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
Riprap Design for Overtopping - Man Do I Need a Martini!
Riprap remains one of the most common and cost-effective means of providing erosion protection, structure stability, and slope stabilization from overtopping flow conditions. Currently, at least twenty-four riprap design relationships and/or synthesis procedures have been formulated since 1936 for sizing stone riprap. Most of these relationships were derived from a limited empirical database extending over a relatively small range of boundary conditions; each expression often targeted for a specific use or application. Commonly, designers extrapolate a stone-sizing relationship beyond the boundary conditions of development to determine an appropriate stone size to resist stone movement, particularly for median sizes greater than 10 in. The intent of this presentation is to highlight the twenty-four relationships/procedures currently available for designer consideration and provide a “road map” for their specific use and/or application for angular stone.
Resistance Equation for Large-Scale Roughness
A theory of flow resistance for large-scale roughness is developed, the main points of which are supported by a flume study from which the outline of a process-based resistance equation is determined. Rivers with large-scale roughness have steep slopes and depths of the same order of magnitude as the bed material size. Flow resistance depends on the form drag of the roughness elements and their disposition in the channel. Theoretical considerations show resistance to be a function of Reynolds number, Froude number, roughness geometry, and channel geometry. These processes are examined using the results of flume experiments based on five different roughness beds and a wide range of flows. Semi-empirical analysis supports the theory and allows the development of a theoretical power law resistance equation which accounts for most of the processes. A summary of the data is presented for use in other research studies.