Table 10.7. Size and Filter Design for Riprap at Spur Noses and Spur Shanks.
T (yrs)
Q (cfs)
K50 (ft)
GK
tr (ft)
1
1
2
1
2
Gf
t f (in.)
(in.)
2
f50 (mm)
f50 (mm)
tf
GF
(a) At Spur Noses
50
51,600
1.1
1.3
2.4
6.0
3.0
8.0
60.0
2.0
7.0
100
62,000
1.3
1.3
2.9
6.0
3.0
9.0
60.0
2.0
9.0
200
72,500
1.4
1.3
3.1
6.0
3.0
10.0
60.0
2.0
9.0
(b) At Spur Shank
50
51,600
0.6
1.3
1.3
6.0
3.0
6.0
20.0
2.0
6.0
100
62,000
0.7
1.3
1.5
6.0
3.0
6.0
20.0
2.0
6.0
200
72,500
0.7
1.3
1.5
6.0
3.0
6.0
20.0
2.0
6.0
Note:
T is the return period, Q is the design discharge, K50 is the design riprap size for which 50 percent is finer by weight, GK is
1
1
1
the gradation coefficient of riprap, tr is the thickness of riprap, f50 , G f , and t f are, respectively, the gravel size for which
2
2
50 percent is finer by weight, the gradation coefficient, and the thickness of the first layer of gravel filter, and f50 , G f ,
2
and t f are, respectively, the gravel size for which 50 percent is finer by weight, the gradation coefficient, and the
thickness of the second layer of gravel filter.
Figure 10.11 shows the suggested spur design. The methods of design are briefly described
below.
In Figure 10.9, Spurs No. 1 and 2 are T-nose spurs, and the others are round-nose spurs. The
angle of spur to the bank is usually 60 to 120. The available literature shows that the angle
for T-nose spurs is normally 90 but the angles for round spurs varies. Mamak (1964) states
that the best results for deflecting flow and trapping sediment load are obtained with spurs
inclined upstream from 100 to 110. However, the study by Franco (1967) showed that for
channelization the normal or angled downstream spurs (60) performed better than the angled
upstream spurs. HEC-23 (Lagasse et al. 2001) suggests that for most applications, spurs can
be oriented at 90 to the bank line, except for the first spur in the spur field. In this case, with a
short radius bend, a moderate downstream angle is justified. Thus, judging from the purpose
of spurs and flow conditions being considered, it is determined that the angles of round spurs
should be constructed about 70 to the bank, angled downstream as shown.
The length of a spur depends on its location, amount of contraction of stream width, and
purpose of the spur. The purpose of the spurs considered in this study is to guide the flow
away from the bank and to provide a flow alignment similar to that of 1965. The lengths of
spurs are determined to serve this purpose (Figure 10.9 and Table 10.7).
The spacing between spurs is primarily related to the length of the spur. In general, the
recommended spacing is from one and one-half to six times that of the upstream projected
spur length into the flow (see HEC-23 for additional guidance). For bank protection in a sharp
bend, a smaller spacing should be used. For the design conditions, the spacing of spurs is
taken as 600 ft (183 m).
The height or elevation of a spur is determined by considering the maximum flow depth above
thalweg level. In order to provide additional protection against the breaking waves, an extra
foot of freeboard was added to the design height of each spur above the existing thalweg level.
The local scour depth at the spur nose is the same as that at the leading portion of the
continuous riprap revetment. The computed local scour depths for different design floods are
given in Table 10.2. The minimum depth of riprap below thalweg level is determined by
considering the local scour depth and the antidune heights, which also gives the design depth
of spur at the spur nose below the existing thalweg level.
10.15
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