FLOOD06380

• County: Statewide
• Community: Adams/Arapahoe/Denver/Broomfield/Douglas/Jeffco
• Basin: Statewide
• Title: Urban Storm Drainage, Criteria Manual Volume 2
• Date: 6/1/2001
• Prepared By: Wright Water Engineers, Inc.
• Floodplain - Doc Type: Educational/Technical/Reference Information

HYDRAULIC STRUCTURES

DRAINAGE CRITERIA MANUAL (V. 2)

AP

Cp_m~ = (~~)

(HS-8)

e

in which:

AP = pressure deviation (fluctuation) from mean (ft)

v;, = incident velocity Oust upstream of jump) (ft/sec)

g = acceleration of gravity (ft/sec2)

Effectively, Cp is a function of the Froude number of the supercritical flow. The parameter varies as a
function of X, which is the downstream distance from the beginning of the jump to the point of interest.

Table HS-3 presents recommended Cp.~ positive pressure values for various configurations. When the
Froude number for the design case is lower than those indicated, the lowest value indicated shouid be
used (do not reduce on a linear relationship) for any quick calculations. The values can be tempered by
reviewing the Cp graphs, a few of which are given in Figures HS-4 through HS-6. Note that the graphs
are not maximum values but are the mean fluctuation of pressure. The standard deviation of the
fluctuations is also indicated, from which the recommended Cp-m~ values were derived.

e

TABLE HS-3
Nominal Limit of Maximum Pressure Fluctuations Within the Hydraulic Jump (Toso 1986)

Jump Condition F roude Suggested
Number Maximum Co
00 slope, developed inflow (boundary layer has reached surface) 3.0 1.0
300 slope, toe of jump at base of chute> 3.8 0.7
300 slope, toe of jump on chute> 3.3 0.8
300 slope, with Type II basin (USBR) 5.0 0.7
300 slope with Type III basin (USBR) 5.0 1.0

> Velocity head increased by elevation difference between toe of jump and basin floor, i.e., depth
at the drop toe.

Figure HS-4 illustrates positive and negative pressure fluctuations in the coefficient, Cp, with respect to the
location where the jump begins at the toe. Figure HS-5 presents the positive pressure fluctuation
coefficient where the jump begins on the face. Figure HS-6 illustrates how the pressure fluctuations vary
in a U.S. Bureau of Reclamation (USBR) Type II or III basin.

For the typical basin layouts given and where the drains are at the toe and connect directly to the

supercritical flow, these pressure fluctuations should not be of great concern. However, when drains

e

HS-18

06/2001
Urban Drainage & Flood Control District