Dry Wells Table 4 illustrates the variable configuration of dry wells depending on soil type. In order to be effective a dry well must drain relatively quickly so that it can be used to store water from consecutive storm events. Figure 1 below shows how with decreasing permeability of soil, a dry well’s ideal dimensions become shorter and broader in order to allow for a greater surface area for infiltration. The required configuration of a dry well in clay loam and clay soils can make it difficult to construct and install under these conditions.
Figure1 Dry-well dimensions by soil type to meet water treatment and retention requirements Optimization of Dry Well Dimensions by Soil Type Dimensions Shown are for Storage of Runoff from 90th percentile storm event in Los Angeles Storm Intensity: I = 0.22 inches/hour Storm Duration: t = 6 hours Design Criteria: 1) Drains within 3 days of storm event 2) Height must not exceed 8 feet 3) Assumes cnstant infiltration rate during storm event
d = 4.4 h = 7.0 d = 4.8 h = 5.8
d = 6.2 h = 3.5
SAND Q=1.2 inches/hour
SANDY LOAM Q=1.0 inches/hour
d = 8.7 h = 1.8
LOAM Q=06 inches/hour
CLAY LOAM Q=0.3 inches/hour
Table 4 Dry Well sizing per 1000 square feet of impermeable drainage area
Soil Type
Sand
Sandy Loam
Loam
Sandy Clay Loam
Pipe Diameter (ft)
4.1
4.1
5.3
7.5
Depth (ft) (h)
5.5
5.8
6.0
6.0
Volume (ft )
71.5
74.8
1332.0
264.0
Â
3
Clay not recommended
Note: Constructability of a dry well in both clay loam and clay soils is limited by the shallow well height and large well diameter
DRAFT
Stormwater Guidelines A-03
05