ST9304-CS 860919ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS AND SCIENTISTS
ALBERT H. HALFF, CHAIRMAN OF BOARD
JOSE L NOVOA, PRESIDENT
RAUL WONG, JR.. SENIOR VICE PRESIDENT
DON O. BROCK, VICE PRESIDENT
MICHAEL K. KOESLING, VICE PRESIDENT
TROY LYNN LOVELL, VICE PRESIDENT
LOUIS M. SALCEDO, TREASURER
WALTER E. SKIPWITH, SECRETARY
GEORGE PRALL, JR.
JEFFREY H. JACOB
DAVID S MORGAN
September 19, 1986
AVO 8173
DALLAS * FT. WORTH
8616 NORTHi2VEST PLAZA DRIVE
DALLAS, TEXAS 75225
214~3~0094
Dallas County
Public Works
161 Commerce Street
Dallas, Texas 75207
Att: Mr. Allen Bud Beene
SEP 19 ;986
3,0UNTY DIRECTOR OF. PUBUC WORKS
Re: Belt Line Road Project 490-402
Dear Mr. Beene:
As requested, we have reviewed the report on the Elm Fork of the
Trinity River at Belt Line Road which was submitted to Dallas County by
Nathan Maier Engineers. The Nathan Maier report states that the Belt Line
Road bridges could be reduced to 300, 260 and 300 feet for the west overflow
channel, main channel, and old Elm Fork channel, respectively. Albert H.
Halff Associates recommends bridge lengths of 320, 280, and _400 feet for
these same bridges. A review of Nathan Maier's report and a discussion of -
the methods and criteria used by Albert H. Halff Associates are included in
this letter.
The hydraulic profiles included in the Nathan Maier Report are
different than those produced by our models. A review of Mr. Maier's report
revealed five reasons for these differences. First, the Nathan Maier models
do not include surveyed sections at Belt Line Road. Field surveys have
found the cross-sectional area of the main channel to be less than what has
been used in the Nathan Maier models. Second, in Nathan Maier's report, the
ending section of the split flow analysis is 1,950 feet upstream of the St.
Louis and Southwestern Railroad (SLSWRR) structures (or 903 feet upstream of
the final split flow section used in the Halff Associates models). In the
Halff Associates study the location for this section was determined using a
1:1 contraction of flow upstream of the railroad bridges. This is the
method which is recommended by the Corps of Engineers. Based on this
criteria, the distance to the final split flow section should be 1,150 feet
upstream of the SLSWRR structures. Third, the method for determining the
flow distribution in the aplit flow analysis is not discussed in the Nathan
Maier report. The flow distribution resulting from the Nathan Maier study
is significantly different from our study. Since the Nathan Maier method is
ALBERT H. HALFF ASSOCIATES, INC.
F. NGINEERS AND SCIENTISTS
Dallas County
September 19, 1986
Page 2
not discussed, we cannot comment on those results. In our study however,
Halff Associates determined the split flow distribution by matching energy
grade lines at the last section of the split flow models. This is the
procedure which is recommended by the Corps of Engineers. The fourth
difference in the profiles results from the flow expansion rate downstream
of the west overflow bridge. Nathan Haler used a 1:1 expansion rate, while
our models use the 4:1 rate as recommended by the Corps of Engineers. The
fifth factor which contributed to differences between the Halff Associates
models and those presented by Mr. Naier is that the Maier study includes
flood plain reclamation projects which have not yet been approved. The
Halff Associates models reflect existing flood plain conditions both
upstream and downstream of the site. In our opinion, these five items must
be addressed before conclusions can be drawn from the Nathan Maier study.
Maintaining non-erosive velocities through each of the Belt Line Road
and SLSWRR bridges is also an important design constraint. The State
Department of Highways and Public Transportation (S.D.H.P.T.) recommends
that channel velocities be limited to a range of 4 to 6 feet per second for
bridge projects, unless the existing channel velocities exceed these values.
In the Halff Associates design, the velocities increase in the west overflow
channel and the old Elm Fork channel, but decrease in the main channel of
the Elm Fork. The velocities through the west overflow bridges increase
from 1.93 to 3.18 feet per second. The velocities through the old Elm Fork
bridges increase from 2.01 to 3.33 feet per second. The velocities through
the Elm Fork main channel decrease from 8.15 to 7.43 feet per second. The
Nathan Maier models indicate that the velocities increase to 7.63 feet per
second and 5.30 feet per second in the west overflow bridge and the old Elm
Fork bridge. The velocity of 7.63 feet per second is higher than that
recommended by the $.D.H.P.T.
In the Nathan Maier report, an increase in water surface elevation of
0.61 feet occurs downstream of the project in the west overflow swale. This
increase is not addressed in the report and no comment is made on how this
affects the adjacent areas. Halff Associates has studied both the water
surface profiles and the channel velocities for the existing condition's
100-year discharge of 44,000 cfs and the fully developed lO0-year discharge
of 55,800 cfs. Our proposed design will result in decreased flood
elevations upstream of the site at every cross section for both flow rates.
Downstream of the project, small increases in the water surface profiles
result due to constriction of the west overflow channel to a 320-foot bridge
opening. The increase varies from 0.03 feet by conventional modeling
techniques to 0.18 feet by split flow analysis. In our opinion, these
increases are minimal. However, these results must be submitted to the
ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS AND SCIENTISTS
Dallas County
September 19, 1986
Page 3
Corps of Engineers, City of Carrollton, Farmers Branch-Carrollton Flood
Control District and the Irving Flood Control District Section III for
review and approval. If the 0.1B-foot increase must be eliminated, the west
overflow bridge length must be increased to approximately 400 feet or
channelization must be performed in the west overflow swale downstream of
the project.
In addition to studying the 100-year discharges, Halff Associates
reviewed the effects of the proposed improvements on the Standard Project
Flood water surface elevations. We have found that the proposed
improvements lower the water surfaces produced by the Standard Project Flood
for both a conventional analysis and a composite split flow profile. We
have attached tables which summarize the water surface elevations and
velocity results from our analysis.
In summary, we recommend minimum bridge lengths of 320 feet for the
west overflow channel, 280 feet for the main channel of the Elm Fork and 400
feet for the old Elm Fork channel. In our opinion, no channelization is
required for the project if these bridge openings are constructed. Some
minor channelization may be required downstream of the project in the west
overflow swale if any of the entities which must review the project require
that the 0.18-foot increase in water surface elevation be eliminated. The
bridge lengths recommended for the main channel and old channel of the Elm
Fork are equal to or slightly larger than the existing railroad bridges
immediately upstream of Belt Line Road. They maintain approximately the
same naturally created cross sectional areas and top widths found in the
channels both upstream and downstream of the project. We do not recommend
disturbing the smooth transition by constricting the flow at Belt Line Road
with shorter bridge openings and then re-expanding the flow to its original
width immediately downstream of the project. This is what would occur if
the bridge openings suggested in the MaSer report were adopted. The west
overflow channel is the only location where we feel that a constriction can
occur without adverse affects. The existing railroad bridge opening is
approximately 1,000 feet long. The developer upstream of the railroad has
encroached on the effective area of this structure. Therefore, an array of
bridge openings were tested for this location. Our studies indicate that a
320-foot bridge is the minimum length of bridge which will prevent
significant increases in the water surface elevations and velocities in the
west overflow channel.
We recommend that Dallas County meet with the U.S. Army Corps of
Engineers, the City of Carrollton. the Farmers Branch-Carrollton Flood
Control District and the Irving Flood Control District Section III as soon
ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS AND SCIENTISTS
Dallas County
September 19, 1986
Page 4
as possible to obtain the appropriate reviews, comments, and approvals of
the project. Please contact us if Dallas County wishes us to arrange these
meetings. As previously discussed with you, we are proceeding with design
based on the bridge lengths recommended by Fmlff Associates.
We hope this letter clarifies our recommendations for bridge lengths
for the Belt Line Road project. If you would like any additional
information concerning this matter, please call.
Sincerely,
ALBERT H. HALFF ASSOCIATES, INC.
Patrick L. Kurtz,
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