Overview of stormwater infiltration

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Stormwater infiltration is the process by which rainfall and stormwater runoff flows into and through the subsurface soil. Stormwater infiltration occurs when rainfall lands on pervious surfaces, when runoff flows across pervious surfaces, and when runoff is collected and directed to a stormwater infiltration Best Management Practice (BMP).

Current stormwater management policies encourage, when appropriate, maximizing the infiltration of stormwater to reduce the volume of runoff discharging to surface waters. In addition to reducing runoff volume, stormwater infiltration helps reduce stormwater pollutant loading to surface waters. Many factors influence the rate and volume of stormwater infiltration including soil characteristics, storage capacity, and vegetation. These and other factors are discussed in further detail in subsequent sections. Once stormwater infiltrates into the soil, it has the potential to enter the groundwater and become part of the subsurface flow. For the purposes of this section, infiltration BMPs are considered those without underdrains.

Basic concepts of stormwater infiltration

Relationship between impervious cover and surface runoff. (Source:Stream Corridor Restoration: Principles, Processes, and Practices (1998). By the Federal Interagency Stream Restoration Working Group (FISRWG)(15 Federal agencies of the U.S.))

Stormwater runoff is considered to be any water that runs off pervious and impervious surfaces after a rainfall or snowmelt event, with a greater percentage running off from impervious surfaces. In urban areas, runoff can constitute approximately 30 to 55 percent of the water budget. In comparison, runoff may constitute as little as 10 percent in forested or rural areas (FISRWG, 1998).

Role of Stormwater Infiltration in the Natural Water Cycle

The increase in impervious surfaces can disrupt the natural water cycle and alter the surrounding environment via the decrease of groundwater recharge and the increase of water directly flowing to surface waters. As a likely consequence, this can lead to a reduction in the baseflow to streams, a decrease in the elevation of the groundwater table, and transport of sediment and pollutant loads into surface waters. A more detailed explanation of stormwater runoff and its effects on the environment can be found here.

Stormwater infiltration in urban areas can be enhanced by the disconnection of impervious surfaces and by the improvement of pervious surfaces such as turf. Impervious surface disconnection is the direction or redirection of stormwater runoff from impervious surfaces (e.g., sidewalks, parking lots, rooftops, etc.) onto pervious surfaces, such as a roof drain discharging onto a lawn. Redirection of impervious surface runoff to pervious areas promotes infiltration and reduces overall site runoff. The reduction in site runoff from impervious surface disconnection can vary considerably depending on many factors including the size of the contributing drainage area, size and infiltration capacity of the soils, vegetation in the area receiving the additional stormwater, slope and site grading and other site conditions. Stormwater infiltration can also be enhanced by improvement of pervious surfaces, such as amending soil with compost to increase the water holding capacity of the soil.

In new developments, stormwater infiltration is often intended to mimic the natural hydrologic cycle. Stormwater infiltration in re-development aims to mitigate changes in the urban water cycle brought about by increases in impervious surfaces caused by the urbanization. Infiltration can achieve the following objectives.

• Decrease peak runoff flow rates
• Decrease the volume of stormwater runoff
• Reduce stormwater pollutant loading to surface waters
• Promote retention and breakdown of contaminants in the soil/act as a pollutant barrier between the land surface and groundwater
• Increase groundwater recharge
• Preserve base flow in streams
• Reduce thermal impacts of stormwater runoff

Typically, stormwater from disconnected impervious surfaces is routed to a stormwater BMP. Infiltration BMPs achieve the above objectives by capturing, retaining, and infiltrating stormwater. Infiltration BMPs include

• infiltration basins,
• infiltration trenches,
• bioretention-bioinfiltration (most often a rain garden),
• permeable pavements, and
• tree trenches/tree boxes (sometimes considered a form of bioretention).

Small and medium sized infiltration BMPs are often located at the beginning of a stormwater treatment train, while larger systems are often placed at the end of the train. These BMPs can also be installed as off-line treatment systems. Infiltration BMPs are most suitable in sites with permeable soils and sufficient separation from the seasonally high groundwater table, bedrock, and polluted sites. While infiltration practices can remove some types of physical and chemical pollutants, careful consideration should be given to implementing stormwater infiltration practices in potential stormwater hotspots (PSH) or at sites with known pollution issues such as brownfields.

Factors affecting stormwater infiltration

Several interrelated factors that control the infiltration of stormwater into the soil/subsurface are described below. It is important to clarify the terms soil and subsurface (these terms are used interchangeably in this report). In general, the soil and/or subsurface is the material that is directly below the ground surface. When an engineered media is used, the soil or subsurface actually refers to the material underneath the engineered media.

• Infiltration rate. The infiltration rate (IR) is one of the main factors affecting the performance of an infiltration BMP. While design rates do exist, it is HIGHLY RECOMMENDED that the rates be measured in-situ. Information on how to find the IR in the field can be found here. Designers should test soil at fully saturated conditions (saturated hydraulic conductivity or Ksat) to obtain the most conservative rate. All design and credit information contained in the MPCA’s stormwater manual is based on the assumption of saturated soil. Average Ksat values are found here, however, they should be used as a reference only.
• Evapotranspiration rate. The evapotranspiration (ET) rate is the sum of evaporation and plant transpiration. It is an important function when the infiltration BMP relies at least in part on vegetation for its stormwater management. The average ET rate in Minneapolis and St. Paul from 2001 to 2010 was 0.16 in/day. It should be noted that this is a general value and is subject to variation due to several coarse and fine-scale conditions. Figure 1.3 provides the estimated mean annual evapotranspiration rate, in centimeters, throughout Minnesota between 1971 and 2000. Detailed records for certain areas of Minnesota are available on the Minnesota Department of Natural Resources Climate Summaries and Publications web site.
• Storm intensity. When the intensity of the storm is greater than the IR of the soil, runoff may occur (WEF, 2012). This is due to the fact that as storm intensity increases, the storage on the surface and in the subsurface macropores is more quickly exhausted and the soils rapidly reach saturation.
• Antecedent soil moisture condition. The antecedent moisture condition, or the moisture condition of the soil prior to the storm event, will affect the space available in the subsurface for infiltration. High antecedent moisture conditions reduce the amount of pore space available for infiltration, which in turn decreases the amount of runoff water entering the soil.
• Soil type. In general, infiltration is more rapid in coarser soils (e.g., medium to coarse sands or gravels) than in fine grained soils (e.g. silts and clays).
• Bulk density of the soil. As the bulk density of the soil increases, the IR generally decreases. The average bulk density of soil based on different land uses or activities can be found here.
• Temperature. Infiltration rates tend to decrease as temperature decreases. This is due to the increase in viscosity of the stormwater at lower temperatures (Lin et al., 2003; Braga et al., 2007; Emerson and Traver, 2008).

Limitations and disadvantages of stormwater infiltration

The following general limitations should be recognized when considering infiltration BMPs.

• Failure can occur due to improper design and/or siting.
• Infiltration BMPs may not be appropriate for sites with soils having a low infiltration capacity.
• Infiltration BMPs may not be appropriate for areas with steep slopes.
• Infiltration BMPs are susceptible to clogging by sediment and other debris, and may require a greater amount of maintenance compared to other BMPs.
• Algae growth within the BMP can block the infiltration of runoff into the subsurface (WEF, 2012).
• Infiltration BMPs are not ideal for stormwater runoff from land uses or activities with the potential for high loads of certain pollutants. The Minnesota CGP prohibits infiltration of runoff from vehicle fueling areas and certain industrial practices.
• Infiltration BMPs may increase the risk of groundwater contamination depending on subsurface conditions, pollution concentrations in the runoff, and aquifer susceptibility.
• Wide-scale implementation is required to significantly reduce the rate and volume of discharge to downstream surface waters, especially in areas with a high density of impervious surfaces (WEF, 2012).