downstream base level (control) of the bridge reach, gravel mining from the stream bed,
diversion of water into or out of the stream, natural lowering of the total system, movement of a
bend, bridge location in reference to stream planform, and stream movement in relation to the
crossing.
Analysis of long-term streambed elevation changes must be made using the principles of river
mechanics in the context of a fluvial system analysis. Such analysis of a fluvial system
requires the consideration of all influences upon the bridge crossing, i.e., runoff from the
watershed to the channel (hydrology), the sediment delivery to the channel (erosion), the
sediment transport capacity of the channel (hydraulics) and the response of the channel to
these factors (geomorphology and river mechanics). Many of the largest impacts are from
human activities, either in the past, the present or the future. The analysis requires a study of
the past history of the river and human activities on it; a study of present water and land use
and stream control activities; and, finally, contacting all agencies involved with the river to
determine future changes in the river.
A method to organize such an analysis is to use a three level fluvial system approach (see
Chapter 9 and HEC-20). This method provides three levels of detail in an analysis: (1) a
qualitative determination based on general geomorphic and river mechanics relationships; (2)
engineering geomorphic analysis using established qualitative and quantitative relationships to
establish the probable behavior of the stream system to various scenarios of future conditions;
and (3) quantifying the changes in bed elevation using available physical process mathematical
models such as BRI-STARS (Molinas 2000) or HEC-6, (U.S. Army Corps of Engineers 1993).
Methods to be used in the three levels of analysis are given in this manual, FHWA's HEC-18
(Richardson and Davis 2001) and HEC-20 (Lagasse et al. 2001).
7.6 GENERAL SCOUR
General scour at a bridge can be caused by a decrease in channel width, either naturally or by
the bridge, which decreases flow area and increases velocity. This is contraction scour.
General scour can also be caused by short-term (daily, weekly, yearly or seasonally) changes
in the downstream water surface elevation that controls the backwater and hence the velocity
through the bridge opening. Because this scour is reversible it is included in general scour
rather than in long-term scour. General scour can result from the location of the bridge with
regard to a bend. If the bridge is located on or close to a bend, the concentration of the flow on
the outer part of the channel can erode the bed.
General scour can be cyclic. That is, during a runoff event the bed scours during the rise in
stage (increasing discharge) and fills on the falling stage (deposition).
General scour from a contraction occurs when the flow area of a stream is decreased from the
normal either by a natural constriction or by a bridge. With the decrease in flow area there is
an increase in average velocity and bed shear stress. Hence, there is an increase in stream
power at the contraction and more bed material is transported through the contracted reach
than is transported into the reach. The increase in transport of bed material lowers the bed
elevation. As the bed elevation is lowered, the flow area increases and the velocity and shear
stress decreases until equilibrium is reached, that is the bed material transported into the reach
is equal to that which is transported out of the reach.
7.7
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