FLOOD01419

• County: Statewide
• Title: Colorado Association of Storm Water and Floodplain Managers 7th Annual Conference
• Date: 9/18/1996
• Prepared For: State of Colorado
• Prepared By: CASFM
• Floodplain - Doc Type: Educational/Technical/Reference Information

to a stable configuration is critical to the hydraulic performance of the structure.

For drops less than 0.6 m (2 feet), graded riprap with a dso of 0.3 m (1 ft.)
minimum (Type M per UDFCD) must be used. For 0.6 to 0.9 m (2 to 3 feet)
drops the minimum <Iso must be 0.45 m (1.5 feet) minimum (Type H per UDFCD).

UDFCD GUIDELINES FOR SELECTION

The UDFCD (UDFCD, 1990) presents guidelines for the selection of grass lined
channels based on channel bed stability, drop height, safety considerations (to
minimize or avoid vertical drops in public areas), and flow.

OTHER DROPS UTILIZED

As previously mentioned, numerous other drop types are used (and mis;Jsed).
Some are used for special situations and may portray future trends.

Slopinll Riprap Drops
Riprap drops continue to
be utilized for small flow -=:::. ~ _

~~~~:: st~!e ~~~~ -'~.' ::--::~ ~:;~

structures. UDFCD's ~~
drop research study
(Taggart. 1986)
established a strong correlation to the Smith work (1965), and recommended
similar guidelines for rock sizing, stringent material gradation testing, and
construction quality control. With this improved guideline riprap drops generally
were more expensive than the other standards. Many of the riprap structures
completed after the research project in the Denver Metropolitan area (Taggart,
1986) do not conform to those guidelines and have developed problems. The
UDFCD no longer helps maintain such drops.

Dr. S. R. Abt, of CSU (1988, 1991) has conducted extensive research :me;
development of riprap design criteria. In rural, highway, and mining situatior.s
such drops are often used because of cost considerations, and because the
inevitable rock movement can be tolerated.

St~ped or SIQping Drops
Utilizinll Inverted Jump
Technology
Conventional hydraulic
jumps, where the reverse
current and turbulence
largely occurs on the
surface, form a hazard to
the inadvertent swimmer and the recreational boater. One of the most successfu7
and versatile technologies to address this issue is called a hydraulic jump 3.1: a.:::
abrupt drop. The abrupt drop refers to a deep underwater drop off in the }j3.s;n
where the supercritical flow is directed horizontally at the surface Gf r:he
downstream pool and the current reversal is underneath (Taggart, 1984). The ;Jew
South Platte River Dam and whitewater bypass at Confluence Park was constructed
by UDFCD using this technology.

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4

Taggart, et al