Conduits

The storm drainage conduit system transports the runoff from the surface collection system (inlets) to the outfall. Conduit connections between points in the network such as junctions and inlets are referred to as "runs." Although it is an integral component, the conduit system is analyzed independently of the inlet system. However, the configuration of laterals and trunk lines is controlled by the locations of all inlet and roadway layouts, minimum cover requirements, and utility and foundation locations.
The longitudinal slope of the conduit affects the capacity. The slope of the subject conduit run is tentatively established during the system planning stage of design. Typically, the slope will be approximately parallel to the surface topography. However, the slope may have to be adjusted to adapt to critical elevations (such as outfall elevations or utilities) to increase capacity or to afford adequate cover for the conduit.
The Department minimum diameter for trunk lines and laterals is 18 inches because of difficulties in the construction and maintenance of smaller sizes. Some designers prefer to limit the minimum diameter to 24 inches. The following recommendations on conduit dimensions should be considered:
  • Larger into smaller conduit dimensions -- Avoid discharging the flow of a larger conduit into a smaller conduit. The capacity of the smaller conduit may theoretically be greater due to a steeper slope; however, a reduction in size almost always results in operational problems and expenses for the system. Debris that may pass through a larger dimension may clog as it enters a smaller dimension.
  • Soffit and flow line placement in conduits -- At changes in conduit size, the soffit (top inside surface), not the flow line, of the two conduits should be aligned. When flow lines are aligned, the smaller pipe often must discharge against a head. It may not be feasible to follow this guideline in every instance, but it should be the rule whenever practicable. It is acceptable to have the entire downstream conduit offset downward because the flow would still not discharge against a head.
  • Conduit length -- The approximate length of conduit should be determined as the inlets and junctions are located. The length is the distance from the centerline of the upstream node to the centerline of the downstream node of the subject conduit run. The length and average flow velocity are used to estimate the travel time within the run. Establish the length of the run during the first phase of the storm drain system design in which the inlets are located.
    NOTE: These lengths are hydraulic lengths of conduit, not pay lengths; the Department standard specifications provide that pay lengths include only the actual net length of pipe and not the distance across inlets or access holes where no conduit actually is placed.