HomeMy WebLinkAboutWPO201300032 Correspondence 2021-08-02John Anderson
From: Dustin Greene <DGreene@roudabush.com>
Sent: Monday, August 2, 2021 1:50 PM
To: John Anderson; Frank Pohl
Cc: Charlie A. Hurt; Jim Taggart
Subject: WPO201300032, Cascadia - Amendment #2
Attachments: HW-D = 1A8.pdf, HW-D = 1.52.pdf; Pond #2 Emergency Spillway Control Section.pdf;
Pond #2 Emergency Spillway Channel Minimum Slope 5%.pdf; Pond #2 Emergency
Spillway Channel Maximum Slope 20%.pdf
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John,
Thanks for your review of the amendment for Cascadia Pond #2.
Is'comment: 10-year capacity for the Broadus Memorial Baptist Church culvert.
The original reduction of the 10-yr storm event from the pre to post developed condition reduced the flow from
27.83 cfs to 8.77 cfs. This is a reduction of almost 70% of the original 10 year flow to the culvert underneath the church
access.
Pre -developed = 27.83 cfs
Post -developed = 8.77 cfs
The capacity of the 30" RCP culvert is 39 cfs. The headwater to depth ratio exceeds 1.50 at 40 cfs (See attached
pdfs). The reduction and the capacity show that this culvert will be able to handle the 10-year flow. There does not
appear to be any erosion around the culvert area further showing that it has not eroded since being built for the church
access in 1994.
2"1 comment: The emergency spillway channel should have the some cross-section as the control section.
I have provided the County an equivalent cross section for the 40' control section and the 16' wide emergency
spillway channel to the toe of the slope for the dam. There are an infinite amount of cross sections that could be used
to convey the 100-yr storm from the control section of the emergency spillway to the toe of slope for the dam. The 100-
year storm conveys 147.43 cfs through the emergency spillway at 356.99' (WSE). The emergency spillway uses the weir
equation for cross sectional area of 44.20 square feet. The emergency spillway channel uses Manning's equation to
determine the required depth to convey the 100-yr storm in the rip rap channel. The slope of the channel reduces the
required cross sectional area required to safely convey the storm, that is why I am calling it an equivalent cross section
as opposed to the same cross section.
Maximum slope of 20% requires a depth of 0.64' or equivalent area of 9.78 square feet for the emergency
spillway channel.
Minimum slope of 5% at the toe of the slope requires a depth of 0.95' or equivalent area of 15.10 square feet
for the emergency spillway channel.
As -built channel = 14', 1.5' deep, OK.
The existing channel is at least 1.5' deep so the existing channel will be able to safely convey the 100-yr storm to the toe
of slope without erosion. The contractor has been instructed to remove all woody plan materials from the emergency
spillway channel and embankment. The contractor has also been instructed to cut the emergency spillway control
section as opposed to providing a diversion on top of the downhill side of the embankment Please let me know if you
have further questions or concerns.
Best Regards,
Dustin Greene, PE
Roudabush, Gale & Associates, Inc
(o) 434-977-0205 ex - 724
(Direct) 434-328-1069
(m)415-420-8238
dgreene@roudabush.com
Physical Address:
999 Second Street SE
Suite 201
Charlottesville, VA 22902
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