HomeMy WebLinkAboutSUB199300105 Calculations 1993-12-16 riE. — 1 e - 93 THU 1 S : 39 COX ! FCHA P - 01
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THE COX COMPANY \ ,4,4CRi,,
Planners • Landscape Architects
Civil Engineers• Urban Donors 4441110
December 16, 1993
Memorandum
To:
re ounty Engineering Department
From: Mr. Fred Missel, ASLA
The Cox Company
Re: Redfields
Phase 2-A Stormwater Detention & Management
After careful review of the impact of development on the channel and culvert directly
adjacent to Phase 2-A of Redfields we have completed the following calculations and
resulting recommendations.
Stormwater Runoff Variables and Calculations:
PRE-DEVELOPMENT
Total Contributing Area(A): 49.1 acres
Time of Concentration: 20 min,
intensity(i): 4 Inches/hr,
Coefficient of Runoff(c): 0.25
Total Runoff (cfs): 49.10 cfs.
POST-DEVELOPMENT
Total Area of Proposed Development (Phase 2-A & 2-B): 11,71 acres
Weighted Coefficient of Runoff for total contributing area(a): 0.30
Total Contributing Area(A): 49,1 acres
Time of Concentration: 20 min,
Intensity(i): 4 Inches/hr.
Coefficient of Runoff(c): 0.30
Total Runoff (cfs): 58,92 cfs,
Delta Pre/Post Development: A 82 cfs
804.29S•7i3I
220 East High Street
Charlottesville, Virginia 22902
DE- - 1 6 - 93 THLI 15 39 COX - KH ii P 0 :2
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Memorandum
Mr. Don Franco
December 16, 1993
Page 2
EXISTING CONDITIONS
Channel;
Field inspection of the channel revealed a stable,well established channel which we believe is
adequate to accommodate existing and proposed runoff. Vegetative growth Is sufficient to
handle the extremely small overall increase In runoff and additional erosion or stream
destabilization Is not probable,
CuIvont.*
The existing culvert is an 18"CMP in fair to poor condition, The estimated slope on the pipe is
approximately 4%. The maximum carrying capacity of this culvert running full (93%) is 22.59 cfs,
It is obvious that the existing pipe was installed for farm purposes without the benefit of
engineering studies and is Insufficient to handle even the existing pre-development channel,
However, It Is Interesting to note, considering the season, that very little, if any, evidence of
wash-over is noticeable across the farm road.
in our opinion,If the existing, undersized culvert, Is now working acceptably. we do not feel the
additional development and its accompanying runoff would have a significant Impact on the
situation, It Is difficult to Imagine that the additional development would have a detrimental
impact on public health, safety and welfare considering the location and current condition of
the culvert. It seems difficult to place the responsibility of upgrading the culvert entirely on the
post-development runoff considering the current inadequacy of the pipe.
If the culvert were to be replaced,the runoff would require a 36" CMP running at 70% capacity
to handle the existing runoff. The calculations reveal that the additional runoff created by
development would not have required on Increase in the culvert size if the existing pipe had
been adequate to handle the pre-development runoff.
As always,thank you for your advice and assistance in this matter, We look forward to hearing
from you in the near future on this issue as well as the Items remaining prior to tentative on phase
2-A.
FAM/aJh
xc: G. Beights. Development Corp. of Virginia
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