4.10 SUMMARY
Computing the bed material sediment transport capacity for a river should be grounded on a
firm understanding of the river channel characteristics, sources of sediment and modes of
sediment movement. Equations that are well suited for the particular river conditions should
be used and, if more than one are well suited, the results should be compared to assess the
range of possible outcomes. For a specific river, it is recommended that the results be
compared with actual measurements.
Selection of an equation for use depends on the data available and scope, objectives, and
resources available for a specific project. The Colby procedure (Section 4.5.4) provides a
reliable quick estimate of the bed material discharge in sand bed streams and the Meyer-
Peter and Mller bed load equation (Section 4.5.1) has been widely used to estimate bed
load (contact load) in coarser bed systems. The power function relationships (Section 4.6)
provide a practical method for quick sediment transport calculations for sites within the range
of conditions for which they were developed. The Yang sand and gravel equations (Section
4.7) are widely accepted and adaptable to both hand calculation and computer solutions.
Finally, many of the sediment transport relations of Table 4.4 are frequently used in sediment
transport investigations and several are represented as options in the code of sediment
transport models such as HEC-6 (U.S. Army Corps of Engineers 1993), BRI-STARS (Molinas
1990, 2000), and SAM (Thomas et al. 2000).
Typical applications of sediment transport relationships include equilibrium slope and
sediment continuity analyses to determine long-term trends in bed elevation change
(aggradation/degradation). These applications are illustrated in HEC-20 (Lagasse et al.
2001). The next section outlines a general sediment transport analysis procedure.
4.11 SEDIMENT TRANSPORTATION ANALYSIS PROCEDURE
4.11.1 Step 1: Determine if Sediment Transport Computations are Necessary
Determine qualitatively if the sediment problem at the bridge is aggradation or degradation.
If degradation, previous scour calculations may be sufficient or a sediment transport analysis
using equilibrium slope or sediment continuity could provide an initial estimate of long-term
degradation (see HEC-20, Lagasse et al. 2001). If aggradation or if more refined
degradation estimates are required, then calculating the quantity and gradation of the
sediment being transported may be necessary.
For important bridges, determining the quantity and gradation of sediment being transported
can be used to:
Check on the previous determination of long-term aggradation or degradation and
contraction scour.
Determine the type of countermeasures to solve a sedimentation problem.
Design debris basins for aggradation problems.
Design a check dam for degradation problems.
Determine if there will be environmental problems upstream or downstream of the bridge
from sediment transport conditions at the bridge crossing.
Estimate the cost of methods to solve the aggradation or degradation problem.
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