Stormwater Management

Water pollution control and management is essential for minimizing pollutant waste discharge into the environment. Contaminated rainwater seepage into the soil can taint streams, rivers, lakes, and groundwater used for drinking water. Additionally, soil contaminated because of improper stormwater management can potentially harm agricultural sites, livestock operations, and wildlife habitats. For facilities managers, a pertinent concern is managing runoff from parking lots and other exterior features of the property.

Stormwater management refers to collecting precipitation and routing it to areas where it will not create a problem for a facility, neighboring properties, or the environment. Stormwater collection systems normally consist of catch basins or drop inlets constructed in low areas of parking lots, lawns, sidewalks, paved pavilions, and promenades. Depending on the local weather history, there may be no collection systems in place at all, whether by design or design oversight. If this is the case and runoff overwhelms the designed sewer capacity, the results can be disastrous.

Most stormwater discharge from a facility and site is considered a direct point source discharge. A direct discharge occurs when stormwater or wastewater is discharged from a facility or site directly to a natural body of water and a point source discharge comes from pipes, sewers, ditches, or anything “confining.” An example of a direct point discharge is when runoff from building roofs, parking areas, and decks is collected and piped to a receiving stream or body of water.

Apart from its unpredictability, stormwater presents facility management with issues that are entirely different from those involved with wastewater, which is piped into a municipal system for treatment. While there are possibilities of on-site treatment or home-grown wells, they are a very small percent. In a rain storm, stormwater runoff from a street or parking lot may be contaminated with gas or oil; some will flow directly to a sewer and some, perhaps, to a retention pond. That won’t be treatable or controllable in the first case, and may or may not be in the second, where dilution is the main offset. Storms seldom produce overt contamination due to oil tanks or the like, but if the possibility exists, management is well advised to undertake spill protection and/or containment design.

Collecting and Directing Stormwater

The storm drain collection system discharges the water to either a municipal stormwater system or an existing watercourse/stream or drainage path. In general, stormwater flows are discharged to streams and watercourses except in highly urbanized areas, which have municipal systems. Municipal systems usually carry stormwater from the streets, ultimately discharging it to a stream or river. In many older systems, stormwater and sanitary sewers are combined, creating a public health hazard during sewer system overflow and flooding. Potential drain or sewer backups are possibilities that workplace managers have to consider in risk assessment and emergency response planning. These are emergencies that can shut down a workplace or an entire community, so hazard communication is something else that must be planned for—from the managers to their site occupants, and from the municipal water provider to the community at large. Collaboration on procedures and communication to enact in the event of a sewer emergency should be part of the facility’s advanced risk assessment planning.

Discharges to streams and watercourses require the use of an energy dissipater, which reduces the velocity of the stormwater to eliminate erosion in the stream channel and at the pipe outlet. Aside from velocity’s effect on the pipes, attention must also be paid to the effect of temperature on the destination watercourse. For instance, 60° Fahrenheit water from a detention pond spewed into a stream at the proper velocity still can introduce a temperature eddy or excursion that is harmful to wetlands and possibly to larger bodies of water.

Oil—Water Separators

Environmental regulations require the use of oil—water separators in areas where there is a risk of petroleum product leakage, such as at gas stations, oil refineries, tank farms, and fuel-oil delivery services. They may also be installed where stormwater discharges into environmentally sensitive areas such as fish or waterfowl breeding grounds. Separators are typically located below ground level and are considered confined spaces (regulated by OSHA), requiring special training prior to personnel entry for inspection, service, or cleaning.

One type of oil—water separator is divided into several chambers by baffle walls. Stormwater enters the influent chamber of the separator through a stormwater inlet pipe located at one end of the tank. The water travels a certain path so that its velocity is reduced to allow the oil to float. The final effluent is discharged at the opposite end of the tank through an elbow outlet pipe that allows only the clearer water to exit. The end of the elbow is positioned a certain distance from the tank bottom so that heavier solids that accumulate in the bottom are not discharged with the effluent. Inspection, maintenance, and cleaning are required on a scheduled basis to ensure that these solids are removed periodically.

Detention and Sedimentation Ponds

When required, properties developed within the last 25 years incorporate detention ponds as part of a compliant stormwater management design. When developed, vegetated property is covered with a large impervious surface area (such as parking areas, roads, and buildings), thus greatly increasing the volume and velocity of stormwater runoff. In addition, the impervious surfaces transport the water more quickly than undeveloped land does.

The regulatory response is the “zero percent increase in runoff” policy for new developments. The policy’s goals are met by constructing detention ponds, which are manmade ponds with an outlet structure designed to let out only a certain amount of water over a given period. During a storm, the runoff from a developed property is discharged to a retention pond, which allows an increased volume of water or runoff to enter the pond, while limiting the amount of water leaving the pond to a constant rate.

Sedimentation ponds are similar to detention ponds. In many cases, the detention pond functions as a sedimentation pond, providing an area for silt and sediment picked up and carried in turbulent stormwater flows to settle out in a controlled area. These contaminants often consist of sand, silt, and sediments that have been swept up off paved and unpaved surfaces during a storm. Silts and sediments are the smallest soil particles. Other contaminants, such as oils and chemicals that are unintentionally discharged to surfaces exposed to runoff, will also be discharged into a sedimentation pond but may not be retained.

Maintenance of these types of ponds is relatively simple, but easy to forget. First, architects and engineers are usually experienced in their design for the region, which includes sizing of the ponds and their piping. Detention ponds must remain empty throughout the year to leave room for the large rain or snow runoff. Consequently, they will contain plant and weed growth, so periodic checks should be made to keep inflow or outflow pipes clear of obstructions.

Also, detention and sedimentation ponds must be located in areas where there is no possibility of contamination from a tank failure or overflow. They can be protected by grading and/or distance, as well as barrier walls or berms. Pumps and controllers should be checked and maintained according to schedules. If there is direct flow from a pond into natural bodies or wetlands, temperature monitoring instruments need to be checked for function and accuracy. Overall, managers should consider these installations to be rarely used, but when they are used, the systems and procedures must function well.

This article is adapted from BOMI International’s course Building Design and Maintenance. More information regarding this course is available by calling 1-800-235-2664.