Problem

Water running down a street reaches a grate or a curb opening, some of it goes into the sewer, and the rest carries on to the next inlet. A model has to say how much goes in — which is a HEC-22 calculation from the spread, the depth at the curb and the inlet's design — and where the rest goes.

OpenSWMM offers two ways to attach an inlet, and they are not interchangeable. A conduit-attribute inlet is a property of a street conduit: [INLET_USAGE] names the conduit, the design and the node the captured water is delivered to, and the capture is booked at the conduit's downstream node. An inlet junction is a node of its own, sitting between two street conduits. This chapter runs both on one street and shows what each makes possible.

Deck

Deck: docs/figures/decks/street_inlets/street_inlet_junction.inp

One street reach draining to a parallel sewer:

J_TOP --ST_A--> J_MID --ST_B--> [IJ1] --ST_C--> OUT_ST     street, at grade
          |            capture     |  capture
          +--> MH1 --SW_1--> MH2 --+--SW_2--> OUT_SEW      sewer, buried

ST_A carries a conduit-attribute inlet — a combination grate and curb opening, two of them — whose capture is delivered to MH1. IJ1 is an inlet junction between ST_B and ST_C carrying a single curb opening, delivering to MH2. The storm is a 2-hour event on a small catchment above the street.

The deck is a copy of examples/inlets/street_inlet_junction.inp; its header shows all four [INLETS] design grammars, including the two-line combination encoding and a custom design pointing at a diversion curve.

Options that select the formulation

An inlet junction is a virtual junction that also carries an inlet, so it inherits every virtual-junction rule: exactly two conduits, the same cross-section on both, zero offsets, and dynamic wave routing.

[INLET_JUNCTIONS]
;;Name  Elev   MaxDepth  InitDepth  Inlet  Node   Count  ...
IJ1     ...                                Curb1  MH2    1
 
[INLET_USAGE]
;;Conduit  Inlet   Node   Number  %Clogged  Qmax  aLocal  wLocal  Placement
ST_A       Combo1  MH1    2       0         0     0       0       AUTOMATIC

with [STREETS] giving the cross-section both conduits use and [INLETS] the designs. A conduit attached to an inlet junction may not also carry an [INLET_USAGE] row — that would capture the same water twice, and the engine refuses it (rule 629). That is why the conduit-attribute inlet sits on ST_A rather than on ST_B.

Results

Figure 5-1

Figure 5-1 Capture and bypass along a street reach with a conduit-attribute inlet and an inlet junction

	Peak approach flow	Peak capture	Average capture	Volume captured	Volume bypassed
`ST_A`, conduit attribute	11.937 cfs	99.00 %	98.22 %	266.6 kgal	2.70 kgal
`IJ1`, inlet junction	0.133 cfs	90.00 %	90.00 %	2.41 kgal	0.28 kgal

Table 5-1 The Street Inlet Flow Summary for the two inlets, as the report prints it

Read the summary's columns carefully. "Peak Flow" is the peak approach flow the inlet saw, not the flow it captured. "Bypass Flow Pcnt" — 88.90 % for ST_A — is the frequency of bypass over the periods with capture, not a share of the volume; the volume share is the last two columns, and by volume ST_A captured 99 % of what reached it. A reader who takes the bypass column as a volume fraction will conclude the opposite of what the run says.

The first inlet does nearly all the work. It sees 269 kgal over the storm and passes 2.7 kgal on. The inlet junction then sees only what got past the first one, captures 90 % of that, and leaves 0.28 kgal to reach the downstream outfall. Panel (a) shows the same thing as flows: the street flow falls from 11.9 cfs to 0.13 cfs to 0.012 cfs across the two inlets, three orders of magnitude on a symmetric log scale.

What the inlet junction adds. The capture itself is the same HEC-22 calculation either way. The difference is that IJ1 is a node, so it

has a head of its own, which the two street conduits see, instead of the capture being booked at a neighbouring node's head;
can flood above the curb and report it as a node would;
takes MH2's surcharge back onto the street as backflow, which a conduit attribute cannot represent at all;
transmits momentum between ST_B and ST_C rather than terminating them.

When the conduit attribute is the right answer. When the inlet is one of a run of many on a long reach, when the sewer below it will not surcharge, and when the street's own hydraulics are not the question. It is cheaper — no extra node, no virtual-junction constraints on the cross-sections — and this run shows it doing the same capture job.

Where to go next

Hydraulics 7 — the HEC-22 capture relations and the inlet-junction formulation
Hydraulics 5 — the street cross-section, its spread and its depth at the curb
Section: [STREETS], Section: [INLETS], Section: [INLET_USAGE] and Section: [INLET_JUNCTIONS] — the grammar of each section
T2 — Streets, Inlets and the Inlet Junction — the same model built in the GUI
Chapter 2 — what happens in the sewer below, once the captured water surcharges it