PA1301-CS140605;;; HALFF
June 5, 2014
AVO 28540 X002 TA02
Board of Directors
Denton County Levee Improvement District No
C/O Julianne Kugle
1980 Post Oak Blvd., Suite 1380
Houston, Texas 77056
Re: Review of Floodplain Study Submittal by Teague Nall & Perkins — Conditional Letter
of Map Revision (CLOMR) for Denton Creek Pedestrian Bridge
Dear Board of Directors,
We have completed an initial review of the Conditional Letter of Map Revision (CLOMR) for
Denton Creek Pedestrian Bridge prepared by Teague Nall & Perkins, dated April 30, 2014, and
received on May 2, 2014. This general review consists of checking the submittal package for
potential impacts to the levee and channel improvement section of Denton Creek operated and
maintained by the Denton County Levee Improvement District No. 1 (DCLID-1), and checking
the submittal package for completeness and compliance with the City of Coppell floodplain
ordinances, regulations, and policies.
The following comments are a result of this review:
DCLID-1
For operation and maintenance purposes, DCLID-1 respectfully request concrete riprap
be used instead of gabion baskets around the abutments of the proposed pedestrian
bridge.
2. Additionally, for operation and maintenance purposes, DCLID-1 respectfully request that
concrete riprap, especially on the levee side of the channel, be placed from the abutment
of the proposed pedestrian bridge to the toe of the levee slope.
3. Please see that attached letter from Terracon pertaining to comments on the Geotechnical
Engineering Study report prepared by CMJ Engineering, Inc. dated May 1, 2014.
City of Coppell Criteria
Full compliance with the City of Coppell's floodplain development criteria requires
information to be provided that demonstrates impacts of the proposed development on
floodplains, floodways, flood elevations and velocities for both the 2 -year and 100 -year
HALFF ASSOCIATES, INC.
1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM
RICHARDSON, TX 75081-2275 FAX (214) 739-0095
;�; HALFF
fully developed storm event. While impacts will be relatively small (if any), please
provide tables which reflect the hydraulic impacts caused by the proposed bridge crossing
on the 2 -year fully developed flood event.
2. Please provide a workmap to the correct scale. The scale bar shown on the map (Exhibit
B.2-1) does not equal "1 inch = 300 feet". Revise and resubmit.
3. Please verify the reach lengths at cross-section 24099 in the proposed conditions model.
Or explain why the reach length differs from that shown in the revised existing conditions
model.
4. Please verify the `n' values used and `n' break locations at cross-section 23906. Based
on current operations and maintenance, the 0.03 `n' value is reasonable on the levee slope
only. Moving the break location from the toe of the levee to the right side of Denton
Creels does not seem reasonable. Furthermore, the clearing of trees and brush for the
proposed pedestrian crossing does justify the lowering of the `n' value in this area;
however, it is viewed as only a short term condition unless routine mowing of the area by
either the City of Coppell or DCLID-1 occurs.
5. Please verify the top of rail elevations used in the model. The upstream side elevations
differ slightly from the downstream side elevations.
6. The report states that calculations at the proposed bridge crossing uses the energy
equation for low flows and the weir equation for high flows, however, in the Bridge
Modeling Approach Editor of HEC -RAS the "Energy" equation is selected for both low
and high flows. Please revise and resubmit.
7. The bridge geometry in the model does not match that shown in Exhibit 2.1. The low
cord at the left abutment is approximately 0.4 ft higher than that shown in the exhibit and
the low cord is approximately 1 foot lower in the model than the exhibit near the apex of
the bridge. Please revise and resubmit.
FEMA Comments
1. Please provide electronic files for all models for Denton Creek. The effective HEC -2
models (multi -profile and floodway) were not provided.
2. Please provide the effective HEC -2 model output with this CLOMR submittal.
3. A table comparing water surface elevations generated by the effective HEC -2 model to
that of the imported effective HEC -2 model into HEC -RAS needs to be included with this
CLOMR. Any water surface elevation discrepancies greater than 0.5 feet between HEC -
2 results and the HEC -RAS import of the HEC -2 model should be investigated and an
explanation for the discrepancies provided.
;�; HALFF
4. Please verify the water surface elevations for the duplicate effective model shown in
Table 2 of the report. Elevation shown at cross-section 32200 does not match model
output. Please revise and resubmit.
5. Please check the discharges used in the floodway models for revised existing and
proposed conditions. Or provide an explanation on why the 100 -year discharges used
differ from those used in the multi -profile model. The report should also include the
output from these floodway models.
6. The 100 -year water surface elevations generated by the floodway models should replicate
the water surface elevations which are generated by the multi -profile model. Please
provide an explanation for the discrepancy or revise the floodway model and resubmit.
7. Please provide an explanation on why the `n' values at cross-section 23906 were not
modified in the FEMA proposed conditions model similar to that of the fully developed
proposed conditions model. Or revise and resubmit.
8. Please provide a workmap to the correct scale. The scale bar shown on the map (Exhibit
1) does not equal "l inch = 300 feet". Revise and resubmit.
9. Please verify the location of the right bank station at cross-sections 24585, 24620, 24660,
and 24700 of the proposed conditions FEMA models. The cross-sectional geometry of
these cross-sections do not differ much from the cross-sectional geometry shown in the
fully developed model, however the location of the right bank stations differ between the
models. Revise and resubmit.
10. Please provide an explanation on why the proposed pedestrian bridge is not reflected in
the proposed conditions floodway model. Revise and resubmit.
11. Please provide an explanation into why channel `n' values differ in the floodway model
when compared to the multi -profile model in the area of the proposed bridge crossing.
12. In the floodway models (revised existing and proposed), the 100 -year encroached profile
should start with a water surface elevation 1 foot higher than the 100 -year un -encroached
profile to account for the maximum potential floodway encroachment downstream of the
project. Please revise the floodway models and resubmit.
This review was performed for the benefit of the Denton County Levee Improvement District
No. 1 and the City of Coppell. This is not a detailed engineering review of any construction
plans, and Halff Associates, Inc. assumes no liability for the adequacy of the planning, design or
construction of the project.
;pp HALFF
Do not hesitate to call me at 214-346-6245 if you have further questions or comments
concerning this review. Please include the reference "AVO 28540" on all correspondence.
Sincerely,
HALFF ASSOCIATES, INC. 0
Russell P. Erskine, P.E., CFM
Project Manager
Attachments (1)
cc: Kyle J. Dykes, P.E.
Teague Nall & Perkins
Mike Garza, P.E.
City of Coppell
John Elias
City of Coppell
June 2, 2014
Halff Associates, Inc.
1201 North Bowser Road
Richardson, TX 75051-2275
Attn: Mr. Russell Erskine, P.E.
RErskineCa�Halff.com
RE: Geotechnical Review Services
Denton County Levee Improvement District No. 1
Trail Crossing at Denton Creek
Coppell, Texas
Terracon Project No. 94145137
Dear Mr. Erskine:
Irerracon
Terracon Consultants, Inc. (Terracon) has completed a review of the following report:
"Geotechnical Engineering Study
Pedestrian Bridge
Andy Brown Park
Coppell, Texas"
May 1, 2014
Prepared by CMJ Engineering, Inc. (CMJ)
CMJ Project No. 131-14-132
Our review was accomplished in general accordance with our proposal number 94140620
provided to Haiff Associates, Inc. on May, 14, 2014 and authorized by signature on May 19, 2014.
This letter summarizes our comments following review of the report. The sole intent for the review
of the CMJ report is to determine if there is a long term impact on an existing levee that may be
caused by the proposed construction of the pedestrian bridge.
PROJECT INFORMATION
We understand that the project consists of design and construction of a pedestrian bridge that
will span Denton Creek along the northeast side of Andy Brown Park, located in Coppell, Texas.
A relatively small earthen levee protects a neighborhood immediately northwest of Denton
Creek, which is owned and operated by Denton County Levee Improvement District No. 1. We
understand that the levee currently provides protection from a 500 -Year storm event with at
least 3 -feet of freeboard.
Terracon Consultants, Inc. 8901Carpenter Freeway, Suite 100 Dallas, Texas 75247
P [214] 630 1010 F [214] 630 7070 terracon.com
Geotechnical Review services Irerracon
Trail Crossing at Denton Creek Coppell, Texas
June 2, 2014 Terracon Project No. 94145137
GEOTECHNICAL REPORT FINDINGS
A total of two (2) borings were drilled, one at each proposed bridge abutment at the site. The
proposed abutments are located along the northwest side of Denton Creek on flop of the levee
and one at the southeast side of Denton Creek. Boring B-1 was drilled at the southeast side to
a depth of approximately 60 -feet while Boring B-2 was drilled at the northwest side to a depth of
45 -feet below existing grade.
The borings encountered varying material ranging from fat clays to sandy lean clays followed by
clayey sands. Clay shale was encountered in Boring B-1 at a depth of approximately 59.5 -feet
below grade. Boring B-2 did not encounter clay shale. Groundwater was encountered during
the field exploration in both borings at a depth of 17 to 24 feet and at 12 to 23 feet during post
drilling measurements.
The following recommendations were provided in the CMJ report:
Deep foundation with either underreamed or straight shaft drilled piers to support the
abutments;
Underreamed Pier Allowable End Bearing Capacity of 4,000 psf at a depth of 15 feet
below grade;
Straight Shaft Pier Allowable End Bearing Capacity is neglected and 500 psf for
compressive skin friction with a required penetration of 15 feet below grade;
Underreamed Pier bearing stratum to be above the groundwater level;
Bearing stratum ranging from clay to silty clay for an underreamed pier and sandy clay,
silty clay or clayey sand for a straight shaft pier;
Soil uplift pressure of 1,200 psf over the upper 12 feet of the pier shaft;
Underwater concrete placement for the straight shaft pier; and
General recommendations concerning site preparation, excavation conditions, wall
backfill, and abutment drainage and erosion protection.
REVIEW COMMENTS
General Comments
The pier recommendations in the CMJ report appear reasonable for the conditions
encountered in the borings;
Below ground work shall not be performed during any flood events (higher than normal
water flows).
CMJ Report Comments
Review comments relating specifically to the CMJ report are presented below:
Responsive ■ Resourceful ■ Reliable 2
Geotechnical Review Services
Trail Crossing at Denton Creek Coppell, Texas
June 2, 2014 Terracon Project No. 94'145137
Irerracon
Page 5 - Caution should be applied if underreamed piers are to be used. Since the site
is located within Denton Creek, the water levels encountered during construction could
differ from the water levels encountered during the geotechnical borings which may
require a change in the planned depth of the pier. Close monitoring of the groundwater
levels during the pier drilling will be required;
Pages 6 to 8 — Caution should be applied for all excavations located within 50 feet of the
levee template. The potential bearing stratum of straight shaft piers is defined on Page
6 to range from clay to sand material. If sandy materia( is encountered for the bearing
stratum, close monitoring the condition of the bearing stratum will be required in order to
provide swift action to prevent a quick condition from occurring.
t LOSURi
We appreciate the opportunity to be of service to you on this project. Please contact us if you have
any questions concerning this letter report, or if we may be of further service.
Sincerely,
Terracon Consultants, Inc.
Saad M. Hineidi, PE
Senior Principal
ResponsiveL Resobm,co ul RGIiabkc
S. Brent McNerne, P.E.
Geotechnical Engineer