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

e. Ltryer thickness. All stones should be contained
within the riprnp layer thickness to provide maximum
resistance against erosive forces. Oversize stones. even in
isolated spots. may result in riprap failure by precluding
mutual support and interlock between individual stones,
causin g large voids that expose filter and bedding
materials, and creating excessive local turbulence that
removes smaller size stone. Small amounts of oversize
stone should be removed individually and replaced with
proper size stones. The following criteria apply to the
riprap layer thickness:

(1) It should not be less than the spherical diameter
of the upper limit W 100 stone or less than 1..5 times the
spherical diameter of the upper limit W 50 stone, which-
ever results in the greater thiclrness.

(2) The thickness determined by (I) above should be
in=d by 50 percent when the riprap is placed under-
water to provide for uncertainties associated with this type
of placement. At one location in the US Army Engineer
Division. Missouri River, divers and sonic sounders were
used to reduce the underwater thickness to 1.25 times the
dry placement thickness.

Section II
Channel Characteristics

3-3. Side Slope Inclination

The stability of riprap slope protection is affected by the
steepness of channel side slopes. Side slopes should ordi-
narily not be steeper than I V on 1..5H. except in special
cases where it may be economical to use larger
hand-placed stone keyed well into the bank. Embankment
stability analysis should properly address soils characteris-
tics, groundwater and river conditions, and probable
failure mechanisms. The size of stone required to resist
the erosive forces of channel now incre:lses when the side
slope angle approaches the angle of repose of a riprap
slope protection. Rapid water-level recession and piping-
initiated failures are other factors capable of affecting
channel side slope inclinalion and needing consideration
in design.

3-4, Channel Roughness, Shape,
Alignment, and Gradient

As boundary shear forces and velocities depend on chan-
nel roughness. shape. alignment. and invert gradient. these
factors must be considered in determining the size of
stone requined for riprap revetment. Comparative cost
estimates should be made for several alternative channel

3-4

plans to determine the most economical and practical
combination of channel factors and stone size. Resistance
coefficients (Manning's n) for riprap placed in the dry
should be estimated using the following fonn of
Slrickler's equation:

n = K [D90 (min) ]1/6

(3-2)

where

K= 0.036. average of all flume data

= 0.034 for velocity and stone size calculation

= 0.038 for capacity and freeboard calculation

D9D<min) = size of which 90 percent of sample is finer.
from minimum or lower limit curve of
gradation specification, ft

The K values represent the upper and lower bounds of
laboratory data determined for bottom riprap. Resistance
data from a laboralOry channel which had an irregular
surface similar to riprap placed underwater show a
Manning's n about 15 percent greater than for riprap
placed in the dry.

Section 11/
Design Guidance for Stone Size

3-5. General

Riprap protection for open channels is subjected to hydro-
dynamic drag and lift forces that tend to erode the revet-
ment and reduce its stability. Undermining by scour
beyond the limits of proteetion is also a common cause of
failure. The drag and lift forces are created by now
velocities adjacent to the stone. Forces resisting motion
are the submerged weight of the stone and any downward
and lateral force components caased by contact with other
stones in the revetment. Stone avaiIability and experience
play a large part in detennining size of riprap. This is
particularly a-ue on small projects where hydraulic param-
eters are ill-defmed and the tolal amount of riprap
required is small.

3-6. Design Conditions

Stone size computations should be conducted for now
conditions that produce the maximum velocities at the
riprapped boundary. In many cases. velocities continue 10