|
<br />I
<br />I
<br />I
<br />I
<br />f
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />-
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />
<br />SECTION I
<br />ADDENDUM
<br />
<br />INTRODUCTION
<br />This discussion highlights advances in technology, useful new references, refinement in analysis, and
<br />the experience gained on drop structures since the 1986 MWE report "Evaluation of and design
<br />Recommendations for Drop Structures in the Denver Metropolitan Area" by McLaughlin Water
<br />Engineers (MWE), Each topic is addressed according to the respective sections of the 1986 MWE
<br />report.
<br />
<br />The refinements cited herein have been considered in preparation of the revised "Hydraulic
<br />Structures" section of the UDPCD criteria manual, to be issued March 15,1990,
<br />
<br />SLOPING RIPRAP DROPS - SECTIONS V, X, AND XII
<br />The report analyzed several algorithms for rock sizing on sloping rip rap drop, and compared them to
<br />parameters measured for sloping riprap drops that had experienced some movement and in some
<br />cases near total failure, MWE found that a higher Shield's parameter P* could probably be used for
<br />the relatively shallow flow (on a sloping drop for grass lined channels), However, MWE found that
<br />a safety factor should be used, and that the approach used by Steven's, 1976 was a very reasonable way
<br />of implementing this for sizing riprap, The net effect of this recommendation, coupled with other
<br />suggestions for improved construction practices, is that much larger size and thicker layer of riprap
<br />would need to be utilized in order to reduce the extent of problems that have occurred.
<br />
<br />Dr. Steven Abt of Colorado State University (CSU), 1986 and 1987 published research results
<br />concurrent to publication of the 1986 MWE report. Testing of sizes up to 6 inch D50 riprap stability,
<br />for flow down sloping faces, Data measured included unit discharge, slope, stone size, interstitial
<br />velocity within the riprap, resistance to flow, effects of the flow on the fIlter blanket, channelization
<br />of flow through the rip rap, and the effects of duration of flow and riprap gradation, Abt reported that
<br />failure occurred with an equivalent Shield's parameter slightly greater than expected using the Shield's
<br />diagram, This agreed with Taggart, 1986; Smith, 1975; and Wang, et. al1985, Abt noted that several
<br />methods including U,S, Army Corps of Engineers (COE), Stevens, and the Bureau of Reclamation
<br />(USER)) predicted much higher values, while the new CSU and Stephenson's method gave results that
<br />agreed with the previous CSU tests. This may be the result of compensating effects since many of
<br />the methods evaluated have applied a low Shield's parameter with a relatively high safety factor (A
<br />low Shield's parameter results in a larger rock requirement), Por the case of shallow flow, such as at
<br />a typical drop structure, a somewhat higher Shield's parameter than 0,06, more like 0,07 to 0,09, should
<br />be used,
<br />
<br />Abt and Wittler, 1988, reported that after consulting with Stevens, a modification could be taken in
<br />the approach to fonnulation of a safety factor based on rock movement force mechanics. That study
<br />recommended an equation which correlated well to observed movement. When compared to Steven's
<br />original formulation at a safety factor of 1.1 (and a conventional Shields parameter), Abt found that
<br />
<br />1-1
<br />
|