Cimarron Pond, King County. A flood test was performed at the Cimarron storm water infiltration pond in King County in April 2001. This pond, which is shown in Figure 4.5, is located in a residential area on the Sammamish Plateau in eastern King County and was constructed in 1981. The basin area draining into the pond is approximately 14.6 acres (0.06 km2). The pond bottom is roughly triangular in shape with side dimensions of approximately 42 feet (12.8 meter) by 85 feet (25.9 meter) by 110 ft (33.5 meter). The pond side slopes were estimated to be 0.6V:1H and depth of the pond is approximately 5 feet (1.5 meter). A breached swale was included in the design for pre-treatment of the stormwater. The top 6 inches (15 cm) of the pond were filled with well-graded, clean sand and ¾-in. to 2 in. (2-cm to 5 cm) washed gravel placed beneath the sand. The average infiltration rate used for the design was 13.3 in/hr (9.4x10-3 cm/s). The photograph included in Figure 4.5, which shows the pond essentially full of water, was taken in January of 2002 after several month of high precipitation. The pre-treatment swale was fully submerged and an over- flow discharge pipe was partially submerged at this time. This overflow pipe discharges into a nearby wetlands, which suggest a relatively high water table. Falling head test infiltrometer tests were conducted at the Cimarron pond in an earlier study (X. Xxxxx, 2000). The measured infiltration rates from these tests was 2.7 inches/hour (1.9x10-3 cm/s). At the time of this earlier study, the soils from the bottom of the pond were found to have a high percentage of fines (12 to 20 percent).
Cimarron Pond, King County. The basic data that were used to estimate infiltration rates at Cimarron pond are shown in Table 4.1 and in Figure 4.19. The information in Table 4.1 describes the flow rates, the duration of the test, and the pond geometry. Figure 4.19 shows the depth of water as a function of time. These depths were measured at 5-minute intervals. The flow rate into the pond was approximately 1220 ft3/hr (150 gpm or 35 m3). The pond was filled for approximately 1.7 hours and the water reached a maximum depth of approximately 13 inches (33 cm). The total volume of water discharged into the pond was approximately 2070 ft3 (59 m3) The test at the Cimarron pond was completed by placing four pressure transducers at different locations on the pond bottom. These transducers were at slightly different elevations, as shown in Figure 4.19. The average of these four transducers was used to calculate the depth of water in the pond and the infiltration rate. Figure 4.20 shows the infiltration rate averaged over a 30-minute period. The maximum infiltration rate was approximately 1.9 in/hr (1.3x10-3 cm/s) and occurred at the end of the filling phase. The infiltration rate dropped to a relatively steady rate of approximately 0.1 in/hr (7.1x10-5 cm/s) after approximately 2 hours. The relatively high- frequency fluctuations may have been the result of precipitation events that occurred during the drainage period. Given these higher-frequency fluctuations, the estimated average infiltration rate was nearly constant after approximately 3 hours, as shown in Figure 4.20. Again, a constant infiltration rate suggests that the hydraulic gradient that causes flow is also approximately constant during this period. Figure 4.21 compares infiltration rates as a function of water level for the rising and falling limbs of the hydrograph. The graph shows that infiltration rates during the rising limb were significantly larger than the infiltration rates during the falling limb.
