a “once and done” affair. Each will require varying degrees of ongoing maintenance. Therein lies the concern for scores of municipalities, property managers, developers, homeowners associations, and others agencies throughout the state that now need to plan for the time and costs of this additional maintenance. Because the required maintenance schedule varies from one technology to another, the selection of appropriate devices must be considered early on in any project’s planning stage. To assist in this selection process, the following is a summary of the types of commonly used stormwater management technologies and their associated maintenance requirements. The summary is divided into two main categories-infiltration devices and water-quality devices-with commercially manufactured and site-built technologies being discussed. While the list is by no means comprehensive, it serves as a general overview of the types of devices that are being used today in successful medium- to large-scale installations.
Infiltration Devices
Pervious Pavement-Perhaps no single stormwater infiltration technology has garnered more attention lately than pervious pavement. Pervious pavement is available in asphalt- and concrete-based products alike, and the maintenance requirements are similar for both types of surfaces. In both products, some portion of the fine particles component is omitted from the mix to allow a more open structure through which water can flow freely in the finished product. Maintenance requirements for these products focus on two main goals. First, a proper maintenance schedule ensures that the pores and open spaces in the paving don’t become clogged. Second, it must ensure that rainwater has somewhere to go once it penetrates the pavement.
The first of these requirements is typically met by the periodic use of a vacuum truck to remove debris from the pavement surface. The Pennsylvania Stormwater Best Management Practices Manual recommends that this be done twice per year, although it may be necessary somewhat more or less frequently, depending upon specifics of the site. It’s also important to plan carefully for winter maintenance when using pervious pavement. While road salt does not pose a problem, sand, cinders, or other aggregate should never be used on pervious pavements, as they will clog the pavement’s pores and be difficult to remove. Because pervious pavements are less resistant to shear stresses than conventional pavements, snow plows should be raised slightly above the paved surface to prevent the top layers of the material from being worn away, and pervious pavements should be avoided in areas of tight turning by heavy vehicles, such as near loading docks.
The second maintenance requirement is met by ensuring that the system is installed properly and that all underdrains and overflow structures are kept freely flowing. Pervious pavements are often installed over a gravel bed that is significantly thicker than that of traditional pavements to allow for the storage of infiltrated runoff. Water in this gravel bed will eventually infiltrate the subsoil and provide groundwater recharge, but it can only do so if the subsoil layer also permits the water’s passage. When installing the paving section, it is important that heavy machinery not compact the subsoil layer and decrease its permeability. It is also important that appropriate geotextile fabrics be used to surround the stone bed to prevent fine soil particles from entering the stone and filling the porous spaces. Pervious pavement systems are typically equipped with overflow systems to ensure that proper drainage is provided for storm events that exceed the stone bed’s storage capacity. As with any storm sewer system, it is important that these drainage facilities be maintained free of debris so that they can function properly.
Vegetated Pavement– In addition to concrete- and asphalt-based systems, a number of companies manufacture products that reinforce turfgrass or other vegetation sufficient to allow vehicular loading. While these products are available in several grades suitable for vehicles from golf carts to large trucks, their implementation has largely been confined to such occasional-use areas as overflow parking lots, emergency access drives, and trails and recreational areas. Because products in this class are often made of recycled plastics, maintenance requirements are minimal. Once the products are installed and vegetation is established, they require only the maintenance that any other vegetated area would require, such as mowing and watering. The important consideration with products of this type is to restrict the frequency and intensity of vehicular traffic in these areas so that vegetation can recover between uses.
Subsurface Storage and Infiltration– Another option available to site designers in meeting Pennsylvania’s stormwater infiltration requirements is the use of any of an ever-expanding array of underground storage devices. This type of technology typically consists of modular plastic units linked together and surrounded by a stone bed, all of which is then buried several feet below the finished grade. While these systems can be designed to function as a “rate only” control device (much like the venerable detention basin), they are often designed as a hybrid rate-and-volume-control device. In this configuration, the lower portions of the storage chamber would not be drained by any means other than infiltration into the subsoil, while the upper portions would contain an overflow structure to allow for the discharge of more intense storm events.
Maintenance requirements for these types of products revolve largely around the need for regular inspection and cleaning. Because storm drains often connect directly to the subsurface facilities, the opportunity exists for leaves, litter, and other debris to find its way into the system. When the underground system is designed with an undrained sump to allow for infiltration, this debris often has no way to exit the system, where it settles and begins to impede permeability at the interface between the modular units and the stone or soil bed.
To allow for proper maintenance, systems must be carefully designed with access points from which a vacuum truck or maintenance worker can access all areas. The frequency with which the system must be cleaned out varies, depending on the application. Roof runoff, for example, contains very little debris compared with that from a commercial parking lot, so a system collecting only roof runoff would require less-frequent maintenance.
Water-Quality Devices
Inlet-Mounted Devices– As easy access points that are installed in just about any land development, storm sewer inlets and manholes serve as logical locations for installing stormwater management devices. Recognizing this opportunity, a number of manufacturers have developed products that fit into these structures and provide some measure of physical or chemical separation of various pollutants. Depending on the application, the devices can be installed in each storm sewer structure, or in selected units at the junction of major branches of the storm sewer systems.
Devices that provide physical separation of contaminants tend to work on one of two principles: screening or settling. Screening devices, such as the Grate Inlet Skimmer Box by Suntree Technologies, use a metal mesh or a series of metal mesh panels through which storm runoff must pass. While they can be combined with chemical absorbents, the mesh panels themselves capture debris of varying sizes with coarse, medium, and fine screens, and form a basket, which can be lifted out of the inlet for cleaning. Other screening products that are not removable are serviced with a vacuum truck or by other means. Settling devices, such as Snout from Best Management Products Inc., are installed in a specially sumped or recessed inlet. By creating this sump and equipping the outlet pipe with a plastic hood, the design forces water to exit from the middle of the water column instead of the bottom. This allows debris that is heavier than water, such as sand, grit, and sediment, to settle to the bottom of the sump, while material that is lighter than water, such as oil and some kinds litter, floats to the top. Inlets equipped with devices of this type must be serviced periodically with a vacuum truck to remove the accumulated debris.
Standalone Devices– In addition to inlet-mounted devices, a number of manufacturers also provide standalone devices that can provide separation of contaminants by a several different means. While screening and settling remain popular methods in standalone devices, there are products on the market that function on other principles.
The Aqua-Swirl device by AquaShield Inc., for example, separates contaminants by a combination of settling and centrifugal force. The device consists of a specially designed cylindrical chamber that is installed inline with the storm sewer piping. The inlet and outlet pipes, as well as internal baffles, are located in such a way that the flow of water spins as it passes through the device, forming a vortex that throws heavier debris toward the outside of the cylinder, where it settles to the bottom. Lighter materials collect in the center of the chamber and float to the top. Cleaned water is permitted to exit after passing beneath a baffle that blocks the passage of floating debris. The device is equipped with an inspection and maintenance port from which the accumulation of debris can be measured and removed by a vacuum truck when it reaches a predetermined depth.
Chemical Absorbent Devices– As with physical separation devices, manufacturers provide a wide variety of chemical absorbent devices that can be installed in either inlet-mounted or standalone configurations. Most devices of this type operate by passing stormwater through a bed of absorbent media that is replaced periodically as it becomes saturated with contaminants removed from the runoff stream. The absorbent media can be composed of activated charcoal, manmade resin beads, leaf compost, granular minerals such as perlite, or any of a number of other materials. The selection of an absorbent media can be tailored to the problem contaminants and can remove anything from oil and nutrient pollution to heavy metals. Because chemical absorbent systems can be costly to install and present an ongoing maintenance cost for media replacement, their implementation has typically been limited to runoff with an unusually high pollutant loading, or to especially sensitive watersheds.
In addition to physical structures and devices, there are a number of methods and techniques that, in combination with the types of devices discussed above, are collectively referred to as BMPs. What constitutes the best management practice for one site may not be best for another, so it’s important to consider all options before committing to a particular type of system. More information on the items discussed in this article, as well as others, can be found in the Pennsylvania Stormwater Best Management Practices Manual.
With an increasing number of new stormwater management devices placed in service each year, a property manager must add their inspection and maintenance to his or her long list of responsibilities. As you can see from the discussion above, this maintenance is centered on one important requirement: cleanliness. Regardless of type, all stormwater management devices must be kept clean to perform their intended function. Perhaps we can all take some comfort, though, in the knowledge that it’s not just the stormwater management devices that we’re cleaning: It’s also the rivers, lakes, and streams that so many enjoy.