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Potential Drift Accumulation at Bridges
Locations on Site with respect to Drift Delivery
The delivery of drift at a highway crossing is localized. Some areas of a site may be entirely free of drift transport, while others receive concentrated delivery of drift. Evaluate the potential for drift delivery at each pier and span (figures 29 and 30).
Figure 29. Location categories relative to local drift delivery.
Figure 30. Flow chart for determining location category.
In a sheltered location, forest lying directly upstream traps transported drift and prevents its delivery to the bridge. Use this category where gaps between trees are much narrower than the average tree height and the width of forest along the direction of flow is more than a single or double line of trees as along a fence line or river bank. If upstream forest is potentially subject to clearing, select a location category assuming the forest's removal.
For the purpose of estimating local potential for drift delivery, group the flood plain, bank top, and bank slope together in a single location category. Locations in this category may be either forested but subject to future clearing, or presently clear of trees. Flood plain includes any area outside the channel inundated in the design flood to a depth sufficient to transport drift. Piers on bank slopes are not significantly more likely to accumulate drift than piers on the flood plain.
Drift can be transported anywhere in the channel, and drift accumulations are more common in channel locations than on banks or flood plains. USGS studies of scour potential have been conducted in humid regions. These studies have defined locations "in the channel" as those in the water when the bridge was inspected, ideally during base flow (Huizinga and Waite, 1994; Bryan and others, 1995). In arid regions, where base flow is relatively low, designate all locations between the bank bases as "in the channel." Anywhere below the top of the bank, judgment is involved in deciding whether an element of the bridge is in the channel. If evidence indicates that drift is delivered to some location below the bank top, assign that location to the "in the channel" category.
In most streams, secondary circulation currents converge at the surface, causing floating material to be transported along a relatively narrow drift path within the channel. Piers located in the drift path are the most common sites of drift accumulation. Such a drift path can be defined in most streams based on observations of small floating material if logs are absent. If such observations are not available, estimate the location of the drift path based on channel characteristics. The width of the drift path is variable, but for the purpose of design, assume a width one-third the channel width unless observations are available.
In a straight reach, the middle of the drift path typically coincides with the thalweg, the thread of the stream, and/or the center of the channel. In a curving reach, the middle of the drift path generally lies between the thalweg and the outside bank of the bend. At a sufficiently high flood stage, the drift path no longer remains confined to the channel. Drift following the submerged outside bank may be swept across the high end of a point bar or across the isthmus between meander bends. Surface flow may direct drift into chutes originating at the outside of bends (Damaskinidou-Georgiadou and Smith, 1986).
The best way to locate the drift path is to observe it during bank-full or higher flow. If high-flow observations are not available, observations during base flow will likely confirm estimates based on channel characteristics. The location of the drift path may move minimally with changing stage, but will be somewhat different at bank-full than at base flow. Observations need not include large pieces of drift, because all floating material responds similarly to the flow pattern.
If direct observation of the drift path is impossible, assign a location to it based on channel characteristics as described above. If the location of the drift path is indefinite, calculate the drift-accumulation potential under several assumed alternate locations for drift path -- for example, the left third of the channel, the middle third, and the right third. If you designate the entire channel "in the channel" and none of it "in the path," you may artificially lower the drift accumulation potential. At worst, drift is scattered evenly over an entire channel, leaving the entire channel "in the path."
If available information indicates that the drift path includes part of the bank or part of the flood plain, call that part "in the path." At high water, the drift path may cross point bars and necks between meanders. It is unlikely to go beyond the meander belt, which is the band of the valley defined by extreme excursions of the meandering channel to the left and right.
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