2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

5.3.2 Determine Infiltration Feasibility

This section provides step-by-step procedures for evaluating the feasibility of infiltration for a site and determining design infiltration rates for BMP design. Each of the following steps is outlined in more detail in the subsequent sections:

Seasonal timing for geotechnical/soils investigations, infiltration testing and groundwater monitoring requirements for infiltration BMPs can impact project schedules. Subsurface investigations shall be scheduled during the wet season, between December and March, whenever possible.

The Developer may choose to perform Steps 2 through 5 concurrently, or in series. Larger projects may benefit from consulting with a licensed professional early in project development. Refer to Figure II-5.1 for a flowchart illustrating these steps for completing an infiltration feasibility assessment.

Step 1: Evaluate Horizontal Setbacks and Site Constraints

Evaluate the following criteria related to limitations, horizontal setbacks, and contaminated soil or groundwater. For any portion of the site that falls within an area that limits or restricts infiltration BMPs, as documented and approved through the Site Assessment and Planning submittal review (Vol I–2.5 Submittal Requirements), further infiltration investigation to meet the On-Site Stormwater Management, Flow Control, or Runoff Treatment requirements is not required. An infiltration feasibility flow chart is presented in Figure II-5.1.

Figure II-5.1. Infiltration Feasibility Flow Chart.

Infiltration Feasibility Flow Chart

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Assess horizontal setbacks and site constraints to determine infiltration feasibility for the site, as follows:

Horizontal Setbacks

For infiltrating bioretention and rain gardens, horizontal setbacks are measured from the vertical extent of the cell or basin (e.g., top of the bioretention soil). For infiltration chambers, horizontal setbacks are measured from the outside bottom of the structure. For all other infiltration BMPs, horizontal setbacks are measured from the edge of the aggregate within the BMP.

Infiltration is not permitted in the following areas:

  • Within 10 feet of property lines (may be reduced from right of way with County approval).

  • Within 10 feet of another infiltration BMP.

  • Within the following setbacks from structures (on and off site):

    • When runoff from less than 5,000 square feet of impervious surface area is infiltrated on the site, the infiltration BMP shall not be within 5 feet from a building without a basement, and/or 10 feet from a building with a basement.

    • When runoff from 5,000 square feet or more of impervious surface area is infiltrated on the site, a building shall not intersect with a slope 1 horizontal to 1 vertical (1H:1V) from the bottom edge of an infiltration BMP. The resulting setback shall be no less than 5 feet from a building without a basement and/or 10 feet from a building with a basement. For setbacks from buildings or structures on adjacent lots, potential buildings or structures shall be considered for future build-out conditions.

  • Within Kitsap County Board of Health specified setbacks for drinking water wells, septic tanks, and drainfields.

  • Within 100 feet from open water features and designated landslide hazards.

  • Within 200 feet from springs and flowing artesian wells used for drinking water supply. Infiltration BMPs upgradient of drinking water supplies shall comply with Health Department requirements.

  • Within 50 feet from the top of any slope over 15%.

Site Constraints

  • Steep Slope or Landslide-Hazard Areas – Infiltration is limited within landslide-prone areas or within a setback of 10 times the height of the steep slope to a maximum of 500 feet above a steep slope area. Infiltration within this area may be feasible provided a detailed slope stability analysis is completed by a licensed engineer or engineering geologist. The analysis shall determine the effects that infiltration would have on the landslide-prone or steep slope area and adjacent properties.

  • Septic Systems and Drain fields – See Table II-5.2 for infiltration setback requirements to onsite sewage systems. Refer to the Kitsap County Board of Health Ordinance 2008A‑01, as now or hereafter amended, for additional setback information.

  • Drinking Water Supply Wells or Springs – See Table II-5.3 for infiltration setback requirements to private and public wells. Refer to the Kitsap County Board of Health Ordinance 2008A‑01, as now or hereafter amended, for additional setback information

  • Contaminated Sites and Landfills –

    • Within 100 feet of a contaminated site or landfill (active or closed). For projects where runoff from 5,000 square feet or more of impervious surface area will be infiltrated on the site, infiltration within 500 feet up-gradient or 100 feet down-gradient of a contaminated site or landfill (active or closed) requires analysis and approval by a licensed hydrogeologist.

    • Infiltration is infeasible where soil and/or groundwater contamination problems have been identified by, including but not limited to, the following:

  • Underground or Above Ground Storage Tanks –

    • Infiltration is infeasible within 10 feet of an underground or above ground storage tank or connecting underground pipes when the capacity of the tank and pipe system is 1,100 gallons or less. (Applicable to tanks used to store petroleum products, chemicals, or liquid hazardous wastes.)

    • Infiltration is infeasible within 100 feet of an underground or above ground storage tank or connecting underground pipes when the capacity of the tank and pipe system is greater than 1,100 gallons. (Applicable to tanks used to store petroleum products, chemicals, or liquid hazardous wastes.)

Step 2: Conduct Subsurface Investigation and Evaluate Vertical Separation Requirements

The applicant may choose to perform Steps 2 and 3 in either order or concurrently (i.e., infiltration testing can be done before or during a subsurface investigation and evaluation of vertical separation requirements). Review G.2.3 Simple Subsurface Investigation for additional guidance, prior to conducting the investigation.

Subsurface Investigations

Subsurface investigations are required to identify subsurface and groundwater conditions that may affect performance of the infiltration BMP. Investigations shall be performed at the location of the proposed BMP or as close as possible, but no more than 50 feet away. The number and type of subsurface investigations required are provided in Table II-5.4. Seasonal timing for infiltration testing and groundwater monitoring requirements for infiltration BMPs can impact project schedules. Subsurface investigations shall be scheduled during the wet season, between November and March, whenever possible. Larger projects may benefit from consulting with a licensed professional early in project development.

This manual includes four types of subsurface investigations:

  • Simple subsurface investigation (limited applications in rural areas only; see Table II-5.4);

  • Standard subsurface investigation;

  • Comprehensive subsurface investigation; and

  • Deep infiltration subsurface investigation.

Subsurface investigation is required for the entire site or portion(s) of the site that have not been excluded based on information reviewed in Step 1.

The type of subsurface investigation required for a project is provided in Table II-5.4 and varies by the impervious surface area infiltrated on site. Subsurface investigation requirements and standards are provided in Appendix G. As indicated in Appendix G, a licensed professional shall conduct the subsurface investigation for standard, comprehensive, and deep infiltration investigation. A licensed professional is not required for simple subsurface investigations.

Projects shall document the results of the required subsurface investigation and evaluation of vertical separation requirements. The information to be contained in this report is provided in Volume II, Chapter 1.

Where feasible and recommended by the licensed professional, deep infiltration BMPs can be used to direct stormwater past surface soil layers that have lower infiltration rates and into well-draining soils. The nature and depth of the soil layers with lower infiltration rates can vary significantly, so the technique required to reach the well-draining soils will also vary. Consult with the licensed professional to determine if deep infiltration is feasible.

Vertical Separation Requirements

Vertical separation requirements shall be evaluated when performing a subsurface investigation. Infiltration BMPs require a minimum vertical separation from the lowest elevation of the BMP to the underlying groundwater table or hydraulically restrictive material. Groundwater elevation data shall be used to evaluate the bottom of the BMP against the vertical separation requirements to determine infiltration feasibility.

The vertical separation requirements for shallow infiltration BMPs depend upon the type of subsurface investigation required and the seasonal timing of the geotechnical exploration conducted to evaluate clearances. See Appendix G for additional information.

A determination of infiltration infeasibility may be approved (without proceeding to Step 3: Conduct Infiltration Testing) if the Geotechnical Analysis/Soils Report (see Volume II, Chapter 1) documents that groundwater seepage or a hydraulically-restrictive material is encountered within the vertical separation requirements specified in Appendix G.

Step 3: Conduct Infiltration Testing

This manual includes five allowed methods of field infiltration testing to determine the measured infiltration rate:

  • Simple Infiltration Test (limited application in rural areas only; see Table II-5.4)

  • Small Pilot Infiltration Test (PIT)

  • Large PIT

  • Deep Infiltration Test

  • Grain Size Analysis (limited application in rural areas for residential only; see Table II-5.4)

The type of infiltration test required for a project is provided in Table II-5.4 and varies by the impervious surface area routed to infiltration BMPs on a site. The required procedures that shall be used for small and large PITs, the Simple Infiltration Test, and Grain Size Analysis are provided in Appendix G. The minimum requirements for the Deep Infiltration Test are provided in Appendix G.

Test reports for the Small and Large PITs, Deep Infiltration Tests, and Grain Size Analysis shall be prepared by a licensed professional. The Simple Infiltration Test does not require a licensed professional.

The minimum allowed infiltration rates are provided in Table II-5.5. The values vary by infiltration BMP type and by the approach that will be used to meet MR #5: On-site Stormwater Management. See Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management for additional discussion of these approaches, which include the List approach and the LID Performance Standard approach.

Step 4: Determine Design Infiltration Rate

The measured infiltration rate determined in Step 3 shall be reduced using correction factors to account for site variability and the number of tests conducted, uncertainty of the test method, and potential for long-term clogging due to siltation and bio-buildup. The corrected infiltration rate is considered the long-term or design infiltration rate and is used for all BMP sizing calculations. Methodology for determining correction factors is provided in Appendix G.

Infiltration BMPs are generally not appropriate for sites that have a design infiltration rate less than 0.3 inch per hour. Project sites with infiltration rates lower than those identified in Table II-5.5 may be used for infiltration of stormwater only with Kitsap County approval on a case-by-case basis.

Step 5: Conduct Groundwater Monitoring, Receptor Characterization, Mounding and Seepage Analysis, and Infiltration Testing for Permeable Pavements Surfaces (as applicable)

The licensed professional shall provide recommendations for, and analysis of, groundwater monitoring, receptor characterization, mounding and seepage analysis, and infiltration testing. See the minimum requirements listed in Table II-5.4. As an exception, all permeable pavement BMPs are required to perform specific infiltration testing per BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual. At a minimum, the infiltration testing shall demonstrate that the infiltration BMP performs at or above the design infiltration rate.

Step 6: Evaluate Use of Infiltration to Meet Minimum Requirements

If infiltration is considered feasible based on the above steps, evaluate the feasibility of specific infiltration BMPs when selecting for On-Site Stormwater Management (Vol II–5.3.3 Select BMPs for On-Site Stormwater Management), Runoff Treatment (Vol II–5.3.4 Select BMPs for Runoff Treatment), and Flow Control (Vol II–5.3.5 Select BMPs for Flow Control).

If the results of the subsurface investigation, infiltration testing, groundwater monitoring, infiltration receptor characterization, groundwater mounding, and/or seepage analysis indicate that adverse conditions could occur, as determined by a licensed professional, the infiltration BMP shall not be built. Groundwater elevation data shall be used to evaluate the bottom of the BMP against the vertical separation requirements to determine infiltration feasibility.