Engineer defends using hydraulic-gradient-adjusted rate to judge infiltration feasibility

During cross-examination at the Feb. 10 Kitsap County hearing on Meadowview, engineering witness Ms. Decker explained the team’s approach to determining whether infiltration best-management practices (BMPs) are feasible on the site.

Kitsap County’s stormwater design manual directs applicants to determine a measured infiltration rate during feasibility studies and to use a design infiltration rate for facility sizing. The appellants’ attorney pointed to manual language and a table that lists a feasibility threshold — a measured infiltration rate of 5 inches per hour or greater for some infiltration BMPs — and asked whether the witness treated the Massman-derived KSAT number as the measured infiltration rate or applied a hydraulic-gradient correction first.

Ms. Decker described three steps used in the project work: (1) estimate saturated hydraulic conductivity (KSAT) from grain-size analysis using the Massman equation; (2) compute a representative or equivalent KSAT across tested strata; and (3) apply a hydraulic-gradient factor to convert KSAT to an estimated infiltration rate appropriate for field conditions. That gradient accounts for how water will move vertically through soil beneath an infiltration facility, she testified. The project spreadsheet produced a computed measured infiltration rate of about 4.3 inches per hour after applying a steady-state gradient to the site KSAT — below the 5-inch-per-hour threshold cited in the manual for some infiltration practices.

When counsel pressed that appendix G language in the manual labels “measured initial KSAT,” Decker said the professional judgment of licensed engineers requires converting laboratory-derived conductivities into realistic infiltration rates by accounting for hydraulic gradient and site-specific conditions. She said relying only on raw KSAT would “drastically overestimate” a site’s infiltration potential and, in practice, leads to failed facilities.

Decker also described the difference between a measured infiltration rate and a design infiltration rate: the latter includes additional correction factors for testing method, clogging potential and long-term performance and is used for final BMP sizing. She told the hearing that preliminary feasibility findings guide later, more detailed field testing that is normally done during final engineering.

Ending: The dispute is partly textual (how to read specific manual language) and partly technical (how to represent subsurface flow). The examiner will weigh the record in a written decision that could address whether the project’s infiltration analysis satisfied county minimums for feasibility.