Experimental investigation of apron installation level on variation of local scour at the downstream of block ramps

Document Type : Research Paper

Authors

• Mahdi Esmaeili Varaki ¹
• sama mirmohammadi ²
• Sahameddin Mahmoudi Kurdistani ³

¹ Department of water engineering, Faculty of Agriculture Science, University of Guilan, Rasht

² Department of water engineering, Faculty of Agriculture Science, University of Guilan, Rasht, Iran

³ IA.ING, Lecce, Italy

Abstract

Keywords

• Apron
• Block ramp
• River restoration
• Roughness
• Scour

20.1001.1.2008479.1402.54.7.4.6

Main Subjects

• سازه‌های آبی

Experimental investigation of apron installation level on variation of local scour at the downstream of block ramps

EXTENDED ABSTRACT

Introduction

One of the structural methods to protect river reaches with unstable bed slope, is the construction of block ramps that decreases flow energy grade line by passing flow via a sloping surface. Application of these structures reduces and stabilizes the upstream river’s bed slope that causes the decreased velocity of flow and corresponding capacity of sediment transport, which leads to stability of the bank of rivers at upstream reaches (Esmaeili Varaki et al. 2021). Several types of research have been conducted on local scour downstream of grade-control structures. Some studies were conducted to investigate the hydraulic characteristics of passing flow over the surface of block ramps, the downstream scour process, and energy dissipation, i.e., Bormann & Julien (1991), D’Agostino & Ferro (2004), Pagliara & Chiavaccini (2006a), Pagliara & palermo (2008), Pagliara & palermo (2009), Pagliara et al. (2012), Ortel & Bung (2015) and Esmaeili Varaki et al. (2021). Furthermore, the design critera and scour countermeasures downstream of block ramps were investigated by Robinson et al. (1998), Pagliara & palermo (2008), Pagliara & palermo (2010), Weitbrecht et al. (2016), and Moayedi moshkaposhti et al. (2022). The main purpose of the current study is to investigate the effect of the macro roughness, and indtalation level of the downstream apron as scouring countermeasures on the maximum scour depth downstream of the block ramp.

Experimental Setup and procedure

All the experiments have been conducted at the Physical Hydraulic Laboratory of the Department of Water Engineering, University of Guilan (IRAN). The experimental setup included a rectangular recirculation flume 8.4 m long, 0.88 m wide, and 1 m deep where the ramp was made in such a way that the height of the structure for all experiments was P = 0.3 m. The flow rate was measured by an ultrasonic flow meter with a precision of ± 0.01 L/s. A flow straightener was placed at the flume entrance to avoid inlet effects. Block ramps were made in three ramp slopes of S₀ = 0.2 (1V:5H) and 0.33 (1V:3H). Smooth block ramp and two values of the macro roughness k_s =1.15 cm and k_s =5.2 cm was used to make block ramps. Furthermore, in case of k_s =5.2 cm, two configurations including compact and stagerd was used to investigate arrangements of large element of surface roughness on scour depth. A rectangular channel with dimensions of 2.5 m long, 0.3 m deep and 0.88 m wide was used as a stilling basin downstream of the block ramp filled with uniform sand of mean particle size d₅₀ = 0.7 mm and G_s = 2.65 that the well-leveled stilling basin bed before starting a test. At the end of each run, temporal scour development was measured using digital camera under different interval time and the final scour was surveyed using a Laser scanner Leica with ±1 mm accuracy.

Results and discussion

Flow passing over sloping surface of block ramps has three distinguished regimes including nappe flow (y<k_s), skimming flow (y>1.5y_s) and transition flow (y_s<y<1.5y_s) (Oertei, 2013). For current study, Flow depth on block ramp was in range of 1.5-3.5 cm that led to cover all flow regimes for meadim and large surface roughness of block ramp.Comparison of the maximum equilibrium scour depth for the block ramp with slope 1:3 indicated that by reduction of the installation level of apron to P/6, d_se/P changed from 0.37 to 0.63 and 0.46 to 0.77, for low and high discharges, respectively. Furthermore, as surface of block ramp changed to compact roughness with k_s =5.2 cm, d_se/P reduced considerably and changed from 0.3 to 0.32 and 0.4 to 0.42, for low and high discharges, respectively. As the slope of the block ramp was reduced to 1:5, the effect of the surface roughness on the reduction of the scour depth became more noticeable. A comparison of results indicated that for compact roughness with k_s =5.2 cm, d_se/P changed from 0.21 to 0.25 and from 0.28 to 0.32, for low and high discharges, respectively.

Conclusion

The comparison of results indicated that there is a direct relationship between the level of the apron level and the scour depth so that by reducing the level of the apron to 1/10 and 1/6 of the block ramp height, the maximum scour depth increases in an average 36 and 41 for slope of 1:3 and 32 and 35 for slope of 1:5, respectively. By increasing the size of surface roughness such that the overpass flow regime changes to nappe flow, the reduction of installation of the level of the block ramp does not noticeable effect on the increase of the scour depth.