4.11 Design Basis & Capacity
a. Design Basis.
1) Sewers shall be designed to carry the peak domestic, commercial and industrial contributions, plus infiltration/inflow from the individual gravity service laterals, sewer mains and manholes. Where more detailed information is not available, new sewer systems within the City may be designed on the basis of the following flows.
a) Residential domestic flows: 100 gal/capita/day (gpcd)
b) Schools, non-residential students & staff: 25 gpcd
c) Commercial, non-residential customers
(1) Restaurant/Cafe: 40 gal/day/seat
(2) Tavern/Bar: 50 gal/day/seat
d) Laundries, self-service: 500 gal/day/machine
e) Infiltration/Inflow
(1) New facilities: Per estimated EPA Design guidelines
(2) Existing facilities: Per estimated or DEQ design guidelines in conjunction with concurrent Sewer Master Plan.
2) Assumed flows from types of establishments not listed above shall be as approved by the Director.
b. Capacity: Sewers should be designed to carry, when flowing full but not surcharged, not less than the following plus existing or planned flows from upstream properties:
1) Lateral and Mainline Sewers:
a) For new installations serving new areas, minimum peak design flow shall not be less than 4 times the design sanitary flow plus I/I allowance.
b) For new installations serving existing sewered areas, minimum peak design flow shall not be less than 4 times the design sanitary flow plus an additional I/I allowance based on existing conditions.
2) Trunk Sewers:
a) For new installations serving new areas, minimum peak design flow shall not be less than 3 times the design sanitary flow plus I/I allowance.
b) For new installations serving existing sewered areas, minimum peak design flow shall not be less than 4 times the design sanitary flow plus an additional I/I allowance based on existing conditions.
3) Roughness Coefficient:
a) A minimum "n" value of 0.013 shall be used in Manning's formula for the design of all sewer facilities regardless of pipe material. The use of higher "n" values for existing pipe may be required by the Director. An "n" value of less than 0.013 will not be considered for approval.
4) It is recommended that design calculations include estimates of average, maximum and minimum daily flows. The submission of design calculations will not ordinarily be required, and engineers should be prepared to substantiate pipe sizes, layout, population estimates, land uses or other design assumptions as may be requested.