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When discharge is high enough to produce critical depth equal to the crown of the culvert barrel, the full flow condition shown in figure III-IB will occur. The outlet velocity reduction is again illustrated in the previous example. In this case, however, it is necessary to determine if the increased culvert dimensions result in brink depth below the culvert crown. When this occurs, the flow area used in the continuity equation is that associated with brink depth, which for this illustration is assumed to be critical depth. Figures III-3 through 1II-8 are included for convenience in determining critical depth for various shapes of culverts. Example: A 3.0-foot CMPdischarging 100 cfs, flowing full with tailwater of 2.0 feet. From Figure 1II-4 critical depth (de) exceeds 3.0 feet. Therefore, the barrel is flowing full to the end. From table 1II-2 with d/D=l, A/D2=0.785 and V=100/.785(9)=14.15 fps. Changing to a 4.0-foot CMP, changes dc to 3.1 feet which is less than D so dc controls outlet velocity. v = FULL 100 = 7.98 and dc/D = 3.1 = .78 .785(16) 4.0 V/V = 1.16 from figure VII-C-3 and V = 1.16(7.98) = 9.25 FULL This is a reduction of about 35 percent instead of the approximate 44 percent indicated in the previous example. When culverts discharge as in figure III-IC and D with critical depth near the outlet, changing the barrel slope will have no effect on the outlet velocity as long as the slope is less than critical slope. Changing the resistance factor will change the depth at the outlet an insignificant degree and will, therefore, not modify the outlet velocity. The initial step is to compute normal depth (tailwater) in the outlet channel, tables III-l and 2 at the end of this section facilitates normal depth calculations with this, Figures III-9 and 10 may be used directly to determine out- let brink depths for rectangular and circular sections, respectively. These figures are dimensionless rating curves 1II-2 e e e ~