Modeling Channel Response to Instream Gravel Mining

Abstract

Sand and gravel on riverbeds have been considered as an attractive (high quality and low cost) source of building material for centuries (Kondolf, 1994; Gill, 1994; Rinaldi et al., 2005). Detrimental effects of in-stream mining have been documented in literature including riverbed degradation or incision (Rinaldi et al., 2005), stream-bank instability (Chang, 1987; Kondolf, 1997), destruction of bridges and channelization structures (Kondolf, 1997; Rovira et al., 2005), etc. On the other hand, removal of in-stream sediment can be beneficial, for instance, it can serve as a maintaining way of the navigation water depths (Fredsoe, 1978). To minimize the detrimental effects and maximize the beneficial impacts, channel response due to gravel mining or dredging has been studied by experiments (Fredsoe, 1978; Kornis and Laczay, 1988; Lee et al., 1993; Lee and Chen, 1996; Neyshabouri et al., 2002), field observations (Kondolf, 1997; James, 2004; Neyshabouri et al., 2002; Rinaldi et al., 2005), and simplified analytical models (Cotton and Ottozawa-Chatupron, 1990). With the rapid development of computational fluid dynamics (CFD) since late 1980s, sophisticated numerical modeling has become a practicable tool for a quantitative understanding of the channel response due to sand and gravel mining (Van Rijn, 1986; Chang, 1987; Yue and Anderson, 1990; Gill, 1994; Cao and Pender, 2004; Chen and Liu, 2009). However, the inherent complexity of sediment transport and channel changes makes the firm, specific prediction of mining effects on rivers impossible at present. For instance, sediment transport around the mining pit area behaviors a distinct non-equilibrium state due to the sharp inflection of streamlines around the upstream and downstream ends of mining pits. However, existing numerical models either choose equilibrium sediment transport formulas or bear much uncertainty due to the various parameters selection which must be introduced to close the non-equilibrium sediment transport formulas. Therefore, specific laws and regulations regarding the safe in-stream mining have not been provided for users and officials despite extensive investigation made in the past (Kondolf, 1997; Neyshabouri et al., 2002). This paper aims to simulate the channel response to instream gravel mining. First, the feasibility of the two-dimensional depth-averaged model (CCHE2D) in modeling mininginduced bed change was examined by comparing the calculated results to the data measured in two sets of published laboratory experiments. Thereafter, the two dimensional model (CCHE2D), with appropriate non-equilibrium adaptation parameters, was applied to examine the impact of deep sand and gravel mining pits (with a depth of 10 meters) on