FLOOD03601

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Evaluation of and Recommendations for Drop Structures in the Denver Metropolitan Area
• Date: 12/1/1986
• Prepared For: UDFCD
• Prepared By: McLaughlin Water Engineers, Ltd.
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Steven's methodology gave results 36% to 56% higher. The new analysis considers the weight
component of the adjoining rock on the rock being analyzed, In effect, it substitutes the coefficient
of friction between rocks for the angle of repose, which appears reasonable for observations in the
field. Nevertheless, the relationships proposed by Abt are very usefuL

In general, sizing methods proposed by the 1986 MWE report and Abt will tend to produce similar
results. Unfortunately, for drop structures this still results in the need for very large riprap and more
expense than other options, Considering the construction difficulties, UDFCD has chosen to
recommend other types of drops,

As another footnote, Simons, Li, and Associates (SLA), 1986, performed a study of scour and riprap
stability at submerged drops, which may be useful for analysis of sloping riprap drops,

SEEPAGE ANALYSIS- SECTION XII
Flow net and computerized seepage flow and pressure analysis is a clear improvement over Lanes
Weighted Creep Analysis because it is much more practical to handle multiple layers, and predict
flows and pressures, Lanes Weighted Creep Analysis is given as a minimum approach. With the
advent of microcomputers and the numerous software packages available, computerized seepage
analysis is preferred.

EFFECTS OF HYDRAULIC JUMP TURBULENT PRESSURE FLUCTUATIONS ON RIGID
STRUCfURES SECTION XI AND XII
At about the same time as the 1986 MWE report was issued, research results on turbulent pressure
fluctuations were published by Toso, 1986, and Bowers, 1988, These reports documented more clearly
that the severe turbulence in a hydraulic jump can pose some special problems often ignored in
hydraulic structure design, Turbulence can cause significant positive and negative pressure
fluctuations along a rigid structure,

A good example of the problem is illustrated by a situation in which an impervious, rigid sloping drop
face is entirely underlaid by a gravel seepage blanket, If the gravel provides a continuous free seepage
path to the bottom of the basin or other locations where the jump will occur, the positive pressure
fluctuations could be transmitted directly to the area under the sloping face. This condition can
destabilize the structure if there is insufficient weight (of water) in the area of shallow supercritical
flow to counter the uplift forces, This exact situation caused a major failure of a large dam spillway
(Bowers, 1988) and is thought to be a contributing factor to failure in smaller drops.

Toso, 1986, presents a thorough discussion and several laboratory studies of this situation, The data
is derived for somewhat higher Froude numbers than is generally encountered in the design of drops
for grass lined channels, Until better information is available, it is advisable to review data that is
close in range to the site conditions considered in design,

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