SITING AN ELEVATED TANK: NOT IN MY BACKYARD

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1 SITING AN ELEVATED TANK: NOT IN MY BACKYARD Scott Huneycutt, P.E., Union County Public Works Benjamin Cownie, E.I.T., Black & Veatch Jeff Coggins, P.E., Black & Veatch Kevin T. Laptos, P.E., Black & Veatch* David Taylor Drive, Suite 240 Charlotte, NC ABSTRACT As water distribution systems grow, pumping, pipeline, and storage infrastructure must be constructed to accommodate system expansion and provide safe and reliable water service to system customers. Water storage facilities provide a source of water to help meet peak hourly system demands, fire flows, and emergency supply. When storage facilities are properly located throughout the distribution system, they serve to minimize required pipeline sizes and associated capital costs. Floating storage facilities also serve to minimize required pumping capacity thereby minimizing associated capital and energy costs. In 2011, Union County Public Works (UCPW) (Monroe, NC) determined that pressure zone modifications should be implemented in their water distribution system to improve water service pressures. A new elevated tank and booster pumping station are planned for a new pressure zone which will provide improved water service to an area of the system that currently experiences low pressures at times. Leaders from the influential community where the elevated tank needs to be sited were reluctant to have an elevated tank located within their jurisdiction. This paper presents some of the challenges UCPW had to face and the engineering approach utilized to identify, evaluate, and justify viable elevated tank sites using hydraulic modeling and GIS support tools. Some of the key engineering criteria included ground elevation, location within pressure zone, and proximity to large diameter mains. Other important criteria in developed and/or populated areas can include site availability and cost, visibility, and public perception. The engineering approach used to screen potential elevated tank sites resulted in a defensible shortlist of viable parcels for acquisition negotiations. KEYWORDS Elevated Storage, Pressure Zone Optimization, Tank Site Selection, Hydraulic Modeling, GIS Analysis, Distribution System Optimization INTRODUCTION Background Union County Public Works (UCPW) (Monroe, NC) first identified the need for a 1.5 MG elevated storage tank in the western portion of their distribution system to improve low pressures (Figure 1) and fire flows as part of their 2005 water master plan update. Initial efforts in the late 2000 s to identify and secure a suitable site for the tank encountered various obstacles including identification of willing sellers, high

2 property acquisition costs, local community acceptance, and most importantly municipal zoning ordinance challenges. UCPW analyzed over twenty-five (25) sites and on two separate occasions secured an option to purchase a suitable site pending municipal re-zoning. In both cases the municipality denied the re-zoning request, citing that the tank will not be in harmony with the area. At this point, UCPW retained the services of Black & Veatch to develop a more definitive engineering approach to siting the tank. Figure 1: Low Pressure Areas in Existing 853 West Zone As part of their 2011 Comprehensive Water and Wastewater Master Plan, UCPW determined that in addition to a new elevated tank, pressure zone modifications should also be implemented in the western portion of their water distribution system to improve water service pressures. Instead of constructing an elevated tank at the existing pressure zone hydraulic grade line (HGL) of 853 feet, an elevated tank with an overflow elevation of 880 feet is now planned along with a booster pumping station to establish a new pressure zone (Figure 2). This new pressure zone HGL requires an elevated tank that is 27 feet greater in height than the previously planned tank. Leaders from the local community where the elevated tank needs to be sited were reluctant to have an elevated tank located within their jurisdiction due to visual aesthetics. Ground/pumped storage alternatives were also considered and evaluated during the system evaluation process. However, the capital and operating costs associated with the required pump station coupled with the greater operational challenges associated with the ground storage alternatives caused UCPW to continue to pursue sites for the elevated storage tank alternative.

3 Objective Figure 2: Creation of New 880 Zone and Expansion of Existing 821 Zone The objective of this paper is to present some of the challenges UCPW had to face and the engineering approach utilized after the 2011 Comprehensive Water and Wastewater Master Plan to identify, evaluate, and justify viable elevated tank sites using GIS and hydraulic modeling support tools. Some of the key engineering criteria included ground elevation, location within pressure zone, and proximity to large diameter mains. Other important criteria for developed and/or populated areas included site availability and cost, visibility, and public perception. The engineering approach used to screen potential elevated tank sites resulted in a defensible shortlist of viable parcels for acquisition negotiations. METHODOLOGY/RESULTS Elevated Tank Site Selection Criteria Prior to initiating identification and evaluation of potential tank sites, various site selection criteria were established to help facilitate the process: 1. Location within Pressure Zone To maximize the hydraulic (pressure and fire flow) benefit to the zone and minimize potential water quality problems, the tank must be located in the central portion of the zone. 2. Distance from Existing 24-Inch Transmission Main To provide adequate hydraulic connectivity to the planned 880 Zone pumping station and minimize the length and cost of required tank connecting main(s), the tank should be located near the existing 24-inch transmission main. Potential tank sites further from the 24-inch main may require additional connecting main at additional cost. 3. Ground Elevation To limit tank height (ground to overflow) to less than 200 feet, ground elevations must be greater than 680 feet. Ground elevations greater than 705 feet are most desirable. 4. Site/Parcel Size To provide adequate clearance for construction and future maintenance, the site/parcel will likely need to be at least 2.5 acres to 4 acres in size, depending on the final tank height and specific site conditions. 5. Site/Parcel Availability and Accessibility The site/parcel needs to be available for acquisition and/or purchase from the current owner, preferably undeveloped, and adequately accessible by construction and maintenance equipment. 6. Parcel Cost The costs associated with acquiring the parcel must be validated by an independent property appraisal. Identification and Screening of Potential Tank Sites Using GIS Tools Using Geographic Information System (GIS) tools and data, all areas within the UCPW s service area that met Criteria 1, 2, 3, and 4 were identified and are shown in Figure 3. Colored areas are used to illustrate the distance from the existing 24-inch transmission main and potential costs associated with additional tank connecting piping. Yellow areas are closest to the 24-inch main, while green and blue areas are further. Light versus dark color shading is used to illustrate required tank height. Dark areas and light areas, regardless of color, will require tank heights (ground to overflow) of less than 175 feet and 200 feet, respectively. Union County s tax parcels with associated acreage for larger parcels are also shown.

4 Figure 3: Potential Elevation Tank Sites Conforming to Site Selection Criteria 1, 2, 3, and 4 At this point in the site evaluation process the number of potential tank sites or parcels was reduced from a total of 981 parcels that met Criteria 1, 2, and 3, to 110 parcels that also met Criteria 4. The County s GIS tax parcel database and aerial photography were further analyzed to determine which of the parcels were undeveloped and accessible consistent with Criteria 5. Figure 4 displays these parcels in red that are suitable for further investigations by the County s real estate acquisition team.

5 Figure 4: Potential Elevation Tank Sites Conforming to Site Selection Criteria 5 At this point in the site evaluation process the number of potential tank sites or parcels was reduced from a total of 110 parcels that met Criteria 1, 2, 3, and 4 to 38 parcels that also appeared to meet Criteria 5. Evaluation of Viable Tank Sites Using Hydraulic Modeling Tools Discussions with property owners resulted in two primary tank site options. Site #1 immediately adjacent to the existing 24-inch transmission main and Site #2 approximately 1.5 miles from the 24-inch main to the northeast as shown in Figure 5. Hydraulic modeling performed during the 2011 Comprehensive Master Plan project previously determined that Site #1 was in an optimal location for the new elevated tank and would meet all of the site selection criteria. With the identification of Site #2 which is considerably further from the existing 24-inch main but has significantly lower parcel acquisition cost, it was questioned if and how much additional piping would be needed to provide an adequate hydraulic connection to the new 880 Zone, especially areas of the zone to the west that are closer to Site #1.

6 Figure 5: Locations of Tank Sites #1 and #2 Hydraulic analyses were performed with UCPW s hydraulic model to evaluate Site #2. The analyses were intended to: 1. evaluate the ability of the planned elevated tank to deliver sufficient water pressure and fire flows to the new 880 Zone; 2. compare the hydraulic performance of Site #2 with Site #1 which is immediately adjacent to the existing 24-inch transmission main; and 3. identify and recommend any additional pipeline improvements associated with the Site #2 location, if needed. Site #2 is located at two parcels, 1.15 acre parcel and a portion (2.59 acres) of a 34 acre parcel. The combined parcels provide a site approximately 3.74 acres in total land area. Currently, UCPW customers in this area are directly served by 8-inch main and 16-inch main ties this area of the system into the existing 24-inch transmission main. In order to evaluate the Site #2 location, benchmark high elevation customer locations were selected in both the Eastern and Western portions of the new 880 Zone. For each location, projected 2030 maximum day / peak hour pressure and maximum day fire flow analyses were conducted to assess hydraulic system performance in both sides of the new 880 Zone. - Eastern Benchmark Customer Location: Butterburr Dr and Cinnamon Dr (El 752 ft) - Western Benchmark Customer Location: Crane Rd and Calumut Farms Dr (El 705 ft) Maximum day / peak hour simulations were conducted utilizing the hydraulic model and assumed system storage was partially depleted. With the new elevated tank at Site #1 immediately adjacent to the existing 24-inch transmission main, the minimum peak hour pressure in the Eastern portion of the 880 Zone was predicted to be 42 psi, with the minimum pressure in the Western portion of the 880 Zone predicted to be 47 psi. With the new elevated tank at Site #2, the minimum pressures in the East and West were predicted to be 47 and 40 psi, respectively. BENCHMARK LOCATION SITE #1 PRESSURE SITE #2 PRESSURE PRESSURE DIFFERENCE Eastern 42 psi 47 psi +5 psi Western 47 psi 40 psi - 7 psi Table 1: Maximum Day / Peak Hour Benchmark Pressure Comparison

7 As shown in Table 1, Site #2 is predicted to generally increase peak hour pressures to the Eastern portion of the 880 Zone and decrease pressures in the Western portion, however, pressures throughout the zone are predicted to remain above the 2011 master plan pressure performance criterion of 40 psi. Maximum day / fire flow simulations were conducted utilizing the hydraulic model with system storage assumed to be substantially depleted at the end of a fire flow event. With the new elevated tank at Site #1, the available fire flow (at a minimum residual pressure of 20 psi) at the Eastern benchmark location was predicted to be approximately 1,250 gpm, with the available fire flow at the Western benchmark location predicted to be 1,950 gpm. With the new elevated tank at Site #2, the available fire flows in the East and West were predicted to be 1,350 gpm and 1,900 gpm, respectively. BENCHMARK LOCATION SITE #1 FIRE FLOW SITE #2 FIRE FLOW FIRE FLOW DIFFERENCE Eastern 1,250 psi 1,350 gpm +100 gpm Western 1,950 gpm 1,900 gpm - 50 gpm Table 2: Maximum Day Benchmark Fire Flow Comparison As shown in Table 2, Site #2 is predicted to generally increase available fire flows to the Eastern portion of the 880 Zone and decrease available fire flows in the Western portion. Site #2 is at a better location than Site #1 to provide fire flows to the remote, high elevation customers in the Eastern portion of the 880 Zone while not substantially adversely affecting customers in the West. DISCUSSION/CONCLUSIONS It was previously recognized and mentioned that potential tank sites far from the existing 24-inch main in the 880 Zone could require additional pipeline improvements to provide an adequate hydraulic connection to existing transmission mains and the rest of the 880 Zone. However, through the use of hydraulic modeling, Site #2 which is very close to a 16-inch main was determined to essentially be hydraulically equivalent to Site #1 which is directly adjacent to the 24-inch transmission main for the system conditions and scenarios discussed above. In general, the Site #2 location is predicted to result in better pressure and fire flow performance in the East, with slightly reduced performance in the West; however, because the Western portion of the 880 Zone will also be served directly by the 880/821 Booster Pump Station, the slight reduction in service due to the elevated tank being moved further to the northeast is largely mitigated. Because the Site #2 location is close to the existing 16-inch main, additional pipeline improvements would not be required to provide adequate zone-wide hydraulic performance. The only pipeline improvement needed would be 700 ft, 20-inch main between the new tank site and the 16-inch main at a nominal project cost of approximately $180,000. While the total acreage of Site #2 (3.74 acres) is well within the recommended site size of 2.5 to 4 acres, GIS analysis also provided a means to assess if the shape of the proposed combined parcel site could accommodate the elevated tank with an adequate buffer radius for construction and future maintenance. UCPW currently has an option to purchase this site pending the re-zoning approval from the municipality. A rendering of how the planned tank could look on Site #2 is shown in Figure 6.

8 Figure 6: Elevated Tank Rendering for Site #2 As discussed in this paper, GIS and hydraulic modeling are invaluable tools to assist with siting an elevated tank. Due to the size and visibility of most elevated tanks, oftentimes water utilities are met with opposition to their construction in developed areas. For these cases, it is especially important for utilities to carefully evaluate storage tank alternatives and location options to provide defensible reasoning as to why the storage tank should be located at the best available location. In addition, GIS and hydraulic modeling are excellent tools for quantifying the benefits (pressures, fire flows, and service reliability) that will be directly realized by utility customers in the local community. ACKNOWLEDGEMENT The authors acknowledge the Union County Public Works department staff for providing system data and information and supporting the study projects which made this paper possible.