Peak Demand Estimates

The maximum hourly or peak-demand flow upon which to base the design of a water distribution system should be determined for each situation. A small residential community, for example, would have characteristics different from a new realty subdivision, central school, or children's camp. Therefore, the design flow to determine distribution system capacity should reflect the pattern of living or operation, probable water usage, and demand of that particular type of establishment or community. At the same time, consideration should be given to the location of existing and future institutions, industrial areas, suburban or fringe areas, highways, shopping centers, schools, subdivisions, and direction of growth. In this connection, reference to the city, town, or regional comprehensive or master plan, where available, can be very helpful. Larger cities generally have a higher per-capita water consumption than smaller cities, but smaller communities have higher percentage peak-demand flow than larger communities.

The maximum hourly domestic water consumption for cities with a population above 50,000 will vary from about 200 to 700 percent of the average-day annual hourly water consumption; the maximum hourly water demand in smaller cities will probably vary from 300 to 1,000 percent of the average-day annual hourly water consumption. The daily variation is reported to be 150 to 250 percent, and the monthly variation 120 to 150 percent of the average annual daily demand in small cities.159 A survey of 647 utilities serving populations of 10,000 or more in 1970 found the mean maximum daily demand to be 1.78 times the average day, with a range of 1.00 to 5.22. Studies in England showed that the peak flow is about 10 times the average flow in cities with a population of 5,000.160 It can be said that the smaller and newer the community, the greater the probable variation in water consumption from the average will be.

Various bases have been used to estimate the probable peak demand at real estate subdivisions, camps, apartment buildings, and other places. One assumption for small water plants serving residential communities is to say that, for all practical purposes, almost all water for domestic purposes is used in 12 hours.161 The maximum hourly rate is taken as twice the maximum daily hourly rate, and the maximum daily hourly rate is 1.5 times the average maximum hourly rate. If the average maximum monthly flow is 1.5 times the average monthly annual flow, then the maximum hour's consumption rate is 9 times the average daily hourly flow rate.

Another basis used on Long Island is maximum daily flow rate = 4 times average daily flow rate; maximum 6-hour rate = 8 times average daily flow rate; and maximum 1-hour rate = 9.5 times average daily flow rate.162

A study of small water supply systems in Illinois seems to indicate that the maximum hourly demand rate is 6 times the average daily hourly consumption.163

An analysis by Wolff and Loos164 showed that peak water demands varied from 500 to 600 percent over the average day for older suburban neighborhoods with small lots; to 900 percent for neighborhoods with 0.25- to 0.5-acre lots; and to 1,500 percent for new and old neighborhoods with 0.33- to 3-acre lots. Kuranz, Taylor, and many others have also studied the variations in residential water use.165

The results of a composite study of the probable maximum momentary demand are shown in Figure 2.10. It is cautioned, however, that for other than average conditions, the required supply should be supplemented as might be appropriate for fire flows, industries, and other special demands.

Peak flows have also been studied at camps, schools, apartment buildings, highway rest areas, and other places.

Dwelling units, residential area

FIGURE 2.10 Probable maximum momentary water demand.

Dwelling units, residential area

FIGURE 2.10 Probable maximum momentary water demand.

The design of water requirements at toll road and superhighway service areas introduces special considerations that are typical for the installation. It is generally assumed that the sewage flow equals the water flow. In one study of national turnpike and highway restaurant experience, the extreme peak flow was estimated at 1,890 gpd per counter seat and 810 gpd per table seat; the peak day was taken as 630 gpd per counter seat and 270 gpd per table seat.166 In another study of the same problem, the flow was estimated at 350 gpd per counter seat plus 150 gpd per table seat.167 The flow was 200 percent of the daily average at noon and 160 percent of the daily average at 6 p.m. It was concluded that 10 percent of the cars passing a service area will enter and will require 15 to 20 gallons per person. A performance study after 1 year of operation of the Kansas Turnpike service areas showed that 20 percent of cars passing service areas will enter; there will be 1.5 restaurant customers per car; average water usage will be 10 gallons per restaurant customer, of which 10 percent is in connection with gasoline service; and plant flows may increase four to five times in a matter of seconds.168

Awwa Water Demand Curve Restaurant
FIGURE 2.11 Estimate curves for demand load. (Source: R. B. Hunter, "Water-Distributing Systems for Buildings," Report BMS 79, National Bureau of Standards for Building Materials and Structures, November 1941.)

Peak flows for apartment-type buildings can be estimated using the curves developed by Hunter.169 Figure 2.11 and Tables 2.8 and 2.9 can be used in applying this method. Additions should be made for continuous flows. This method may be used for the design of small water systems, but the peak flows determined will be somewhat high.

At schools, peak flows would occur at recess and lunch periods and after gym classes. At motels, peak flows would occur between 7 and 9 a.m. and between 5 and 7 p.m.

It must be emphasized that actual meter readings from a similar type establishment or community should be used whenever possible in preference to an estimate. Time spent to obtain this information is a good investment, as each installation has different characteristics. Hence, the estimates and procedures mentioned here should be used as a guide to supplement specific studies and aid in the application of informed engineering judgment. Peak demands and

TABLE 2.8 Demand Weight of Fixtures in Fixture Units0

Fixture or Group6


Type of Supply

Weight in Fixture

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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  • phillipp
    Why is peak flow taken in environmental engineering?
    4 years ago
  • toby
    What is peak demand in environment engineering?
    3 years ago

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