North Lake Dam -- Slope Flattening vs IrrigationIn response to our discussion regarding the maintenance of the
downstream slope of the main dam, John and I have done some analysis to
estimate the cost of flattening the downstream slope in order to abandon
the existing irrigation system.
To achieve a downstream cut/fill balance and a stable slope of 5:1, the
Main Dam would need to be lowered to elevation 509 ft-msl, approximately
6 feet from the existing crest. This would require removal and
demolition of the crest road and wave wall (similar to what we had
planned for originally). We would also include replacement of the two
crest piezometers (also planned for originally) and three toe
piezometers, as well as the extension of the drain pipes for the sand
blanket drain.
All of this comes to a total estimated cost of about $573,000. It
should be noted that the number for the extension of the blanket drains
is a very rough estimate and would need to be further refined if this is
the avenue the City would like to pursue. See the table below for a more
detailed estimate:
Item
Quantity
Units
Unit Cost
Item Cost
Dirt Work - Cut/Fill
38,800
CY
$6
$232,800
Demolition - Crest Road
28,000
SF
$5
$140,000
Demolition - Wave Wall
2,000
LF
$50
$100,000
Replace Piezometers
5
EA
$10,000
$50,000
Extend Blanket Drains
1
LS
$50,000
$50,000
Total Cost
$572,800
Again, this option would flatten the downstream slope of the Main Dam
to 5:1 and lower the effective crest to 509 ft-msl, which would allow
for the existing irrigation system to be abandoned and removed. The
other alternative is to leave the Main Dam at its current slope and
crest elevation (leaving the road and wave wall in place) and
modify/replace the irrigation system.
Please let us know how you’d like to move forward.
Patrick Miles, P.E.
Water Resources
Freese and Nichols, Inc.
4055 International Plaza, Suite 200
Fort Worth, Texas 76109
817-735-7236
817-735-7483 fax
www.freese.com
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