Jeff Selck, Utah Valley University
Physical Sciences
Stream discharge through narrow, deep slot canyons can be a major source of groundwater recharge in the arid Southwest. Various state and federal agencies use the empirical Manning Equation to predict the discharge through artificial slots created for diversion of rivers around coal mines. However, it is not obvious that the Manning Equation could be applied to slot canyons or artificial slots as the data base used for development of the Manning Equation did not include either natural streams or artificial structures for which most of the friction occurs along the sides of the channel. The objective of this research is to develop an empirical formula for estimating the Manning roughness coefficient for flow through narrow, deep slots. The objective is being addressed by measuring stream discharge through natural slot canyons in southern Utah that are fed by perennial streams, springs or dam outlets. Based on measurements at ten sites along eight streams, the best estimate for the Manning roughness coefficient is n = 0.873nJ exp(5.108A/w2) where A is stream cross-sectional area, w is stream width, and nJ = 0.39S0.38R-0.16 is the roughness coefficient estimated by Jarrett (1984) for high-gradient streams, in which S is slope of the stream bed and R is hydraulic radius (ft). The new formula estimates stream discharge with a mean accuracy of 44%. On the other hand, Jarrett’s (1984) formula underestimates stream discharge by 1-2 orders of magnitude for aspect ratios A/w2 in the range 0.6-0.7. The new formula will be refined by additional measurements on slot canyons, artificial diversions, and a laboratory hydraulics bench. It is hoped that the new formula will lead to a more realistic design for artificial slot diversions.