SCOUR DOWNSTREAM OF A CORRUGATED APRON

Abstract

Hydraulic structures mostly built across channel or canal must have to be protected against local scour or erosion occurring in vicinity of structures founded on loose soil. The scour occurs when shear stress possessed by the flowing stream of water exceeds critical shear stress of bed material. To minimize scour several structural measures have been proposed in literature. In Irrigation structures we usually provide loose concrete blocks as apron or launching apron downstream of solid apron or baffle blocks, sills, chutes etc. Various studies have been carried out on scour and flow characteristics in scour hole. H. Rouse, (1940) performed pioneering investigation on scour due to jet. The scour due to 2D horizontal jet was investigated by Laursen, (1952); Tarapore,(1956); Hogg et al.(1997). Ali and Lim, (1986) computed velocity distribution in scour hole theoretically. Uyumaz, (1988) studied the effect of different size of sediment, tail water depth, total discharge and head on scour depth and finally he found that scour depth is inversely proportional to D900.4. Dey and Sarkar (2006) studied the variation of normalized scour depth (ds/a) with normalized median diameter (D50/a). Rajaratnam and MacDougall (1983) as well as Dey and Sarkar, (2006) studied the effect of tailwater on scour depth. The influence of reducing scour depth downstream of a solid apron under submerged horizontal jet was studied by Dey and Sarkar (2006b). N. Rajaratnam and Ead, S.A., (2002) studied the flow characteristics of submerged jump on corrugated apron first time. Later, Ali et al., (2014) studied the effect of corrugated apron on scour due to submerged jets. They used different sets of triangular strips of corrugated apron and spaced corrugations also. Spaced triangular corrugated aprons minimize the scour depth and length iii | P a g e of fine sand by 63.4% and 30.2%, and for coarse sand by 44.2% and 20.6% in comparing with classical jump. The triangular corrugated apron is not practical in field and with passage of time triangular pointed crest gets eroded and changes into sinusoidal corrugation shape. Therefore, it was felt necessary to study variation in scour parameters by varying dimensions of sinusoidal corrugation instead of triangular corrugation. The present study of scour downstream of corrugated apron was carried out for two different flow conditions. In phase I, scour in free flow condition when flow is subcritical and in phase II, scour in submerged hydraulic jump condition was studied. In phase I, use of corrugated apron gives adverse results, use of corrugated apron increases the scour, rather than decreasing. Analyzing turbulence intensity in plane and corrugated apron in free flow jet condition, it was found that turbulence intensity for corrugated apron at every section of scour bed and apron is higher than plane apron. In phase II when flow is submerged hydraulic jump, scour depth reduces significantly in comparison with scour due to plane solid apron. The results are analyzed graphically and equations have been proposed for prediction of scour depth, scour length and point of maximum scour depth for corrugated apron in submerged hydraulic jump condition which are valid for range 1.26≤F≤5.5 of Froude number and 1.56≤s/a≤8.0, 0.33≤t/a≤1.75, 3.56≤yt/a≤11.00 for s/a,t/a,yt/a. Maximum and minimum reduction in scour depth and length was found to be 78.95 %, 12.73% and 83.15%,10.75% respectively. Optimum dimension of corrugated apron was found to be S=0.07 m and t=0.025 m which give minimum depth and length of scour for tested range of parameters. Similarly, the proposed equation was statistically evaluated. Thus, based on literature review and present study, corrugated apron can serve as the one of the best scours preventing appurtenances.