The routing path or the major drainage system is that part of the storm drainage system which carries the runoff, which exceeds the capacity of the designed drainage facilities. The major drainage system shall have the capacity to carry runoff from a storm with a return period of not less than one hundred (100) years without causing significant threat to property or public safety.

It is not economically feasible to size a storm sewer system to collect and convey more than the frequent storm runoff. However, runoff which exceeds the capacity of the storm sewer system must have a route to follow. Essentially, the complete drainage system of an urban area contains two (2) separate drainage elements. While the storm sewers belong to the design system, surface drainageways must be provided for the major flow from more intense storms.

The intent of planning for the major drainage element is to ensure stormwater runoff, which exceeds the capacity of the design drainage system, has a route to follow which will not cause a major loss of property or any loss of life. Street rights-of-way are a common choice for conveying major drainage flows.

The major storm runoff is routed through the drainage system to determine if the combined capacity of the routing path and storm sewer system is sufficient to maintain surface flows within permissible limits. The capacity of the conduit at any given point is assumed to be the same for the major storm as for the initial design storm for preliminary design purposes. If the major storm runoff exceeds the combined capacity of the street and storm sewer drainage system, revision in the major drainage design is required.

Where the street is designated as the major drainage way, the depth of flow shall not exceed twelve (12) inches at gutter line for local and collector streets, and shall not exceed six (6) inches in depth at crown for arterial streets. The same maximum depth criteria will apply where a major drainageway crosses the street.

Routing of the major storm at culvert locations shall be at low areas or sags of vertical curves of streets. Elevations for the design of the street shall be such to permit the major storm to flow across the street and to prevent damage to any existing or proposed building structure.

Where a major drainageway is located outside a street right-of-way, easements will be provided and a grading plan will be submitted with detailed engineering plan submission. The grading plan will include elevations along the routing path and other elevations necessary to show that the major storm is contained within the planned area. The grading plan shall be submitted and filed with the Engineer.