HomeMy WebLinkAboutAPPENDIX D - Draft 07 29 2020 Norton East Ranch Wastewater Memo V1
July 29, 2020 Kevin Spencer Norton Properties Greg Stratton Kilday & Stratton via email Re: Norton East Ranch Wastewater Analysis
Dear Kevin and Greg:
We have compared the wastewater flow projections from the 2009 Sanderson Stewart Lift Station Design Report to revised flow values we have developed for the overall project area based on the platted development, proposed development and the revised zoning. In summary, the revised updated flow values are less than the original design and as such the lift station has capacity for the development as proposed. Original Design Values The following wastewater flows are from the 2009 Sanderson Stewart Report:
Zoning Area (ac)
Dwelling Unit Duty (du/ac)
Dwelling Units
Population Duty (per/du) Population
Ave. Day Qww Flow Duty (gpd/ac)
Ave. Day Qww Flow (gpd)
B-P 24.63 5.2 128 2.11 270 960 23,645
R-O 63.59 5.2 331 2.11 698 980 62,318
R-3 52.84 6.5 343 2.11 725 1220 64,465
R-2 21 5.2 109 2.11 230 980 20,580
R-4 39.97 10.4 416 2.11 877 1950 77,942
Phase 1 40 n/a 314 2.11 663 89 58,966
Total 242.03 1,641 3,463 307,916
Based on a peaking factor of 3.39 and an allowance for infiltration, the peak flow developed by Sanderson and Stewart was 750 gpm. Proposed Buildout of Development Figure A is included as an attachment to this letter and outlines the various phases that have been developed, that are planned for development, and that are proposed to be rezoned. Updated flow estimations have been computed based on platted lots, proposed development plans, and proposed zoning.
The following table summarizes the estimated flows from the platted areas: Area Number of Units (1) Flow Per Unit (gpd) Total (gpd) Phase 1
Single Family 35 140 4,900
Multi Family 52 140 7,280
Phase 2
Single Family 30 140 4,200 Phase 2A Single Family 18 140 2,520 Multi Family 28 140 3,920
Phase 3A
Single Family 48 140 6,720
Phase 3B
Single Family 30 140 4,200 Phase 3C Single Family 30 140 4,200
Phase 4
Single Family 9 140 1,260
Multi Family 96 140 13,440
Total 376 52,640
Note 1. Assumed one unit per single family lot and multi-family lot unit count taken from final plat park table. The following table summarizes the estimated flows form the areas with proposed development plans:
Area Number of Units (1) Flow Per Unit (gpd) Total (gpd)
Phase 5 Single Family 138 140 19,320 ADU/Duplex (30%) 41.4 140 5,796 Phase 3A Remainder
Single Family 150 140 21,000
Total 329.4 46,116
Note 1. The Phase 5 unit count is based on number of lots plus an allowance for 30% of the lots to have an ADU or a duplex unit. The Phase 3A Remainder area is based on a unit count provided by Kilday & Stratton based on a proposed development plan.
The following table summarizes the estimated flows form the areas that are proposed to be rezoned: Area Number of Acres Flow Per Unit (gallon/acre/day) Total (gpd)
R-5 Zoned 8.72 2,100 (1) 18,312 REMU Zoned 8.49 2,100 (2) 17,829 B-2M Zoned 65.34 2,840 (3) 185,566 Total 77.59 221,707
Note 1. For R-5 a density of 15 dwelling units per acre was utilized for a total of 2,100
gallons per acre. City does not have a standard value.
Note 2. For REMU the City standard uses the same value as R-4 zoning which is 1,456
gallons per acre per day. This has been increased to 15 dwelling units per acre for a
total of 2,100 gallons per acre based on mixed use potential.
Note 3. For B-2M a base level of 2,000 gallons per day per acre was used based on B-2
zoning and an additional 840 gallons per day per acre were added to account for an
estimated residential component at 6 units per acre. This results in a total of 2,840
gallons per day.
Based on these three tables the total flow from the development is estimated to be 320,463
gallons per day. Based on a peaking factor of 3.25 and a base infiltration rate of 25 gpm the peak
flow rate is calculated to be 748 gpm.
Evaluation of Flow Projections
We evaluated the reasons why the flows are projected to be lower than what was first anticipated
even though the areas generally south of Fallon Street will probably be developed at higher
densities. The following items were identified:
• Change in City Flow Calculations
o The 2009 calculations were based on 2.11 people per dwelling unit and 89 gpd per
person for a total of 188 gpd per unit.
o The 2015 Wastewater Facility Plan updated these values to 2.17 people per
dwelling unit and 64.4 gpd per person for a total of 140 gpd per unit.
o For the overall development this allowed for densities to increase 34% at the same
total flow.
• Developed and Planned Areas are Less Dense than Planned (generally north of Fallon
Street)
o Based on the areas that are developed or fully planned, the estimated the number
of units was extrapolated to be 1,126 from the 2009 calculations.
o This area as platted and planned will have an estimated number of units of 705.
o This is reduction in density of 37%.
Norton Properties, LLC – City of Bozeman Wastewater Agreement
Norton Properties, LLC, entered into Wastewater Collection System Agreement in 2014 that
generally addressed sewer service to the Norton Annexation property. The agreement states that
the City will provide adequate capacity to the allow the property to develop at densities allowed
by zoning that was in effect at the time of the agreement. It is our understanding that the zoning
in effect in 2014 was the same that existed in 2009 and was used as a basis for Sanderson
Stewart’s evaluation.
At the time of the agreement, the 2009 Sanderson Stewart Lift Station Design Report had been
approved by the City of Bozeman which suggests that the 750 gpm peak flow value in the report
was acceptable representation of what would be allowed for a peak flow.
The agreement has requirements for participating in offsite improvements if the property is served
by the Cattail Creek Drainage Zone. This is currently the case and the developer participated in
constructing sewer improvements in Baxter Lane and the Cottonwood Road right of way to
provide added capacity for the property.
The agreement also states that if the property is served by the Baxter Creek Drainage Zone that
the developer generally has no obligations to participate in offsite improvements. Once the City
of Bozeman has completed the Davis Lift Station and Norton Outfall project the development will
be served by the Baxter Creek Drainage Zone.
Actual Flows and Lift Station Modifications
We have contacted the City of Bozeman to get flow readings from the Norton Lift Station to confirm
that the actual uses are in line with the projected uses. As of the date of this letter we have not
received this information.
In order to allow for the buildout of the development, the Norton Lift Station will need to be
upgraded as generally outlined in the Sanderson Stewart 2009 report. It is recommended that
once actual flow values are available that an evaluation be completed on when upgrades may be
needed based on the projected development schedule.
If you have any questions or need additional information regarding this evaluation, please contact
us.
Sincerely,
James Nickelson, PE Senior Engineer Enclosures Figure A Wastewater Collection System Agreement