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Emergency
Spillway Design Problem
During recent Erosion and Sediment Control Program
evaluations of Conservation Districts in the region it was discovered that a
large percentage of engineering firms were submitting incorrect design
calculations for vegetated emergency spillways. The problem is associated with
the use of the weir equation Q=CLH1.5. The “C” value used was
always 2.8, regardless of the design characteristics of the spillway. This error
may be from a mis-interpretation of the State’s Erosion and Sediment Control
Design Manual. On page 55 it reads “an acceptable alternative is the use of
the weir equation: Q=CLH1.5 when this option is used, the maximum
values of “C” should be 2.8”.
The weir equation was an alternative to using the NRCS’ Engineering
Field Manual Chapter 11. Unfortunately, this alternative required you to use
another section in this same manual. In order to design an emergency spillway
without a copy of Chapter 11. The Department of Environmental Protection staff
has developed a table to assist you in selecting the proper “C” value.
We encourage all plan designers to correct this issue on their projects.
The under sizing was explained to us as being significant. We welcome your
comments on this issue. Please feel free to contact Ed Magargee at (610)
892-9484
View
"C" Value Table
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