Reply to the ‘Discussion by S.K. Mazumder on “Riverbank protection with Porcupine systems: development of rational design methodology” by M. Aamir and N. Sharma (2015)’

TLDR: Aamir et al. as mentioned in this paper investigated the sediment entraining efficiency of RCC Porcupine field, regardless of whether the sediment being captured by the screens was taken from the bed or injected externally, and found that sediment deposition was considerably more near the side wall of the flume than in the mid-section, as shown in Figures 10−17.

Abstract: © 2016 Indian Society for Hydraulics CONTACT Mohammad aamir aamir.dce2014@iitr.ac.in, mohdaamir.amu@gmail.com This paper replies to the ‘Discussion of Riverbank protection with Porcupine systems: development of rational design methodology’; S.K. Mazumder (ISH Journal of Hydraulic Engineering, 24th July 2015, vol. 22, no. 1, pp. 68–69). The authors would like to thank the discusser on his keen interest in the paper and for raising pertinent points for discussion. Experiments have been performed by Aamir and Sharma (2015) in a flume of limited width i.e. 0.86 m; therefore, the effect of retards on the velocity of flow and hence their efficiency in capturing sediment cannot be studied without providing intrusions of more than 50%. Although, this is in violation with the maximum permissible encroachment in the field, as pointed out by the discusser, laboratory constraints demanded the exclusion of this design parameter. However, this experimental program was designed to study the sediment entrainment capacity of the screens and hence, it was the velocity of flow which was under consideration, irrespective of the width of the channel. Figures 3 and 6 are just indicative of the placement of Porcupine field in the channel, without showing the actual number of retards for each case. For a given flow condition, i.e. discharge, sediment concentration and water depth, different combinations of Lr/Ls ratio have been analyzed, some being too close together while others being sufficiently far away from each other. The spacing between the screens does not depend on the length to be protected, rather the width of the required protection decides the Lr/Ls ratio to be adopted. Porcupine field was placed starting at a section 3 m from the upstream end of the flume. Techniques were provided for achieving fully developed flow in the test section, such as flow straighteners/dampeners, perforated wall and rough bed surface in the upstream section, which led to the full development of flow in as small length as 2.5 m. Therefore, sediment was injected at a section 2.5 m from the upstream end of the flume, in order to achieve sediment-laden flow in the fully developed region. Although, sediment was injected uniformly over the entire width of the Porcupine field, it was found that the sediment deposition was considerably more near the side wall of the flume than in the mid-section, as shown in Figures 10–17. Hence, the effect of sediment fall velocity would have been overcome by the presence of the screens, which apparently played a much larger role in the deposition of sediment. It is obvious that in actual field condition, bed and bank materials move and sediment is entrained by the flow, as mentioned by the discusser, but the present study was limited to analyze the sediment entraining efficiency of Porcupine systems, regardless of whether the sediment being captured by the screens was taken from the bed or injected externally. BDF values given in Table 3 have not been fixed arbitrarily. Average and maximum BDF values were determined from BDF vs. PFLF curves for each condition. Close examination of these values of average and maximum BDF suggests that for high submergence and low sediment concentration, the average BDF values vary in the range of 0.05–0.15, and for medium concentration and submergence, these vary in the range of 0.1–0.2. Similarly, for low submergence and high sediment concentration it varies from 0.2 to 0.3. These values have directed to fixing thresholds for the three design objectives of erosion control, moderate reclaim and high reclaim. Owing to the heavy weight of RCC Porcupines, they do not require anchoring into the bed and field experience suggests that they are quite stable even in flood season. However, bed can be protected by mattress against possible scour around the Porcupines. Consider a Porcupine unit having each member bar of 2 m length and 0.15 m × 0.15 m cross section. This Porcupine unit is placed in such a manner that its face normal to flow direction acts as an equilateral triangle. Therefore, gross area can be calculated as the area of an equilateral triangle of side 2 m, which comes out to be 1.73 m2. Now, three member bars will provide complete obstruction to flow, whereas one bar will appear to be vertical in the direction of flow, its effective height being (22 – 12)1/2 = 1.73 m. Hence, the complete area of obstruction can be calculated as [{3 × (2 × 0.15)} + (1.73 × 0.15)] which gives 1.15 m2. Therefore, the area of openings become (1.73 – 1.15) = 0.58 m2. Hence, permeability of Porcupines is calculated to be 0.58/1.73 = 0.34 or 34%, which is almost within the range given by Lagasse et al. (1995). It is to be noted that permeability increases with increase in the length of the member bars.