Sediment Transport at River Contractions

Abstract

River contractions might occur because of either natural controls or hydraulic works. This paper aims to provide new insights on the temporal evolution of the morphological patterns at river contractions based on experiments at laboratory scale. Laboratory tests were performed in a 20 m long straight channel having a rectangular cross section, 1.0 m wide and 1.0 m deep. The erodible bed reach was made by almost uniform sediment consisting of either rough sand or medium gravel. The contraction model was 0.5, 1.0, 2.0, or 3.0 m long. Three contraction ratios = b/B = 0.9, 0.8, and 0.7 were considered, with b width of the contracted reach and B width of the approach channel. The experiments were performed in clear-water scour regime and steady flow conditions. Typically, they were of long duration (up to 15 days) to achieve conditions of dynamic equilibrium, but runs of short duration were also carried out to acquire suitable experimental data at the earlier stages of the bed forms development. Compared to previous studies by the author on the same topic, new results are provided on bed morphology characteristics, also considering literature data, and predictive models on the timedependent development of the bed degradation in and downstream of the contracted area.

Morphological Patterns at River Contractions

“…The flow discharge was measured using an orifice plate inserted into the supply pipe with an accuracy of ±3%. A conventional point gauge with an accuracy of ±0.1 mm was used to survey the water surface while a so-called shoe gauge, e.g., [21,23] was used for monitoring bed levels. Typically, around 500 bed-level data were collected at the end of each run while about 10-15 measurements only along the channel centreline at the contraction zone and downstream were collected during the run for avoiding disturbances to the flow and bed evolution.…”

“…The present paper aims to provide a contribution to this context. Starting from previous and partial results by the Authors (Oliveto and Marino [21,22], Oliveto [23]), new and definitive equations are drawn up also on the basis of the experimental works by Gill [6], Webby [2], and Dey and Raikar [3]. The proposed time-dependent predictive models would describe to a greater extent the typical bed forms that develop at and downstream of the contracted area.…”

The Impact of River Contractions on the Bed Morphology under Unsteady Flows

“…l/B≤1 according to [5]) and long contractions. More details on the experiments and main test conditions and observations are given [14][15][16][17]. In this study six new experiments were carried out under unsteady flow.…”

“…All the above parameters relate to the approach flow and to a specific time-step t. Moreover, the reader is referred to the previous papers [14][15][16][17] for the definition of Fd and Fdi. It can be noted that Re was always greater than (or close) to 10 5 (i.e.…”

“…New special laboratory experiments are presented. They, along with previous ones carried out at University of Basilicata, Italy, Oliveto and Marino, and Oliveto [14][15][16][17] have allowed to develop predictive models on the temporal evolution of the bed profile, at and downstream of the contracted area, under both steady and unsteady flows.…”