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. Select pad radius, R (m).
1.5 b. for most piers and 2.0 b. for most abutments.
Use a larger pad radius if:
· uncertain about angle of allack
· channel degradation could expose footing,
· uncertain about approach flow velocity
· surface area of existing scour hole is significantly large than pad.

If more than one Toskane pad is present in the stream cross section, check the
spacing between the pads. If a distance of 1.5 m or less exists between pads,
extend the width of the pads so that they join.

5. Select the number of Toskanes per unit area from the nomogram on Figure 6.2 or
Toskane detail sheet Figure 6.3.

a. Determine the protection pad thickness. Pads with randomly placed units
have to be a minimum of two layers thick.

b. For a two layer pad with a filter, select a pad thickness from the nomogram or
Toskane detail sheel.

6. If bed material is sand, gravel, or small cobbles, add a cloth or granular filter. Toe in
or anchor the filter. If the filter is granular, the d85 of the filter material directly below
the Toskane layer can be read from the nomogram or Toskane detail. Additional
layers of filter, that may be needed based on the gradation of the bed material, can
be designed according to standard requirements. Additional guidelines on the
selection and design of filter material can be found in Brown and Clyde (HEC-11)
(1989) and Holtz et al. (FHWA HI-95.038) (1995).

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

A bridge over Blue Creek has a single pier located on the outside of a bend (see Figure
6.4). The pier is round nosed and is 1 m wide and 6 m long. The footing is not exposed and
bed material is cobbles and gravel. The average velocity directly upstream of the bridge
during high flow is 2.5 m/s and has an angle of attack of 150.

6.8