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

G.3.2 Simple Infiltration Test (SIT)

The Simple Infiltration Test is a small-scale infiltration test procedure adapted from the Rain Garden Handbook for Western Washington (Hinman et al. 2013).

The Simple Infiltration Test does not require a licensed professional and may only be applied for project sites located in rural areas, outside the UGA and UAs in accordance with Table II-5.4.

The Simple Infiltration Test is not allowed for projects with no offsite point of discharge. These projects shall use a Small PIT. See G.3.3 Small Pilot Infiltration Test (Small PIT).

G.3.2.1 Procedure

If testing is performed during the wet season (November through March), only one test is required. If the test is performed during the dry season (April through October), two tests must be performed in same hole within 2 days, with the beginning of each test spaced 24 hours apart.

  1. Dig a hole a minimum of 2 feet deep. Preferably, the depth of the hole should be measured from the bottom of the BMP but at a minimum shall be measured from the proposed site finished grade. The hole shall be at least 2 feet in diameter.

  2. Record the type and texture of the soil. If the soil is primarily fine-grained such as silt or clay, or is glacial till, infiltration may not be feasible.

  3. At the same time that you dig your test hole, check for high groundwater by using a post hole digger to excavate a hole to the minimum subsurface investigation depth, as provided in G.2.3 Simple Subsurface Investigation, approximately 5 feet from the test hole. If standing water or seeping water is observed in the hole, measure the depth to the standing water or seepage.

  4. Pre‑soak period: Add 12 inches of water to the hole. This can be measured using a ruler, scale, or tape measure. Be careful to avoid splashing, which could erode the sides of the hole or disturb the soil at the base of the hole.

  5. Record the depth of water in the hole in inches.

  6. Record the time water was added to the hole.

  7. Check and record the time and depth of water in the hole on an hourly basis for up to 2 hours. Estimate the infiltration rate in inches per hour by calculating the drop in water level in inches for each hour. Based on the lowest of these measurements, determine which time interval to use for the infiltration test by following these guidelines:

    • 3 inch per hour fall, check at 15‑minute intervals.
    • 3 inch to 1 inch per hour fall, check at 30‑minute intervals.
    • <1 inch per hour fall, check at hourly intervals.

  8. Infiltration Test: Fill the hole back up to a depth of 12 inches. Check and record the time and depth of water in the hole at regular intervals based on the time interval determined during the presoak period for a total of six measurements. If the hole empties prior to the six measurements, refill and continue recording until you have recorded six measurements.

  9. Calculate measured infiltration rate. Refer to Table II-5.5 for minimum infiltration rates for each type of infiltration BMP. Using the collected data, estimate the measured infiltration rate in inches per hour by calculating the drop in water level in inches for each hour data was collected during the infiltration test. There should be a total of six values. The lowest calculated value is the measured infiltration rate in inches per hour

  10. Mark test locations on site map.