The Virginia – American Water Company provides public water service to over 22,000 residential, commercial, industrial, and “other” customers within the city limits of Alexandria, Virginia.  The Alexandria District obtains its source of supply from interconnections with the Fairfax County Water Authority (FCWA).  It also has limited well capacity.  The system is required to provide usable distribution storage volume equal to 25% of the projected maximum day deliveries from FCWA for demand equalization plus an additional volume for fire protection.

The District distribution storage is provided by five facilities.  Three of these are reservoirs, one is a ground level storage tank, and one is an elevated tank.  Preliminary analysis of the District Water Demands indicates that the existing storage capacity of the system exceeds requirements through the year 2010.  However, some components of the storage system have reached the limits of their design life and are currently in need of rehabilitation.

Project teams will be required to analyze the available data on system use and storage needs and the current condition of the existing facilities.  Recommendations are to be made regarding rehabilitation, abandonment, and replacement of the storage facilities to provide the most cost effective and reliable storage system for the Alexandria District.  Past studies have placed particular emphasis on the Duke Street facilities (described below).  Options considered for these facilities have included repair or replacement of reservoir covers and liners, repair of embankments, retirement of one or both reservoirs, upgrades to the reservoirs, and the construction of alternate storage at the site.

Description of Existing System

 At the end of 1994, the Alexandria District provided public water service to about 22,100 residential, commercial, industrial, and “other” customers within the city limits of Alexandria, Virginia.  Average annual system delivery increased from 14.47 mgd in 1984 to 15.38 mgd in 1994.  The historical maximum day demand of 20.44 mgd occurred during the summer of 1988.  Average annual system delivery is projected to increase to 16.9 mgd, 17.4 mgd, and 17.9 mgd by the year 2000, 2005, and 2010 respectively.  Corresponding maximum day demands are projected to increase to 23.1 mgd, 23.8 mgd, and 24.5 mgd.  These demand projections are used to establish storage requirements and demand conditions for the distribution system.  The increased demand is primarily associated with development plans for three areas of the city: Potomac Yards (Main Service Area), Cameron Station Military Reservation (Jefferson Park High Service Area), and the Carlisle Project (Main Service Area).

As illustrated on Figure 1, the Water Company obtains its source of supply from ten metered interconnections with the Fairfax county Water authority (FCWA).  With one exception, water from the FCWA is supplied at the hydraulic grade for the respective service areas, which are approximately 410 feet, 340 feet and 160 feet for the BRHSA, JPHSA, and MSA, respectively.  Supply to the MSA is reduced from a hydraulic grade of about 210 feet at the MSA Telegraph Road metering point by tow pressure reducing valves located in a vault adjacent to the Duke Street Pumping Station.  The Water Company also has an emergency interconnection in the BRHSA that may be used to supply water to the Arlington County water system.

On an annual basis, about 78% of the water is delivered to the system through the BRSHA metering points, 20% through the MSA metering points, and 2% through the JPHSA metering point.  About 40, 25, and 35 percent of the demand is allocated to the BRSA, JPHSA, and MSA, respectively.  Excess supply to the BRHSA may be delivered to the JPHSA and MSA through several pressure regulating valves.  The pressure regulating valves are set to establish an average hydraulic grade of about 340 feet in the JPHSA and maintain the Main hydraulic grade at about Elevation 160.
 
The overflow elevation of the Payne Street elevated tank establishes the hydraulic grade for MSA at 160 feet.  This hydraulic grade results in maximum and minimum static pressures of 48 psi and 67 psi for customers in the service area at ground elevations ranging from 6 feet to 50 feet above sea level.  There is a significant variation in ground elevations for both the JPHSA and BRHSA.  An average hydraulic grade of 340 in the JPHSA results in maximum and minimum static pressures of 140 psi and 60 psi at ground elevations ranging from 10 feet to 202 feet above sea level.  The range of ground elevations in the BRHSA from 55 feet to 272 feet results in maximum and minimum static pressures of 154 psi and 60 psi for an average hydraulic grade at Elevation 410.

In addition to purchased water from the FCWA, VAWC maintains two wells at the Duke Street site with a combined capacity of 1 mgd.  These wells were constructed during the 1970’s and are used as a supplementary supply to the FCWA interconnections.  Since the Water Service Area Agreement was negotiated in 1982, the operating cost of supplying water from the wells is higher than the purchased water cost.  Consequently, the wells are currently exercised once a month and pumped to waste.  As a supplementary supply, the wells can be pumped to the JPHSA directly or to the Duke Street Reservoirs.  Chemical treatment facilities are provided for the wells, including sodium hypochlorite, fluoride, and a phosphate solution for corrosion control.

Distribution storage totaling 22.7 MG is provided by five facilities.  Three of these are reservoirs, one is a ground level storage tank, and one is an elevated tank.  Water from the ground storage tank and reservoirs are delivered to the respective service area through associated booster stations.  A summary of information on the storage facilities within the Alexandria system is presented in Table 1.  Note that the effective storage is less than the indicated values because of limitations of the system pumps and reserve water required to prevent vortexing.

There is no distribution storage located within the JPHSA.  However, distribution storage for this service area is available from the BRHSA via existing transmission mains and pressure reducing valves.  In addition, pumped storage may be provided from the Duke Street reservoirs via a 3.6-mgd pump at the Duke Street pumping station.  With the exception of the Payne Street elevated tank, the overflow elevations of the storage facilities are below the hydraulic grades established by the FCWA interconnections and PRV settings within the Alexandria distribution system.

The Duke Street Pumping Station is designed to deliver water from the Duke Street Reservoirs to the JPHSA and MSA Service areas.  As illustrated on Figure 2, the reservoirs may be filled from the JPHSA, Duke Street wells, or MSA.  The reservoirs are filled from the JPHSA or Duke Street wells via a pressure sustaining valve located on the west side of the reservoirs.  The reservoirs can be filled from the MSA via a flow control valve located in a meter vault at the Duke Street Pumping Station.  Three pumps are installed at the Duke Street Pumping Station with a total rated capacity of 9.6 mgd.  Pump No. 1 with a rated capacity of 3.6 mgd at 290 feet TDH supplies the JPHSA through a 16-inch discharge main.  Pump Nos. 2 and 3 have rated capacities of 4 mgd (126 feet TDH) and 2 mgd (100 feet TDH), respectively.  Water from these pumps is delivered to the MSA via a 16-inch discharge main, which increases to a 20-inch main at the system entry point.  The pumping station is equipped with an emergency generator.  The usable volume of the Duke Street reservoirs is limited to the quantity of water these pumps can deliver in an 18-hour period.

Chemical treatment at the Duke Street Pumping Station consists of gaseous chlorine, gaseous ammonia, and zinc orthophosphate for corrosion control.  Water entering the Duke Street Reservoirs is chloraminated and can be rechlorinated if there is an insufficient chlorine residual in the water exiting the reservoirs.  All water entering the MSA from the FCWA Telegraph Road interconnection is treated with zinc orthophosphate.

The St. Elmo Pumping Station delivers water from the St. Elmo Reservoir to the system in the northern part of the MSA.  Two identical pumps with rated capacities of 750 gpm at 180 feet TDH are installed in the pumping station.  However, electrical design limitations preclude simultaneous operation of the pumps.  One pump is capable of emptying the reservoir’s estimated usable volume of 0.57 MG in about 12 hours.

The Braddock Road Pumping Station delivers water from the Braddock Road Reservoir to the BRHSA.  The station is equipped with three pumps having a total rated capacity of 18 mgd and an emergency generator.  The rated capacities of the individual pumps are 4 mgd, 6 mgd, and 8 mgd versus 170 feet TDH.  The pumps were designed to meet peak period demands and provide fire flow capacity to the BRHSA and JPHSA.  The total pumping capacity is capable of delivering the estimated usable volume of 4.63 MG in about 6 hours if necessary.

Storage Requirements

 Distribution storage requirements for the Alexandria District consist of demand equalization and fire protections for the BRHSA, JPHSA and MSA.  The Water Company must maintain usable storage within each service area for demand equalization equal to 25 percent of the projected maximum day deliveries from the FCWA.

 Historically, the City of Alexandria’s Fire Department has been proactive in identifying deficiencies and recommending storage and pipeline improvement concepts to be considered for the water supply system.  In the past, the City has expressed concern with respect to the maximum fire flow demands used by the Water Company to assess improvement needs.  For example, the Fire Department has noted the occurrence of 10,000 gpm to 12,000 gpm fires in the downtown and west-end warehouse sections.  According to the 1980 ISO Fire Suppression Rating Schedule, these flows should be delivered for a minimum duration of 4 hours.  Fire reserve requirements adopted by the Water Company typically reflect the ISO requirements, i.e., maximum flows of 3,500 gpm for a 3-hour duration in each service area.

 The minimum volumes determined based on demand equalization and fire suppression should be provided within each service area via gravity or pumping, or in combination with surplus storage from a higher service area gradient.  Reliance on augmenting storage assumes that the existing interconnections and transmission capacity would allow adequate water transfer between service areas.

Condition of Duke Street Reservoirs and Potential Modifications

 Existing liners and covers for the Duke Street reservoirs were installed in 1979 and are approaching their estimated useful life of approximately 20 years.  The installed cost for the existing liners and covers was about $640,000 for both reservoirs.  Since that time Water Company expenses for operating and maintaining the reservoirs have not been significant.  However, a number of problems have occurred that should be addressed with respect to the possible replacement of the reservoir covers and liners within the next several years.

 The most serious of these concerns is the instability of the embankment and reservoir foundation.  In late 1979 and early 1980, slope erosion occurred along a substantial length of the northern upper embankment of Reservoir No. 2.  As a result, about 120 to 150 feet of the cover/liner anchor curbing deflected toward the reservoir.  In addition, a portion of the toe of the embankment had moved into the reservoir beneath the liner.  This subsidence was attributed to superficial seepage of ground water rather than leakage through the liner.  (Pre-construction drawings for the duke Street site indicate that two streams once passed through the service area, including one near the location of the slope failure on Reservoir No. 2).  Measures that were taken to intercept the seepage and prevent additional settlement and potential damage to the liner included the installation of an underground drainage system in the vicinity of the slope failure and construction of a concrete anchor curb in more stable soil.

 During a recent site visit, no additional deflection of the replacement concrete anchor curb was readily visible, although the slope of the embankment appeared to have flattened in the vicinity of the earlier subsidence.  Construction costs for a new liner and cover should include allowances for additional sitework relative to this problem.  Depending on the extent of slope degradation, such sitework may include the replacement of unstable soil, the construction of a deeper anchor curb, and/or the installation of sheet pile to serve as an additional barrier to water flow beneath the slope.

 It is believed that the large volume of storage provided by the Duke Street reservoirs is more than the minimum requirement.  Water quality concerns can occur due to inadequate turnover of the water in the reservoirs, including the presence of bacteria associated with the loss of chorine/chloramine residual, THM formation and nitrification.  There is a contamination risk (via bird droppings, for example) of ponded rainwater or stored water leaking though tears in the reservoir covers.  Improvements to the reservoir design with respect to inlet/outlet configuration, baffling, surface water collection and removal system, and provisions for additional sampling locations within the reservoirs would enhance the overall integrity of the water quality protection provided by replacement flexible liners and covers on the Duke Street reservoirs.  Alternate cover/liner materials should be considered.  In addition, construction of rigid frame structures such as clear span or column-supported aluminum roofs should be considered to eliminate concerns with contamination through tears, possible reduced maintenance costs, and longer service life.

 As shown on Figure 2, the primary inlet/outlet for the Duke Street reservoirs consists of a 24-inch pipe to Reservoir No. 1 and a 16-inch pipe to Reservoir No. 2.  The 16-inch and 24-inch pipes tie to a common 24-inch pipe from the Duke Street Pumping Station.  Under normal operating conditions (reservoirs emptied and filled to and from the MSA), these pipes represent common inlets and outlets to the reservoirs.  During the installation of the original covers and liners, the 24-inch inlet/outlet pipe was relocated to the bottom of Reservoir No. 1.  The elevation of the 16-inch pipe in Reservoir No. 2 is not known.  However, the Water Company has indicated that flow of water form the inlet/outlet piping has impacted the reservoir cover.  This is some indication that the pipe is located close to the maximum water surface elevation.
 In addition to the primary inlet-outlet pipes, the reservoirs may also be filled from 12-inch pipes entering the west side of each reservoir.  The water supplied to these pipes may be obtained form the JPHSA or the Duke Street wells via a pressure sustaining valve installed near the perimeter of Reservoir No. 2.  The Water Company periodically uses these inlets to circulate water through the reservoirs.  This is intended to alleviate stagnant water conditions resulting from operation of the reservoirs in the common inlet/outlet configuration.  However, water entering the reservoirs from the JPHSA cannot be chloraminated for disinfection and reduction of THM formation potential.

 A 12-inch pipe and an 8-inch pipe also connect Reservoir Nos. 1 and 2 through a common berm.  Therefore, the reservoirs normally operate at the same level.  These pipes are located at a maximum depth of about 5 feet below the maximum water surface elevation (97 feet).  The reservoirs also have 8-inch drain pipes on the reservoir bottoms as well as overflow pipes.
 Other storage alternative may be considered.  These could include retirement of one or more reservoirs.  Other alternatives might include retirement of reservoirs in combination with expansion of others or replacement of reservoirs with additional above ground storage.  Pump modifications may also be required depending on the alternative considered.

Scope-of-Work

Work during this semester will be devoted to planning.  The objective is to understand the water distribution system and storage requirements and come to a recommendation regarding how to best meet those storage requirements.  Getting to the point of making a recommendation will require each team to generate and evaluate multiple alternatives and the costs related to those alternatives.  Enough design work will need to be performed to allow a reasonably accurate cost estimate (you have to define “reasonably accurate” for yourself).  Alternatives will not be taken to a final design level this semester.  Next semester you will continue to develop and refine a selected alternative to produce construction documents.
Below are items to be considered during the planning of modifications to the water system.

? System storage requirements and the best way to provide for these requirements including retirement of facilities, expansion of facilities and replacement of facilities
? Replacement of the cover/liner system at the Duke Street Reservoir(s) including alternative materials for a flexible system and a rigid system
? Embankment stability concerns
? Modifications to the reservoirs to address water quality concerns
? Pipe relocations as needed

You will not be able to reach a decision without considering costs (both initial and long term).  However, cost will rarely be the only factor in a decision.  Other factors could include system reliability, community concerns, previous investments, neighborhood impacts, etc.