StormReady
Sandbagging at Woodside - DRAFT PAGE
Many buildings are on higher ground than others with finished floor elevations at 33 feet elevation or above. (The highest flood elevation in Woodside occurred in 1986 and was 34 feet.) For these buildings a quick method would be to place sandbags about one foot high at the entrances to the residences. This could prevent immediate damage, but would not prevent water from seeping slowly through the walls into the residence.
The number of sandbags needed for each building is based on building a "half-pyramid" wall around each building (Figure 1). The height of the wall is assumed to start 6" below the finished floor and is built 6" above the predicted flood elevation. The base of the sandbag wall should be 1.5 times the height of the wall. At least 6 mil plastic sheathing should be laid underneath the wall and up the sides of the building. If possible the sheathing and sandbags should be "keyed" into the ground. Plywood should be placed to protect doors, windows, and vents to the height of the sandbag wall.  Table 1 provides an estimate of the height of the sandbag wall needed to protect each building for a predicted flood elevation. Table 2 provides an estimate of the number of sandbags needed to build the wall to protect the building from the predicted flood elevation. Table 3 provides an estimate of the amount of sand needed to build the wall. These estimates of sandbag wall heights, sandbags and sand are provided to give the building residents an idea of the effort and resources needed to protect their homes from flood damage. Each building resident will need to evaluate their ability to provide adequate protection for their homes in the time available.
In the past the most effective first use of sandbags at Woodside was at the entrance to Woodside East preventing flooding from Strong Ranch Slough over Northrop Avenue. In December 2006 DWR constructed a retaining wall along the south bank of Strong Ranch Slough immediately upstream and downstream of Bell Avenue. This wall provides flood protection up to 32 feet elevation, which is about the same elevation as the top of banks of Sierra Branch Creek. So the first flooding now can come from either Northrop Avenue or Sierra Branch Creek. If sandbags are placed along the Northrop Avenue side, the sandbag line may well have to be wrapped along Sierra Branch to be effective. Although the accuracy of current mapping for Woodside is sufficient for study purposes, it may not define adequately paths where water can enter first. Additionally, the type and location of a storm can cause Strong Ranch Slough to be higher than Sierra Branch, or vice versa. Sandbagging efforts for this method are best left up to the Woodside Operation Center and Property Manager.
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