2006 Water Comprehensive Plan - City of Bellevue

Fire Flow VolumeDefinitionWAC 246-290-230 and DOH design criteria require that new or expanding water systems havecapacity to provide design fire flows during maximum day demand conditions, that is, duringthe highest demand day of the year. It is also a requirement that the analysis be done under theconditions where both equalizing and fire flow storage components have been depleted .Required fire flow volume is calculated by multiplying the design fire flow by a time ofduration, defined as 1 hour for each 1,000 gpm increment of design fire flow. As an example, adesign fire flow of 3,000 gpm would require a minimum time of duration of 3 hours, resulting ina minimum fire flow volume of 540,000 gallons (0.54 MG).CriteriaFire volume criteria was based on the largest fire flow the system can reasonably be expected toprovide within each operating area, determined to be 5,500 gpm in each operating area based onpreviously conducted fire flow analysis. Therefore, required fire storage in each operating areais 1.65 MG. Bellevue Fire Department experience shows that the probability of occurrence ofmultiple large fires in the city is extremely small, except in the case of a large seismic event.However, because of limited hydraulic transfer capacity between the three operating areas,particularly across I-90, separate fire volume components are provided for each operating area.Emergency and Reserve (Standby) VolumeDefinitionEmergency and reserve (or standby) volume is required to supply reasonable system demandsduring an interruption to normal supply or an emergency, such as a major pipeline failure, poweroutage, or other system interruption. A key concept is that establishment of reserve storagevolume involves planning for reasonable system outages, those that can be expected to occurwithin normal planning contingencies. Major system emergencies, such as those created by anearthquake, are covered under emergency system operations planning, because additionalconstraints can be imposed on water usage, thereby reducing anticipated demands.The required emergency and reserve storage volumes depend on the system demand, theduration of the system outage, and the available remaining supply capacity to the system at thetime of the emergency. Emergency volume is the most difficult of the three storage componentsto quantify because it involves subjective evaluation of system emergency scenarios that, bytheir nature, are difficult to predict and evaluate. DOH has published design guidelines thatquantify target storage volumes on the basis of number of equivalent residential connections.For systems that have a single source of supply, the guidelines recommend a standby storagevolume of at least 800 gallons per connection per day. This target volume may be reduced forsystems that have multiple sources of supply and/or redundant supply facilities; however, totalstandby volume may not be less than 200 gallons per day per equivalent connection.8-7