Northlake Water Supply Study Final Technical Memorandum
2
I. Existing Information
The following information on the existing conditions of North Lake was obtained from the
“North Lake Dam Operation and Maintenance Manual” written in September 2015.
Required Pool Elevation: 484 ft-msl
Surface Area @ Required Pool Elevation: 273 acres (11,891,880 sqft)
Capacity @ Required Pool Elevation: 2900 acre-feet (944 Mgal)
Spillway Crest Elevation: 484 ft-msl
II. Estimated Water Demand
The demand is composed of evaporation and irrigation demand. Drought, peak year irrigation
demand was provided for the new Cypress Waters development by the City of Coppell.
Evaporation and precipitation data was obtained from measurements recorded at the DFW
International Airport between 2001 and 2015 as listed on the National Weather Service Weather
Forecast Office. The evaporation data was obtained from measurements in Dallas County
between 2000 and 2014 as listed on the Texas Water Development Board website. Two methods
were used to estimate the required supplemental water supply volume. Method 1 is generally a
straightforward approach of summing precipitation, evaporation and irrigation demand for the
area encompassed by the lake. Method 2 utilizes the Integrated Storm Water Management
(iSWM) system as developed by the NCTCOG, and considers the entire watershed that
contributes runoff to Northlake.
Method 1: Historical Weather Data
The Annual Volume, corresponding Average & Peak Daily Flow Rates required to maintain the
normal pool elevation of 484 ft-msl were calculated for maximum, minimum and average annual
precipitation for maximum, minimum and average annual evaporation. To ensure a conservative
estimate, the following was assumed:
Precipitation is captured within the Lake perimeter only. The watershed is not included.
Evaporation occurs when water levels in the lake are at required pool elevation which
represents a surface area of 273 acres.
Worst case scenario includes peak irrigation demand of 1,250 ac-ft per year, with 15% of
total taken in 3 peak months (June-August)
Average conditions include irrigation demand assumed to be half of peak demand.
The estimated Daily Flow Rates, using the assumptions above, are calculated for annual
precipitation conditions as measured between 2001 and 2015 in Dallas-Fort Worth. The
evaporation data was obtained from measurements obtained between 2000 and 2014 in Dallas
County only.
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BEST CASE CONDITIONS
Minimum
Evaporation (2007)
Maximum
Precipitation (2007)
Cypress Waters
Irrigation Annual
Demand Annual Vol. to
maintain Normal
Pool Elev. (Mgal)
Average Daily
Flow Rate Req.
(MGD)
Peak Daily
Flow Rate Req.
(MGD) Measured
(in.)
Total
(Mgal)
Measured
(in.)
Total
(Mgal) (Ac-ft) (Mgal)
50.70 375.85 50.05 371.03 0.0 0.0 4.82 0.01 0.02
AVERAGE ANNUAL CONDITIONS
Average
Evaporation
Average
Precipitation
Cypress Waters
Irrigation Annual
Demand Annual Vol. to
maintain Normal
Pool Elev. (Mgal)
Average Daily
Flow Rate Req.
(MGD)
Peak Daily
Flow Rate Req.
(MGD) Measured
(in.)
Total
(Mgal)
Measured
(in.)
Total
(Mgal) (Ac-ft) (Mgal)
57.88 429.07 36.14 267.93 626 204 365.14 1.00 1.83
WORST CASE CONDITIONS
Maximum
Evaporation (2011) Minimum
Precipitation (2005)
Cypress Waters
Irrigation Annual
Demand Annual Vol. to
maintain Normal
Pool Elev. (Mgal)
Average Daily
Flow Rate Req.
(MGD)
Peak Daily
Flow Rate Req.
(MGD) Measured
(in.)
Total
(Mgal)
Measured
(in.)
Total
(Mgal) (Ac-ft) (Mgal)
69.75 517.07 18.97 140.63 1,250 407 783.44 2.15 3.92
Method 2: iSWM™ Monthly Water Balance
This analysis utilizes the Integrated Storm Water Management (iSWM) system as developed by
the NCTCOG. Unlike the annual averaging used in Method 1 above, this method uses a monthly
water balance approach that considers additional factors such as the watershed runoff, and
spillway overflow. The water balance equation is below, with an explanation of variables and
assumptions:
∆ܸൌܴܲܤെܫെܧെܧ௧ െܱ
∆V is change in volume – or in this case the supply required to keep Northlake at level.
P is monthly precipitation – using the same worst case conditions as used in Method 1, the
monthly precipitation was from 2005.
R is the runoff – a drainage basin of 1,675 acres was delineated from existing topography and the
storm drain systems to calculate runoff.
B is baseflow – baseflow contributions are negligible for ponds outside of a stream system and
non-existent for Northlake, thus was assumed to be 0.
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I is infiltration into the soil – infiltration is negligible since Northlake is situated in the fatty clays
overlying the Eagle Ford Shale which have very low infiltration rates, and was assume to be 0.
E is monthly evaporation - using the same worst case conditions as used in Method 1, the monthly
evaporation was from 2011.
Et is evapotranspiration – evapotranspiration is only considered when wetland vegetation
dominates, and was assumed to be 0.
O is overflow – the monthly surplus over the normal pool capacity which is lost over the spillway,
and subtracted from the water balance if 485 ft-msl is assumed to be maintained.
The graphical results for the supplemental daily flow are shown below in Figures1 and 2, and the
complete water balance table is provided in Attachment A.
Figure 1 – Supplemental Pumping Rates by Month for Average Case Conditions (Average Rainfall, Average Evaporation, & Half
of Maximum Irrigation Demand)
Figure 2 – Supplemental Pumping Rates by Month for Worst Case Conditions (Minimum Rainfall, Maximum Evaporation &
Maximum Irrigation Demand)
0.0
0.5
1.0
1.5
2.0
2.5
Supplemental Daily Flow (MGD)Required Supplemental Supply Rate
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Supplemental Daily Flow (MGD)Required Supplemental Supply Rate
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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The worst case scenario monthly pattern including Cypress Waters maximum irrigation demand
is estimated to require a total annual supplemental supply of approximately 573 Mgal/year, with
a peak supply rate of approximately 3.71 MGD (July) to maintain normal pool capacity
at 484 ft-msl.
The average monthly pattern including Cypress Waters average irrigation demand is estimated to
require a total annual supplemental supply of approximately 64 Mgal/year, with a peak supply
rate of approximately 1.36 MGD (July) to maintain normal pool capacity at 484 ft-msl.
In an average year, the minimum daily supplemental flowrate required per Method 1 and 2 is
estimated to be between 1.8 and 1.4 MGD respectively. In a drought year, the maximum
supplemental flow rate, or peak design flowrate required, is estimated to be 3.9 to 3.7 MGD
respectively. The difference in results between the methods is primarily that 2 considers the
entire watershed for Northlake, and thus should be considered more accurate.
III. Cost Analysis
The two primary components of cost are the infrastructure improvements and the water purchase
cost for each option. Since the facilities must be sized to convey the maximum rate possible, a
flowrate of approximately 4.0 MGD, or 2800 gpm was assumed per the discussion above.
Infrastructure Costs
Treated Water – This option proposes that flushing stations be installed in the southern sector,
which is the area west of Northlake south of Southwestern Blvd. This area is in the Northlake
watershed and treated water flushed to the storm drain system would feed Northlake. This area
also experiences water quality issues due to low usage and flushing would likely improve the
flow in the pipe network and likely the water quality in the distribution system in this area. The
flushing stations should be constructed to convey water from the water system to the storm drain
system that flows to the lake. A flushing station should be equipped with an automatic SCADA
controlled valve, meter, air gap, dechlorination chemicals and feed equipment. Three stations
with 1,000 gpm discharge capacity each were assumed for adequate supply and redundancy.
Treated Water Flushing Stations
Flushing station piping & air gap connection $ 40,000
Dechlorinating station $ 60,000
Meter Vault with automatic valve $ 75,000
SCADA and electrical $ 25,000
Sub-total $ 200,000
Sub-total (for three stations) $ 600,000
Contingency (25%) $ 150,000
Engineering (20%) $ 150,000
Total for Three stations $ 900,000
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Raw Water – For a raw water supply, portions of the existing Northlake Pump Station (NPS)
and pipeline could be used to deliver raw water from the Elm Fork of the Trinity River to
Northlake as was done by Dallas Power and Light since the 1950’s. Due to the age, condition
and size of the existing equipment it is recommended that, at a minimum, the existing pumps and
electrical equipment be replaced. The costs presented in this memo assume that the existing
intake structure, pump deck and influent channel can be reused with little modification or
rehabilitation. However, it is possible that an entirely new intake structure and influent channel
may be required. A full evaluation of the intake structure and survey of the adjacent river
channel is recommended.
The NPS currently has two 28 MGD pumps and one 14 MGD pump to convey water to
Northlake via a 42” pipeline. The proposed maximum flowrate is approximately 4.0 MGD,
which would have a maximum velocity of less than 0.6 feet per second in a 42-inch pipeline.
This would allow sediment to accumulate, and ultimately pipeline blockage. To achieve non-
settling velocities of three to five feet per second, a smaller pipeline of approximately 12-14 inch
diameter is recommended. This newer smaller pipeline could be installed either on top of or
inside of (sliplininig) the existing pipe.
Northlake Raw Water Pump Station
Intake improvements and dredging $ 250,000
Pumps & piping modifications $ 300,000
Demolition of ex. Station and transformer removal $ 250,000
SCADA & electrical $ 150,000
24” Transmission main (Sliplining of ex. 42”) $ 800,000
Sub-total $ 1,750,000
Contingency (25%) $ 437,500
Engineering (20%) $ 437,500
Total $ 2,625,000
Groundwater – The alternative to purchasing water is a groundwater well field supply. This
option only presents cost for the infrastructure and annual O&M, as groundwater is free if
available. Upon review of the Texas Water Development Board’s groundwater database, and
calls to local drillers, a 6” well has been reported to produce up to 100 gpm and a 10” well may
produce up to 200 gpm. To achieve a 4.0 MGD rate, or 2,800 gpm, fourteen 10” wells would be
required. In addition to the wells, a network of piping would be required to bring the water from
each well to an outfall structure on the lake, or multiple outfall structures as needed to limit the
piping runs.
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Well Field
Water wells1 (14~10” wells at $400,000 each) $ 5,600,000
Piping and related infrastructure2 $ 350,000
Well pad site & access road $ 700,000
SCADA & electrical $ 700,000
Sub-total $ 7,350,000
Contingency (25%) $ 1,837,500
Engineering (20%) $ 1,837,500
Total $11,025,000
1. Assumes 200 gpm safe yield per well, for 4.0 MGD supply
2. Assumes 20,000 LF of 4"-6" connecting piping to multiple outfalls.
Water Cost
Water is currently obtained per the Wholesale Treated Water Contract between the City of Coppell
and the City of Dallas dated October 27th, 1987. Per the contract, the rates established are subject
to changes by the Dallas City Council. Below is a table reflecting wholesale water rates for treated
and untreated water from DWU per their “Cost Study” dated June 2017.
Rates Table
Proposed Wholesale Rates
Regular Untreated Water $ 1.02
Interruptible Untreated Water $ 0.4761
Treated Water Demand (per MGD/year) $ 280,458
Treated Water Volume $ 0.4565
Treated Water Flat Rate $ 2.2094
Rates Description
Regular Untreated Water: Raw water pumped from the source (lake or river).
Interruptible Untreated Water: Raw water during flood stage.
Treated Water Demand + Volume: Demand charge paid for operational costs plus
volume charge paid per thousand gallons of water pumped.
Treated Water Flat Rate: Payment required for volume taken. If take exceeds 1
MGD, then the contract may be transferred to a “Treated Water Demand/Volume”
rate.
Method 2
(iSWM water balance)
Annual
Vol.
(Mgal) Raw Water
Treated Water
Demand +
Vol. Rate Flat Rate
Best Case Weather Year 5 $ 5,100 $ 1,124,115 $ 11,032
Average Weather Year 64 $ 65,280 $ 1,151,048 $ 141,209
Worst Case Weather Year 580 $ 591,600 $ 1,386,602 $ 1,279,712
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Total Costs
The annual costs, not including any capital expenditure or debt service, consist of water
purchase, power and Operations and Maintenance (O&M), and are summarized in the table
below. An average weather year is assumed and can vary for drought or wet years. O&M is
assumed to be 1% of the capital cost expenditure.
Annual Estimated Cost
Treated Water Raw Water Well Water
Water 1 $ 1,151,048 $ 65,280 $ 0
Power Cost3 $ 0 $ 9,000 $ 58,000
O&M4 $ 9,000 $ 26,250 $ 110,250
Total $ 1,160,048 $ 100,530 $ 168,250
See next table for reference notes
Total Estimated Cost (20 yr. Present Value)
Treated Water
(Demand + Vol.
Rate)
Raw Water Well Water
Water 1 $ 23,020,960 $ 1,305,600 $ 0
Infrastructure $ 900,000 $ 2,625,000 $ 11,025,000
Debt interest2 $ 298,000 $ 869,000 $ 3,650,000
Power Cost3 $ 0 $ 180,000 $ 1,160,000
O&M4 $ 180,000 $ 525,000 $ 2,205,000
Total $ 24,398,960 $ 5,504,600 $ 18,040,000
1. No change in water rates assumed over 20 year analysis.
2. Interest on debt assumed to be 3% per annum, for 20 year debt obligation,
rounded up to nearest $1,000.
3. 5 cents/kWh assumed for electrical rate.
4. Assumed to be 1% of capital cost times 20 years.
Technical Memorandum
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IV. Discussion and Recommendations
The treated water option has the lowest infrastructure cost, but it is significantly higher than the
other options due to the cost to purchase the treated water from DWU, which is mostly due to the
increase in the demand charge ($280,458 per MGD). If water cannot be obtained at the “Flat
Rate”, it is recommended that treated water be eliminated from consideration.
Groundwater would be a less expensive water supply option than treated water on an annual
basis, but the infrastructure cost is high is due to the cost of drilling and completing multiple
water wells and the related wellfield piping. This infrastructure could be less if the yield of the
wells is higher than estimated, but the specific yield of any well cannot be determined with
certainty until it is drilled and tested. It was assumed each well should be spaced 1,000 feet apart,
but a hydrogeologist should be consulted before exploring this option further. No costs for
easements or right-of-way were included and should be considered for access to well sites.
From both a long term and short term financial perspective, a raw water supply appears to be the
most attractive. The raw water purchase cost may be less if the “Interruptible Rate” can be
applied, but it is only likely in the event of a wet year when the supplemental supply to
Northlake will be low. Although there are some unknowns regarding the condition and feasibility
of reusing the existing Northlake raw water pump station and pipeline, it costs significantly less
than the other two options, and may present less risk compared to wellfield development.
City of Coppell Northlake Water Supply StudyProject Number 3757‐00Attachment AMaximum surface area of pond (acres) 289Minimum surface area of pond (acres) 273Drainage area (acres) 1675Capacity at normal pool elevation (ac‐ft) 3199Capacity at normal pool elevation (Mgal) 1042Minimum lake elevation (ft‐msl) 484Minimum lake capacity (ac‐ft) 2946Spillway elevation (ft‐msl) 485Average annual rainfall (in) 33.87Average annual evaporation (in) 57.89Watershed efficiency 0.31Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecDays Per Month 31 28 31 30 31 30 31 31 30 31 30 31Monthly Precipitation (in) 2.83 2.24 3.52 2.80 4.84 3.64 2.00 1.92 2.91 3.17 1.90 2.10Monthly Precipitation (ac‐ft) 68.08 53.98 84.87 67.55 116.61 87.55 48.28 46.16 70.01 76.43 45.69 50.47Monthly Evaporation (in) 2.54 2.58 3.88 4.85 5.09 6.64 7.50 7.82 6.03 4.823.542.60Monthly Evaporation (ac‐ft) 61.08 62.23 93.48 116.88 122.63 160.03 180.61 188.40 145.17 116.00 85.19 62.50Runoff (ac‐ft) 92.14 73.06 114.87 91.42 157.83 118.50 65.35 62.48 94.76 103.45 61.84 68.31Irrigation (ac‐ft) (625 ac‐ft total) 57.29 57.29 57.29 93.75 93.75 93.75 57.29 57.29 57.29Delta (ac‐ft) 99.14 64.81 48.97 ‐15.21 94.52 ‐47.73 ‐160.73 ‐173.51 ‐37.69 6.59 ‐34.95 56.28Starting Lake Volume (ac‐ft) 3199.00 3199.00 3199.00 3199.00 3183.793199.00 3151.27 2990.54 2946.00 2946.00 2952.59 2946.00Ending Lake Volume (ac‐ft) 3199.00 3199.00 3199.00 3183.79 3199.00 3151.27 2990.54 2817.03 2908.31 2952.59 2917.64 3002.28Ending Lake Volume (Mgal) 1042.40 1042.40 1042.40 1037.44 1042.40 1026.85 974.47 917.93 947.68 962.10 950.71 978.30Supplemental Supply (ac‐ft) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 128.97 37.69 0.00 28.36 0.00Supplemental Supply (Mgal) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 42.02 12.280.009.240.00Supplemental Daily Flow (MGD) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.36 0.41 0.00 0.31 0.00Notes:Total Annual (Mgal) 63.551) Precipitation data downloaded from NOAA for Dallas/Fort Worth, (average between 2001‐2015)2) Evaporation data downloaded from TWDB for Dallas County, (average between 2001‐2015)3) Baseflow is assumed to be 0 as most ponds have negligible baseflow and would have been estimated from observations4) Evapotranspiration is assumed to be 0 because it is only critical when wetland vegetation dominates 5) Infiltration is assumed to be 0 because lake sits on top of the fatty clays of the Eagle Ford Shale6) Assume pond starts at normal pool elevation7) Assume contract requires City of Coppell to keep Northlake at least 1ft below normal pool elevation8) All methods and equations from iSWM Hydrology Chapter 4 "Water Balance" Average Monthly Water Balance Under Average Rainfall and Evaporation Conditions to Maintain Lake Elevation of at least 484 ft‐mslHydrological & Physical Data for PondMonthly Water Balance 0.00.20.40.60.81.01.21.41.6Supplemental Daily Flow (MGD)Required Supplemental PumpingJanFebMarAprMayJun JulAugSepOctNov Dec
City of Coppell Northlake Water Supply StudyProject Number 3757‐00Attachment AMaximum surface area of pond (acres) 289Minimum surface area of pond (acres) 273Drainage area (acres) 1675Capacity at normal pool elevation (ac‐ft) 3199Capacity at normal pool elevation (Mgal) 1042Minimum lake elevation (ft‐msl) 484Minimum lake capacity (ac‐ft) 2946Spillway elevation (ft‐msl) 485Minimum annual rainfall (in) 18.97Maximum annual evaporation (in) 69.75Watershed efficiency 0.31Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecDays Per Month 31 28 31 30 31 30 31 31 30 31 30 31Monthly Precipitation (in) 4.33 1.62 2.17 0.56 3.35 1.14 0.74 2.46 1.36 0.89 0.02 0.33Monthly Precipitation (ac‐ft) 98.51 36.86 49.37 12.74 76.21 25.94 16.84 55.97 30.94 20.25 0.46 7.51Monthly Evaporation (in) 2.23 2.39 4.46 6.38 5.61 8.23 9.09 10.34 8.89 5.98 4.1 2.04Monthly Evaporation (ac‐ft) 50.73 54.37 101.47 145.15 127.63 187.23 206.80 235.24 202.25 136.05 93.28 46.41Runoff (ac‐ft) 141.14 52.81 70.73 18.25 109.20 37.16 24.12 80.19 44.33 29.01 0.65 10.76Irrigation (ac‐ft) (1,250 ac‐ft total) 114.58 114.58 114.58 187.50 187.50 187.50 114.58 114.58 114.58Delta (ac‐ft) 188.92 35.29 ‐95.94 ‐228.73 ‐56.80 ‐311.64 ‐353.34 ‐286.58 ‐241.56 ‐201.37 ‐206.75 ‐28.15Starting Lake Volume (ac‐ft) 3199.00 3199.00 3199.00 3103.06 2946.002946.00 2946.00 2946.00 2946.00 2946.00 2946.00 2946.00Ending Lake Volume (ac‐ft) 3199.00 3199.00 3103.06 2874.33 2889.20 2634.36 2592.66 2659.42 2704.44 2744.63 2739.25 2917.85Ending Lake Volume (Mgal) 1042.40 1042.40 1011.13 936.60 941.45 858.41 844.82 866.57 881.25 894.34 892.59 950.79Supplemental Supply (ac‐ft) 0.00 0.00 0.00 71.67 56.80 311.64 353.34 286.58 241.56 201.37 206.75 28.15Supplemental Supply (Mgal) 0.00 0.00 0.00 23.36 18.51 101.55 115.14 93.38 78.71 65.62 67.37 9.17Supplemental Daily Flow (MGD) 0.00 0.00 0.00 0.78 0.60 3.38 3.71 3.01 2.62 2.12 2.25 0.30Notes:Total Annual (Mgal) 572.801) Precipitation data downloaded from NOAA for Dallas/Fort Worth, minimum year 2005 (between 2001‐2015)2) Evaporation data downloaded from TWDB for Dallas County, maximum year 2011 (between 2001‐2015)3) Baseflow is assumed to be 0 as most ponds have negligible baseflow and would have been estimated from observations4) Evapotranspiration is assumed to be 0 because it is only critical when wetland vegetation dominates 5) Infiltration is assumed to be 0 because lake sits on top of the fatty clays of the Eagle Ford Shale6) Assume pond starts at normal pool elevation7) Assume contract requires City of Coppell to keep Northlake at least 1 ft below normal pool elevation8) All methods and equations from iSWM Hydrology Chapter 4 "Water Balance" Worst Case Monthly Water Balance Under Minimum Rainfall and Maximum Evaporation Conditions to Maintain Lake Elevation of at least 484 ft‐mslHydrological & Physical Data for PondMonthly Water Balance 0.00.51.01.52.02.53.03.54.0Supplemental Daily Flow (MGD)Required Supplemental PumpingJanFebMarAprMayJun JulAugSepOctNov Dec