FLOOD06847

• County: Statewide
• Basin: Statewide
• Title: Bridge Scour and Stream Instability Countermeasures
• Date: 7/1/1997
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

SteD 2: Determine the side slope correction factor, K1:

From Figure 4.5 the reduction factor for a 1V:2.5H side slope is:

K1 = 0.73

K1 = 0.67

(for Block Size 1)
(for Block Size 2)

SteD 3: Determine the factor of safety for blocks placed on the channel side slope:

r (945 .
SF = 2. SF.) K1 - (1) 0.73 = 2.9 (for Block Size 1)
r 0 235.2
r 1085 .
SF = 2.(SF.)K1 = (1) 0.67 = 3.5 (for Block Size 2)
r 0 209.8

Factor of Safety Equations Example

Given: In addition to the hydraulic conditions given above, the following block
characteristics are provided.

Block Submerged R1 R2 Ra R4 tJ.Z co to'
Size Weight (mm) (mm) (mm) (mm) (mm) (mm) Pa
(N) (N/m2)
1 127 76 223 122 223 12.7 329 958.0
2 148 76 223 122 223 12.7 329 1102.0
'to determined from testing.

NOTE: For computations, variables have been converted to meters.

SteD 1: Compute factor of safety parameters

e = tan-1(~) = 21.80
2.5

(side slope angle)

A = tan-1( 0.~39) = 2.230

T) = ~ = 235.2 = 0.246
to 958

Tl = ~ = 209.8 = 0.190
to 1102

(bed slope angle)

(stability number for Block Size 1)

(stability number for Block Size 2)

conservatively assuming that FL = Fo then:

4.11