FLOOD09874

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Use of Riprap for Bank Protection
• Date: 6/1/1967
• Prepared By: USDOT
• Floodplain - Doc Type: Project

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2.1.2 - Extent of Stone Blanket

The upper vertical limit of the protective cover should extend above
design high water. The allowance tor freeboard depends upon the veloci-
ties near the riprap cover and, at some locations, upon the height of
waves that might be generated on the water surface. Established sod
above the stone protection will provide considerable protectiOll from
floods which overtop the riprap cover.

Where the stream channel is composed of sand or silt, bank protection
should extend a mini_ vertical distance of 5 feet below the streambed on
a continuous slope with the embankment (figure 3A). On the outside of
curves or sharp bends, scour is particularly severe, and the toe of the
bank protection should be placed deeper than in straight reaches. Where a
toe trench cannot be dug, the riprap blanket should terminate in a stone
toe at the' level of the streubed (figure 3B). The toe provi'des material
which will tall into a scour hole and thus extend the blanket.

On large rivers or tidal estuaries having a couiderable depth of
flow at low water stages, the Corps ot Engineers carries the stone protec-
tion 5 teet vertically below mean low water and OIIIi ts the toe. The stone
blanket should be keyed into a berm when a toe or toe trench is not pro-
vided. The purpose of the toe protection is to prevent undermining, not
to support the blanket. Unless the protection has sufficient stability to
support itself on the embankment slope, the protection cannot be considered
adequate.

'l'he bank protection should extend both upstream and downstreu from
the pOints of reverse curvature on the outside of a curved channe1. Bank
protection is usually not required on the inside of the curve unless
return of overbank flow creates a scour problem. On a straight channel,
bank protection should begin and end at a stable feature in the bank if
practicable. Such features might be outcroppings of erosion resistant
materials, trees, vegetation, or other evidence of stability. When a
stable feature does not exist, cutoffs should be provided (figure 4). If
the protective cover is long, intermediate cutoffs might be required to
reduce the hazard of cOlllp1ete tailure of the stone blanket.

2.1.3 - Thickness of Stone Blanket

The thickness of the stone blanket should be at least equal to the
maximum size stone (section 3.1.1).

2.2 - Design of Hand-Placed R!prap

Hand-placed riprap was at one time considered superior to dumped stone,
and both the size of stone aDd the thickness of the hand-placed stone blanket

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