Hustle & Flow

Proactively managing stormwater runoff requires the right tools for handling flow, storage, and time

• By John R. Kastrinos, Mark D. Kelley, Michael C. Alfieri
• Mar 01, 2006

Managing stormwater infiltration is an ongoing and increasing need. Factors influencing stormwater filtration management include:

• The need to protect undersized, aging drainage infrastructure from unnecessary stormwater loads;
• Increased public awareness and interest in sustaining aquifer recharge and baseflow to streams and rivers, particularly in basins that are stressed because of increased water demands caused by development;
• Increased interest among developers and the architectural/engineering community in Low Impact Development/Green Building initiatives that focus on minimizing the impact of impervious surfaces.

Infrastructure related to conveyance of stormwater and wastewater is stressed in many communities that rely on systems that were built in the 1950s, if not earlier. Protecting these aging facilities from unnecessary stormwater loads is essential to protect the public from flooding hazards and to extend the useful life of the infrastructure, resulting in a potential rehabilitation/replacement cost savings.

In densely developed regions, water resources are stressed to or beyond capacities. Streamflows can dwindle or dry up altogether during peak demand periods in the summer. Communities are often forced to mandate water use restrictions, explore additional water resources, or import water from outside of the basins. Stormwater infiltration can help recharge the groundwater baseflow that sustains these resources.

Communities are responding to these needs by actively pursuing conservation efforts and setting limits on development through zoning and placing specific requirements or limits on stormwater runoff and infiltration. While it has been standard procedure for many years to design systems to control runoff such that further development does not increase peak runoff rates, more communities are now requiring zero increase in runoff volume over pre-development conditions. Also, there is increased public interest in reducing the impacts of urban and suburban sprawl through "green building" approaches, including reducing runoff through stormwater storage and infiltration.

Regardless of the objectives, project owners, engineers, and geologists need to collaborate closely in identifying areas suitable for stormwater infiltration and evaluating alternatives for balancing stormwater outflow, infiltration, and storage with available area and the required or regulatory-driven timeframes for infiltration.

Data Needs to Define Infiltration
Infiltration data needs are very dependent on state and local regulations or bylaws. At a minimum, it is essential to accurately characterize the site hydrogeology by defining soil types, measuring depth to groundwater, documenting or estimating depth to seasonal-high groundwater, measuring the depth to bedrock and other semipervious-to-impervious strata, and defining seasonal water table variations. Field testing is necessary for both the unsaturated and saturated zones, as the volume of water infiltrated into the ground must flow vertically downward through the unsaturated zone and then flow laterally away from the infiltration point.

Estimating groundwater flow patterns across a site is important because adverse effects, such as groundwater mounding (groundwater rise in response to infiltration) can be reduced by orienting infiltration areas parallel to the prevailing hydraulic gradient (flow direction). Other site characteristics to be considered include: the proximity of infiltration areas to hill slopes; homes and wetlands, which may have specific setback requirements; and subsurface structures, such as basements, which are susceptible to groundwater seepage from infiltration systems.

Field Methods for Data Collection
Field investigations typically include hand augers, test pits, test borings, and monitoring wells. Tests used to evaluate soil infiltration capacity include percolation tests, basin flooding, permeameters, double-ring infiltrometers, borehole slug tests, and grain size distribution analysis. One of the most cost-effective methods is field reconnaissance by an experienced geologist or hydrogeologist. Spending only a day or two in the field can reveal a wealth of information on areas of shallow bedrock, shallow groundwater, and soil characteristics, including drainage properties. This will help guide the more costly field investigations that follow.

Data Interpretation
Stormwater infiltration is typically not controlled by pumps and valves but rather by soil infiltration rates and infiltration area. The infiltration rate is the amount of water per surface area and time units that penetrates the soil. Infiltration data needs to be comprehensive enough to inspire confidence in their validity, and they need to be interpreted with care. Infiltration rates -- with appropriate safety factors to account for long-term clogging -- and typical soil heterogeneity are critical for determining field dimensions and bottom elevations to comply with limits on stormwater runoff rates and/or volumes.

Above the water table, in the unsaturated zone where infiltration takes place, designers typically assume that infiltration occurs under a gradient of 1. Once the soil is saturated, however, the hydraulic gradient is typically a small fraction of 1 (often in the range of 0.001 to 0.01) and infiltration rates drop dramatically. If this is not considered, a system may be significantly undersized with respect to area. Conversely, if infiltration rates are underestimated, a system may be oversized such that more infiltration occurs than is necessary, putting neighboring structures (i.e. basements) at risk of groundwater seepage.

The thickness and areal extent of soils in the proposed infiltration area must also be evaluated because, regardless of infiltration rates, restrictive features such as bedrock or low-permeability soils (i.e. silt and clay) may severely limit infiltration capacity. Thus, it is critical that testing data be interpreted in the context of the larger-scale site stratigraphy and regional geology.

Groundwater mounding may significantly influence an infiltration system design. If a stormwater system depends on infiltration rate rather than storage to control peak runoff rates or volume, excessive mounding may prevent infiltration and result in greater runoff. In these cases, the engineer must work closely with the hydrogeologist or groundwater hydrologist to come up with alternative designs to meet the overall objectives. This may involve increasing the infiltration system area, raising the elevation of the system to increase available subsurface storage; distributing the stormwater over multiple systems rather than concentrating it in a single area; or building adequate storage facilities to extend the infiltration period and reduce the infiltration rate.

While infiltration rates and areas, hydraulic conductivity, and depth to seasonal-high groundwater are critical variables, mounding analysis must also consider hydraulic boundaries. Barrier boundaries, such as areas with shallow bedrock or low-permeability soils, increase mound height because these transmit very little groundwater. Recharge boundaries, such as wetlands and surface waters, can serve as sinks for the infiltrating water, mitigating mound height between the recharge boundary and the infiltration system. The closer the boundary is to the infiltration area, the greater effect it has on the extent and magnitude of groundwater mounding.

The scale of many small projects does not typically allow for support of a sophisticated groundwater flow model an extensive network of observation wells, piezometers, and field pilot testing. Simple analytical models or numerical models, with assumptions applied based on professional experience, are indispensable tools. These simplified models can be used to "bracket" solutions within a reasonable range of hydraulic parameters and to evaluate the effects of design modifications on infiltration and groundwater mounding.

Planning and Communication Key to Success
Ongoing communication between the engineer, designer, and hydrogeologist is needed to address the often-conflicting goals of providing adequate storage, controlling outflow to storm drainage systems, and infiltrating sufficient water volume at a sufficient rate to reduce runoff without creating or exacerbating groundwater seepage in neighboring structures. Ideally, geologic/hydrogeologic characterization will be conducted early in overall project planning since subsurface conditions drive the design of stormwater infiltration systems. By anticipating the site limitations early, minor adjustments can be made to impervious surface area, site grading, and layout to accommodate areas best-suited for infiltrating stormwater. Collaborating about project and site issues, regulatory compliance needs, and community expectations can resolve these issues to benefit the owner as well as the community.

This article originally appeared in the 03/01/2006 issue of Environmental Protection.