Discussion of “Protecting Vertical-Wall Abutments with Riprap Mattresses” by A. H. Cardoso and C. M. S. Fael

The authors are to be commended for their experimental program and for the attempt to provide guidelines for safe design of scour countermeasures by means of launching aprons. This is a technical field in which the existing literature is still relatively scarce. In this discussion, some remarks will be given about the upstream width wu of the launching apron refer to Fig. 12 of the paper under discussion. The authors did not report specific experimentation in this respect. Indeed, the upstream width of the mattress has received relatively scarce attention in works on riprap countermeasures. Experimental evidence has shown that for short abutments as well, the values of the time-averaged shear stress distribution in the corner region upstream of the abutment face are lower than that in the incoming reach Rajaratnam and Nwachukwu 1983; Molinas et al. 1998; Dey and Barbhuiya 2005. If the phenomenon is modeled at threshold conditions as is typically done in clear-water scour experiments, no scour would be expected upstream of the abutment face in all the regions with stress lower than that in the upstream reach. Experimental evidence on sediment motion during the scour process at short and intermediate abutments Radice et al. 2006,2008 has shown that at the beginning of the erosion process, the particle motion is most intense at the abutment nose where scour starts. In contrast, grain motion is almost negligible at the junction between the duct and abutment walls junction region; here, an appreciable rate of scour depth is observed only after the scour hole whose point of maximum depth is typically at the obstacle nose has reached a significant transverse slope. In terms of design of scour countermeasures, protecting the sediment bed around the abutment nose might be sufficient to prevent the scouring process. An experimental test of local scour at a protected abutment was performed to check this issue. A rectangular duct with a length of 5.8 m and a width of 0.40 m was used. The recess section of the duct was filled with uniform plastic grains with a characteristic size of D = 3.6 mm and a relative density of 1.43. Water discharge was 18.5 l / s, corresponding to the threshold value for incipient particle motion in the undisturbed bed. The abutment model was a vertical plate with a length of 10 cm and a thickness of 1 cm Fig. 1. The experimental installation has been described in detail by Radice et al. 2009, who performed an identical scour experiment without abutment protection. In that study, the possibility of comparing scour data taken under freesurface and covered flows was also demonstrated. In the present experiment, the sediment bed around the abutment nose was protected by means of a square plate as shown in Fig. 1. The plate dimension was 20 cm along both the flow and transverse directions; plate thickness was 1 cm. The plate was roughened gluing on its upper surface sediments identical to those of the loose bed; thus the resulting thickness of the countermeasure was 1.4 mm. The described protection device is actually a rough collar installed at the level of the original sediment bed. However, at scour initiation the collar is likely to behave similarly to a launching apron with an elevation identical to that of the protected sediment bed. Therefore, the following observations on scour initiation should not be significantly affected by the different configuration of the countermeasure. At the beginning of the process, some sediment motion was evident in the junction region Fig. 2. In this region the timeaverage shear stress is expected to be smaller than the threshold value; it is therefore reasonable to assume the motion events as due to the peak stress values. During the motion events, the grains assumed directions opposite to that of the incoming flow, consistent with the structure of the principal vortex. The sediment particles moved downstream of the abutment passing over the protection plate. The grain motion in the junction region resulted in a weak but nonnegligible scour rate. Some particles coming from the upstream reach can be recognized. This is consistent with the definition used here of incipient particle motion, which corresponds to a weak sediment transport condition dimensionless sediment transport rate per unit width= 6 10 �5 . After about 3 min of run time Fig. 3, the junction-region scour depth exceeded the thickness of the protection plate and the sediments