In other sections of this chapter, rainfall runoff was controlled by either adjusting the velocity of runoff (Check Dams section & Energy Dissipaters section) or adding protective measures within the channel (Channel Lining section & Outlet Protection section). These measures reduce the amount of damage caused by runoff. An alternative method of controlling damage caused by runoff is to not allow water to become "runoff." It is possible to slow runoff to a rate that allows the water to infiltrate the ground. Obviously, such measures have a practical application only on a small scale; But a number of these applications installed within a single drainage basin can have a significant impact.
Infiltration devices also improve the quality of water entering our environment. Runoff from parking lots, streets, drives and roads contain grease, oil, antifreeze, and other pollutants. As it rains, these pollutants are picked up by storm water runoff and carried downstream. Unless removed, these pollutants can cause problems for aquatic life and for cities who rely on the stream or river for drinking water. When runoff is forced to infiltrate the ground, these pollutants are trapped in the top layer of soil where they can be picked up and disposed of properly.
Infiltration basins temporarily store storm water runoff until it infiltrates the surrounding ground through the bottom and sides of the basin. Such basins are made by constructing an embankment or excavating into an area of relatively permeable soils. these basins can be natural depressions in an open area or serve an auxiliary use such as a recreational facility. Infiltration basins, depending on the drainage area, can vary from the size of a front yard up to 50 acres.
Infiltration basins operate in either of two ways:
When a basin is on-line, it is designed to capture the storm water runoff entirely. As storm water exceeds the basin's capacity, excess water overflows the basin at a pre-determined location and continues downstream. On-line basins are not as efficient as off-line basins because the continual flow of water into the basin flushes pollutants through the basin and on downstream. On-line basins are generally used for groundwater recharge systems.
Off-line basins are designed to divert the more polluted storm water into a basin and allow the remaining storm water to pass. Typically, pollutants are picked up by the first flush of storm water runoff. The first 1/2 to 1 inch of rainfall will wash off most of the accumulated pollutants. Any runoff after this initial amount is relatively clean. Off-line basins are smaller because they are designed only for the initial amount of runoff. Since they are small, they are easily incorporated into the landscaping surrounding a parking lot or drive. These basins are installed in natural or excavated grassy depressions, recreational areas, and landscaped islands within a parking lot. Off-line basins do, however, require maintenance. Trapped sediment containing pollutants must periodically be removed and disposed.
Infiltration basins must be located over soils with hydraulic conductivity rates allowing trapped runoff to infiltrate the ground within a determined amount of time. Typically, this time is 24 to 72 hours. The seasonal high-water table should be at least 3 feet from the basin's floor allowing ample distance for pollutants to be removed before entering the water table. Bedrock should be at least 4 feet from the floor of the infiltration basin.
The site of an infiltration basin should be flat and in an area where the surrounding slopes are less than 20%. A soils test is important. The county soils survey will indicate if the selected area has soil types with high percolation rates. Percolation tests are routinely completed to determine the length of lateral fields for septic systems used for rural residences. This same test will provide an initial determination if the site in question is acceptable as an infiltration basin. Infiltration basins should be 50 feet or further from water wells and septic systems.
Once a site with suitable soil is found, it is marked and protected from traffic. Excavation of the basin should have side slopes no steeper than 4 feet horizontal to 1 foot vertical. Following the excavation, the bottom and sides are plowed to break the compaction caused by the excavation equipment. Once plowed, the basin should be protected from all unnecessary traffic. The sides and bottom are planted with a dense turf or native grass. Obviously, the grass type should be water-tolerant. Vegetation in the basin not only protects against erosion, but keeps the bottom from being compacted and filters out additional pollutants.
Channels flowing into the basin are protected from erosion by lining the channel with grass or crushed limestone. Erosion upstream of the basin will fill the basin with sediment and reduce its efficiency.
In general, site planning issues for an infiltration trench are the same as an infiltration basin. As with basins, the infiltration trench must be carefully designed, installed and maintained. Infiltration trenches are very susceptible to clogging, and once clogged very difficult to unclog. Unfortunately, trenches are "out-of-sight, out-of-mind" and offer no continual reminder to be maintained.
Infiltration trenches can be located on the surface or buried beneath the ground. These infiltration devices serve areas up to 10 acres in size and are especially appropriate in urban areas where land costs are high. An infiltration trench is shown in Figure 21. These trenches consist of a long, narrow excavation ranging in depth from 3 to 12 feet. The trench is backfilled with large-sized crushed limestone. Open spaces around the crushed limestone allow for storage of the first flush of collected storm water runoff. Stored runoff infiltrates the surrounding soil either through the sides or bottom of the trench. Perforated storm sewer pipe is installed in the trench to draw storm water from the infiltration trench's inlet. Storm water passes through the pipe's perforations and into the voids surrounding the crushed limestone. A filter fabric surrounds the pipe keeping the crushed rock from entering the pipe.
Trench bottoms must be at least 4 feet above the seasonal high-water table level to prevent pollutants from entering the water table.
Figure 21 Infiltration Trench
Crushed limestone in a surface infiltration trench extends up to the ground surface. Storm water flows overland to the trench and down into the voids around the crushed limestone. A grass buffer next to the surface trench removes some of the sediment carried by the storm water. While surface trenches are susceptible to sediment accumulation, their accessibility makes them easy to maintain. When plugged, a backhoe removes the rock and new, clean rock is added. Surface trenches are appropriate for use in highway medians, parking lots, narrow landscape areas, and residential areas.
Where surface trenches receive runoff from the surface, underground trenches receive runoff from inlets. The inlet and underground trench are in separate locations and are connected by a pipe. The inlet is specifically designed to prevent larger sediment, oil and grease from clogging the voids around the crushed limestone in the trench. These special inlets include oil/grease traps, sediment catch basin, and baffles to reduce the amount of sediment, leaves and other debris. An additional filter, such as a grass buffer or vegetated swale, may be needed to further protect the infiltration trench.
One popular type of underground infiltration trench is an ex-filtration system. In this system, storm water runoff is diverted into an over-sized pipe installed in an infiltration trench. This pipe is perforated and surrounded by large sized crushed limestone. The first flush of runoff is stored in the pipe and ex-filtrates out into the voids surrounding the crushed rock. Storm water then infiltrates into the soil.
Maintenance of this system is difficult and expensive because of the inaccessibility of the system, especially when installed beneath a parking lot.
A second type of underground infiltration trench is a dry well system. This system is used to store and infiltrate roof runoff. In a dry well system, the downspout from the roof gutter is extended into an underground trench installed at least 10 feet from the building's foundation. Rooftop gutter screens are installed to trap particles, leaves and other debris.
Infiltration trenches must be inspected regularly. Inspection after major storms for ponding water tells if the trench is clogged. An observation well next to the trench indicates if water is entering the soil. Finally, maintaining a grass buffer next to the trench will improve its effectiveness.