G. Verstraeten et J. Poesen, Estimating trap efficiency of small reservoirs and ponds: methods and implications for the assessment of sediment yield, PROG P GEO, 24(2), 2000, pp. 219-251
Throughout the world, several millions of small ponds exist for water suppl
y, irrigation, flood control or to control water quality downstream. The re
duced flow velocity in these ponds causes sedimentation of transported part
icles. For most ponds this is a negative impact as their retention capacity
decreases due to sedimentation processes. Sediment volumes in small ponds
can be used to reconstruct sediment yield values and to study the spatial v
ariation in sediment yield over large areas. Especially in developing count
ries, this technique can be very helpful in establishing large data sets on
sediment delivery as there are often no resources for expensive monitoring
programmes. However, when such studies are undertaken, one has to take int
o account the efficiency of the pond in trapping sediments. This trap effic
iency is dependent on the characteristics of the inflowing sediment and the
retention time of the water in the pond, which in turn are controlled by p
ond geometry and runoff characteristics. Because trap efficiency is one of
the most important properties of a pond or reservoir, it has been studied f
or quite some time. This article provides an overview of the different meth
ods available to estimate the trap efficiency of reservoirs and ponds. The
first set of methods are empirical models that predict trap efficiency, mos
tly of normally ponded large reservoirs using data on a mid to long-term ba
sis. These models relate trap efficiency to a capacity/watershed ratio, a c
apacity/annual inflow ratio or a sedimentation index. Today, these models a
re the most widely used models to predict trap efficiency, even for reservo
irs or ponds that have totally different characteristics from the reservoir
s used in these models. For small ponds, these models seem to be less appro
priate. They also cannot be used for predicting trap efficiency for a singl
e event. To overcome these restrictions, different theoretical models have
been developed based on sedimentation principles. These can be very simple,
such as the overflow rate method, but also very complex when runoff and se
diment are routed through a pond with incremental time-steps. The theoretic
al-based models are probably more capable of predicting trap efficiency for
small ponds with varying geometric characteristics, and some of them also
provide data on effluent sediment concentrations and quality. However, when
reconstructing sediment yield values using sedimentation rates over a peri
od of a few months to a few year (the mid-term basis), one needs a trap eff
iciency value for the whole period, not for one single event. At present on
ly limited research has been done on establishing mid-term trap efficiency
models based on theoretical principles. This is probably the most important
gap in trap efficiency research.