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Infiltration Modeling

Regulatory Approach to Infiltration

The Dane County infiltration standard is modeled after the Wisconsin Department of Natural Resources (DNR) standard. The standard is based upon requiring a percentage of precipitation infiltrated in the predevelopment condition (also known as predevelopment infiltration) to be infiltrated in the post development condition. 90% of the predevelopment infiltration must be infiltrated for all sites. The county utilizes the same “stay-on” approach as the DNR for modeling. Stay-on is defined as all precipitation that does not runoff. Therefore, stay-on includes evaporation, plant transpiration, and recharge. While commonly referred to as an infiltration standard, it is technically a stay-on standard.

The county’s regulatory approach differs from the DNR approach in two main ways. First, no event-based goals for infiltration are included in the county’s standard. Dane County’s standard is based solely on an average annual goal and requires the use of continuous flow modeling. Second, there are no limits or “caps” placed on the amount of site area that must be dedicated to infiltration practices to meet the County’s standard. If more than 2% of a development must be dedicated to meet the stay-on goal, the Dane County standard allows designers to alternatively achieve a target average annual recharge goal. The target stay-on and target recharge goals are discussed in the following sections.

Meeting the Infiltration Performance Standard

The intent of the infiltration standard in ch. NR 151, Wis. Adm. Code, is to encourage infiltration of runoff. This requirement is tempered by a series of exemptions and exclusions for the purpose of minimizing the risk of groundwater contamination and addressing the practicality of implementation. These exemptions and exclusions were never intended to be evasive tools for developers and designers to avoid infiltration altogether.

Developers and designers need to seek practical and sometimes innovative methods to meet infiltration requirements. Where infiltration standards are unable to be fully realized, then developers and designers need to meet the standards to the Maximum Extent Practicable (MEP). MEP is a term that provides flexibility in meeting a standard (or requirement). However, there needs to be unique site-specific reasons why a project is unable to fully meet a standard. If full attainment of a standard is impractical due to unique site conditions, then the standard is to be achieved to the furthest degree practical. For example,

  • If a portion of a site is not acceptable for infiltration due to poor soils or high groundwater, directing runoff via gravity flow to other areas of the site that are suitable needs to be considered.
  • If a shallow layer of clay soil is underlain by sandy soils suitable for infiltration then excavation of the clay layer may be warranted.
  • If the only area on a site suitable for an infiltration basin is located up-slope of proposed impervious areas and the impervious areas have no other reasonable location, the designers are not required to pump water to meet the infiltration requirements in NR 151. However, decentralizing infiltration practices and installing rain gardens or other smaller practices around the site must be considered as a viable alternative.

Proper implementation of NR 151 will require that some land or parcels will be needed for storm water management. The economic considerations regarding the loss of developable land are not a reasonable justification to prevent full attainment of a standard. The developer and designer shall not skew data or sampling methods to realize a predetermined outcome or rely on the exemptions and exclusions identified in ch. NR 151 to avoid infiltration, but rather they shall seek ways to maximize infiltration to the MEP.

Infiltration Practice Design

Infiltration practices must be designed to meet the average annual infiltration goals included in the Dane County Erosion Control and Stormwater Management Ordinance. Infiltration practice design takes into account the physical characteristics of the site such as: water table, soil types, limiting layers, and tributary land use. In addition, infiltration practices may serve a dual purpose by mitigating thermal impacts. Bioretention basins are often designed to achieve the county requirement for oil and grease treatment and sediment removal.

The design of all infiltration practices designed to meet ordinance requirements shall comply with the information in this manual and applicable DNR conservation practice standards.

Target Stay-On Requirement

Before an infiltration practice can be designed and sized the target stay-on requirement must be determined. The target stay-on is dependent upon the predevelopment curve number (CN). Target stay-on for residential and non-residential developments, for a range of predevelopment CNs, can be determined using Figure 1.

Figure 1: Target stay-on requirements (based on RECARGA modeling)

The target stay-on requirement is given in depth (inches per year), but it may be advantageous to convert this depth to a volume (cubic feet per year). A conversion equation is provided below.


Tv = (Td / 12) * As

Where
Tv = Target stay-on volume, ft3
Td = Target stay-on depth, inches
As = Area of site, ft2


Target Recharge Requirement

If more than 2% of a development must be dedicated to meet the stay-on requirement, the designer may instead chose to design infiltration practices to meet Dane County’s recharge goal. The "native" recharge rate for Dane County is determined by the Wisconsin Geological and Natural History Survey's report. Site specific recharge rates can be determined by turning on the "Groundwater Recharge Identify" layer in the LWRD Viewer and clicking on the site location. At the time of publication, the only accepted model for determining the amount of recharge achieved by an infiltration practice is RECARGA. When designing practices to meet the recharge goal, at least 2% of a development must be dedicated to infiltration practices.

Approved Models for Calculating Infiltration and Recharge

RECARGA

The RECARGA model was developed to provide a design tool for evaluating the performance of bioretention facilities, raingarden facilities, and infiltration basins. The model is made available through the DNR website, and can be downloaded free of charge.

RECARGA is an approved model for calculating infiltration and recharge.

Calculating Recharge

The recharge calculation is based upon the RECARGA model output, ‘Recharge’. An example recharge calculation is provided below.


RPost = RF+RT

Where
RPost = Post development recharge depth
RF = Calculated facility recharge depth (from RECARGA)
RT = Calculated tributary recharge depth (from equation below)

RT = Pp * RNative

Where
Pp = Percent pervious
RNative = Site recharge rate, in/yr (from LWRD viewer)


WinSLAMM

WinSLAMM (Source Loading and Management Model for Windows) was developed to evaluate nonpoint source pollutant loading in urban areas using small storm hydrology. The model and parameter files are made available through the DNR website.

The model uses series of normal rainfall events to calculate the pollutant loading and runoff volumes generated. The user is able to route runoff through a approved control devices, such as infiltration basins, bioretention basins, wet basins, pervious pavement, or catchbasins to determine how effectively these devices remove pollutants and infiltrate stormwater.

WinSLAMM is an approved model for calculating infiltration and sediment control.

Modeling Assumptions and Guidance

  • As with the use of any modeling software, it is important to understand the underlying principals and assumptions used to generate the model’s results. Designers utilizing RECARGA or WinSLAMM modeling software are expected to read and understand the user manual prior to submitting plans that include modeling results.

  • The DNR conservation practice standard 1002 must be used to determine all design infiltration rates.