Drainage

Drainage inlets placed at low points collect runoff and direct it into a storm-drain system, channel, or other outlet. Their performance depends on inlet location and elevation, interception capacity, upstream grading, downstream conveyance, and whether debris or sediment restricts flow. An inlet may be clear and unobstructed yet still experience ponding or bypass when the runoff reaching it exceeds its interception capacity or the downstream system cannot convey the flow.

Drainage-capacity analysis considers both the runoff reaching the system and the ability of the drainage components to collect and convey it. Relevant factors can include drainage area, rainfall characteristics, site slope, inlet configuration, pipe or channel dimensions, obstructions, outlet conditions, and downstream restrictions. Hydrologic and hydraulic calculations can then be compared with surveys, photographs, and the site's documented history of ponding, overflow, or flooding.

Small elevation differences can change the path that surface runoff follows across a site. Grading may direct water toward drainage inlets, away from structures, across adjoining property, or into unintended low areas. Surveys, spot elevations, contours, grading plans, and field measurements can document the site's actual elevations and slopes and help assess whether the constructed grade is consistent with the drainage path observed during storms.