Dry Weather Flow in SWMM 5

Subject:   Dry Weather Flow in SWMM 5

Dry weather flow can be added to any node in SWMM 5.  The dry weather flow is computed as the average flow * the monthly pattern * the daily pattern * hourly pattern * the weekend daily pattern to give the Dry Weather Flow at any time step (Figure 1).   Since the four types of patterns (Figure 2) are all multiplied together then for Saturday and Sunday the hourly pattern and the weekend hourly pattern will both be used.   This will have the effect of overestimating the flow if the multipliers are greater than 1 and underestimating the flow if the multipliers are less than one.  You should enter the  Pattern X for the Weekend Hourly Pattern in SWMM 5  where

X  = Weekend Hourly Pattern / Hourly Pattern

So that when the pattern X is multiplied by the Hourly Pattern the program will use the intended Weekend Pattern.

Figure 1.  How Dry Weather Flow is Computed in SWMM 5

Figure 2.  The Four Types of Time Patterns in SWMM 5.0.022