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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5. LOCATION - Ideally, a short weir should be placed a distance (S) upstream from the
location where the midstream tangent flow line (midstream flow line located at the start of
the curve) intersects the bankline (PI). Additional bendway weirs are then located based on
the site conditions and sound engineering judgment. Typically, the weirs are evenly spaced
a distance (S) apart.

6. SPACING - Bendway weir spacing is influence by several site conditions. The following
guidance formulas are based on a cursory review of the tests completed by WES on
bendway weirs and on tests completed by MRD on underwater sills. Based on the review,
bendway weirs should be spaced similarly to hardpoints and spurs. Weir spacing is
dependent on the streamflow leaving the weir and its intersection with the downstream
structure or bank. Weir spacing (S) is influenced by the length of the weir (L), and the ratios
of weir length to channel width (W) and channel radius of curvature (R) to channel width.
Spacing can be computed based on the following guidance formulas:

( R )0.8 ( L )0.3
S=1.5L W W

(laGrone 1995)

S = (4 to 5)L

(Saele 1994)

Maximum Spacing (Smax) is based on the intersection of the tange.nt flow line with the
bankline assuming a simple curve. The maximum spacing is not recommended, but is a
reference for designers. In situations where some erosion between weirs can be tolerated,
the spacing may be set between the recommended and the maximum.

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S~ =R(1-(1-~Jr

(laGrone 1995)

Results from the spacing formulas should be investigated to determine that the weir spacing,
length, and angle will redirect the flow to the desired location. Streamlines entering and
exiting the weirs should be analyzed and ctrawn in planfonn.

7. LENGTH OF KEY - Bendway weirs like all bankline protection structures should be keyed
into the bankline to prevent flanking by the flow. Typically the key length (LK) is about half
the length of the short weirs and about one fifth the length of the long weirs. Tests
conducted by MRD found that lateral erosion between spurs on nearly straight reaches
could be estimated by using a 20 degree angle of expansion (see Figure 1.3). The
following guidance formulas for LK were therefore developed. These fonnulas compute
minimum LK and should be extended in critical locations. The need for a filter between the
weir key and the bank material should also be determined. Guidelines for the selection,
design, and specification of filter materials can be found in Brown and Clyde (HEC-11)
(1989) and Holtz et al. (FHWA HI-95-Q38) (1995).

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

LK = S tan(200) - L

(LaGrone 1995)

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