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I I I I I 16 of the importance and functioning of these two drainage systems. The function of the initial system is to handle runoff so that it wil elim- inate inconvenience. Beyond this, however, the maJor system fulfills the main damage-prevention function. The maJor system will operate whether or not it has been designed and whether or not urban Improve- ments are situated wisely in regard to it. In nearly all cases it is economically impossible to provide an initial drainage system of suf- ficient capacity to obviate the need for the maJor drainage system. I I I I I I I The work in this assignment deals exclusively with the major drainage system. The study did not include a review of the initial drainage system. Once the major drainage system is planned, designed and con- structed, the initial drainage system consisting of storm sewers and small channels falls into order more easily. The future cost of storm sewer works In Boulder will be only a small fraction of the amount that would be spent without a good major drainage system. In many places storm sewers will be unnecessary because of the existence of the maJor drainage works. It should be kept in mind that any money spent On maJor drainage will be more than saved in the future because of the savings On the storm sewer systems. ALTERNATES STUDIED The basic alternates which were developed and studied and presented to City representatives with advantages and disadvantages ere 8S follows: I. GREGORY CREEK 1. Upstream from Baseline Remove old cottonwood up to traIlhead, pave road or stabilize gravel surfacing, re-establish channel from trailhead to Base- line, and develop sedIment retention area immediately up- stream from Baseline Avenue. I I I I I I I 2. Baseline to Pennsylvania Avenue Remove all signIfIcant obstructions and construct road culverts or brIdges where needed. Several private driveways require re- construction over channel. Several re-alignments are required because of unfortunate home sitlngs. ,orne check dams are required because of the steep average grade of 8 per cent. 3. PennsylvanIa to Boulder Creek a. 7th Street Conduit Chute. A box culvert or CMP concrete pipe conduit would be laid in 7th Street, with a cross- sectional area of about 80 to 100 square feet.* A debris basin is required west of 7th Street to keep damaging rocks out of the conduIt. A stIlling basin would be constructed on the south bank of Boulder Creek. Low flows would follow existIng channel. * Planned as 40 to 100 SQ. ft.