Stabilization of Angular-Shaped Riprap under Overtopping Flows
Riprap is mostly used to prevent erosion by flows down the steep slopes in river engineering. A total of 53 stability tests performed on angular riprap with a median stone size ranging from 15 to 278 mm and slope ranging from 1 to 40% are used in this study. The existing equations for the prediction of medium size of angular stones are checked for their accuracy using the available data. Predictions of median size using these equations are not satisfactory and results show deviation by more than ±20% from the observed values. A multivariable power regression analysis is performed to propose a new equation relating the median size with unit discharge, bed slope, riprap thickness and coefficient of uniformity. The proposed relationship satisfactorily predicts the median angular stone size with ±20% error. Further, the required size of the rounded stone is more than the angular stone for the same unit discharge and the ratio increases with unit discharge and also with embankment slope of the riprap. Keywords—Angularity, Gradation, Riprap, Stabilization
Hydraulic Performance of a Steep Single Layer Riprap Drainage Channel
Highway drainage channels often approach slopes of 0.5 (50%). Single layer riprap-lined channels at this slope have been constructed for highway drainage and appear to perform satisfactorily, but no quantitative information is available to guide their hydraulic design. A 1/6 scale laboratory hydraulic model, using a single layer of sized crushed limestone, was constructed to determine the relationship between depth of flow and discharge. The flow range investigated took place at depths near or below the top of the riprap, since riprap instability occurred at greater depths. Standing waves and localized hydraulic jumps dominated the surface of the flow, which held streamwise-averaged Froude numbers in a narrow range near the critical condition for those flow depths for which the channel is designed. This resulted in a simple relationship between unit discharge and depth of flow that applies to the model and prototype channels.
Resistance to Flow in Steep Rough Streams
A theory of resistance to flow in steep, rough streams is developed that takes into account the existence of a highly turbulent wake zone near the bed of a very rough boundary. Two zones are identified in the flow field. The first one, close to the top of the bed, contains overlapping wakes generated by protruding roughness elements. In the second zone, located above the first one, the velocity distribution is described in terms of a logarithmic profile. Flume data are used to check the validity of this formulation. The compound velocity profile allows for equations to obtain Darcy‐Weisbach and Chezy factors. The proposed approach is compared with the results of four previous theories using data obtained from several mountain rivers. Although flow in actual mountain rivers is more complicated than the idealized case treated in the study, the formulation provides equations that allow accurate prediction of flow resistance with a mean error of ±15%.