Infiltration Map Project

Summary
A key modern stormwater management strategy for addressing the impacts of development is to infiltrate as much rainfall and snowmelt into the ground as possible, thereby reducing overland runoff and replenishing groundwater supplies. In collaboration with Dane County, DNR, and UW-Madison, relative infiltration maps have been developed for Dane County. The maps are meant to be used as a screening tool early on in the planning/design/development process to identify relatively high infiltration areas, as well as areas that might be enhanced through engineering techniques (e.g., replacement with engineered soils). While the maps do not replace the need for site specific analysis, they do provide a useful planning and decision-making tool for infiltration and stormwater management. They also help promote discussion of innovative methods and design techniques to enhance infiltration, and identify potential retrofit opportunities in previously developed areas.

If you would like to provide feedback about this project, or have additional questions, Please contact Mike Kakuska at 608-266-9111 or email MikeK@CapitalAreaRPC.org

Map 1 shows relative infiltration as it occurs naturally. Map 2 presents enhanced infiltration that could result from engineering practices tapping into deeper sand and gravel deposits. Map 3 shows areas where the potential for engineered infiltration practices may be the greatest. These areas show the greatest improvement in infiltration capacity between the natural and engineered states.

Map 1. Relative Natural Infiltration
Shows relative infiltration as it occurs naturally.
Click on Township areas for detailed maps
Click here to download the complete set of Relative Natural Infiltration maps (19 meg. WinZip Archive).

The maps promote various opportunities and strategies that can be used to help minimize the impacts of future development and possibly retrofit previously developed areas. Areas with naturally high infiltration should be used to recharge the groundwater to the greatest extent possible. They may also be prime locations for regional stormwater facilities that could be used to infiltrate stormwater generated in other parts of the watershed. Other areas, such as clay soils with low permeability, are less suitable for infiltration. Stormwater generated in these areas could be reduced on site to some extent, such as through rain gardens, but the majority will likely need to be routed to facilities down-gradient. These facilities would need to be adequately sized to accommodate the rates and volumes of stormwater generated by the proposed development. In some areas, primarily where shallow bedrock exists, permeability is so highly variable that more detailed investigation is needed.

Overall, the greatest infiltration opportunities seem to lie primarily in the glaciated eastern two-thirds of the county. It is interesting to point out that for nearly every large-scale development that might be proposed in the area there is an infiltration area located nearby that could be used to great advantage. The overall purpose of these maps, therefore, is to highlight these areas early on as important elements of site design, so that they may be more fully utilized for water quality protection and groundwater recharge. While the maps do not replace the need for more in-depth analysis for a particular site, they do provide a useful planning tool to encourage the incorporation of innovative stormwater management practices into urban design.

Map 2. Relative Engineered Infiltration
Presents enhanced infiltration that could result from engineering practices tapping into deeper sand and gravel deposits.
Click on individual Townships for detailed maps.
Click here to download the complete set of Relative Engineered Infiltration maps (16 meg. WinZip Archive).

Methodology
The maps were derived from NRCS soil information using relative cumulative scores based on soil permeability, depth to water table, depth to bedrock, and slope (Table 1). Depth to bedrock and slope were considered less important and weighted accordingly. Map 1 represents relative infiltration as it occurs naturally. Permeability is scored based on the most limiting soil layer in the soil column, measured to a depth of five feet. Map 2 represents enhanced infiltration that could result from engineering practices. Permeability is scored based on the permeability of the soil layer below the depth of five feet. This assumes aggressive practices to replace shallower, less permeable soils layers with more permeable (i.e., engineered) soils to a depth of five feet.

Map 3 indicates areas where infiltration enhancement potential may be the greatest. These areas show the greatest difference in scores between the natural and engineered states, highlighting opportunities where more permeable soils (e.g., sand and gravel deposits) may be found deeper in the soil column. The wetland/floodplain areas include soils with shallow depth to groundwater or potential for flooding, and are generally not conducive to infiltration practices.

Table 2 contains soil permeability data and relative scoring used in the analysis. The data was obtained from the Natural Resource Conservation Service SSURGO tabular soils data for Dane County (downloaded 1/23/06) and scored according to the criteria in Table 1. Natural scores were based on the most limiting soil layer in the soil column (to a depth of five feet). Engineered scores were based on the permeability of the soil layer below a depth of five feet. This assumes aggressive practices to replace less permeable soils in shallower layers. Enhanced scores were calculated as the difference between Natural and Engineered scores. Notice that soils with highly variable permeability were given an arbitrary score of 13 to separate them out as a single group. Because their permeability was so highly variable, these soils could not be rated. In most cases this group possesses shallow depth to bedrock with permeability values ranging across two or more criterion groups and, very often, across the full criterion spectrum. These areas will require special consideration and site-specific investigation. As presented here, they suggest added caution is needed – infiltration may be either limiting, as in the case of solid bedrock; or excessive, as in the case of solution cavities, cracks, and other karst features.

Table 3 contains the criterion scores (using Table 1) and cumulative totals for each soil unit presented in the maps. Notice that the total scores in Table 2 do not include slope. Slope was calculated internally by the computer, creating many individual polygons using the 2000 digital elevation model (obtained from the Dane County Land Information Office, two-foot resolution) and then mapped.

Map 3. Relative Enhanced Infiltration Potential
Shows areas where the potential for engineered infiltration practices may be the greatest. These areas show the greatest improvement in infiltration capacity between the natural and engineered states.
Click on individual Townships for detailed maps.
Click here to download the complete set of Relative Enhanced Infiltration Potential maps (10 meg. WinZip Archive).