​SWMM 5.1.007 computes flow incorrectly for surcharged weirs (e.g. a sewer system CSO regulator) when Can Surcharge = YES is selected (this is the default). Discharge is incorrectly calculated as zero when a weir is surcharged. The problem can be fixed by restoring a few lines of code from version 5.0.022 to the weir_validate function. The desired behavior is described in Engine Update 5 for version 5.1.007:

“A new Weir parameter was added that specifies if the weir can surcharge using an orifice equation or not. Surcharging was the only option in SWMM 5.0 but was switched to no surcharging in earlier 5.1 releases. This new parameter accommodates both closed top weirs that can surcharge and open channel weirs that cannot.”

My understanding of how the code should look is shown below, along with an example model. I expect this will be fixed in the next EPA release. This also affects PCSWMM, as that program uses the EPA .dll.

Suggested resolution:

void  weir_validate(int j, int k)
//
//  Input:   j = link index
//           k = weir index
//  Output:  none
//  Purpose: validates a weir's properties // {
    int    err = 0;
    double q, q1, q2, head;

…snip…

    // --- compute an equivalent length
    Weir[k].length = 2.0 * RouteStep * sqrt(GRAVITY * Link[j].xsect.yFull);
    Weir[k].length = MAX(200.0, Weir[k].length);
    Weir[k].surfArea = 0.0;

               // --- find flow through weir when water level equals weir height
               head = Link[j].xsect.yFull;
               weir_getFlow(j, k, head, 1.0, FALSE, &q1, &q2);
               q = q1 + q2;

               // --- compute equivalent orifice coeff. (for CFS flow units)
               head = head / 2.0;  // head seen by equivalent orifice
               Weir[k].cSurcharge = q / sqrt(head);

}

Example illustrating the problem:

[TITLE]
SWMM 5.1.007 issue
When Surcharge = YES for weir, no flow occurs when weir is surcharged In this example, Q=10 cfs with Surcharge = NO, 0 cfs with Surcharge = YES. Version 5.0.022 yields 15 cfs

[OPTIONS]
FLOW_UNITS              CFS
FLOW_ROUTING            DYNWAVE
START_DATE              10/02/2012
START_TIME              00:00:00
END_DATE                10/03/2012
END_TIME                00:00:00
REPORT_STEP             00:15:00
ROUTING_STEP            0:00:30

[JUNCTIONS]
;;Junction        Invert      MaxDepth    InitDepth   SurDepth        Aponded
1                 10          0           0           0           0
2                 1           0           0           0           0

[OUTFALLS]
;;Outfall         Invert      Type        Stage Data        Gated
3                 0           NORMAL                        NO

[CONDUITS]
;;Conduit         From Node         To Node           Length          Roughness   InOffset    OutOffset   InitFlow    MaxFlow
2                 2                 3                 400             0.013       0           0           0           0

[WEIRS]
;;Weir            From Node         To Node           Type              CrestHt     Qcoeff      Gated       EndCon      EndCoeff        Surcharge
1                 1                 2                 TRANSVERSE        2           3.33        NO          0           0           YES

[XSECTIONS]
;;Link            Shape             Geom1             Geom2           Geom3       Geom4       Barrels
2                 CIRCULAR          3                 0           0               0           1
1                 RECT_OPEN         1                 3           0               0

[INFLOWS]
;;Node            Inflow            Time Series       Type          Funits      Fscale      Baseline    Pattern
1                 FLOW              ""                FLOW        1.0           1.0         15

[COORDINATES]
;;Node            X-Coord                 Y-Coord
1                 1476.938                7497.547
2                 2183.513                6133.464
3                 2968.597                4661.433