Resolutions-1997-021

If the five-year return frequency flow exceeds the capacity of a 48-inch pipe, open channel flow may be used in lieu of an enclosed system upon approval of the City Engineer. Generally, this will be approved only where a stream or creek exists. C. The overland flow route shall be designed to convey the 100-year return frequency flow assuming the pipe is inoperative. Such routes shall be designed to convey the flows without damage to property and shall be clearly designated on the construction drawings and final plat or site plan. D. All design flows shall be calculated using commonly accepted engineering practices appropriate for the size of the drainage area under consideration. All designs shall consider existing and fully developed conditions and use the larger of the two flows. E. The minimum size for any storm sewer receiving surface water is 12 inches in diameter, excluding storm water management control structures. The minimum size for subsurface drainage pipes receiving only ground water and storm water management control structures, is 6 inches in diameter. F. All storm sewer pipes shall have a slope which will give a mean velocity when flowing full of not less than 2.0 feet per second based on Manning's formula using a minimum "n" value of 0.013. G. Where velocities in a pipe are greater than 15 feet per second are calculated, special provisions shall be made to protect against displacement, abrasion or shock. H. All area intakes (excluding street intakes) shall include trash racks and be designed assuming 25% blockage by debris. I. The maximum slope for storm sewer outlet lines shall be 10%. J. If R.C.P. is utilized for outlets lines having a slope greater than or equal to 8%, all joints must be tied together. The flared end section and last two pipe joints shall be tied together in all instances. K. All pipe outlets must extend to an existing, established drainage channel. If there are no existing channels within the property being developed the developer shall convey the storm water through an underground piping system to the nearest established drainage channel. 9.04 OPEN CHANNEL FLOW A. The erosion potential of the soil shall be evaluated with regard to anticipated velocities. Appropriate measures shall be taken to protect the soil and/or reduce velocities to prevent erosion. B. The channel capacity in conjunction with possible over-bank flow shall be designed to safely convey the 100-year return frequency flow. If over-bank flow is considered, its impact must be evaluated and area of conveyance protected by easement.