# An improved method for calculating the Manning roughness coefficient for estimation of stream discharge through slot canyons in southern Utah

## Location

Eccles Conference Center

## Event Website

http://water.usu.edu

## Start Date

4-2-2014 4:00 PM

## End Date

4-2-2014 4:15 PM

## Description

Stream discharge through slot canyons is potentially a significant source of groundwater recharge in the arid Southwest. However, these canyons are normally dry except during flash floods, making stream velocity nearly impossible to measure. The discharge could be estimated using the empirical Manning Equation, which requires only the channel geometry, water depth, slope of the stream surface, and an empirically-derived roughness coefficient. The objective of this research was to develop an empirical formula for calculating the Manning roughness coefficient n for flow through slot canyons by carrying out the first measurements of n in slot canyons. Stream discharge was measured at 13 slot canyon sites on tributaries of the Escalante, Paria and Virgin Rivers in southern Utah that are fed by perennial streams, springs or dam outlets. Based on these measurements, the best estimate for the Manning roughness coefficient n is n =(0.871 ± 0.269)n_J exp[(4.38 ± 0.72)B] where B is the aspect ratio defined as B = A/w^2, where A is cross-sectional area and w is width, and n_J = 0.39 S^0.38 R^(-0.16) is the roughness coefficient estimated by Jarrett (1984) for high-gradient streams (S > 0.002), in which S is slope of the water surface and R is hydraulic radius (ft). The ability of the formula of this study to predict discharge improves dramatically, as compared to the formula of Jarrett (1984), as B exceeds 0.2. According to the formula of this study, the previous best estimate of the discharge of the late Pleistocene Bonneville Flood of 3.30 × 10^7 cfs is an overestimate by a factor of 1.85, based upon an underestimation of n.

An improved method for calculating the Manning roughness coefficient for estimation of stream discharge through slot canyons in southern Utah

Eccles Conference Center

Stream discharge through slot canyons is potentially a significant source of groundwater recharge in the arid Southwest. However, these canyons are normally dry except during flash floods, making stream velocity nearly impossible to measure. The discharge could be estimated using the empirical Manning Equation, which requires only the channel geometry, water depth, slope of the stream surface, and an empirically-derived roughness coefficient. The objective of this research was to develop an empirical formula for calculating the Manning roughness coefficient n for flow through slot canyons by carrying out the first measurements of n in slot canyons. Stream discharge was measured at 13 slot canyon sites on tributaries of the Escalante, Paria and Virgin Rivers in southern Utah that are fed by perennial streams, springs or dam outlets. Based on these measurements, the best estimate for the Manning roughness coefficient n is n =(0.871 ± 0.269)n_J exp[(4.38 ± 0.72)B] where B is the aspect ratio defined as B = A/w^2, where A is cross-sectional area and w is width, and n_J = 0.39 S^0.38 R^(-0.16) is the roughness coefficient estimated by Jarrett (1984) for high-gradient streams (S > 0.002), in which S is slope of the water surface and R is hydraulic radius (ft). The ability of the formula of this study to predict discharge improves dramatically, as compared to the formula of Jarrett (1984), as B exceeds 0.2. According to the formula of this study, the previous best estimate of the discharge of the late Pleistocene Bonneville Flood of 3.30 × 10^7 cfs is an overestimate by a factor of 1.85, based upon an underestimation of n.

https://digitalcommons.usu.edu/runoff/2014/2014Abstracts/39