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Potential Drift Accumulation at Bridges
Stream Characteristics and Drift Storage
Transport and storage of drift depend on discharge, channel characteristics, and the size of drift pieces relative to the channel dimensions (Triska and Cromack, 1980; Bilby, 1985; Harmon and others, 1986; Bilby and Ward, 1989; Smock and others, 1989; Benke and Wallace, 1990; Robison and Beschta, 1990b). The relation of the length of typical drift pieces to the channel width is critical in determining the rate of transport and the type and amount of stored drift in the channel. The depth and slope of the channel also affect its ability to move drift. Islands, secondary channels, and flood plains influence transport and can be important sites of accumulation (Sedell and Duval, 1985).
Large woody debris in stream channels typically decays slowly. Waterlogged drift decays much more slowly than comparable drift that periodically dries out (Harmon and others, 1986; Chergui and Pattee, 1991). In the Pacific Northwest, where most of the work that addresses this topic has been done, the rate of decay of large woody debris in streams ranges from 1 percent to 3 percent per year, and much of the wood in streams is decades or even centuries old (Keller and Tally, 1979; Harmon and others, 1986; Andrus and others, 1988; Murphy and Koski, 1989; Gippel and others, 1992). Elsewhere, the rate may be higher, but even small woody debris may persist for years (Ward and Aumen, 1986; Golladay and Webster, 1988; Hauer, 1989; Chergui and Pattee, 1991). Because the dominant source of large woody debris in small channels is the infrequent fall of mature trees which then decay very slowly, streams flowing through mature forests typically contain much more large woody debris than those flowing through young second-growth forests (Sedell and others, 1988).
The narrow channels of first- and second-order streams rarely transport large drift (figure 12). Most drift is transported within the channel rather than in the flood plain. The size of drift is critical in determining whether it can be transported. Logs longer than the width of the channel typically become lodged across the channel, and rarely move without being broken into smaller pieces. In small channels with large amounts of stable woody debris, pieces with a length more than half the channel width are relatively stable. Exceptions to this pattern include steep streams subject to debris torrents, and valleys where drift is transported outside the channel over a cleared, deeply inundated flood plain.
Figure 12. Narrow channel bridged by fallen trees.
In Bonnie Creek on Prince of Wales Island in southeast Alaska, a stream reach readily transported drift up to half the channel width in length, and the investigators determined that "...pieces with lengths about equal to bank-full width can be transported distances more than several channel widths downstream at high flow" (Lienkaemper and Swanson, 1980; Swanson and others, 1984). In streams draining old-growth timber in western Washington, Bilby and Ward reported the relation between mean length of woody-debris pieces in the channel and channel width, over a range of channel widths from 4 to 20 m (13 to 66 ft), to be:
Mean length = ( 0.43 * Channel width ) + 3.55 m. (Bilby and Ward, 1989)
Although trees undergo limited transport in narrow channels, these streams have been documented to transport drift (Gregory and others, 1985). Scour-potential studies recorded drift accumulations at bridges with upstream channels as narrow as 3 to 4 m (10 to 13 ft) (Noel Hurley, USGS, written commun., 1992; Ron Thompson, USGS, written commun., 1992; Bernard Helinsky, USGS, written commun., 1992; G.W. Parker, USGS, written commun., 1993). Few channels narrower than 3 m (10 ft) were included in these studies; thus, drift accumulations may be equally common in narrower channels.
In most streams of intermediate size, typically third- and fourth-order, large floods move some of the large woody debris as drift. Most drift occurs in sizable log jams containing several large pieces and typically spanning the channel (Diehl and Bryan, 1993). The abundance of drift stored in the channel typically decreases with increasing channel width, but the average size of drift pieces increases. Although the amount of drift present per kilometer of channel length in an intermediate stream is less than in a small stream, enough drift remains within the channel to form channel blockages at downstream bridges.
One consequence of the rare occurrence of drift transport in streams of intermediate width is that they typically contain a significant amount of large woody debris that has accumulated during the decades or centuries since the last major flood. The channel-wide jams that contain most of the drift may eventually be broken up and transported. Large floods can mobilize this inventory of debris and transport it downstream to bridges (O'Donnell, 1973; Gannett Fleming Corddry and Carpenter, Engineers, 1974).
In low-gradient streams, the force of flowing water on stored drift is less than in high-gradient streams with the same channel dimensions. Stored drift can be abundant in large, low-gradient channels (Benke and Meyer, 1988; Benke and Wallace, 1990). Some channels have so little gradient that even floating logs are not transported any appreciable distance. The minimum slope or velocity necessary to move drift has not yet been established.
In the scour-potential studies, drift accumulations at least as wide as the bridge span were recorded in channels with bank heights as low as 0.6 m (2 ft). These studies include only a few bridges with bank heights less than 0.6 m (2 ft); therefore, that depth may not represent a minimum depth for drift transport. Drift accumulations were about as common in channels with a bank height of less than 1 m (3 ft) as in deeper channels. The width of shallow channels with extensive drift ranged from 3 m (10 ft) to 17 m (56 ft).
A shallow channel may transport only drift that is much smaller than its width would suggest. Potential for transport of large logs is low for sites along channels in which flow is never deep enough to float such logs above the bed. The depth sufficient to float a log is about the diameter of the butt plus the distance the roots extend below the butt. This is roughly 3 to 5 percent of estimated log length in the case of typical large logs observed at drift-study sites. Where large trees have fallen into shallow channels, they remain in place or move a short distance and turn parallel to the flow with their root mass upstream. However, if flow deep enough to float large logs off the bed can occur, the transport of a large inventory of logs stored in the channel is possible.
Where the depth of a wide channel is adequate to float large logs, drift may be stored on mid-channel bars and point bars, at island heads, or in pools along the base of the outside banks of bends (Wallace and Benke, 1984). Mid-channel bars may lie downstream from a zone of surface convergence so that floating drift is directed over them at high flow stages. When these bars have brush growing on them, or are only shallowly inundated, drift may form accumulations on their highest areas, promoting further sedimentation and bar growth. Stored drift in pools can easily be mobilized during a flood. Channel-wide drift accumulations are known to have occurred in wide channels, but such accumulations are presently rare (Young, 1837; Triska, 1984).
Islands may intercept drift, forming island-head jams or accumulations on trees growing on the island crest. Island-head accumulations grow in the upstream direction through accretion of additional drift and promote island growth in the downstream direction through sedimentation (Crockett, 1955; Helmericks, 1968; Gippel and others, 1992).
In most wide streams, typically fifth-order and larger, not much drift is stored within the channel, and nearly all drift entering the main channel is transported by frequent floods. Most drift accumulates outside the channel on islands, on forested bars, in flood-plain forests, and in sloughs (McFadden and Stallion, 1976; Sedell and Duval, 1985; Malanson and Butler, 1990; Chergui and Pattee, 1991). Observations of stored drift in or near large channels confirm that the channel has the potential to deliver abundant drift to a highway crossing downstream.
Flow patterns change as a river floods. As stage rises, more islands and chutes (secondary channels) are present. At the downstream end of bends, surface flow emerging from the zone of surface convergence in the bend may direct concentrated drift across an inundated point bar rather than across the channel to the outside of the next bend. Bridge piers on cleared point bars and flood plains have potential to collect drift (Klingeman, 1971; I. Nagai, California Department of Transportation, written commun., 1992). When inundated, forested point bars may collect large accumulations of drift, especially where the pattern of flood flow directs the drift path into the woods.
Chutes may receive most of the drift in the river if they begin on the outside of a bend, especially toward the lower end of the bend (Damaskinidou-Georgiadou and Smith, 1986). Chutes too narrow or too shallow to transport the drift they receive become sites for accumulation (McFadden and Stallion, 1976). An accumulation on the head of the island separating the secondary and main channels may grow upstream and block the chute entrance.
When the depth of flood-plain inundation exceeds roughly one-third the channel depth, the zone of surface convergence in the channel becomes discontinuous or ceases to exist, and most surface flow follows the axis of the valley (Toebes and Sooky, 1967; Elliot and Sellin, 1990; Knight and Shiono, 1990). The distance between trees in a typical flood-plain forest is much less than the length of an average log, so the forest acts as a trash rack (Kochel and others, 1987). Accumulations on the upstream sides of flood-plain trees were observed at several sites during this study. Except where trees are sparse, flood plains remove more drift from the river than they add (Benke and Wallace, 1990).
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