Inlets In Storm Sewer Design
Presenter: Ashley Wilson, PE
Organization: Ameritex Pipe
Description: Stormsewer design is multi-faceted. It addresses placing, alignment, spacing and capacity. Access hole location and spacing criteria have been developed in response to storm drain maintenance requirements. Spacing criteria are typically established based on a local agencies past experience and maintenance equipment limitations.
PLACEMENT
Let’s talk about placement of inlets from a horizontal curvature perspective before we talk vertical curvature and general guidelines. As water follows the curbline it will remain in the gutter as long as the capacity remains. However on a vertical curve where the radius is short enough, the storm water may want to flow towards the centerline of the roadway, spreading into the roadway as sheet flow. Therefore a good practice is to place a drop inlet at the point of curvature and/or the point of tangent. That allows us to catch the water before it goes across the around the corner or across the street if the road is graded to allow that. The graphic on the slide hopefully provides you a visual of where that takes place along the curb-line.
ALIGNMENT
Where possible, storm drains should be straight between access holes. However, curved storm drains are permitted where necessary to conform to street layout or avoid obstructions. Pipe sizes smaller than 4.0 ft should not be designed with curves. For larger diameter storm drains deflecting the joints to obtain the necessary curvature is not desirable except in very minor curvatures. Long radius bends are available from many suppliers and are the preferable means of changing direction in pipes 4.0 ft in diameter and larger. The radius of curvature specified should coincide with standard curves available for the type of material being used.
SPACING
Access hole location and spacing criteria have been developed in response to storm drain maintenance requirements. Spacing criteria are typically established based on a local agencies past experience and maintenance equipment limitations. At a minimum, access holes should be located at the following points:
• Where two or more storm drains converge
• Where pipe sizes change
• Where a change in alignment occurs
• Where a change in grade occurs
