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DRAINAGE CRITERIA MANUAL RUNOFF study and subbasins in the study area. A field check and possibly field . surveys should be made for each basin. At this stage of planning, the possibility of the diversion of transbasin waters should be investigated. Transbasin diversions out of the study area should also be kept in mind. The engi neer shoul d be very cauti ous when reduci ng a des i gn flow due to a transbasin export, particularly for the major storm analysis. See Colorado Drainage Law in this manual for comments on liability concerning transbasin waters. The major storm drainage basin does not always coincide with the minor storm drainage basin. This is often the case in urban areas where a low flow will stay next to a curb and follow the lowest grade, but when a large flow occurs the water will be deep enough so that part of the water will overflow street crowns and flow into a new subbasin. For an example of how to apply the Rational Method refer to the STORM SEWER part of this manual. 3.8 Major Storm Analysis When analyzing the major runoff occurring on an area that has a storm sewer system sized for the initial storm, care must be used when applying the Rational Method. Normal application of the Rational Method assumes that all of the runoff is coll ected by the storm sewer or gutter. In the initial design the time of concentration is, in part, dependent upon the flow time in the sewer. However, during the major runoff the sewers should be fully taxed and cannot accept all the water flowing to the inlets. This additional water then flows by the inlets and continues overland, generally at a significantly lower velocity than the water in the storm sewers. This requires an analysis of different concentration times between underground flow and overland flow. This difference in travel times provides opportunities for the storm sewer to continue flowing full for a longer period and to, in effect, carry significant portions of the major runoff. The basis for this increased benefit is that the excess water from one inlet will flow to the next inlet downhill, using the overland route. If that inlet too is fully taxed, the water will often cDntinue on until capacity is available in the storm sewer. The analysis of this aspect of the interaction between the storm sewer system and the major storm runoff is complex. The most useful procedure for this analysis is the routing of hydrographs through the two routes concurrently. 5-1-84 . .