USE OF THE MANNING EQUATION FOR THE DESIGN OF HIGH-GRADIENT CANALS

The Manning Equation is an empirical equation for the estimation of stream or canal discharge based on the water depth, channel geometry, slope of the water surface, and an empirically-derived Manning roughness coefficient. The problem with the use of the Manning Equation is that structures are being designed that have parameters outside of the database that was used to develop formulae for estimating the Manning roughness coefficient, including the high-gradient canals that are common in Utah. Although formulae exist for steep mountain streams, natural streams tend to have a relationship among slope, depth and sediment size that would not necessarily apply to canals. The objective of this study is to develop a new methodology for estimating the Manning roughness coefficient in high-gradient canals, which will allow for proper canal design that will avoid overflow or canal failure. The objective is being addressed by combining measurements on high-gradient canals of discharge, water depth, channel geometry, and slope of the water surface, in order to solve for the Manning roughness coefficient. All study sites are in Utah County, Utah, and include earthen, concrete and rock-lined canals. Gradients of measured canals have ranged from 0.2% (typically defined as the lower limit for high-gradient streams) to as high as 36.7%. Measured values of the Manning roughness coefficient have been in the range 0.034-0.066 with mean 0.049, with little dependence on the slope or the canal material. By contrast, published values for the Manning roughness coefficient based on the material are in the range 0.013-0.050 with mean 0.022, and published formulae for the Manning roughness coefficient in high-gradient streams vastly overestimate the measured Manning roughness coefficient in canals, especially at the higher slopes. The preliminary interpretation is that many of the high-gradient canals are flowing in the supercritical regime, which has eroded the canal walls, for example, to expose the aggregate beneath the trowel finish of a canal wall. In fact, in many canal walls, it is possible to see a fairly sharp boundary separating the erosive effects of supercritical flow (shallower flow) from subcritical flow (deeper flow). Further results will be reported at the meeting.