Friction Factors for Flow through Cracks in Concrete Hydraulic Structures

Flow through cracks in a variety of concrete structures can be problematic for water resources infrastructure. Design of drainage systems to collect and remove such flow requires estimating flow rates through cracks. When the crack is wide or the flow path is relatively short, entrance losses dominate, and discharge may be estimated using orifice equations that neglect friction losses. However, for relatively narrow and long cracks with rough interior surfaces, friction losses can significantly affect discharge. This article reanalyzes data previously collected from tests of flow through a cracked concrete beam to obtain Darcy-Weisbach friction factors and presents them relative to the Reynolds number of the flow through the crack, similar to the Moody diagram commonly used to estimate friction factors in pipe flow. The data are also used to develop an empirical equation for predicting friction factors and to determine the effective roughness of the cracked concrete surfaces. Observations are made regarding unexpected behavior of friction factors for the combination of low Reynolds number and extremely large relative roughness, which is outside the bounds of most previous research on flow resistance.