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

(I) Windrow revetments: riprap placed at top of
bank

(2) Trench-fIll revetments: riprap plxed at low
water level

(3) Weighted riprap toes: riprap placed at intersec.
tion of channel bottom and side slope

Trench-fill revetments on the Mississippi River have
successfully launched to protect for a vertical scour depth
of up to 50 ft. On gravel bed stre:lms. the use of
Iaunchable stone is not as widely accepted as in sand bed
Stre:lms. Problems with using Iaunchable stone in some
gravel bed rivers may be the result of underestimating
SlOne me, scour depth, or Iaunchable stone volume
because the concept of Iaunchable stone has been success-
ful on several gravel bed rivers.

3-11. Revetmell1 Toe Protection DesIgn

The following guidance applies to several alternative
methods of toe protection iIIuslrated in Plate 43.

a. Method A. When toe excavation can be made in
the dry, the riprap layer may be extended below the exist-
ing groundline a dist:lllce exceeding the anticipated depth
of scour. If excavation quantities are prohibitive, the
concept of Method D can be ad:lpted to reduce
excavation.

b Method B. When the bottom of the channel is
nonerodible material, the normal riprap should be keyed
in at Stre:lmbed level.

c. Method C. When the riprap is to be placed under-
water and little toe scour is expected (such as in straight
reaches that are not downsa-eam of bends. unless stre:lm
is bmided), the toe may be placed on the existing bottom
with height a and width c equal to I.5T and 5T,
respectively. This compensates for uncertainties of under-
water placement.

d. Method D. An extremely useful technique where
water levels prohibit excavation for a toe section is to
place a Iaunchable section at the toe of the bank. Even if
excavation is practicable. this method may be preferred
for cost savings if the cost of eXIra stone required to
produce a launched thickness equal to or gre:uer than
1.5T is exceeded by the cost of excavation required to
carry the design thickness T down the slope. This con-
cept simply uses toe scour as a substitute for mechanical
excavation. This method also has the advancge of

EM 1110-2-1601
1 Jul 91

providing a -builtin" scour gage. allowing easy monitoring
of high-flow scour ani! the need for additional stone rein-
forcement by visual inspection of the remaining toe stone
after the hi gh flow subsides or by surveyed cross sections
if the toe stone is underwater. It is readily ad:1ptable to
emergency protection, where high flow and the require.
ment for quic.l:: action make excavation impractical.
Shape of the stone toe is not critical. For lrench.tiJl
revetments. the height of the stone section is generally
one.half to one times the length. For weighted riprap
toes. heights as low as 2 times the bank protection
thickness have been used. Providing an adequate volume
of stone is critical. To compute the required Jaunchable
stone volume for Method D, the following assumptions
should be used:

(1) L:lunch slope" IVan 2H. This is the slope
resulting from rock launched on noncohesive material in
both model and prototype surveys. Launch slope is less
predictable if cohesive material is present. since cohesive
material may fail in large blocks.

(2) Scour depth = existing elevation - maximum
scour elevation.

(3) Thickness after launching" 1.5 times the thick.
ness of the bank revetment T .

Using these assumptions, the
volume" I.ST times launch slope length

= I.5T times scour depth limes rr

= 3.35T (scour depth)

Add a safety fxtor if data to compute scour depth are
unreliable. if cohesive bank material is present. or if
monitoring and maintenance after construction cannot be
guaranteed. Guidance for a safety factor is lacking, so to
some extent it must be determined by considering
consequences of failure.

3-12. Delivery and Placement

Delivery and placement can affect riprap design. See
EM 1110.2.2302 for detailed guidance. The common
methods of riprap placement are hand placing; machine
placing. such as from a skip, dragline. or some fonn of
bucket: and dumping from lrUcks and spreading by bull-
dozer. Hand placement produces the most stable riprap
revetment because the long axis of the riprap particles are
oriented perpendicular to the bank. It is the most expen-
sive method except when stone is unusually costly and/or

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