Inlet Calculations

Stormwater Studio has the ability to design and analyze a system of HEC-22 style inlets along with your underground pipe system. It has built-in capability to analyze

Drop Curb
Drop Grate
Generic (Known Capacity)

Inlets are located at the upstream end of a Line and can be in a sag or on a longitudinal slope and can be of any size. The purpose of this analysis is to determine the amount of flow a particular inlet can capture, the ponding depth, inlet and gutter spread widths, the amount of flow that is bypassed, and what affect it has on downstream inlets.

The program has design features that will size inlets to capture 100% of the flow. To simplify this process, Stormwater Studio assumes that all inlets have common n-values. These values are specified on the Ribbon menu Inlet Design tab. Gutters, however, contain unique n-values values for each Line.

Unique Clogging factors can be specified for inlets containing a curb opening or a grate, on grade or in a sag.

Stormwater Studio follows the methodology of FHWA HEC-22 for inlet interception capacity calculations.

Flow Development

Stormwater Studio uses a separate procedure for determining Qs for inlets than that for the overall pipe network. The Qs for inlets are computed using the Rational formula, Q = CiA. But the intensity is based on the individual catchment’s Inlet Time, not the cumulative Tc. The C x A term is for the individual catchment’s drainage area and corresponding runoff coefficient. This Rational method Q is labeled as “Q Catchment”, “Incr Q” and “Q = CiA” in the reports. Known Q’s are automatically added to overland flows.

Carryover and Bypassed Q’s

Q’s for individual inlets are developed from two components. The first is from the catchment’s drainage area or runoff. The second component is from excess flows, or non-captured flows from upstream inlets. This non-captured flow is called “Carryover”. It is called “Bypassed” when an inlet cannot capture 100% and sends a portion of the total flow off site or to another specified target inlet.

Carryover and Bypass flows are both non-captured flows. To an inlet, carryover is an incoming flow and bypassed is an outgoing flow. These flows are labeled as “Q carryover ” and “Q bypassed” on the reports. Note that Lines with a manhole or no inlet will bypass all carryover flows to its downstream Line.

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