FLUVIAL MORPHOLOGY AND SEDIMENT SAMPLING 8.11
suspended sediment, although this is not normally achieved unless sediment size is rather
small. If below a river confluence, the gaging site should be far enough downstream that
complete lateral mixing occurs between the tributaries. Areas of backwater should be
avoided because this affects both the stage-discharge and the velocity-discharge
relationship. For sand-bed rivers in which the ratio of total load to suspended load
depends on channel characteristics, the measurement site should normally have a
configuration similar to the normal stream geometry.
Gaging stations must be accessible during floods, so sediment stations are frequently
located at highway bridges which provide both access and a measurement platform
spanning the river. As a disadvantage, bridges normally occur at constricted reaches, can
collect debris, can alter streamlines, and in sand-bed rivers can experience significant
scour. Samples need to be collected at the same cross-section location throughout the
period of record. Avoid sites where channel realignment, a new bridge, or other
construction affecting the channel is anticipated over the life of the sampling program
(Edwards and Glysson, 1988).
8.2.4 Sampler Designations
Samplers developed by the U.S. Federal Inter-Agency Sedimentation Program are
designated by the prefix US, letters, and a two digit number. The letter designators are D
= depth integrating, H = hand held, P = point integrating, BM = bed material, BP =
battery pack, U or SS = single stage, and PS or CS = pumping type. The last two digits
indicate the year the sampler was developed.
8.2.5 Depth-Integrating Sampler
Depth-integrating samplers (Fig. 8.10a) are designed to continuously withdraw an
isokinetic sample while transiting a vertical. A sampling station is illustrated in Fig.
19.17. The sampler consists of a bottle fitted with nozzles of various sizes to control the
rate of inflow, an exhaust tube to release air from the sample bottle as water enters, and a
streamlined "fish" to hold the bottle and maintain the nozzle oriented directly into the
current. Both the nozzle and the exhaust tubes are sized to achieve isokinetic flow
velocity. For small streams, a sampler attached to the end of a rod is manipulated by hand
while the user is wading. Larger samplers for deeper water are raised and lowered by
winch. All samplers should be oriented parallel to the flow prior to immersion of the
sampler nozzle into the water at the initiation of sampling. The resulting sample is
discharge-weighted in the vertical and can be multiplied by the mean flow velocity at the
same vertical to obtain the load. The total load across the entire stream is computed by
sampling and summing the loads at several verticals.
For round-trip depth-integrated sampling, the sampler is lowered through the water
column to the bottom and then raised again at transit rates which are constant in each
direction but which need not be equal. The downward transit velocity should be slower
than the upward velocity to minimize the possibility of the sampler digging into the bed,
and sampler direction should be reversed as soon as it strikes bottom to avoid
oversampling the higher-concentration zone near the bed. In sand-bed rivers, duplicate
samples should be taken and visually compared for large differences in the amount of
sand, and a third sample should be taken if necessary. The transit rate must be adjusted so
that an adequate sample size is collected, and yet the bottle must not be allowed to fill
completely. Once the bottle is filled, water will continue to circulate, entering the nozzle