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

4. Since the engineer is confident about the flow velocity and angle of attack, and the
channel is not expected to experience any vertical instability, a pad radius of R = 1.5b.
was chosen.

Pad Radius, R = 1.5(2.5) = 3.75 m.

The Toskanes will be installed around the pier, a horizontal distance of 3.75 m from the
wall of the pier.

5. From Figure 6.2 or 6.3, the number of
Toskanes per unit area for the 100 kg
Toskane size with a pad thickness of 2Du is
7.08 Toskanes/m2 Total area of the pad
(see Figure 6.5) is:

Area = 2(5(3.75)) + (1t(4.252 - 0.52)) = 93.5
m2.

#Toskanes = 7.08(93.5) = 662 Toskanes.

The pad thickness is 2Du = 900 mm.

6. Since the bed material is cobbles and
gravel, a granular filter is added beneath
the pad of Toskanes. The dss of the filter
directly beneath the pad of Toskanes is 95
mm. The cobbles and gravel are
sufficiently large so no additional filter
layers are required.

Area of Pad

~.-

0.5.
r- 3. 75m.,
1

5m

J
~

Figure 6.5 Area of Pier Pad

Design Example for a Bridge Abutment (Fotherby & Ruff 1995)

The bridge at Blue Creek in Figure 6.4 has vertical wall abutments with wing walls. During
normal flows the west abutment extends 0.6 m into the flow, but during high flows it
obstructs 2.4 m of the flow (normal to the flow field). The embankment slope is at1 H:1V.
The east abutment does not obstruct the flow even during high flows.

1. Determine the velocity value, Vv (m/s).
The abutment is located near the bank, outside of the thalweg, C1 = 0.9.
Since the abutment has wing walls, Gs = 0.85.
The T oskane pad is installed so that the top of the pad is level with the bed, Gh = 1.0.
A randomly installed pad of Toskanes is selected, C; = 1.0.

Vv= 2.5(0.9)(0.85)(1.0)(1.0)(1.5) = 2.87 mls.

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

6.10