DR1101-CS130501 (2) FREESE wg�as, ri
4055 International Plaza,Suite 200 • Fort Worth,Texas 76109 • 817-735-7300 • fax 817-735-7491 • www.freese.com
May 1, 2013
Mr. Kyle Hodges, P.E.
Texas Commission on Environmental Quality
TCEQ Region 4-Dallas/Fort Worth
2309 Gravel Drive
Fort Worth, Texas 76118-6951
Re: Proposed Modifications to North Lake Dam
Dam#TX00832
Dear Mr. Hodges:
Freese and Nichols, Inc. received a letter from TCEQ dated March 11, 2013, requesting additional
information regarding the proposed modifications at North Lake Dam. The following paragraphs form
our response to each of these comments, and the updated Construction Plans and Specifications are
included as attachments to this letter, along with the Design Report, which includes the Geotechnical
Investigation Report.
1. Design Report
a. Spillway Trash Rack—The discharge rating curve was recalculated taking into consideration
a reduction in the weir length of the spillway crest of up to 18%. While the discharge rating
curve was altered slightly by these considerations, the peak water surface elevation for the
100-year event and critical PMF increased by 0.05 feet and 0.02 feet, respectively. This
increase has no impact on design criteria for the dam and adjacent development, so no
changes to the spillway weir or trash rack are planned. However, the discharge rating curve
and hydrologic model results were updated in the Design Report.
b. Roadway Access —While the existing asphalt roadway will be removed under the proposed
modifications, no public access is allowed currently or after modifications. It is, and will
remain, within a fenced controlled access area. Explanation to this effect will be provided in
the Design Report.
c. Embedment Material—Granular embedment will not be used for the box conduit.The plans
are correct as shown with "Compacted Fill" consisting of on-site clay or select fill as
embedment material. The geotechnical report will be revised to reflect the proper
embedment material. Also,see response to Item le.
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May 1, 2013
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d. Shale Backfill — Although the Geotechnical Report does not prohibit the use of shale as
trench backfill, the design team chose to exclude its use due to the potential for differential
settlement and the difficulty in processing the material. Clarification will be provided where
appropriate.
e. Conduit Drainage Provisions — Due to low head conditions, high plasticity clays, and a long
seepage path, the design team has not included any provisions for drainage (i.e., finger
drains or gravel collectors) along the proposed conduit.
2. Construction Plans
a. Sheet C-10—The entire conduit is intended to be cast-in-place. Clarification will be made to
this effect on the appropriate details.
b. Low-Flow Valve—A low-flow valve is not deemed necessary for the proposed modifications,
as such a valve would only allow for lowering the lake an additional five feet. Should these
five feet be necessary in an emergency situation, lowering the lake may be accomplished by
pumping over the spillway crest or partial removal of the concrete weir wall.
c. Included Note — The following note will be included where appropriate: "Plans and
specifications shall not be substantially or materially altered without prior written approval
of the TCEQ's Executive Director."
3. Hydrologic and Hydraulic Analyses—FNI will respond to these comments upon receipt from TCEQ.
4. Past Issues/Deficiencies/Recommendations
a. History of Embankment Slides — Under the proposed modifications, significant portions of
the dam will be removed in the vicinity of the spillway area and portions of the crest may be
lowered as borrow material for backfill, reducing the loading on the embankment and its
slopes. The future owner of the dam, the City of Coppell, is aware of these issues and
further communication will be made regarding the necessary maintenance and dam safety
concerns. No additional survey of the control monuments has yet been performed, pending
the ownership transition. However, a plan for periodic surveying of the crest will be included
in the Operation and Maintenance Plan. It is possible that the embankment will also be used
as a borrow source in the future for other projects. The Operation & Maintenance Plan will
include proper restrictions on any future borrowing. The O&M Plan will be compiled near
the end of construction, and a copy of this document will be provided to TCEQ if so desired.
The potential for future borrowing precludes the use of permanent monuments. Also,
settlement is not expected to be a significant issue after construction of the proposed
modifications, limiting the useful information available from permanent monuments.
b. Abandoned Piezometer — Piezometer P-105 has been effectively replaced by nearby
piezometer P-105A, but it has not yet been plugged and abandoned. Piezometers P-105 and
P-105A, along with piezometer P-102, which is also located on the crest of the dam, will
each be plugged and abandoned prior to the lowering of the embankment crest.
Piezometers P-101, P-103, and P-104 are located at the downstream toe of the dam and will
not be removed or affected during construction. Two new piezometers will be installed on
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May 1, 2013
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the crest of the dam after the embankment has been lowered to serve as replacements for
piezometers P-102 and P-105A. A new instrumentation sheet is provided for the updated
construction plans, as well as appropriate specifications. No new action levels have yet been
determined for the lower lake level. These levels will be determined in conjunction with the
development of the O&M Plan near the end of construction, as mentioned previously. A
copy of the O&M Plan will be provided to TCEQ, if so desired.
Please feel free to contact me with any questions or concerns you may have at (817) 735-7284, or via
email at jlr @freese.com.
Sincerely,
fry
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John L. Rutledge, P.E.
Water Resources Design
cc: Ken Griffin, City of Coppell
Attachments:
Design Report for North Lake Dam Improvements
Construction Plans for North Lake Dam Improvements
Construction Specifications and Contract Documents for Modifications to North Lake Dam and Spillway
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