Research papersEffect of unsteady flow conditions on scour features at low-head hydraulic structures
Introduction
The scour mechanism occurring downstream of low-head control structures is an important topic that has been widely analyzed in recent decades. In particular, the analysis mainly focused on the hydraulics and the scour characteristics in the stilling basin. Low-head structures have been found effective in controlling sediment transport and, at the same time, they are able to guarantee a reduced impact on the ecosystem. Therefore, many traditional structures have been re-converted into more eco-friendly anthropic works, such as block ramps, rock grade control structures, stepped gabion weirs, cross-vane, W-weirs, etc.
One of the first systematic studies investigating erosive process occurring in a movable stilling basin was conducted by Veronese (1937). The author analyzed the effect on the scour characteristics of the stilling basin geometry. Namely, he conducted a series of experimental tests in prototype channels to investigate the effect of symmetric enlargement of the stilling basin on scour morphology. More recently, Bormann and Julien (1991) analyzed several grade control structures configurations, varying both the model scale and the geometry of the structure itself. They concluded that the scour process depends on both the structure geometry and on the stilling basin characteristics. Based on the similarities between scour process due to plunging jets and downstream of grade control structures, Bormann and Julien (1991) and, successively, D’Agostino and Ferro (2004) showed that the diffusion length of the flow entering the stilling basin is a fundamental parameter. Furthermore, as it was also shown for scour due to plunging jets (see for example Rajaratnam, 1981, Rajaratnam and Macdougall, 1983, Breusers and Raudkivi, 1991, Hoffmans and Verheij, 1997, Hoffmans, 1998, Mason and Arumugam, 1985, Pagliara et al., 2010, Pagliara et al., 2012a, Pagliara et al., 2015), the geometry of the structure was found a significant parameter influencing the scour morphology because of the different entering flow inclinations. Among eco-friendly hydraulic structures, block ramps and rock chutes constitute a peculiar typology. Namely, they are characterized by a complex hydraulic behavior due to the rough sloped bed that contributes to dissipate an appreciable amount of energy (Pagliara and Palermo, 2011). In particular, these structure typologies exhibit some similarities with both stepped chutes and rock sills, in terms of both hydraulic behavior and dissipative mechanism. Nevertheless, the presence of a downstream mobile stilling basin contributes to amplify the energy dissipation, as a hydraulic jump generally occurs in correspondence with the structure. In general, eco-friendly structures exhibit substantial similarities in terms of scour process, due to their geometry that is characterized by relatively small dimensions (low height and/or mild surface slopes). In particular, a detailed analysis of the scour mechanism can be conducted in the case for which the structure is stable. Therefore, incipient movement conditions of the stones constituting the different eco-friendly structure typologies has received considerable attention (Parker et al., 1982, Whittaker and Jaggi, 1986, Robinson et al., 1997, Hoffmans, 2010). Furthermore, the main parameters influencing the scour phenomenon (tailwater, stilling basin material and geometry, protection sills, etc.) were carefully analyzed in order to understand the dynamics of the erosive mechanism both in clear water and live bed conditions (Pagliara and Palermo, 2011, Oertel et al., 2011, Pagliara et al., 2012b). The previously mentioned studies on block ramps were useful to understand the similarities and differences among other several low-head structures, as shown by Pagliara and Mahmoudi Kurdistani (2015) and Pagliara et al. (2016). In particular, stepped gabion weirs exhibit similarities with both stepped chutes and block ramp. They are characterized by different flow regimes: skimming, nappe and transition flow regimes. In addition, these different flow regimes deeply influence the scour mechanism in the downstream stilling basin. Pegram et al., 1999, Peyras et al., 1992 and Pagliara and Palermo (2013) analyzed this structure typology in detail. In particular, Pagliara and Palermo (2013) focused on the scour mechanism occurring downstream of stepped gabion weirs and rock grade control structures classifying the onset conditions of the different flow regimes (see also Rajaratnam, 1990, Essery and Horner, 1978, Peyras et al., 1992, Ohtsu et al., 2004). Namely, Pagliara and Palermo (2013) analyzed both the hydraulics and the scour process downstream of different stepped gabion weir configurations. They studied four structure configurations: permeable and impermeable isolated structures and structures with both permeable and impermeable upstream filtering layer having the same height of the stepped gabion weir. Their analysis was conducted for constant discharges up to the equilibrium scour conditions. Similarly, rock sills share similarities with other low-head hydraulic structures, including rock chutes, W-weirs, J-hook, etc. In particular, rock sills can assume different shapes according to their hydraulic functioning and the location in which they are inserted. Detailed studies on this type of structure were conducted by Bhuiyan et al. (2007) and Scurlock et al. (2012). They proved that 3D rock structures can substantially improve fish habitat and, at the same time, they can dissipate a relevant amount of flow energy. Therefore, in this study two different types of rock sills were taken into consideration, i.e., straight rock sills (which are common in river restoration projects) and curved rock sills (which can be considered representative of a larger variety of 3D low-head structures). The aim of the present paper is to analyze the hydraulics and the scour evolutions in the presence of both stepped gabion weirs and rock sills under unsteady flow conditions. In addition, this paper aims to answer to the following questions:
- 1)
Under unsteady flow conditions, is it always correct to select the peak discharge to evaluate the maximum scour depth, using relationships valid for steady flow conditions?
- 2)
If the answer to the previous question is negative (as it will be shown), is there a minimum duration of the unsteady flow event that can cause the same scour features of a steady event characterized by a constant discharge equal to the peak discharge? Equivalently, when should the peak discharge of the unsteady event occur to obtain the same maximum scour depth of a steady event with constant discharge equal to peak discharge?
To answer these two questions, experiments were conducted for different peak discharges varying both the time steps for the discharge increase/decrease and the total duration of the test. Furthermore, in order to minimize scale effects, experimental tests were conducted in two different channels and with different cohesionless materials. This analysis focused on the scour process evolution due to different flow conditions. It was experimentally proven that there is a significant similarity between the tested structures, i.e., it was shown that for certain inflow conditions, the non-dimensional time to reach the equilibrium configuration is essentially the same for all the low-head structure geometries tested in the present study. This implies that the answer to the first question is “no” (as it is valid for certain inflow conditions and not always), whereas the answer to the second question is “yes”. Finally, an applicative example is provided in order to better illustrate the proposed methodology.
Section snippets
Experimental facilities
Two dedicated models were built at the hydraulic laboratory of the University of Pisa. Namely, experimental tests regarding stepped gabion weirs were conducted in channel 1, which is characterized by the following geometric characteristics: 0.30 m wide, 0.60 m deep and 6 m long. Whereas experimental tests with rock sills were conducted in channel 2 (0.50 m wide, 0.90 m deep and 8 m long). The stepped gabion weir was made of uniform rounded stones whose median diameter was D50 = 12 mm. Stones were kept
Hydraulics of low-head structures
The three different flow regimes of stepped gabion weirs are quite similar to those occurring on stepped chutes: Nappe Flow, Transition Flow and Skimming Flow regime. The Nappe Flow regime is characterized by flow plunging on the successive step (Fig. 1a). The Skimming Flow regime (Fig. 1c) is characterized by a coherent flow structure streaming on a pseudo-bottom. Horizontal axial vortexes occur between the steps and prominent flow re-circulation takes place below the pseudo-bottom. This last
Conclusion
This paper analyzed the effect on maximum scour depth of the inflow conditions. Namely, the analysis was conducted in the presence of both stepped gabion weirs and rock sills. References tests, under steady flow conditions, were conducted in order to obtain the reference values of the maximum scour depth occurring in the downstream stilling basin. The same tests were repeated under unsteady flow conditions. The hydrograph adopted to simulate the unsteady flow conditions was characterized by a
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