Estimating interstitial discharge and velocity in flow in riprap and gabion engineering applications
Date
2019
Authors
Keene, Anthony, author
Thornton, Christopher, advisor
Scalia, Joseph, advisor
Williams, John, committee member
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Abstract
Interstitial flow is a difficult hydraulic process to measure and predict. Interstitial flow does not follow the same laws as seepage flow in small-grain media (i.e. Darcy's Law), because flow regimes in aggregate rock are often transitional or turbulent at a mild slope. Flow paths and local velocities in open cavities of a rock layer are dynamic, and instrumentation is difficult to place in rock for physical measurement. Due to the dynamic and complicated nature of interstitial flow, limited tools are available for engineering flow through aggregate rock. Flow in aggregate rock is relevant to many hydraulic engineering applications, including riprap and gabions used in designs for drainage, earth retention, and rockfill structures. Riprap and gabion published design guidelines are derived from external flow conditions and often neglect interstitial flow. Discharge in rock directly influences internal forces that can transport loose rock or strain a gabion mattress structure, interstitial velocity also directly influences bed shear stress. However, despite the importance of interstitial velocity and discharge for design, riprap and gabion design guidelines are developed primarily for rock stability. There is a need for interstitial discharge as design criteria; estimating the discharge capacity of aggregate rock can be useful in applications where drainage for a design flow is relevant. Data from laboratory prototype gabion mattress tests are used in tandem with data collected in a previous study on riprap to develop two simple design equations to predict interstitial velocity and interstitial discharge per unit area of a rock layer. A multivariate nonlinear regression was performed as a function of the following key parameters in a rock system: rock size for which 50% of rock is finer than, D₅₀, rock size for which 10% of rock is finer than, D₁₀, coefficient of uniformity (D₆₀/D₁₀), acceleration due to gravity, and bed slope. The regressions yield a coefficient of determination of 0.97 for both interstitial velocity and interstitial discharge predictive equations. Equations are suited for use in rock layers with nominal sizes from ¼-in to 5-in on bed slopes up to 0.15 ft/ft.
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Subject
drainage
gabions and riprap
hydraulic engineering
erosion control
aggregate rock
geotechnical engineering