LOADING: CONCEPTS AND CRITERIA 127
(b) Secondary loads
SEDIMENT LOAD
The gradual accumulation of significant deposits of fine sediment, notably
silts, against the face of the dam generates a resultant horizontal force, Ps.
The magnitude of Ps, which is additional to water load Pwh, is a function of
the sediment depth, z 3 , the submerged unit weight sand the active lateral
pressure coefficient, Ka, i.e.
PsKa sz^23 /2 (kN m^1 ) (3.10)and is active at z 3 /3 above plane X–X. ( s (^) s (^) w, where (^) s is the
sediment saturated unit weight, and
Ka
wheresis the angle of shearing resistance of the sediment.)
Values of (^) s18–20 kN m^3 ands30° are representative, yielding
an equivalent fluid unit weight, i.e. Ka s, of approximately 3.0 kN m^3.
Accumulated depth z 3 is a complex time-dependent function of suspended
sediment concentration, reservoir characteristics, river hydrograph and
other factors (Section 4.5). Accurate prediction is inhibited by major
uncertainties, but sediment load is seldom critical in design other than for
smaller flood control dams and its introduction is not universal.
HYDRODYNAMIC WAVE LOAD
The transient hydrodynamic thrust generated by wave action against the
face of the dam, Pwave, is considered only in exceptional cases. It is of relat-
ively small magnitude and, by its nature, random and local in its influence.
An empirical allowance for wave load may be made by adjusting the static
reservoir level used in determining Pwh. Where a specific value for Pwaveis
necessary a conservative estimate of additional hydrostatic load at the
reservoir surface is provided by
Pwave (^2) wH^2 s (3.11)
(Hsis the significant wave height, i.e. the mean height of the highest third
of waves in a sample, and is reflected at double amplitude on striking a
vertical face. Wave generation on reservoirs is discussed in Section 4.4.)
ICE LOAD
Ice load can be introduced in circumstances where ice sheets form to
appreciable thicknesses and persist for lengthy periods. In such situations
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