FLOOD03643

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Hydraulics of Steep Gradient Streams
• Date: 6/8/1982
• Prepared For: State of Colorado
• Prepared By: Colorado Geological Survey
• Floodplain - Doc Type: Educational/Technical/Reference Information

LIMITATIONS OF THE PREDICTION AND REGIME EQUATIONS

The following restrictions need to be observed when using the

previously developed equations to predict the Manning's n (equation 10),

the velocity (equation 11), and the discharge (equation 12) of steep-

gradient streams:

1. The equations are applicable to natural main channels having

stable bed and bank materials (gravels, cobbles, and boulders) and

average values of the roughness coefficients in equation 5.

2. The equations can be used for slopes from 0.002 to 0.04 and

for hydraulic radi i from 0.5 to 7 ft (0.15 to 2.1 m). The upper limit

on slope is due to a lack of verification data available for the slopes

"
of steep~gradient streams. Discharges used to develop these equations

ranged from low flows up to discharges equivalent to a flood having

o

about a 25-year recurrence interval. Equation 10 appears to under-

estimate channel roughness and equations 11 and 12 overestimate the

velocity and discharge of low flows. The differences in flow depths of

flows having a 25- to 100-year recurrence interval are generally quite

small so that extrapolation to larger flows should no~ be too much in

error as long as the bed and bank 'material remain fairly stable. Results

of the regression analyses indicated that for hydraulic radius greater

than 7, n did not vary significantly with depth.

3. The energy-loss coefficients due to acceleration or decelera-

tion of velocity in a contracting o'r expanding reach of channel were

assigned the values 0 and 0.5, respectively. Analysis of the data

indicates that moderate to severe natural channel expansions cause large

energy losses and therefore the expansion coefficient for these reaches

should be increased.