FLOOD03313

• County: Denver
• Basin: South Platte
• Title: Drop Structures in the Denver Metropolitan Area
• Date: 12/1/1986
• Prepared For: UDFDC
• Prepared By: McLaughlin Water Engineers, Ltd.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION II
BAFFLED APRON DROPS

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INTRODUCTION

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The UnIted States Bureau of Reclamation (USBR) has developed design standards for
a reinforced concrete chute with baffle blocks on the sloping face of the drop.
They are commonly referred to as baffle block or baffle chute drops. There are
two excellent references, Hydraulic Design of Stilling Basins and Energy Disslpators
(ref. 34), and DeSIgn of Small Canal Structures (ref. 1), that should be used for the
design of these structures. Also, Design of Small Dams (ref. 7) presents information
on Lanes WeIghted Creep theory which can be used for simplified seepage analysis
of these and other drops.

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The design is normally recommended for a unit width flow (q) of 35 cfs!ft or less
with the maximum (q) not to exceed 60 cfs!ft. In relationship to the channel Width,
this results in a well matched design for the UDFCD grass-lined channel criteria.

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The hydraulic concept IS that all of the flow repeatedly encounters obstructions

that are of a nominal heIght eqUIvalent to crItical depth, leading to faIrly complete

energy loss because of the momentum loss assOCIated With reorientation of the flow.

Normally, a minimum of 4 rows of teeth are utIlized to achIeve dIsturbance of the

flow and dIssipatIOn of energy. GUidelines are gIven for SIZing and spacIng the

blocks. DeSIgning for proper approach velOCitIes IS CrItICal to structure performance.

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EffectIvely there are fixed costs regardless of drop height for approach walls,
minimum length of side walls, downstream transition walls and a minimum length of
sloping apron. The baffle chute becomes more economical with increasing drop height.

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This design is quite flexible in adaptation, once the hydraulic principals are understood.

For instance, for low drops, designs which use two rows of baffles on the slope and

two in the horizontal area below have been successful. Also, the apron can have

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