2006 Water Comprehensive Plan - City of Bellevue

Demand. The quantity of water obtained from the water supply source over a given period oftime to meet the needs of domestic, commercial, industrial, and public use. It also includesfirefighting water, system losses, and miscellaneous other water uses. Demands are normallydiscussed in terms of flow rate, such as million gallons per day (mgd) or gallons per minute(gpm). Flow rates can be described in terms of a volume of water delivered during a specifictime period. Flow rates pertinent to the analysis and design of water systems are:Average Daily Demand (ADD). The total volume of water delivered to the watersystem in a year divided by the number of days in the year.Maximum Month Demand. The total volume of water delivered to the water systemduring the month of maximum water use divided by the number of days in the month.Maximum Day Demand (MDD). The volume of water delivered to the water system onthe year's maximum water use day.Peak-Hour Demand (PHD). The volume of water delivered to the water system in thehour of maximum use, usually during the maximum day.Equalizing Storage. Equalizing storage is the total volume needed to satisfy peak systemdemands in excess of the hydraulic capacity of the supply system. Equalizing volumerequirements are greatest on the day of maximum system demand. Equalizing storagecompensates for demands over the average flow rate supplied at system inlets by filling up whenwater demands are lower than the average inlet flow rate and then emptying when demands arehigher.Fire Flow. The rate of flow of water available to fight fires.Fire Storage. Reservoir capacity required to sustain fire flows, over a given period of time.Head. A measure of pressure or force exerted by water. Head is measured in feet and can beconverted to pounds per square inch (psi) by dividing by 2.31.Head Loss. Pressure reduction resulting from pipeline wall friction, bends, physicalrestrictions, or obstructions as water flows through a pipe. Head loss is determined by pipediameter, internal roughness, and water velocity.Hydraulic Elevation. The height of a free water surface above a defined datum; the heightabove a datum to which water in a pressure pipeline would rise in a vertical, open-end pipe.Peaking Factor. A dimensionless parameter used to express the relationship among MaximumDay Demand, Peak-Hour Demand, and Maximum Month Demand, as a ratio to the AverageDay Demand. The peaking factor used for design purposes is often based on the most severepeaking trends observed on a historical basis.1-3