Significant changes in vegetation cover affect the amount of sediment delivered from the
watershed to the river system. One of the most common techniques to study the sources of
sediment in a watershed is to employ aerial photography and remote sensing techniques
coupled with ground investigations. The utilization of remote sensing techniques enables the
skilled observer to determine which areas of the watershed are stable and which are
unstable. Viewing the total watershed from this perspective and using water and sediment
routing techniques, it is possible to evaluate the sediment yield as a function of time.
9.2.8 Flow Alignment
In order to design a safe crossing or longitudinal encroachment, it is necessary to consider
the flow alignment in detail. The direction of flow must be considered as a function of time.
The position of the thalweg will vary with low, intermediate and high stages. The changing
characteristics of the river with stage, such as the change in velocity distribution, the position
of the thalweg, and the river form can have a significant effect on the intensity of attack on the
approaches, the abutments, the piers and embankments. The detailed study of the behavior
of the river over time and with varying discharge is necessary for proper design of training
works. Two-dimensional computer modeling, or in some cases physical modeling, as
discussed in Chapter 5 can provide the detailed hydraulic data to support design. Only with
this type of information can one adequately consider the intensity of attack, the duration of
attack and the necessity for training works to make the river system operate within a range of
conditions acceptable at the crossing or encroachment. Certainly, changes over time at a
particular crossing affect the channel geometry, the geometry of the crossing itself, general
scour and local scour. If the characteristics of the flow and how they vary with time are
known, then the information in Chapter 6 can be utilized to design against excessive
contraction and local scour in order to make the highway functional with minimum
maintenance over the life of the project.
9.2.9 Flow on the Floodplain
Up to this point the concern has been principally with flow in the main channel. However,
design floods usually flow in both the main channel and on the floodplain. Only by studying
the characteristics and geometry of the river and the floodplain can one determine the type of
flows that are apt to occur on the floodplain. This particular topic should be studied in
adequate detail so that the magnitude and intensity of the flows on the floodplain can be
approximated. The characteristics of flow on the floodplain are especially relevant to the
design of longitudinal encroachments. As an example, consider a sinuous channel. At flood
stage there is a tendency for the water to flow in the main channel in such a way as to
develop chute channels across the point bars. Often, the water spills over the outside of the
bends onto the floodplain. Flow conditions on the floodplain and in the main channel can be
greatly different at flood stage than at low flow, and this must be taken into consideration.
Again, 2-dimensional computer modeling, as discussed in Chapter 5, can provide detailed
hydraulic data on overbank and floodplain flows.
9.4
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