Pollution of Gravel spawning grounds due to fine sediment

Gary Parker, Panayiotis Diplas

Research output: Book/ReportOther report

13 Scopus citations

Abstract

Laboratory experiments which model poorly sorted gravel-bed streams were conducted to examine certain phenomena associated with these streams. The existence and role of the pavement, sediment transport, the accumulation and retention of fines in the bed, and the effect of the fines accumulation in the stream behavior are the phenomena included in this study. Two flumes were used. The first one operated as a recirculating system. It was 16.75 m long and 0.3 m wide with transparent walls. The narrow width of this channel suppressed any tendencies for the formation of bedforms. The second one operated as a feed system. It was 12 m long and 0.53 m wide with opaque walls. This flume allowed for the development of alternate bars. In total, seven series of experiments were performed. On the average, six experiments were conducted for each series. The first experiment of each series was conducted with a given mixture of bed material in the absence of fines. This mixture constitutes a scaled-down version of a rather typical bed material observed in natural gravel-bed streams. Fine material was introduced into the flume during the subsequent experiments. The amount of fines introduced was progressively increased, while the channel bed gradually changed from gravel to sand. At the end of each series the bed became completely covered with fines. It was found that the pavement is a mobile bed phenomenon, which acts as a regulator that renders all available grain sizes to be of nearly equal mobility. This is accomplished by overrepresenting the percentage of large grains exposed to the flow. Flow visualization provided evidence of a correlation between grain motion and the turbulent bursting phenomena. Field data, obtained from the literature, are used to study the bedload transport in paved gravel-bed streams. The stream examined in this case is Oak Creek, a small gravel-bed stream draining the east slope of the Coast Range, Oregon. The bed material of this stream, is divided into ten ranges of grain size. The concept of a similarity transformation is used as a means of collapsing the individual bedload relations, developed empirically for each of the size ranges, into a single curve. The similarity parameter used here renders the bedload relations of nine out of ten size ranges almost perfectly similar. This parameter incorporates the variation in mobility as well as the hiding ·effects due to the difference in the size of the grains. A bedload relation is then developed which accounts for the effects of sediment grading. The same approach is used also for the data obtained from the present study, supporting the conclusions obtained from the analysis of the Oak Creek data. The mechanics of fines infiltration into the bed is also described. While the fines accumulated into the bed did not affect the water surface slope or the channel depth, the amount of fines in the subpavement and pavement correlated well with the bedload transport rate. A model describing this process is in order. Initially, the fines collect in the immediate substrate, reducing the mobility of the grains in the pavement
Original languageEnglish (US)
StatePublished - Jun 1985

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