G.3.1 Description

A crucial element of BMP design is the long term (design) infiltration rate of the native soils. In order to determine the design infiltration rate, the designer must first determine the measured (initial) saturated hydraulic conductivity (K_sat).

This section provides procedures for the following infiltration testing methods to determine the measured (initial) K_sat, as required in Vol II–5.3.2 Determine Infiltration Feasibility:

• Simple Infiltration Test (SIT)

• Small Pilot Infiltration Test (PIT)

• Large PIT

• Deep infiltration test

• Grain Size Analysis

To determine which infiltration test method is required for a project, refer to Table II-5.4 in Vol II–5.3.2 Determine Infiltration Feasibility.

If possible, perform infiltration testing at the location of the proposed infiltration BMP. Infiltration testing results from a nearby location within 50 feet of the proposed infiltration BMP may be approved at the discretion of the licensed professional. If the infiltration testing is performed more than 50 feet from the final infiltration BMP location due to existing site conditions (e.g., existing structure at location of proposed BMP) and greater than 5,000 sf is infiltrated on the site, then acceptance testing is required (see Vol II–5.3.2 Determine Infiltration Feasibility).

If variable soil conditions are observed at the site, multiple infiltration tests are recommended in the different soil types.

A simplified and detailed approach are presented to use the initial K_sat to determine the design infiltration rate of the native soils (G.4 Calculation of Design Infiltration Rate of the Native Soils). The design infiltration rate is used to size the infiltration BMP, including verification of compliance with the maximum drawdown time of 48 hours. The test method may be modified due to site conditions if recommended by the licensed professional and the reasoning is documented in the report. Any modifications to the proposed test method should be approved by Kitsap County.