FLOOD08875

• County: Boulder
• Basin: South Platte
• Title: Analysis of a High Hazard Flood Zone
• Date: 10/1/1987
• Prepared For: Boulder
• Prepared By: David Love & Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The weights (W) of individuals considered in this analysis varied from 30 pounds to 240
pounds. This range is based on observation of weight data obtained through various agencies
(Reference 1 and 2).

Buoyancy forces (B) are based upon the weight of water displaced by the design monolith at a
given depth (d).

B = (Thickness) x (Face) x (d) x oywater

This force is a product of the volume of the individual below depth "d", and the specific weight of
water (y), which is approximately 62.4 pounds per cubic foot (pel) under a wide range of
conditions. Note that the design monolith volume was varied according to the individual's weight
as discussed later in this analysis.

The dynamic force (P) of the water is based upon its velocity component normal to the face of
the monolith, a coefficient of drag representing losses due to friction and loss of momentum, and
the density of water.

P = (Cd) x (p) x (V212) x (An) x (SI)

The coefficient of drag (Cd) is a function of shape
and /low regime. Values of Cd were obtained from the
"Colorado Floodproofing Manual" (Reference 3) and
represent losses induced by drag on rectangular objects.
The density of water (p) is approximately 1.94 slugs per
cubic foot (scl) for purposes of this study. The velocity
(V) component contributing to this force represents the
average velocity parallel to /low in the vicinity of the
monolith. This is actually the resultant of the theoretical
velocity distribution, which varies from a minimum value
at ground level (GL) to a maximum value near the water
surface (Figure 2). The resultant velocity actually acts
Figure 3 through a point slightly below half the depth of /low
(Reference 4), but for the purpose of this analysis it will
be assumed to act through the midpoint of the depth (Figure 4), which is a conservative
approximation. The area nonnalto /low (An) was again varied according to weight. A safety factor
(SI) was used to conservatively reflect the degree of uncertainty in the assumptions on which this
equation is based. Note that this factor of safety is applied in both the slippage and the toppling
moment analysis.

Forces due to friction (Ff), acting in the opposite direction of the applied dynamic water
force, are a product of the resultant weight of the monolith [the weight (W) of the individual minus
the buoyancy force (E)], and an assumed coefficient of friction (~).

Ff= ij!) x (W - B)

The static coefficient of friction (for bodies at rest) is normally about 0.7 for dry surfaces such as
concrete and asphalt paving. For the purposes of this analysis, ~ was varied from a low value of
0.3 for slippery surfaces to 0.7 as an upper limit.

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