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

e. Select a random or pattern installation for the protection pad. A random
installation refers to the units beings dumped into position. In a pallern
installation, every Toskane is uniformly placed to create a geometric pallern
around the pier.

C,= 1.0, Random Installation
C, = 0.9, Pattern 1 - 2 Layers with Filter
C, = 0.8, Pattern 2 - 4 Layers

f. Calculate the Velocity Value:
Multiply the average approach flow velocity and coefficients by a safety factor
of 1.5.

v, = 15VoC,C,ChC,

2. Calculate adjusted structure width, b. (m).

For a pier:
a. Estimate angle of attack for high flow conditions.
b. If the angle is less than 50, use pier width b as the value b..
c. If the angle is greater than 50, calculate b.:

bo = Lsina +bcosa

where L = length of the pier (m), b = pier width (m), b. = adjusted structure
width (m), ex = angle of attack.

d. If a footing extends into the flow field a distance greater than:
0.1 . Yo (approach flow depth)
use footing width instead of pier width for b.

e. For an abutment:
Estimate the distance the abutment extends perpendicular to the flow (b)
during high flow conditions.

if b'5. 1.5 m, then b. = 1.5m.
If 1.5 m '5. b '5. 6 m, then b. = b.
if b. :2: 6 m, then b. = 6 m.

3. Select a standard Toskane size, Du, using the design equation or nomogram on
Figure 6.2 using the calculated velocity value, Vv, and the adjusted structure width,
b.. Du represents the equivalent spherical diameter of riprap that would be required.
This parameter can be related to dimensions of the Toskane by Du= 0.622H, where
H is the length of the Toskane (see Figure 6,3). It should be noted that the
nomogram reports results in centimeters (cm) and the equation presented in Figure
6.2 solved for Du has the units of meters (m) when all other parameters are in
meters.

Check the b.IDu ratio using the diameter, Du, of the standard Toskane size. If the
ratio> 21, select the next largest size of Toskane. Repeat until ratio < 21.

6.5