DR1101-SY110806 LETTER OF FREESE
TRANSMITTAL ■ Z t
4055 International Plaza, Suite 200 • Fort Worth, Texas 76109 • 817 - 735 -7300 • fax 817- 735 -7491
PROJECT: CPL11192 DATE: August 8, 2011
TO: Mr. Ken Griffin
ADDRESS: City of Coppell
255 East Parkway
Coppell, Texas 75012
PHONE NO.: 972 - 304 -3680
BY: John L. Rutledge, P.E.
CC: Russ Springer, P.E.
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1.1 North Lake Dam Spillway Modification
City of Coppell
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North Lake Dam Spillway Modification
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Prepared by:
FREESE AND NICHOLS, INC.
4055 International Plaza, Suite 200
Fort Worth, Texas 76109
817 - 735 -7300
CPL11192
North Lake Dam Spillway Modification
City of Coppell r� NI CHOL S
TABLE OF CONTENTS
1.0 Introduction 1
1.1 North Lake Dam 1
2.0 Hydrologic Model Development 3
2.1 Hydrologic Parameters 3
2.2 Elevation- Storage Data 6
2.3 Discharge Rating Curves 7
2.4 Frequency Model Results 9
2.5 PMF Model Results 10
3.0 Modification Alternatives 12
3.1 Alternative 1 ..12
3.2 Alternative 2 ..13
3.3 Alternative 3 ..13
3.4 Geotechnical Review 14
4.0 Environmental Permitting Review 17
4.1 Permanently Lowering the Normal Pool Elevation 17
4.2 Activities Within the New Jurisdictional Pool Area 18
4.3 Activities Not Within the New Jurisdictional Pool Area 18
4.4 Mitigation 18
4.5 Currently Functioning Spillway 18
4.6 Conclusions 18
4.7 Recommendations 19
5.0 Summary and Conclusions 20
August 2011 i
North Lake Dam Spillway Modification IIIMIFREESE
City of Coppell NICHOLS
LIST OF TABLES
Table 1— Curve Number Calculation 6
Table 2 — Basin Parameters 6
Table 3 — Elevation- Storage Data 7
Table 4 — Discharge Rating Curve 9
Table 5 — Frequency Precipitation Depths 10
Table 6 — Frequency Model Results 10
Table 7 — HMR -52 Point Rainfall Depths 11
Table 8 — PMF Model Results 11
LIST OF FIGURES
Figure 1— Location Map 2
Figure 2 — Drainage Basin Map 4
Figure 3 — Land Cover Data 5
Figure 4 — Plan View of Alternative 3 15
Figure 5 — Cross Section View of Alternative 3 16
APPENDICES
Appendix A — References
Appendix B — Hydrologic Parameters
Appendix C — Discharge Rating Curve Calculations
Appendix D — Opinion of Probable Construction Costs
August 2011 ii
North Lake Dam Spillway Modification
rm FRE ESE
City of Coppell HI CHOLS
1.0 INTRODUCTION
In March of 2011, Freese and Nichols, Inc., (FNI) was retained by the City of Coppell, Texas, to provide
design alternatives for the lowering of the conservation pool of North Lake, which is owned by the City
of Coppell and located between the cities of Coppell and Irving. The current conservation pool elevation
is 510.0 ft -msl. The City desires to lower this elevation according to one of two proposed plans — the first
to lower the conservation pool by 20 feet to elevation 490.0 ft-msl and the second to lower it by 30 feet
to elevation 480.0 ft-msl. This report summarizes the results of the hydrologic analysis, geotechnical
analysis, and the review of environmental permitting issues, and presents three preliminary design
alternatives.
1.1 NORTH LAKE DAM
North Lake Dam, which forms North Lake, is currently owned and operated by TXU Generation
Company, L.P. The reservoir once served as a cooling pond for a steam electric generating plant.
However, the dam is now owned by the City of Coppell, and proposed development around the lake has
called for the lowering of the conservation pool elevation. North Lake Dam was authorized by the State
of Texas, Permit No. 1864, on April 23, 1957.
North Lake Dam, completed in August 1957, is located on the South Fork of Grapevine Creek in the
Trinity River Basin in northwestern Dallas County. It is situated on the southeast side of the City of
Coppell between Interstate 635 and East Belt Line Road. The general location of the dam and reservoir is
shown in Figure 1.
August 2011 1
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North Lake Dam Spillway Modification
FREESE
City of Coppell IF. NICHOLS
2.0 HYDROLOGIC MODEL DEVELOPMENT
The hydrologic model for North Lake Dam was created in HEC -HMS' and consisted of two total drainage
basins, as shown in Figure 2. The total drainage area modeled is approximately 2.61 square miles, or
1,670 acres. One basin represents the area that drains to the lake, while the other basin represents the
surface of the lake itself. The overall basin was delineated from two -foot contours from the 2001
NCTCOG dataset. Because the conservation pool will be lowered from the existing level, the drainage
basin representing the lake surface was approximated at elevation 490.0 ft-msl from the 2008
volumetric survey of the lake. While the size and shape of the lake surface area may change with re-
grading and development, this basin is assumed to represent an appropriate approximation. The same
basin was applied for the option that lowers the conservation pool elevation to 480.0 ft-msl. A finalized
hydrologic model will be developed in the final design of the spillway modification.
2.1 HYDROLOGIC PARAMETERS
The HEC -HMS model incorporates the NRCS Curve Number and Unit Hydrograph methods for each
basin. In this model, the curve numbers were based on hydrologic soil classifications and land cover. The
soils dataset was obtained from the NRCS Soil Survey Geographic Database (SSURGO), and land use
classification was determined from a combination of the 2005 NCTCOG Land Use Data and the
proposed master plan for future development. Spatial information about the land use classifications is
presented in Figure 3. Table 1 provides the matrix used in determining the curve number for each basin.
All soils in the basin are in Hydrologic Soil Group D. The curve numbers shown in Table 1 represent both
Antecedent Moisture Condition (AMC) II, which is incorporated in the model for frequency storms, such
as the 100 -year event, and AMC III, which is used to simulate a worst -case scenario with the ground full
saturated for the Probable Maximum Flood (PMF) events.
August 2011 3
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North Lake Dam Spillway Modification
City of Coppell
rim NICH
Table 1— Curve Number Calculation
Land Curve Curve
Classification
Use Number Number
Code Description (AMC II) (AMC III)
111 Single Family 87 94
112 Multi - Family 92 96
121 Office 95 98
122 Retail 95 98
123 Institutional 93 97
131 Industrial 93 97
142 Roadway 98 99
171 Parks /Recreation 80 90
300 Vacant 89 95
500 Water 100 100
The only input into HEC -HMS for the NRCS Dimensionless Unit Hydrograph is a lag time, which is
calculated based on basin conditions, such as hydraulic length and average slope, according to the NRCS
TR -55 Method. The lag time for North Lake was computed by FNI during a previous study for the
development of an Emergency Action Plan (EAP). Table 2 provides a summary of the hydrologic
parameters for the basin representing the area outside of the lake surface. Detailed calculations for the
hydrologic parameters are provided in Appendix B. The basin representing the lake surface was given no
runoff loss or hydrograph transformation parameters, so as to reflect the instantaneous nature of the
direct runoff.
Table 2 — Basin Parameters
Area Lag Time Curve Curve
Basin Number Number
(mi ) (min) (AMC II) (AMC III)
Basin 2.02 18.6 90.5 95.6
Lake 0.59 - -- - -- - --
2.2 ELEVATION - STORAGE DATA
Elevation- storage data for the reservoir was approximated with the one -foot contours generated from
the 2008 volumetric survey to calculate the available storage in the reservoir. The elevation- storage
relationship was used in the hydrologic model for routing both the frequency storm events and the PMF
and is shown in Table 3.
August 2011 6
North Lake Dam Spillway Modification
City of Coppell
IF. 2 111ICHOLS
Table 3 — Elevation - Storage Data
Elevation Storage
(ft -msl) (acre -ft)
450 0
460 95
470 520
480 1,937
482 2,391
484 2,908
486 3,491
488 4,142
490 4,862
492 5,656
494 6,525
496 7,477
498 8,511
500 9,631
502 10,842
504 12,148
506 13,555
508 15,091
510 16,718
512 18,386
514 20,109
516 21,895
2.3 DISCHARGE RATING CURVES
North Lake Dam currently has a single spillway structure located in the right abutment of the
embankment on the northeast corner of the dam. Information regarding the dimensions and elevations
of the spillway was taken from original construction drawings. The existing spillway has a crest elevation
of 510 ft -msl with a total length of 200 feet. The spillway approach is lined with rock riprap and the crest
is an asphalt roadway. The spillway discharges through a concrete -lined chute along the downstream
toe of the dam into a stilling basin then down to the South Fork of Grapevine Creek.
Three modification alternatives were analyzed for this study and will be discussed in greater detail later
in this report. Each of these alternatives proposes removing the existing overflow spillway structure and
replacing it with some sort of riser and conduit system. A portion of the existing concrete chute, along
with the stilling basin, will remain in each proposed alternative.
August 2011 7
North Lake Dam Spillway Modification IFERIIICHOLS FREESE
City of Coppell
Alternatives 1 and 2 have the normal pool elevation at 490.0 ft-msl, and Alternative 3 has the normal
pool elevation at 480.0 ft -msl. Alternative 1 consists of a riser with a 100 -ft weir length in a labyrinth
configuration. This was selected in order to keep the 100 -year flood to less than a 2 -ft rise, as preferred
by the developer. This is discussed in more detail in the next section of this report. The conduit is an 8 -ft
reinforced concrete box. Alternative 2 consists of a square riser with an effective weir length of 30 feet.
The conduit is a 5 -ft reinforced concrete box. Alternative 3 consists of the same riser and conduit
combination as Alternative 2. No practical configuration was found to keep the 100 -year flood to Tess
than a 2 -ft rise with the starting water surface elevation at a normal pool of 480 ft-msl. The discharge
rating curves were calculated using both weir and pressure flow equations and are shown in Table 4,
with detailed calculations presented in Appendix C.
August 2011 8
North Lake Dam Spillway Modification FREESE
City of Coppell 'FINN ?NICHOLS
Table 4 - Discharge Rating Curve
Alternative 1 Alternative 2 Alternative 3
Elevation Discharge Elevation Discharge Elevation Discharge
(ft-msl) (cfs) (ft-msl) (cfs) (ft-nisi) (cfs)
490.0 0 490.0 0 480.0 0
490.5 116 490.5 34 480.5 34
491.0 329 491.0 95 481.0 95
491.5 603 491.5 171 481.5 171
492.0 926 492.0 258 482.0 258
492.5 1,291 492.5 352 482.5 352
493.0 1,553 493.0 453 483.0 453
493.5 1,574 493.5 551 483.5 516
494.0 1,595 494.0 558 484.0 522
495.0 1,635 495.0 571 485.0 534
496.0 1,674 496.0 583 486.0 546
497.0 1,713 497.0 596 487.0 557
498.0 1,751 498.0 608 488.0 569
499.0 1,788 499.0 620 489.0 580
500.0 1,824 500.0 632 490.0 591
501.0 1,859 501.0 643 491.0 602
502.0 1,894 502.0 655 492.0 612
503.0 1,928 503.0 666 493.0 623
504.0 1,962 504.0 677 494.0 633
505.0 1,995 505.0 687 495.0 643
510.0 2,152 510.0 739 500.0 691
515.0 2,299 515.0 787 515.0 819
2.4 FREQUENCY MODEL RESULTS
The 100 -year frequency - or 1% annual chance - storm event was analyzed for each of the North Lake
Dam spillway modification alternatives. The hydrologic model described in the preceding sections was
implemented in analyzing this event. Curve numbers were set to AMC 11, and initial abstractions were
calculated automatically by HEC -HMS. These assumptions represent normal conditions, as would be
expected prior to a storm event of this nature. The precipitation data was obtained from the Integrated
Stormwater Management (iSWM) Manual developed by NCTCOG for the north - central Texas region.
These values are presented in Table 5. Each storm event was assumed to have a duration of 24 hours.
August 2011 9
North Lake Dam Spillway Modification
Fp
City of Coppell
1 NI
Table 5 - Frequency Precipitation Depths
Frequency Precipitation (in)
(yrs) 5 min 15 min 60 min 2 hr 3 hr 6 hr 12 hr 24hr
1 0.44 0.85 1.40 1.66 1.83 2.10 2.40 2.64
2 0.50 1.00 1.73 2.08 2.28 2.64 3.12 3.60
5 0.60 1.24 2.24 2.76 3.06 3.60 4.20 4.80
10 0.66 1.40 2.60 3.22 3.60 4.26 4.92 5.76
25 0.76 1.63 3.07 3.84 4.32 5.16 6.12 7.20
50 0.84 1.81 3.45 4.36 4.89 5.88 6.96 8.40
100 0.93 2.00 3.86 4.90 5.55 6.72 8.04 9.60
500 - -- 3.00 4.86 6.12 6.99 8.76 11.04 13.68
These precipitation depths serve as input data into the hydrologic model, and were routed through the
model as described previously. The results from the 100 -year storm event runs are shown for each
alternative in Table 6.
Table 6 - Frequency Model Results
Modification Normal Peak Peak Peak
Alternative Pool Elevation Inflow Outflow
(ft-msl) (ft -msl) (cfs) (cfs)
Alt 1 490 491.97 7,856 904
Alt 2 490 492.34 7,856 323
Alt 3 480 483.58 7,856 517
As noted previously, the size for Alternative 1 was chosen to limit the 100 -year rise to 2 feet. Such an
alternative was not feasible at a normal pool elevation of 480 ft -msl.
2.5 PMF MODEL RESULTS
The Probable Maximum Flood (PMF) is defined as the greatest flood to be expected, and the Probable
Maximum Precipitation (PMP) is theoretically the greatest depth of rainfall for a given duration that is
physically possible over a given size storm area at a particular geographic location. Generally, the rainfall
depth is calculated for the ten square miles of the watershed which receive the highest intensity rainfall.
Hydrometeorological Report No. 52 (HMR -52), developed by the U.S. Army Corps of Engineers, was
used to determine the rainfall for each basin. PMP estimates were taken from Hydrometeorological
August 2011 10
North Lake Dam Spillway Modification FREESE
City of Coppell riaNICHOLS
Report No. 51 and distributed according to HMR -52 to obtain average rainfall depths over the various
drainage areas.
HMR -52 calculates rainfall depths for storm durations ranging from five minutes to seventy -two hours.
Table 7 lists the point rainfall depths calculated by HMR -52 for storm durations from one hour to 72
hours. Because the total drainage area is less than ten square miles, the same rainfall depths were
applied over the entire drainage area. Additionally, the total rainfall depth was distributed according to
the temporal distribution described by the TCEQ Hydrologic and Hydraulic Guidelines for Dams in
Texas»
Table 7 — HMR -52 Point Rainfall Depths
Storm Depth
Duration (in)
(hr)
1 16.63
2 20.72
3 23.92
6 29.95
12 36.05
24 41.26
48 46.11
72 48.87
The PMF was modeled for each modification alternative, as described previously, with flood routing
started at both normal pool elevations. According to TCEQ regulations, the North Lake Dam is classified
as an intermediate -size, high- hazard structure. As such, North Lake Dam is required to pass 100% of the
PMF to be in compliance with the TCEQ regulations. The critical PMF duration for Alternative 1 was 24
hours, while the critical duration for Alternatives 2 and 3 was 48 hours. Table 8 contains the results of
these PMF model runs.
Table 8 — PMF Model Results
Modification Normal Critical Peak Peak Peak
Alternative Pool Duration Elevation Inflow Outflow
(ft-msl) (hrs) (ft -msl) (cfs) (cfs)
Alt 1 490 24 497.93 6,955 1,748
Alt 2 490 48 499.96 4,129 632
Alt 3 480 48 494.25 4,129 636
August 2011 11
North Lake Dam Spillway Modification
NICHOL
City of Coppell a
3.0 MODIFICATION ALTERNATIVES
Alternatives were developed for two different normal pool elevations — 480 ft-msl and 490 ft-msl. A
total of three modification alternatives were analyzed. Alternatives 1 and 2 have the normal pool
elevation at 490.0 ft-msl, and Alternative 3 has the normal pool elevation at 480.0 ft-msl.
Each of these alternatives proposes removing the existing overflow spillway structure and replacing it
with a riser and conduit system. All three alternatives utilize the same proposed location of the riser
structure and proposed alignment of the conduit. Figure 4 shows a plan view of Alternative 3 as an
example of the proposed location and alignment. Figure 5 shows a cross section view of the spillway for
Alternative 3, including the excavated approach channel. Alternative 3 was selected as the example
because it represents the most significant amount of excavation due to the lower normal pool elevation.
A portion of the existing concrete chute will remain in place for each proposed alternative. The conduit
will run from the riser through the right abutment of the dam, where it will bend to the alignment of the
existing chute until the flowline of the conduit daylights into the chute. The conduit will have a headwall,
and discharges will flow through the existing chute to the existing stilling basin, which will also remain.
3.1 ALTERNATIVE 1
Alternative 1 consists of a concrete riser with a 100 -ft weir length in a labyrinth configuration with a
crest elevation of 490.0 ft -msl. This was selected in order to keep the 100 -year flood to Tess than a 2 -ft
rise, as preferred by the developer. The conduit is set to be an 8 -ft by 8 -ft reinforced concrete box, so as
to maintain proper hydraulic behavior during the 100 -year flood event. The riser will be controlled by
weir flow up to elevation 492.8 ft-msl, at which point the conduit will flow full and be controlled by
pressure flow.
An approach channel will be excavated to elevation 483.0 ft-msl with a 2 -ft layer of rock riprap on the
bottom, side slopes, and around the riser structure. The width of the approach channel will be 44 feet
with 3:1 side slopes. During construction, a portion of the existing ground surface will remain in place to
act as a coffer dam to provide a dry working area for the construction of the riser and conduit. This will
then be excavated and lined with riprap as mentioned previously.
The estimated construction cost of Alternative 1 is $2,429,200. A detailed Opinion of Probable
Construction Costs for each alternative is given in Appendix D.
August 2011 12
North Lake Dam Spillway Modification
FREESE
City of Coppell NICHOLS
3.2 ALTERNATIVE 2
Alternative 2 consists of a square concrete riser with 10 -ft by 10 -ft interior dimensions and 12 -inch thick
walls. Because of the need for backfill around the conduit, the effective weir length is 30 feet. The crest
of this riser is also at elevation 490.0 ft-msl. This riser was selected as a simpler alternative to the
labyrinth weir design of Alternative 1, and the 100 -year flood rise is only 2.34 feet. The conduit is a 5 -ft
by 5 -ft reinforced concrete box. The riser will be controlled by weir flow up to elevation 493.5 ft-msl, at
which point the conduit will flow full and be controlled by pressure flow.
An approach channel will be excavated to elevation 483.0 ft -msl with a 2 -ft layer of rock riprap on the
bottom, side slopes, and around the riser structure. The width of the approach channel will be 44 feet
with 3:1 side slopes. During construction, a portion of the existing ground surface will remain in place to
act as a coffer dam to provide a dry working area for the construction of the riser and conduit. This will
then be excavated and Tined with riprap as mentioned previously.
The estimated construction cost of Alternative 2 is $2,170,300. A detailed Opinion of Probable
Construction Costs for each alternative is given in Appendix D.
3.3 ALTERNATIVE 3
Alternative 3 consists of the same riser and conduit combination as Alternative 2. The riser has interior
dimensions of 10 -ft by 10 -ft with a crest elevation of 480.0 ft -msl. The conduit is a 5 -ft by 5 -ft reinforced
concrete box. The riser will be controlled by weir flow up to elevation 483.3 ft-msl, at which point the
conduit will flow full and be controlled by pressure flow.
No practical configuration was found to keep the 100 -year flood to Tess than a 2 -ft rise with the starting
water surface elevation at a normal pool of 480 ft-msl. The weir length necessary to maintain this little
rise is not feasible for a riser and conduit system. The 100 -year flood rise for Alternative 3 is 3.58 feet.
An approach channel will be excavated to elevation 473.0 ft -msl with a 2 -ft layer of rock riprap on the
bottom, side slopes, and around the riser structure. The width of the approach channel will be 44 feet
with 3:1 side slopes. During construction, a portion of the existing ground surface will remain in place to
act as a coffer dam to provide a dry working area for the construction of the riser and conduit. This will
then be excavated and lined with riprap as mentioned previously.
August 2011 13
North Lake Dam Spillway Modification
City of Coppell
MI N S
ICHOL
The estimated construction cost of Alternative 3 is $3,143,200. A detailed Opinion of Probable
Construction Costs for each alternative is given in Appendix D. As mentioned previously, Figures 4 and 5
show the plan view and cross section view, respectively, of Alternative 3.
3.4 GEOTECHNICAL REVIEW
A preliminary review of the geotechnical stability of the embankment demonstrated that the
lake could be drawn down from its current levels to approximately elevation 488.0 ft-msl without any
concerns for the stability of the upstream face. Reductions below that will need some delay, but would
be expected to be within the timeframe of construction process and can be handled by the contractor.
Precise limits will depend on the final selected configuration and the additional geotechnical exploration
and laboratory testing that will be needed during the final design.
August 2011 14
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CITY OF COPPELL F&N JOB NO
DATE CPL1 t 192
A & NORTH . LAKE SPILLWAY MODIFICATION E JUNE 20"
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North Lake Dam Spillway Modification
FREESE
City of Coppell ?NICHOLS
4.0 ENVIRONMENTAL PERMITTING REVIEW
North Lake currently has a surface area of approximately 800 acres at an elevation of 510 feet msl and a
maximum depth of about 40 feet. Grapevine Creek is a tributary of the Elm Fork Trinity River, a
Relatively Permanent Waterbody (RPW). Therefore, the South Fork of Grapevine Creek, a tributary of
Grapevine Creek, is a jurisdictional water of the U.S.
The City of Coppell (City) has purchased the lake and the surrounding area and has requested that
Freese and Nichols provide a permitting evaluation discussing the permanent lowering of the reservoir's
normal pool elevation to either 480 feet msl or 490 feet msl in order to accommodate a planned
residential development.
4.1 PERMANENTLY LOWERING THE NORMAL POOL ELEVATION
According to David Madden, Project Manager for the U.S. Army Corps of Engineers (USACE), any activity
involving the placement of fill material in a water of the U.S. would require a Section 404 (Clean Water
Act) Permit. For example, construction of a new spillway structure within the existing normal pool (510
feet msl) area would require a Section 404 Permit. However, Mr. Madden also stated that if a
jurisdictional reservoir were to be modified to permanently lower the normal pool elevation without the
placement of fill within the existing normal pool area, no Section 404 permit would be required.
Regardless of how the normal pool elevation is permanently lowered, a new jurisdictional limit would be
established when the newly exposed land surface completely dries out, establishing a new normal pool
elevation, shoreline, and jurisdictional limit, based on the elevation of the new permanent spillway.
If the permanent placement of fill within the current normal pool area were necessary to establish the
new pool elevation, such as in the construction of a new spillway and /or placement of rock riprap, there
are two options available for pursuing USACE authorization for the project: a standard individual permit
(IP) or a modified IP (Letter of Permission 1 (LOP -1)).
An IP application process would involve the submittal of detailed project plans and a mitigation plan for
adverse impacts to waters of the U.S., as well as a public review and comment period and consultation
with state and federal resource agencies.
The application procedure with LOP -1 is similar to that of submitting an IP, except that the USACE would
not require public notification. They would still afford the resource agencies an opportunity to review
August 2011 17
North Lake Dam Spillway Modification
City of Coppell rr7 ?NICHOLS
and comment on the project. The USACE would have the option of rejecting the LOP -1 procedure and
require an IP submittal instead if they deemed that the project was not in the public's best interest.
4.2 ACTIVITIES WITHIN THE NEW JURISDICTIONAL POOL AREA
Any work performed within the new normal pool area involving fill activities would require a permit,
most likely an IP, from the USACE. Activities can be conducted if performed in such a way that avoids
more than incidental fallback of dredged material. Discharge of more than incidental fallback is
considered a fill activity by the USACE. For example, excavation with a back hoe bucket may be
considered to have only incidental fallback, but excavation with an earthmover or bulldozer could be
considered by the USACE to involve more than incidental fallback (i.e., involves the placement of fill) and
therefore would require a permit.
4.3 ACTIVITIES NOT WITHIN THE NEW JURISDICTIONAL POOL AREA
Construction activities outside the new normal pool area of the reservoir, including the placement of fill
material, would not require a Section 404 Permit once the area outside the new normal pool area has
completely dried out.
4.4 MITIGATION
Permitted activities causing permanent impacts to jurisdictional waters of the U.S., such as the
placement of fill material within the existing or new normal pool areas, would require compensatory
mitigation. In this scenario, the purchase of mitigation bank credits may be a practicable alternative to
permittee responsible mitigation. Open water credits at the Trinity River Mitigation Bank cost
approximately $50,000 to $100,000 per acre of impact, depending on the quality of the water resource
impacted (i.e., low or medium versus high quality open water habitat).
4.5 CURRENTLY FUNCTIONING SPILLWAY
A 42 -inch diameter pipeline that originally served to transport water from the Elm Fork Trinity River (Elm
Fork) to North Lake for use in the generation of electric power has been functioning as a spillway since
early 2010. The pipeline invert elevation is 502.25 feet msl, and any overflow would flow directly to the
Elm Fork. It may be possible to construct the proposed permanent spillway on land above this elevation
that has been de- watered by the 42 -inch pipeline.
4.6 CONCLUSIONS
The construction of a new spillway structure within the footprint of the existing normal pool area (510
feet msl or below) would require a Section 404 permit. Once the normal pool elevation has been
August 2011 18
North Lake Dam Spillway Modification
City of Coppell
NICHO
permanently lowered, construction above the new normal pool elevation /jurisdictional limits can occur
when the newly exposed land has completely dried out.
If the USACE agrees that the normal pool elevation is now 502.25 feet msl, based on the invert elevation
of the 42 -inch diameter pipeline that apparently now controls the water surface elevation, then the
construction of the proposed spillway may not impact waters of the U.S. and may not require a Section
404 permit.
4.7 RECOMMENDATIONS
A Pre - Application meeting with the Fort Worth District USACE to discuss the method described above
for constructing a new spillway within the existing normal pool area (510 feet msl or below), or above
the existing "functional" normal pool area elevation (502.25 feet msl), is recommended.
Construction activities within the proposed new normal pool area by the developer would require a
Section 404 permit. It may be in the City's best interest to have the developer pursue their own USACE
permit for such activities. Were the City to obtain a permit for the developer's construction activities,
and then the developer violated the terms of the permit, the City could be held liable.
August 2011 19
North Lake Dam Spillway Modification
City of Coppell ra NICHOL
5.0 SUMMARY AND CONCLUSIONS
The proposed design alternatives for the North Lake spillway, as described above, provide three viable
options for achieving the goal of lowering the conservation pool of the lake. Each of the three
alternatives has been modeled according to State criteria using the best available information. The
Opinion of Probable Construction Costs estimates that the cost for Alternative 1 would be $2,429,200,
the cost for Alternative 2 would be $2,170,300, and the cost for Alternative 3 would be $3,143,200.
A review of environmental permitting issues concluded that a Section 404 permit may be required for
the construction of the new spillway because it would be within the existing normal pool area. However,
the normal pool elevation may be deemed to be 502.25 ft-msl, rather than 510 ft -msl, based on the
flowline of the 42 -inch pipe that is now open and controls the water surface elevation. A meeting with
the Fort Worth District USACE will be necessary to make the final determination. In either case, once the
lake has been lowered to the new normal pool elevation and the land has fully dried out, any
construction above the new normal pool level will not require a permit. Construction below the new
normal pool level will require a Section 404 permit, and it is recommended that this be obtained by the
developer, rather than the City.
August 2011 20
North Lake Dam Spillway Modification
r .FREESE City of Coppell Z COS
Appendix A
References
August 2011
North Lake Dam Spillway Modification
F
City of Coppell
1
References
1. U.S. Army Corps of Engineers, Hydrologic Engineering Center: Hydrologic Modeling
System HEC -HMS - User's Manual Version 3.5, Davis, California, August 2010.
2. North Central Texas Council of Governments (NCTCOG): 2001 Digital Elevation Contours.
< http:// gis .nctcog.org/contours_2001.asp >.
3. Lina T. Ramey and Associates, Inc. Bathymetric Survey of North Lake. [computer map].
Dallas, Texas, September 2008.
4. "Soil Data Mart." NRCS Soil Survey Geographic (SSURGO) Database.
<http: / /soildatamart.n res.usda.gov >.
5. North Central Texas Council of Governments (NCTCOG): 2005 Land Use GIS Data.
<http: / /www. nctcog.org/ris /demographics /landuse.asp >.
6. Billingsley Company. Cypress Waters Draft Masterplan, September 22, 2009.
7. Freese and Nichols, Inc. North Lake Dam Emergency Action Plan. Fort Worth, Texas,
March 2005.
8. North Central Texas Council of Governments (NCTCOG): integrated Stormwater
Management (iSWM) Technical Manual, April 2010. <http: / /iswm.nctcog.org/
technical_manuaLasp >.
9. U.S. Department of Commerce, National Oceanic and Atmospheric Administration and
U.S. Department of the Army, Corps of Engineers: Hydrometeorological Report No. 52,
Application of Probable Maximum Precipitation Estimates, United States East of the
105th Meridian, Washington, D.C., 1982.
10. U.S. Department of Commerce, National Oceanic and Atmospheric Administration and
U.S. Department of the Army, Corps of Engineers: Hydrometeorological Report No. 51,
Probable Maximum Precipitation Estimates, United States East of the 105th Meridian,
Washington, D.C., 1978.
11. Texas Commission on Environmental Quality (TCEQ): Hydrologic and Hydraulic
Guidelines for Dams in Texas, January 2007.
12. Texas Administrative Code, Title 30, Part I, Chapter 200, Subchapter B, Rule §299.11,
Effective January 2009.
August 2011
North Lake Dam Spillway Modification
City of Coppell INIIIIIciNICHOLS
Appendix B
Hydrologic Parameters
August 2011
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North Lake Dam Spillway Modification FREESE
City of Coppell F
Appendix C
Discharge Rating Curve Calculations
August 2011
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North Lake Dam Spillway Modification
FREESE
City of Coppell riiW NICHOLS
Appendix D
Opinion of Probable Construction Costs
August 2011
OPINION OF PROBABLE FREESE
CONSTRUCTION COSTS , ? NICHOLS
NORTH LAKE SPILLWAY MODIFICATION -- ALTERNATIVE 1 6/13/2011
ESTIMATOR CHECKED BY ACCOUNT NO
JPM WBG CPL11192
ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL
EXCAVATION
Conduit - Excavation 42,640 CY $ 7.00 $ 298,500.00
Conduit - Concrete Removal 1,240 CY $ 75.00 $ 93,000.00
Conduit - Trench Safety 590 LF $ 5.00 $ 3,000.00
Approach Channel - Excavation 21,560 CY $ 7.00 $ 151,000.00
PRINCIPAL SPILLWAY
Intake Tower - Reinforced Concrete 65 CY $ 1,000.00 $ 65,000.00
Intake Tower - Structural Backfill 2,850 CY $ 30.00 $ 85,600.00
Intake Tower - Trash Racks 1 LS $ 35,000.00 $ 35,000.00
Intake Tower - Barrier Warning System 1 LS $ 25,000.00 $ 25,000.00
Conduit - 8'x8' Reinforced Concrete Box 590 LF $ 800.00 $ 472,000.00
Outlet Channel - Headwall 1 LS $ 15,000.00 $ 15,000.00
Rock Riprap - 18" max size 3,340 CY $ 75.00 $ 250,500.00
GENERAL ITEMS
Mobilization 1 LS $ 75,000.00 $ 75,000.00
Erosion & Sediment Control 1 LS $ 150,000.00 $ 150,000.00
Care of Water 1 LS $ 150,000.00 $ 150,000.00
SUBTOTAL: $1,868,600
CONTINGENCY & TECHNICAL SERVICES 30% $560,580
PROJECT TOTAL $2,429,200
Cost Estimate.xlsx 6/13/2011 2:11 PM Page 1 of 3
OPINION OF PROBABLE FREESE
CONSTRUCTION COSTS I aIVICHOLS
NORTH LAKE SPILLWAY MODIFICATION -- ALTERNATIVE 2 5/12/2011
ESTIMATOR CHECKED BY ACCOUNT NO
JPM WBG CPL11192
ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL
EXCAVATION
Conduit - Excavation 41,220 CY $ 7.00 $ 288,600.00
Conduit - Concrete Removal 1,240 CY $ 75.00 $ 93,000.00
Conduit - Trench Safety 590 LF $ 5.00 $ 3,000.00
Approach Channel - Excavation 21,560 CY $ 7.00 $ 151,000.00
PRINCIPAL SPILLWAY
Intake Tower - Reinforced Concrete 30 CY $ 1,000.00 $ 30,000.00
Intake Tower - Structural Backfill 2,160 CY $ 30.00 $ 64,800.00
Intake Tower - Trash Racks 1 LS $ 35,000.00 $ 35,000.00
Intake Tower - Barrier Warning System 1 LS $ 25,000.00 $ 25,000.00
Conduit - 5'x5' Reinforced Concrete Box 590 LF $ 650.00 $ 383,500.00
Outlet Channel - Headwall 1 LS $ 10,000.00 $ 10,000.00
Rock Riprap - 18" max size 3,340 CY $ 75.00 $ 250,500.00
GENERAL ITEMS
Mobilization 1 LS $ 67,000.00 $ 67,000.00
Erosion & Sediment Control 1 LS $ 134,000.00 $ 134,000.00
Care of Water 1 LS $ 134,000.00 $ 134,000.00
SUBTOTAL: $1,669,400
CONTINGENCY & TECHNICAL SERVICES 30% $500,820
PROJECT TOTAL $2,170,300
Cost Estimate.xlsx 6/13/2011 2:11 PM Page 2 of 3
OPINION OF PROBABLE FREESE
CONSTRUCTION COSTS , 4NICHOLS
NORTH LAKE SPILLWAY MODIFICATION -- ALTERNATIVE 3 5/12/2011
ESTIMATOR CHECKED BY ACCOUNT NO
JPM WBG CPL11192
ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL
EXCAVATION
Conduit - Excavation 66,290 CY $ 7.00 $ 464,100.00
Conduit - Concrete Removal 1,420 CY $ 75.00 $ 106,500.00
Conduit - Trench Safety 730 LF $ 5.00 $ 3,700.00
Approach Channel - Excavation 39,800 CY $ 7.00 $ 278,600.00
PRINCIPAL SPILLWAY
Intake Tower - Reinforced Concrete 30 CY $ 1,000.00 $ 30,000.00
Intake Tower - Structural Backfill 4,170 CY $ 30.00 $ 125,100.00
Intake Tower - Trash Racks 1 LS $ 35,000.00 $ 35,000.00
Intake Tower - Barrier Warning System 1 LS $ 25,000.00 $ 25,000.00
Conduit - 5'x5' Reinforced Concrete Box 730 LF $ 650.00 $ 474,500.00
Outlet Channel - Headwall 1 LS $ 10,000.00 $ 10,000.00
Rock Riprap - 18" max size 5,070 CY $ 75.00 $ 380,300.00
GENERAL ITEMS
Mobilization 1 LS $ 97,000.00 $ 97,000.00
Erosion & Sediment Control 1 LS $ 194,000.00 $ 194,000.00
Care of Water 1 LS $ 194,000.00 $ 194,000.00
SUBTOTAL: $2,417,800
CONTINGENCY & TECHNICAL SERVICES 30% $725,340
PROJECT TOTAL $3,143,200
Cost Estimate.xlsx 6/13/2011 2:11 PM Page 3 of 3