ST9401-CS 950412MEMORANDUM
TO: Ken Griffin
FROM: Ulys Lane
DATE: 4-12-95
SUBJECT: DENTON TAP ROAD
W&A NO.: 94093
In our meeting on Tuesday at the Texas Department of
Transportation we discussed our firm preparing a watershed map
which defined the drainage area generally west of Denton Tap
Road. We discussed defining the watershed limits to agree with
physical conditions and to agree with the drainage improvements
planned by the city of Lewisville for Denton Tap Road north of
the proposed Spur. This watershed was anticipated to be smaller
than the watershed currently planned by the state for design of
the proposed 3-6'X 5' culvert. Also, Truman Edminster offered to
contact his client to obtain authorization to evaluate directing
part of the watershed west of Denton Tap Road to the existing 6'X
6' culvert located in the Spur just east of Denton Tap Road.
We have revised the watershed limits to agree with the limits
discussed in the meeting and a copy of the watershed map is
enclosed. Watershed "B" is the watershed anticipated to be
directed to the proposed 3-6'X 5' culvert. Watershed "A" is the
watershed planned to be directed to the east to the existing 6'X
6' culvert. The attached watershed calculations indicate the 100
year storm runoff amounts based on the city's criteria. We
estimate the available hydraulic grade from Denton Creek to Spur
553 is approximately 0.16% assuming an enclosed system is laid
along the roadway from the Spur to Denton creek. Any bends in
the culvert will reduce the available H.G.L. and we suggest
curves in lieu of bends. We anticipate the enclosed drainage
facility required would be a double 6'X 5' box section based on a
Mannings roughness coeifficent of 0.12. We indicated the
approximate location of the double 6'X 5' box section alignment
on the attached drainage area map. The double box section can be
reduced from 5' high to 4' high if approximately 92 cfs of runoff
can be diverted to the existing culvert to the east. There is
limited head room available in Denton Tap Road and low depth
culverts are required. Reducing the culvert height from 5' to 4'
will provide more clearance over the culvert and reduce costs for
crossings.
If the culvert alignment is directed to Denton Creek through
the Vista Ridge property along the existing ditch location, the
length of culvert can be reduced 150' and a more efficient
culvert section can be used. The shorter length will increase
the available hydraulic grade line to 0.20%. The most cost
effective option would be to direct the culvert along the
existing ditch section either by a permanent improvement or a
temporary ditch. The state previously offered to extend the
proposed culvert to the exiting north most cross culvert in
Denton Tap Road which is approximately 1100' north of Denton
Creek. Listed below is rough cost estimates for various culvert
sizes. Alignment A is along Denton Tap Road to Denton Creek and
Alignment B is along the existing ditch in Vista Ridge's
property.
culvert size
Alignment "A"
Alignment "B"
3-6' X 5' $606,000 $526,000
2-6' X 5' $430,500 $374,000
2-6' X 4' $373,500 $325,000
Our calculations indicate the culvert size can be reduced by one
barrel size throughout the entire length up to the north end of
the proposed Spur. Perhalps the state can credit some of the
reduced culvert size costs to assist in improvements to the
downstream culvert. Perhalps the adjacent property owners can
assist in the costs in some mannor which reduces the cost to the
city similiar to cross drainage improvements.
We have also included copies of the preliminary plan and
profile sheets for proposed Denton Tap Road to provide an idea of
the.possible road alignment. These plans are very preliminary
and have not been reviewed by the city as yet. Please contact me
if you need any additional information. Copies of this
memorandum and the preliminary plans described have been
forwarded to the parties noted below.
Copy to:
End of Memo
Kenneth Jo Shirley
Truman Edminster
Bill Anderson