EVALUATION OF THE RELATIONSHIP BETWEEN SATURATED HYDRAULIC CONDUCTIVITY AND GRAIN-SIZE DISTRIBUTION OF RECENT ALLUVIUM AND PLEISTOCENE GLACIAL OUTWASH SAND AND GRAVEL DEPOSITS, PUGET LOWLAND, WASHINGTON

Impervious surfaces such as roads, parking lots, and sidewalks prevent rain from infiltrating into the ground, resulting in surface stormwater runoff, flooding, and increased pollution in streams and rivers. Low impact development (LID) stormwater control measures such as bioretention and infiltration systems are designed to capture and retain runoff, thereby reducing the hydrologic impact to land and resources. Understanding how the infiltration rate is controlled by the saturated hydraulic conductivity (Ks) beneath facilities is critical for the appropriate design of stormwater infiltration systems. Most correlation methods and empirical formulas that relate Ks and soil grain-size distribution are based on controlled laboratory tests. Our method, however, was based on in-situ field testing of undisturbed soils in their natural environments. We analyzed the relationship between field-determined Ks and grain-size distribution for over 100 sites (200 paired-samples) using Recent alluvium and Pleistocene glacial outwash sand and gravel deposits in the Puget Lowland region of Washington State. The advance outwash has lower Ks than the Recent alluvium/recessional outwash by a factor of 2 to 4 for similar grain-size distributions. Glacial over-consolidation in the advance outwash results in compaction of the soil framework, reducing permeability. Therefore, when using a grain-size distribution approach to approximate the Ks, a distinction should be made between glacially over-consolidated and normally consolidated deposits.