has to be used in time steps (1–10 years depending on the accuracy
required), as at the end of each period the reservoir volume will be
decreased by the amount of the settled sediment, and thus the trap effi-
ciency in the next period will be decreased.
It has to be appreciated that the curve in Fig. 4.2, when applied to
the reservoir as a whole, does not take into account the shape of the reser-
voir (width to depth and depth to length ratios) or the graded sediment
transport; these parameters can be included only in more refined mathe-
matical modelling (Reeve, 1992).
A useful concept for comparing different projects, as far as the depo-
sition of sediment in the reservoir is concerned, is the so-called half-life of
a reservoir, i.e. the time required to lose half of the storage volume
(Worked example 4.2).
The results obtained by the application of the trap efficiency curve
have to be treated with caution as they can differ considerably from the
results of more detailed mathematical modelling which usually gives less
favourable results.
202 DAM OUTLET WORKS
Fig. 4.2 Reservoir trap efficiency (Brune, 1953). The capacity and the
volume of the annual inflow must be in the same units of measurement.
The upper enveloping curve is to be used when inflowing sediment is
highly flocculated or coarse. The lower enveloping curve is to be used
when inflowing sediment is colloidal or fine
