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I I I I I I I I I I I I i I I I I I I I 36 SECTION VI DESIGN CRITERIA Major drainage is the cornerstone of an urban storm runoff system. The major drainage system will operate whether or not it has been planned and designed, and whether or not urban development is wisely located in respect to it. Thus, the major drainage must be given high priority when considering drainage improvements. The maJor system for the lOO-year runoff often includes many features such as natural and artificial channels, long underground conduit outfalls, street, property line drainage easements, and other water carrying routes. It is closely allied to, but separate from, the initial drainage system consisting of storm sewers, curbs and gutters, and drainageways. A good major system can reduce or eliminate the cost of an underground storm sewer system. An III conceived major system can make a storm sewer system very costly. The 2 or 10-year runoff can flow in the major system, but only a portion of the 100- year runoff will flow in the initial drainage system. Routing of the outfall is usually a matter of following the natural valley thalweg. In these cases the routing is a more straightforward matter, and essentially it need only be defined by mapping. In many urbanized areas, however, there is no thalweg, or the thalweg has been filled and built upon. In these cases, it is necessary to determine many factors before a route can be chosen, The preliminary design phase of master planning is second in impor- tance only to route choosing and the concept stage. It Is here where decisions are made as to design velocities, location of structures, means of accomodating conflicting utilities and irrIgation ditches, and the potential alternate uses in order to create greater benefIts to the community and to reduce drainage costs, It is also here where decisions are made as to the use of detentIon storage in order to reduce peak rates of flow. These decisions have a direct bearing on the design rates of flow and thus the size of the storm drainage works required. The design criteria utilized for this storm drainage pilot planning study are those contained in the Denver Regional Council of Govern- ments Urban Storm Drainage Criteria Manual, Volumes 1 and 2.