DR9303-CS 920710MEMO
Alan D. Ratliff, City Manager
YA
ROM: Kenneth M. Griffin, P.E., City Engineer
SUBJECT: Barbara Austin Drainage
DATE: July 10, 1992
As requested, I have investigated the drainage problem at the northeast comer of Moore and
Sandy Lake (Barbara Austin Property). The problem, as stated by Ms. Austin in her May 5,
1992 letter to the Mayor, City Council and Staff, was that the amount of water coming onto her
property, since development of property to the south of her, has created problems with erosion,
washed out her driveway and killed several large oak trees.
The ultimate solution, as identified in the Albert H. Halff Study, is the installation of a 66" RCP
pipe headed northward from Sandy Lake in the Moore Road right of way and outfalling into the
existing concrete channel on the west side of Moore Road. During a meeting with the Council
subcommittee on drainage, I was instructed to look at possible solutions to the drainage problem
as it affects the Austin property. Survey shots have been taken of the existing culverts in the
area and the existing ground on the east side of Moore Road headed northward from Sandy Lake.
The general thought was whether or not it was possible to construct a temporary ditch northward
from Sandy Lake parallel to Moore Road and have it outfall into the existing channel. The only
way to outfall into the existing channel would be to construct a portion of the proposed concrete
pipe drainage system under Moore Road to the east side of Moore Road and then cut a ditch to
outfall into that pipe.
There is approximately 81 acres of drainage area going through the Austin property. Based on
the development south of her, this is approximately 400 CFS of drainage. To construct an open
barrow ditch that would handle that much drainage would necessitate cutting a ditch with 3:1 side
slopes that would range from 4 to almost 12 foot deep. This equates to a maximum top width
of about 71 feet. Because of the large oak trees in this area, it is not feasible to construct a
channel of this magnitude on a temporary basis.
I then looked at alternative solutions. In trying to maintain a 3:1 side slope and only pick -up the
increase in drainage caused by the development of the property to the south of the Austin
property, I looked at the drainage area coming onto the Austin property that has developed
recently, i.e., the elementary school, the fire station, and the residential development at the
southeast comer of Moore and Sandy Lake Road. In a developed condition, that area contributes
about 270 CFS of water to the drainage basin. I then looked at what that area would contribute
in its undeveloped condition. If it were still undeveloped, it would probably contribute about 150
0
Memo to Alan D. Ratliff, City Manger
July 10, 1992
Page 2
CFS to the drainage basin. I then looked at a channel parallel to Moore Road that would
accommodate the increase in the runoff, i.e. about 120 CFS. To construct a channel with the
typical 3:1 side slopes would necessitate cutting a channel that ranges from 3.2 feet deep to 4.4
feet deep. The slope on the channel would be flat until it approached the reinforced concrete
pipe and then it would drop approximately 6 feet into the pipe. The maximum top width of the
channel with 3:1 side slopes would be about 26 foot.
I then looked at a channel section with 2:1 side slopes. The steeper side slopes are not real
maintainable, however, on an interim basis, they might be acceptable so as to not create such a
wide ditch that several large oak trees would have to be eliminated. The ditch section would
range from 3.1 feet deep to 4.8 feet deep. The maximum top width of the channel would be
about 19 feet. It also would drop approximately 6 foot into the reinforced concrete pipe.
The construction items associated with the interium solution would necessitate the excavation of
the channel, removal and replacement of the concrete paving across Moore Road to install the
concrete pipe and approximately 250 linear foot of 66" RCP coming out of the concrete channel
in a southeasterly direction across Moore Road to the location of where the earthen channel
would begin. The estimated cost of this solution would be approximately $70,000.00. This
solution does not include the cost for right of way. Obviously, the City cannot expend funds on
private property. If this solution is considered for further evaluation, my recommendation would
be that we attempt to get Ms. Austin to dedicate all necessary right of way along Moore Road
prior to the construction of any drainage improvements in that area.
There is also a structure in the general area of where the future street would be and where the
proposal channel would be. This structure would have to be eliminated. I do not know if it is
currently being used. If it is, then that could pose a problem.
Attached to this is a sketch showing the proposed location and alternatives discussed.
I will be available to answer any questions at the council meeting.
KMG/bd
AUSTIN
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