ST9401WA-CS 951106WIER & ASSOCIATES, INC.
November 6, 1995
Mr. Ken Griffin, P.E.
Assistant City ManagedCity Engineer
City of Coppell
255 Parkway Boulevard
P.O. Box 478
Coppell, TX 75019
214/304-3683 · FAX 214-304-3673
Re:
W&A #94-093; Denton Tapp
Road Reconstruction;
Coppell, Texas
Dear Ken:
Several weeks ago we met regarding the city review comments on the
Conceptual Plans for proposed Denton Tap Road. At that meeting we learned
that the city council had decided to delay construction of the roadway project but
would proceed with the 16" water line construction ahead of the bridge
construction. The original design concept would provide placement of the 16"
water line on the proposed bridge crossing. An alternate method of installing the
water line across the creek would need to be established. Also, several
alterations were discussed in the bridge crossing design to accommodate
expanded hike and bike trail capabilities. We were asked to prepare revised
conceptual designs for the waterline crossing ahead of bridge construction and
revised bridge details for the hike and bike trail additions. This letter report and
accompanying revised conceptual plans include our submittal for the water line
and bridge changes. The conceptual plans attached are discussed in the
following:
I. 16" WATER LINE CROSSING AT DENTON CREEK:
We have evaluated two options including a below grade crossing at the
creek and an elevated crossing. The feasibility of a below grade crossing is
discussed first below:
A. BELOW GRADE CROSSING: The water line crossing options at the
creek are impacted by the flow conditions established by water released from
Mr. Ken Griffin, P.E.
W&A #94-093
November 6, 1995
Page 2 of 6
Lake Grapevine. Currently flow is released from the lake continuously even in
dry periods. We do not believe a below grade crossing should be attempted
unless flow can be stopped or reduced to a very low flow during construction.
Construction options with flow in the creek would probably require an individual
Section 404 permit to dam up one side of the creek and lay pipe from side to
side. This construction procedure would be difficult. The flow releases from the
lake when above normal pool are regulated by the Corp. of Engineers Lake
Control Division out of the Forth Worth District Office. The flood flow releases
from area lakes are monitored at the Elm Street Bridge Crossing in downtown
Dallas. Once the lake is at or below normal pool level the flow releases are
considered to be low flow conditions and the volume is set by the City of Dallas
and Lake Cities Municipal Utility District downstream to meet raw water usage
demands. Flood flows are always higher than low flows. Low flows from Lake
Grapevine and Lake Lewisville are combined to provide the required low flow
volume to the cities downstream. Low flows from Lake Grapevine can be as low
as 14cfs up to about 70cfs with typical low flows at 40cfs. Low flow periods
generally occur in late summer and mid to late winter.
We contacted the Corp of Engineers Lake Control Division by telephone
and obtain a summary of flows for the past two years which is attached to this
letter report. I discussed with the Corp of Engineers staff the feasibility of
transferring the low flow requirement from Lake Grapevine to Lake Lewisville for
a short period of time to allow downstream construction in the creek. We were
advised that the City of Dallas could be requested to reduce low flows down to
as low as zero flow. The Corp staff indicated this has happened before for short
periods for construction on the creek. We contacted John Wilson of the City of
Dallas Water Utilities Department to confirm the procedure for requesting zero
low flows. Official approval will be required but the City of Dallas will probably
agree to a zero flow condition. We contacted Mr. Bill White of Park Cities
Municipal Utility District regarding permission for zero flow. Mr. White indicated
Park Cities would be agreeable but would also need an official written request.
If zero low flow can be obtained from the city of Dallas and Park Cities for
a week, we believe a crossing can be installed below the creek. The benefits of
a low water crossing constructed with no flow are as follows:
The crossing will be less costly.
The trench will be located upstream of the proposed bridge
crossing and if backfilled with lean concrete, it can assist to protect
the upstream bridge embankments from scour.
The negative aspects of a below grade crossing are as follows:
Mr. Ken Griffin, P.E.
W&A #94-093
November 6, 1995
Page 3 of 6
It will be necessary to coordinate the construction along with Lake
Grapevine seasonal low flow conditions.
It will be necessary to obtain a zero low flow permit from the City of
Dallas, Lake Cities and the Corps of Engineers.
The pipe will not be accessible for maintenance or for inspection.
Other public utilities could utilize an above grade elevated
crossing.
We believe an elevated crossing is more desirable since weather
conditions and approvals from other agencies will not be required and the pipe
will be accessible for maintenance.
B. ELEVATED CROSSING: We evaluated the design concept for an
elevated crossing and promptly determined that a preliminary design of the
proposed bridge crossings would be necessary in order to locate the elevated
bridge crossing horizontally, vertically, establish toe abutment location, and to
locate the piers. The elevated crossing must fit into the future phased bridge
construction process and the piers should be in line with the proposed bridge
piers. Piers out of line with the future bridge would increase the probability of
hang-up of trees and large debris unless the elevated crossing is located away
from the proposed bridge. The attached typical bridge cross section and bridge
plan indicates the location we recommend for an elevated pipe crossing. The
crossing would be west and slightly upstream of the existing bridge. In the future
the pipe crossing would be between the separated bridges for the main lanes of
the divided roadway. In this location, the pipe crossing would be hidden from
view for most vehicles by the future bridge railing. Vehicles with high seats such
as trucks and vans could see the pipe crossing. The pipe crossing could be
used for other public utilities. If an elevated crossing is installed, we would
suggest requiring GTE transfer the existing five 4" fiberglass conduits on the
existing bridge to the pipe crossing ahead of the future roadway construction.
Other public utilities could attach to the elevated crossing clearing their
underground facilities ahead of the future roadway construction.
The elevated crossing would be accomplished by two wide flange beams
supporting the pipe for the long spans with cross angels for lateral support. We
expect the elevated crossing to cost approximately $75,000.
It is our recommendation to construct the elevated pipe crossing which
will require preliminary designing the bridge to confirm the pier locations,
horizontal pipe crossing location abutment location and column cap elevation.
We have prepared the attached preliminary elevations and plans for the bridge
to reflect our preliminary bridge design efforts to date. The bridge design is
discussed in more detail later on in this lette~report.
Mr. Ken Griffin, P.E.
W&A #94-093
November 6, 1995
Page 4 of 6
II. HIKE AND BIKE TRAIL MODIFICATIONS:
In our last meeting you advised us the city would require 18' wide hike
and bike tails under the bridge on both sides and a crossing over the east future
bridge. We contacted Michael Cart regarding the bridge sections he prefers and
incorporated his recommendations into the bridge sections attached. We can
discuss any alterations you prefer to Mr. Carr's recommendations in a future
meeting.
III. PROPOSED BRIDGE CROSSING PRELIMINARY DESIGN:
We performed a preliminary hydraulic study on the proposed bridge
section after completion of the conceptual plans. In our last meeting we advised
you that the bridge width proposed in the conceptual plans (which matches the
existing bridge opening) would result in high velocities exceeding 12 feet per
second for the fully developed 100 year flood event. We believe the bridge
section area should be increased to reduce velocities at the bridge location.
The anticipated scour depths for channel contraction, piers and abutments are
increased proportional to the velocity. The hike and bike trail modifications will
increase the bridge section and reduce velocities somewhat. The attached plan
indicates the bridge section we recommend. We have performed considerable
effort and research into evaluation methods for protection of the bridge from
scour. Widening the bridge section will decrease velocities and reduce potential
scour depths.
The borings at the site performed by EmCON after completion of the
conceptual plans are indicated on the attached bridge elevations. The borings
at the bridge location indicate the materials along the creek bank are soft
relatively erosive sandy clays and sand and the depth to hard shale for
foundations is relatively deep. We have estimated the anticipated maximum
scour depths at the pier locations and conclude that the scour must be mitigated
by structural protection. The abutments outside of the piers will need to be lined
and the inside of the piers protected from scour.
Another consideration in the design materials utilized is the presence of
ground water roughly below elevation 449. We have discussed the specific
design options utilizing Gabion baskets with Craig Olden of the Olden Company
and researched other options. The preliminary design concept we propose
would include concrete riprap on the hike and bike trail and the abutment above
the trail. Below the hike and bike trail to the Column locations 12" to 18" thick
gabions or cable concrete are proposed to allow the water table to freely drain
while protecting the slope. At the Column locations we propose a concrete
Mr. Ken Griffin, P.E.
W&A #94-093
November 6, 1995
Page 5 of 6
header at grade. Inside the columns along the main channel a means will be
needed to protect the columns from scour. The constant flow in the creek
channel along with the soft bank materials and ground water table will make
construction of conventional measures difficult to install. We indicate sections
for installation of gabion baskets cable supported concrete riprap and steel
sheet piles. We estimate the costs for the three options to be as follows:
Gabion Baskets - $91,000
Cable concrete - $75,000
Sheet piles with weathering steel - $102,000
Sheet piles without weathering steel - $75,000
Corrosion resistant steel and non-corrosion resistant steel costs are
provided above. Attached is information regarding the corrosion resistance of
steel sheets. This literature indicates that sheet piles driven in undisturbed soil
and in fresh water conditions provide a long service life. Corrosion resistant
metals can be used for additional safety. The sheet pile option we believe is the
best option for the reasons listed below:
3.
4.
5.
The work will not require major excavation of the channel bank
which may require an individual Section 404 permit and permit
preparation costs.
Coordination with various cities to control lake low water flow will
not be required.
The deepest protection for maximum potential scour depth will be
provided.
As scour occurs, the channel section will enlarge to the greatest
area reducing velocities which will reduce potential scour depth.
The bank edge will need to be removed during construction and
replaced. The soft bank materials will encourage erosion once
replaced.
Sheet piles are more structurally stable than gabions or cable
concrete. The wire mesh on gabion baskets will deteriorate and
rock will eventually be lost. Cable concrete can float if edges
erode.
A Section 404 General Permit is in place for Roadway Crossings (No. 14
Roadway Crossing) which allows up to 0.30 acres of impact on 404 jurisdictional
property. We anticipate the sheet pile option will meet General Permit
requirements but the other options may impact areas exceeding 0.30 acres.
Considering the cost and construction aspects we believe the sheet pile
method to be the best option.
Mr. Ken Griffin, P.E.
W&A #94-093
November 6, 1995
Page 6 of 6
We have prepared several exhibits attached to demonstrate the items
discussed in this letter report. We hope to meet with you soon to discuss these
items in more detail.
Very truly yours,
ULYS~N~E III, P.E.,'R.P.L.S.
UTL:ga
C:
Rick Wieland, w/plan enclosures
Howard Pafford, w/plan enclosures
Michael Carr, w/plan enclosures
Attachments:
Conceptual Plan
Summary of Flows
Typical Bridge Cross Section
SUMMARY OF FLOWS
LAKE GRAPEVINE OUTLET SPILLWAY
Month
Sep-95
Aug-95
Jul-95
Jun-95
May-95
Apr-95
Mar-95
Feb-95
Jan-95
Dec-94
Nov-94
Oct-94
Sep-94
Aug-94
Jul-94
Jun-94
May-94
Apr-94
Mar-94
Feb-94
Jan-94
Dec-93
Nov-93
Oct-93
Sep-93
Aug-93
Jul-93
Jun-93
May-93
Apr-93
Mar-93
Feb-93
Comments
34 cfs avg. ~ 610 cfs one day
20 to 30 cfs
60 to 70 cfs
Flood releases up to 500 cfs
15 cfs
13 to 17 cfs
200 to 300 cfs
Flood Flow
Flood Flow
Flood - some days at 65 cfs
Flood water most of month - 70 cfs some days
65 cfs - some Flood Flow
Flood Flow - 65 cfs latter part of month
Flood Release
40 cfs
Flood Flow
Flood Flow
Flood Flow
Flood Flow
44 cfs two days - rest Flood Flow
44 cfs
60 cfs to 44 cfs
60 cfs
40 to 73 cfs
Flood Flow
Flood Flow
Flood Flow - one day 36 cfs
Flood Flow
Flood Flow
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