Mother Nature’s soil is the best of all urban stormwater Best Management Practices (BMPs). Precipitation infilitrates the soil, reduces runoff, filters and captures most pollutants, recharges groundwater, and maintains a diverse, self-sustaining biological community.
Conservation of natural soils and urbanization are diametrically opposed. When impervious surfaces are created on an urban site, the burden of additional runoff and pollutant removal must be borne by the remaining pervious areas. However, the capacity of those remaining areas to absorb these impacts is largely unknown.
Soil can be a very effective filter, but is also subject to failure, particularly as it becomes occluded (or clogged) with sediment in runoff.
The fine-grained (clay/silt) fraction of sediment is primarily responsible for clogging, although other materials (e.g., leaf litter) and processes (e.g., freeze/thaw) may affect the rate and location of clogging. When clogging occurs, incident precipitation, along with runoff directed to these areas, will no longer infiltrate and must either pond, run off, or evaporate.
Some Low Impact Development (LID) BMPs have been implemented to collect relatively “clean” sources of runoff, such as rooftop runoff.
These LID-BMPs are sometimes referred to “Rainfall-LIDs,” and include green roofs, cisterns, rain gardens in residential yards, and planter boxes next to commercial buildings. Other types treat “dirty” sources of urban stormwater from impervious surfaces such as streets, parking lots, and outside-storage areas. Sometimes referred to collectively as “Runoff-LIDs,” these types include porous pavement, infiltration trenches/basins, grassed buffers/swales, porous-landscape detention/bioretention, roadside rain gardens/tree boxes, and underground-infiltration galleries.
Rainfall-LIDs are being used in Combined Sewer Overflow (CSO) areas where runoff-volume mitigation is cost-effective compared to storage/treatment costs.
In stormwater systems, Rainfall-LIDs may reduce runoff and associated streambank erosion, but do not contribute substantially to pollutant reduction, whereas Runoff-LIDs do. Because they remove pollutants, these Runoff-LIDs, including those that are infiltration-based, require maintenance. Routine maintenance activities are limited to trash and debris removal and cleaning of overflow structures.
More-extensive, or rehabilitation, maintenance includes raking the infiltration surface to agitate deposited sediments.
Restoration, or replacement, maintenance is required if infiltration capacity is no longer available, vegetation/mulch is damaged beyond repair, or flooding or drought cause catastrophic failure. Retrofitting, or BMP additions, although not considered maintenance, may be required due to improper sizing because of incorrect design assumptions, faulty material selection and/or construction, or failure to meet performance objectives.
Although landscape-based retention and filtration BMPs have been used for decades, their widespread implementation on urban sites has dramatically increased in the past couple of years.
Given the likely time lag of 5 to 10 years before many of these facilities will require restoration maintenance, there is a tendency to consider only routine activities, such as those performed by landscape maintenance contractors, in life-cycle cost estimation.
However, it is critical that the full costs of maintenance be considered when selecting and designing stormwater systems. Regular inspection of the storage and infiltration capacity of BMPs must also be conducted to ensure that design assumptions are being met. As more information is gained on Runoff-LIDs, more-accurate comparisons can be made on the performance, longevity, maintenance and capital costs between BMPs based upon infiltration, filtration, sedimentation, and non-structural methods.