Weir or orifice — why it matters
A sag inlet behaves as a weir at shallow ponding (flow spills over the perimeter) and as an orifice once submerged (flow squeezes through the open area). Capacity scales differently in each regime — d¹·⁵ versus d⁰·⁵ — so using the wrong one over- or under-states capacity exactly where it matters: at the design ponding limit. The calculator computes both and takes the governing (lower) value through the transition.
Gulf practice note: dust and litter clog grates fast in arid climates with infrequent rain. A 50% clogging factor on sag grates is normal practice in KSA and UAE criteria — some authorities require checking the sag with one inlet fully blocked. Sag points trap water with nowhere else to go: under-capacity here is how underpasses flood.
What this check does not replace
- Spread analysis on grade — inlets on a slope intercept only part of the gutter flow; spacing comes from allowable spread (typically lane-edge limits), not sag capacity.
- Bypass accounting — carryover from upstream inlets accumulates; the sag inlet receives everything missed above it.
- The network behind the inlet — an inlet only works if the pipe beneath accepts the flow; surcharge turns capacity to zero. Check pipes with the Manning calculator, peaks with the Rational Method.
Full road drainage design — spread, spacing, network, 2D verification of the sag — is exactly the scope we deliver to Stage 4 IFC on Gulf highway and urban projects.