FLOOD03351

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Hydraulic Design of Flood Control Channels
• Date: 7/1/1991
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2-1601
1 Jul 91

Kl = side slope correction factor (see d(l) below)
g = gravitational constant. lengtMirne2

This equation can be used with either SI (melric) or
non-SI units.

c. Safety factor. Equation 3-3 gives a rock size that
should be increased to resist hydrodynamic and a variety
of nonhydrodynamic-imposed forces and/or unconlr01Iable
physical conditions. The size increase can best be accom-
plished by including the safety factor, which will be a
value greater than unity. The basic safety factor is Sf =
1.1. The basic safety factor may have to be increased in
consideration for the following conditions:

(I) Imposed impact forces resulting from logs,
uprooted lreeS. loose vessels, ice. and other types of large
floating debris. Impact will produce more damage to a
lighter weight riprap section than to a heavier section.
For moderate debris impact, it is unlikely that an added
safety factor should be used when the blanket thickness
exceeds 15 in.

(2) The basic stone slZmg parameters of velocity,
unit weight of rock. and depth need to be detertn ined as
accurately as possible. A safety factor should be included
to ccmpensare for small inaccuracies in these parameters.
If conservative estimates of these parameters are used in
the analysis. the added safety factor should not be used.
The safety factor should be based on the anticipated error
in the values used. The following discussion shows the .
importance of obtaining nearly correct values rather than
relying on a safety factor to correct inaccurate or assumed
stone sizing parameters. The average velocity over the
toe of the riprap is an estimate at best and is the param-
eter to which the rock size is the most sensitive. A check
of the sensitivity will show that a 10 percent change in
velocity will result in a neatly 100 percent change in the
weight limits of the riprap gradation (based on a sphere)
and about a 30 percent change in the riprap thickness.
The riprap size is also quite sensitive to the unit weight of
the rock to be used: a 10 percent change in the unit
weight will result in a 70 percent change in the weight
limits of the riprap gradation (based on a sphere) and
about a 20 percent change in the riprap thiclrness. The
natural variability of unit weight of stone from a stone
source adds to the uncertainty (EM 1110-2-2302). The
rock size is not nearly as sensitive to the depth parameter
as to the other two parameters.

(3) Vandalism and/or theft of the stones is a serious
problem in urban areas where small riprap has been

3-6

placed. A WSo<min) of 80lb should help prevent theft
and vandalism. Sometimes grouted stone is used around
vandalism. prone areas.

(4) The completed revetment will contain some
pockets of undersized rocks, no matter how much effort is
devoted to obtaining a well-mixed gradation throughout
the revetment. This placement problem can be assumed
to occur on any riprap job to some degree but probably
mare frequently on jobs that require stockpiling or addi-
tional handling. A larger safety factor should be consid-
ered with stockpiling or additional hauling and where
placement will be difficult if quality conlrOl CatDlOl be
expected to address these problems.

(5) The safety factor should be increased where
severe freeze.thaw is anticipated.

The safety factor based on each of these considerations
should be considered separately and then the irlrgest of
these values should be used in Equation 3-3.

d. Applications

(1) The outer bank of straight channels downslream
of bends should be designed using velocities computed for
the bend. In projects where the cost of riprap is high, a
channel model to indicate locations of high velocity might
be justified. These coefficients are applicable to a thick-
ness of 1D100(max). Equation 3-3 has been developed
into Plate 37, which is applicable to thicknesses equal to
1D1OO<max), Y, of 165 pcf. and the basic Sf of 1.1.
Plate 38 is used to correct for values of other than Y. of
165 pcf (when D30 is determined from Plate 37). The
KI side slope factor is nonnally defined by the relation.
ship of Carter, Carlson. and Lane (1953)

Kl =

sin2 e
sin" <il

(3-4)

1 -

where

e = angle of side slope with horizonlal

<il = angle of repose of riprap material (normally 40 deg)

Results given in Maynard (1988) show Equation 3-4 to be
conservative and that the repose angle is not a constant
40 deg but varies with several factors. The recommended
relationship for KI as a function of e is given in