Infiltration

Bioretention
Rain Gardensa raingarden with drainage directed to the planting area
Rain gardens are vegetated surface depressions with amended soils used to capture, slow, infiltrate, and treat runoff from impervious surfaces, including rooftops, streets, parking lots and driveways. Rain gardens can be modified to adapt to site constraints including available area and infiltration rates of native soils. The goal of this practice is to infiltrate stormwater runoff, not store it like a pond. Properly designed and installed rain gardens provide volume control, groundwater recharge, and provide an aesthetic alternative to more traditional “gray” infrastructure. 

Engineered Bioretention
In some locations (such as adjacent to roads or in a highly developed area) rain gardens need expert design to avoid overflow or damage to infrastructure. When rain gardens include an underdrain, engineered soils, or other special features, they are more commonly referred to as “bioretention galleries.” The function is largely the same though: to treat stormwater runoff through infiltration.

New research indicates that rain gardens with underdrains should avoid the use of compost except for targeted application around plant roots during installation or use low-phosphorus compost mixes. This will limit the potential for overflow water with high nutrient content to overflow from the garden into the storm sewer system. 


stone lined infiltration trench between a sidewalk and roadway

Infiltration Trenches
Infiltration trenches are linear practices with highly permeable media used to quickly infiltrate stormwater. This practice is similar in function to a dry well, (link to dry well page) but can be used along roadways and the perimeter of parking lots to intercept and quickly infiltrate runoff. These infiltration systems are highly versatile and can be applied from small residential areas to commercial and industrial sites where runoff from parking lots, roads, and sidewalks can be quickly infiltrated. Because runoff from roadways can carry pollutants that could clog an infiltration trench and/or contaminate shallow ground water, a vegetated filter strip is recommended to buffer an infiltration trench from a paved surface where possible.
 
Design Considerations
Infiltration trenches should be designed to hold water no longer than 24 hours in order to avoid a potential mosquito breeding ground. A 30-inch deep soil mix consisting of 50% topsoil and 50% sand should be used for water quality trenches and galleries. If native soils are not conducive to infiltration, an under-drain system should be installed beneath the soil layer to avoid long periods of standing water. Infiltration trenches are generally a minimum of three feet in depth and backfilled with amended soil or loose aggregate.

Pervious/Porous Pavement
Pervious pavement refers to any of a range of paving types that allow water to pass through - including pervious concrete, porous asphalt, permeable paver blocks and reinforced turf structures. Pervious pavement provides stormwater infiltration and/or storage with a surface of structural pavement that performs similar to impermeable load-bearing alternatives. Development of new technologies that fit this description is an active field of research and performance in different loading, salt, and maintenance scenarios varies widely. University of New Hampshire Stormwater Center studies these technologies in Northeast conditions. 

woman bending to feel pervious pavement on pathway

Properly installed and maintained pervious pavement has life-span similar to conventional pavement options, and in some case much longer. Because water drains through the surface and into the subsurface bed, freeze-thaw cycles do not tend to adversely affect pervious pavement.

Pervious pavement is most susceptible to failure due to errors in construction and improper maintenance. It is therefore of paramount importance that a maintenance plan be completed and agreed upon by all invested parties prior to project construction. During construction, the following should be limited:

  • Compaction of underlying soil
  • Contamination of stone subbase with sediment and fines
  • Tracking of sediment onto pavement
  • Drainage of sediment laden waters onto pervious surface or into constructed bed

 


Dry Wells
A Dry Well is a subsurface storage facility that temporarily stores and infiltrates stormwater runoff. Simply an excavated pit pilled with uniformly graded stone or a prefabricated storage chamber or pipe segment with perforations, dry wells accept runoff from roofs or other impervious surfaces. The stormwater is discharged via infiltration into surrounding soils. In the event that the Dry Well is overwhelmed in an intense storm event, an overflow mechanism (connection to larger infiltration area or rain garden) will ensure that additional runoff is safely conveyed downstream.

By capturing runoff at the source, Dry Wells can dramatically reduce the increased volume of stormwater generated by the roofs of structures. Though roofs are generally not a significant source of runoff pollution, they are still one of the most important sources of new or increased runoff volume from developed areas. By decreasing the volume of stormwater runoff, Dry Wells can also reduce runoff rate and improve water quality. As with other infiltration practices, Dry Wells may not be appropriate for “hot spots” or other areas where high pollutant or sediment loading is expected without additional design considerations. Dry Wells are not recommended close to drinking water source protection areas or subsurface sewage disposal systems.


Level Spreadersa level spreader installed along a swale
Level spreaders are devices which may be installed to more evenly distribute runoff over a stabilized surface, calming and decentralizing the erosive nature and velocity of concentrated stormwater flows. By slowing and spreading the flow, these structural elements allow for infiltration and greater practice performance. Level spreaders can be used as the first practice in a working system of Green Stormwater Infrastructural practices.


Vegetated Filter Strip
Vegetated filter strip is a permanent, maintained strip of vegetation designed to slow runoff velocities and filter out sediment and other pollutants from stormwater. Filter strips require the presence of sheet flow across the strip, which can be achieved through the use of level spreaders. Frequently, filter strips are designed where runoff is directed from a parking lot into a stone trench or a naturally vegetative strip. They provide good performance at a low cost, as well as providing aesthetics and habitat benefits. Vegetated filter strips can be applied as a first flush treatment for parking areas where flow is directed toward green space. It can also be used to slow velocity from a level spreader as the flow works its way down slope.


Planter Boxesman kneeling to inspect a planter box at the base of a tree while others look on
Planter boxes can be referred to by several names - stormwater planters, flow-through planters, or contained planters are among the most popular. These contained planting areas capture stormwater runoff and treat it through bioretention that is elevated above grade level.


Soil Restoration
Healthy soils retain more water and support healthy plant and microbial communities. Improving soil health is the single most impactful thing we can do to reduce stormwater runoff and improve overall ecosystem health at all scales. 
Maintaining or restoring healthy soils can be done via the following simple methods: 

  • Amend beds with compost when re-landscaping
  • Annual mulching of existing landscape beds with shredded leaves, woodchips, bark, or compost builds soil health, prevents weeds, keeps plant roots warmer in winter and cooler in summer, and can cut summer irrigation needs by 50%
  • On lawns, mulch-mowing or “grasscycling” (leaving the clippings to feed the soil) reduces fertilizer needs, and build lawn health
  • Top-dress with compost to improve poor lawns, ¼ to ½ inch of compost raked in after aeration in spring or fall
  • Avoid overuse of soluble fertilizers and pesticides which can damage beneficial soil life. Always get a soil test before adding any fertilizer