- erosion protection at the interface between under-layer and embank-
ment fill; - separation, to prevent the protection layer penetrating the fill.
The principal characteristics of the outer face protection system which
influence the under-layer are:
- flexibility, controlling the degree to which inter-layer contact can be
maintained; - permeability, determining the extent to which hydraulic conditions
are transferred into the under-layer.
The size of stone required for rock armouring protection is related to the
size and number of waves which the armouring is intended to withstand.
These are in turn a function of sustained wind speed and time. The under-
lying criteria adopted for design purposes are those which give an appro-
priate combination of return period and possible damage to the armouring
having regard to the risk category of the dam (see Section 4.2 and Table
4.1). In most cases only generalized wind data will be available (as pre-
sented for the UK in ICE (1996)); allowance should be made for enhanced
wind speeds over an open unobstructed body of water, and it may be
prudent to add a further margin to allow for uncertainty.
The principal parameters used in the design of rock armouring are
the significant wave height, Hs, i.e. the mean height of the highest 33% of
waves in a suitably extended wave train. Hsis thus a function of effective
fetch,F, i.e. the unobstructed wind path determined across an arc normal
to the axis of the dam, and the sustained wind speed, U, necessary to gen-
erate the wave train. (Reference should be made to Section 4.4 and Fig.
4.1 and to ICE (1996) with regard to estimation of fetch, F, and for design
curves of the relationship linking predicted significant wave height to fetch
and sustained wind speed, and also to Chapter 14.)
If the adjacent landforms and reservoir shape are such that a greater
effective fetch oblique to the dam axis may reasonably be predicated
consideration should be given to this in estimating Hs. In this event
consideration should then be given to the consequent obliquity of waves
breaking against the face of the dam. There is limited evidence that for a
given significant wave height damage is reduced when waves impinge
obliquely to the face.
The stone size required for rock armouring is normally expressed in
terms of D 50 , i.e. the mass of the median stone size. An empirical rule for
the size of stone necessary for stability under wave action is given by:
M 103 Hs^3 (2.12)whereM(kg) is the mass of stone required and Hsthe significant wave
height (m).