1000 1100 1200 1300 1400 1500 Hours

Third and fourth weeks of July

FIGURE 1.7 Hypothetical monitored flow derived for the month of July.

1.7.1 Average Daily Flow Rate

If we had two years worth of data, then the average daily flow rate would be the mean over the two-year period. If we had three, four,... or Z years worth of data, then the average daily flow rate would be the mean over the three-, four-,., or Z-year period. The means obtained from the long years of records would approximate the long-term means. If Z is a very large number such as 30 years, the long-term mean obtained would really be the true mean.

Now, as mentioned, a field survey has a very serious drawback: the length of the survey is very short. Any survey will not last for 30 years, for example. Nonetheless, a field survey can still be accurate compared with other methods of determining design flows. It is to be noted that before any grant for the construction of sewage facilities was given to any community as required by the Clean Water Act of 1972, the Environmental Protection Agency required a field survey.

With this limitation in mind, let us now derive the average daily flow from Figures 1.6 and 1.7 using the probability distribution analysis. Remember that the average corresponds to the 50% probability. The figures contain, all in all, 61 average daily flow rate values. (The total number of days added for the months of June and July.) In order to apply the probability distribution analysis, the average daily flow rates for each of these days must be calculated. This may done easily by graphical integration.

Figure 1.8 shows the general scheme for performing a graphical integration applied to flow rates of the first day of June. As indicated, the integration is performed by

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