2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

operate and maintain, plus they increase the water age and as such should not be unnecessarily
oversized.
Capacity must be established and documented by an engineering study using the following basic
criteria:
The planning horizon for demand projections must be not less than 20 years. For
new facilities, the planning horizon should be 50 years or more.
Volume should be sufficient to deliver design peak hourly demand at 30 psi to the
pressure zone(s) served. This volume requirement may be reduced when the source
water facilities have sufficient capacity with standby power for pumping to the
reservoir and/or when another storage reservoir can be used to supplement peak
demands of the zone(s). During fire flow conditions, the combination of storage and
delivery system capacity must be adequate to provide water at the required flow
rate and a minimum 20 psi in the main.
To determine the emergency /standby storage component, identify the reasonable
emergencies, define the duration and level of service during the emergency, then
apply SPU’s reliability criteria as described in the current WSP.
Water quality impact of storage and design considerations to enable regulatory
compliance throughout the life of the facility.
5.9.1.3 New or Modifications/Expansions of Existing Storage Facilities
A. General Considerations for New Facilities
The following are general considerations for planning and preliminary design of new
storage facilities or for modifications, expansions of existing facilities, retirements of
facilities or downsizing facilities:
Hydraulic grade line of the water supply system
Pressure zones served by the storage facility
Sizes and capacities of transmission or feeder mains and, where applicable,
booster pumping stations, that supply the storage facility (existing and future)
Sizes and capacities of distribution mains in the pressure zones served by the
storage facility (existing and future)
Availability and type of discharge points for overflow and drain-down water
from the storage facility
Geotechnical and seismic characteristics
Vehicle access for all anticipated vehicle and equipment types
Security
Fire services (fire flow, emergency engine fill points post-earthquake)
Land ownership and future use by City of Seattle departments
Environmental impacts to adjacent properties
5-48 SPU Design Standards and Guidelines