FLOOD00110

• County: Statewide
• Title: Bridge Scour and Stream Instability Countermeasures
• Date: 7/1/1997
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Floodplain Report/Masterplan

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FLOW

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S tan(200)

20 0 Expansion
Angle

L

Banldine

LK

LK= S tan(200) - L

Figure 1.3 Length of key for mild bends

When the channel radius of curvature is small (R < 5W) and S < Utan(200)

L (W)O.3(S)O.5
LK =- - -
2 L R

(laGrone 1995)

NOTE: LK should not be less than 1.5 times the total bank height.

The NRCS guideline for length of key (LK) for short weirs or barbs (Saele 1994) is to key the
barb into the bank a minimum distance of 2.4 m (8 It) or 4 (0100) which ever is greater.

8. TOP WIDTH. The top width of the weir may vary between 1 m and 4 m (3 and 12 It), but
should be no less than (2 to 3)"0100, Weirs over 9 m (30 It) in length will have to be built
either from a barge or by driving equipment out on the structure during low flows. Structures
built by driving equipment on the weir will need to be at least 3 to 5 m (10 to 15 It) wide.
Side slopes of the weirs can be set at the natural angle of repose of the construction
material (1V:1.5H) or flatter.

9. NUMBER OF WEIRS - The smallest number of weirs necessary to accomplish the
project purpose should be constructed. The length of the weirs and the spacing can be
adjusted to meet this requirement. Typically. not less than three weirs are used together on
unrevetted banks.

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10. CONSTRUCTION - Construction of the bendway weirs are typically conducted during
low flow periods for the affected river. Construction methods will vary depending on the
size of the river. Construction on larger rivers may be conducted using a barge which would
allow the rock to be placed without disturbing the bankline. For rivers where a barge is not

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