Balsam 7-11 Pond, Kitsap County. In September 2000 a flood test was performed at the Balsam 7-11 stormwater infiltration pond in Kitsap County. The infiltration pond ( Figure 4.3) is located in east central Kitsap County, south of Xxxxxxxx Inlet. The site is located at the eastern end of a convenience store parking lot adjacent to a major thoroughfare. The estimated area of the basin that drains into the pond is 7 acres (0.03 km2) (Xxxxxxx, 1998). Stormwater that discharges into the pond comes from both residential and commercial developments. The water outlet pipe is located on the south end. Litter (such as wrappers, old shoes, and other articles of clothing) was found in the bottom of the pond. A chain-link fence encloses the pond. The pond was designed in 1977. The pond bottom is rectangular, 7 feet (2.1 meter) wide by 53 feet (16.1 meter) long. The side slopes were surveyed to be 0.7V:1H and the depth of the pond is approximately 7 feet (2.1 meter). There is no pre-treatment for the stormwater that discharges into the pond. The pond bottom and side slopes are dense with wetland plants. Sediment build-up has been removed from Balsam 7-11 within the last three years (Xxxxxxxx, X., 2001).
Balsam 7-11 Pond, Kitsap County. The basic data that were used to estimate infiltration rates at the Balsam site are shown in Table 4.1 and Figure 4.11. The flow rate into the pond during the filling stage was approximately 1000 ft3/hr (125 gpm or 28 m3/hr). The pond was filled for approximately 2 hours and the water reached a maximum depth of approximately 31 inches (79 cm). The total volume of water discharged into the pond was 2290 ft3 (64.8 m3). Similar to the pond in Xxxxx County, the drainage phase can be divided into two parts. During the first part of the draining phase, the water level decreased exponentially. This occurred between hour 2 and hour 6. Between hour 6 and the end of the test, the water level declined linearly. Figure 4.12 shows the infiltration rate averaged over 30-minute periods. The maximum infiltration rate was approximately 12 in/hr (8.5x10-3 cm/s) and occurred at the end of the filling phase. The infiltration rate fell to approximately 2 in/hr (1.4x10-3 cm/s) during the later part of the test. The linear part of Figure 4.11 demonstrates that the infiltration rate was roughly constant after approximately 6 or 7 hours. This infers a constant gradient during the second stage of the drainage phase. The water level in the pond decreased by approximately 15 inches (38 cm) during the last 6 hours of the drainage stage when the infiltration rate had reached a relatively steady state. This supports the idea that the water level in the pond may have been a significant driving force for infiltration.
