G.4.1 The Simplified Approach to Calculating the Design Infiltration Rate of the Native Soils

The simplified approach was derived from high ground water and shallow pond sites in western Washington, and in general will produce conservative designs. This approach can be used when determining the trial geometry of the infiltration BMP and for small BMPs serving short plats or commercial developments less than 1 acre of contributing area. Designs of infiltration BMPs for larger projects should use the detailed approach (described below) and may have to incorporate the results of a ground water mounding analysis as described in G.7 Groundwater Mounding and Seepage Analysis.

A ground water mounding analysis is advisable for BMPs with drainage areas smaller than 1 acre if the depth to a low permeability layer (e.g., less than 0.1 inch per hour) is less than 10 feet.

Using the simplified approach, estimate the design (long‑term) infiltration rate as follows:

• Use any of the options detailed in G.3 Determining the Measured (Initial) Ksat to estimate the initial K_sat.

• Assume that the K_sat is the measured (initial) infiltration rate for the native soils.

• Determine the design infiltration rate by adjusting the initial infiltration using the appropriate correction factors, as detailed below.

Design Infiltration Rate = Measured Infiltration Rate x CF

A correction factor (CF) is applied to the measured infiltration rate to calculate the design infiltration rate. The design infiltration rate shall be used when sizing infiltration BMPs using the design criteria outlined in Vol II–5.4 BMP Design.

Correction factors account for site variability, number of tests conducted, uncertainty of the test method, and the potential for long‑term clogging due to siltation and bio‑buildup. The specific correction factors used shall be determined based on the professional judgement of the licensed engineer in the state of Washington or other professional, considering all issues that may affect the infiltration rate over the long term, subject to the approval of Kitsap County.

Site variability and number of locations tested (CF_V)

The number of locations tested must be capable of producing a picture of the subsurface conditions that fully represents the conditions throughout the proposed location of the infiltration BMP. The partial correction factor used for this issue depends on the level of uncertainty that adverse subsurface conditions may occur. If the range of uncertainty is low—for example, conditions are known to be uniform through previous exploration and site geological factors—one pilot infiltration test may be adequate to justify a partial correction factor at the high end of the range.

If the level of uncertainty is high, a partial correction factor near the low end of the range may be appropriate. This might be the case where the site conditions are highly variable due to conditions such as a deposit of ancient landslide debris, or buried stream channels. In these cases, even with many explorations and several pilot infiltration tests, the level of uncertainty may still be high.

A partial correction factor near the low end of the range could be assigned where conditions have a more typical variability, but few explorations and only one pilot infiltration test is conducted. That is, the number of explorations and tests conducted do not match the degree of site variability anticipated.

Uncertainty of test method (CF_t)

This criterion represents the accuracy of the infiltration test method used. Larger scale tests are assumed to produce more reliable results (i.e., the Large PIT is more certain than the Small PIT).

Degree of influent control to prevent siltation and bio‑buildup (CF_m)

High uncertainty for this criterion may be justified under the following circumstances:

• If the infiltration BMP is located in a shady area where moss buildup or litter fall buildup from the surrounding vegetation is likely and cannot be easily controlled through long‑term maintenance.

• If there is minimal pre‑treatment, and the influent is likely to contain moderately high Total Suspended Solids (TSS) levels.

• If influent into the BMP can be well controlled such that the planned long‑term maintenance can easily control siltation and biomass buildup, then low uncertainty may be justified for this criterion.

For design of bioretention and permeable pavement facilities, the design guidance provided in Vol II–5.4 BMP Design shall be used to determine correction factors.

The overlying bioretention soil mix provides excellent protection for the underlying native soil from sedimentation. Accordingly, the correction factor for the sub‑grade soil does not have to take into consideration the extent of influent control and clogging over time. The correction factor to be applied to in-situ, small-scale infiltration test results is determined by the number of tests in relation to the number of bioretention areas and site variability. Refer to Table G.5. Correction factors range from 0.33 to 1 (no correction) and are determined by the licensed professional that performed the infiltration testing.

Table G.5. Correction Factors to be Used with In‑Situ Saturated Hydraulic Conductivity Measurements to Estimate Design Rates.

Issue	Partial Correction Factor
Site Variability and Number of Locations Tested	CFV = 0.33 to 1.0
Uncertainty of Test Method
Simple Infiltration Test	CFt = 0.40
Small‑scale PIT	= 0.50
Large‑scale PIT	= 0.75
Grain Size Method	= 0.40
Degree of Influent Control to Prevent Siltation and Bio Buildup	CFm = 0.9

The Total Correction Factor shall then be calculated as follows:

CF_T = CF_V x CF_t x CF_m

The design infiltration rate (K_satdesign) is calculated by multiplying the initial K_sat by the total correction factor:

K_satdesign = K_satinitial x CF_T