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EM 1110-2-2300
31 Jul 94

D 15B >5D 15E (C-1) of downstream slope protection. A slope of approxi-
mately 1 vertical on 3 horizontal is about the steepest on
D 15B <5D 85E (C-2) which mowing and fertilizing equipment can operate
efficiently. In arid or semiarid regions where adequate
D 85B >D 15R /5 (C-3) turf protection cannot be maintained, outer embankment
zones composed of soils susceptible to erosion (silts and
D 15B = the 15 percent passing the size of the bedding sands) may be protected with gravel or rock spall blankets
at least 12 in. thick, have berms with collector ditches
D 85B = the 85 percent passing the size of the bedding provided, and have collector ditches at the embankment
toe.
D 15E = the 15 percent passing the size of the material to
be protected (2) Where the downstream slope is exposed to
tailwater, criteria used to establish the required upstream
D 85E = the 85 percent passing the size of the material to protection should be used for that portion of the slope
be protected exposed to wave action. Alternatively, a rock toe may be
provided, extending above the maximum tailwater
D 15R = the 15 percent passing the size of the riprap elevation.

An intermediate filter layer may be required between the f. Alternative slope protection. Alternative slope
bedding and riprap to prevent washout of the bedding. protection designs that are functional and cost effective
Bedding layers over erosion-resistant clay materials need may be used. Factors that influence the selection of slope
not be designed to meet the criteria of Equation C-1 or protection are embankment damage, materials from
Equation C-2 but must still satisfy Equation C-3. Each required excavation, availability and quality of offsite
design should produce a specification that defines material quarries, and turfs. A greater thickness of quarry-run
sources, gradations, and layer thickness to economically stone may be an option to relatively expensive graded
provide the riprap and bedding layers required to protect riprap. Some designers consider the quarry-run stone to
the embankment. have another advantage: its gravel- and sand-size compo-
nents serve as a filter. The gravel and sand sizes should
d. Embankment crest. The top of dam elevation is be less by volume than the voids among the larger stone.
usually selected much earlier in the design process than is Not all quarry-run stone can be used as riprap; stone that
the slope protection. When the slope protection design is gap graded or has a large range in maximum to mini-
has been selected, the top of dam elevation should be mum size is unsuitable. Quarry-run stone for riprap
reviewed to ensure that runup computations (from para- should be limited to D /D < 7. Additional information
85 15
graph C-2b(4)) are consistent with the type of protection is available in EM 1110-2-1601. A careful analysis
to be provided. The slope protection near the top of the should be made to demonstrate the economics of using
dam must ensure embankment safety and security to the alternative.
downstream areas. Each embankment dam should be
reviewed to determine the needed crest elevations of the (l) Upstream slope.
upstream slope. Intermittent overtopping by wave runup
may be acceptable where access to the top of the dam is (a) Class I zone. One alternative to riprap is to use
not necessary during occurrence of the maximum sur- riprap-quality, quarry-run stone dumped in a designated
charge pool and when the crest and downstream slope zone within, but not at, the outer slope of the embank-
consist of material that will not experience damaging ment. The dumped rock is spread and then processed by
erosion. The slope protection provided at the near crest a rock rake operating in a direction perpendicular to the
elevations of the upstream slope may vary for different strike of the exterior slope. Rock raking will move the
reaches but must be stable for the design wave used to larger stones in the zone contingent to the exterior slope
establish the top of dam elevation. of the embankment. The quarry-run stone that remains in
the dumped zone serves as a bedding. The size of the
e. Downstream slope protection. stone in the outer layer can be partially controlled by the
blasting techniques, quarry handling of material, and by
(l) Where an adequate growth of grass can be main- the tooth spacing on the rock rake. The outer zone of
tained, vegetative cover is usually the most desirable type large stone should produce a thickness (normal to the

C-3