Fixing problems

Use the Problems window to catch model mistakes before a run, read SWMM's errors and warnings in plain language after it, and zoom to the objects involved.

The Model Check / Problems window has two tabs. Tools > Model Check / Problems... opens it on Model check; Report > Problems opens it on Last run. Each tab shows how many errors, warnings and notes it holds.

Read a problem row

The Model Check / Problems window on its Model check tab with 2 warnings: Manhole inverts below every outfall (28 objects) and Conduits running uphill (Link RM1). The second is open: the outlet end is higher than the inlet end, which in a gravity sewer is almost always a level or drawing-direction mistake. How to fix: check the inverts and offsets at both ends; reverse the link if it was drawn the wrong way. A Link RM1 button zooms to it. Open SWMM report and Run SWMM at the bottom. — A problem row: what is wrong, how to fix it, and a button that finds the object.

A red bar marks an error (SWMM will stop, or the results cannot be trusted), amber a warning (SWMM runs but changes or weakens the results), blue a note.
The row names the problem, the SWMM error or warning number when SWMM has one, and the object or the number of objects involved. A count such as x28 means several objects or report lines were folded into one row.
Click the row to open it: what it means, a How to fix list, and a button for each object. Click an object button to select it and centre the map on it. Long lists show the first 40 objects, with Show all for the rest.
For a row read from SWMM's report, the original line is shown at the bottom, after SWMM:.

At the bottom, Open SWMM report opens the status report and Run SWMM runs the model and switches to the Last run tab.

What Model Check looks for

Model check runs again on every edit, so the list is always current. It catches the mistakes that make SWMM refuse a sewer or storm drain model, or run it and quietly change it.

Check	Grade	SWMM code
Duplicate IDs	Error	Error 207
Links joined to nodes that do not exist	Error	Error 209
Subcatchment outlet name is ambiguous (the ID is both a node and a subcatchment)	Error	Error 108
No outfall in the network	Error	Error 145
Outfall with more than one link	Error	Error 141
Nodes with no links	Warning
Manhole inverts below every outfall	Warning
Conduits with zero length	Error	Error 111
Conduits with no roughness	Error	Error 113
Links with no cross section	Error	Error 117
Cross sections with zero size	Error	Error 211 or 119
Conduits running uphill	Error with kinematic wave or steady routing, warning with dynamic wave	Error 115
Flat conduits	Warning	Warning 04
Conduit ends below the manhole invert	Warning	Warning 03
Slopes below the Minimum Conduit Slope	Warning	Warning 05
Conduits that fall more than their length	Warning	Warning 08
Offsets above the manhole depth	Warning	Warning 02
Pipe crown above ground level	Warning	Warning 02
Pipe size decreases downstream	Warning
Subcatchments without a rain gauge	Error	Error 209
Subcatchments without an outlet	Error	Error 209
Imperviousness outside 0 to 100%	Error
Subcatchments with zero area or width	Error
Routing step too long for the shortest conduits (dynamic wave only)	Warning, or a note when Variable Time Step is on

The routing step check works out, for each conduit running full, the longest step that keeps it stable (the Courant condition) and lists the conduits that need a shorter step than the one set, with the step to use.

What Last run lists

After a run, the Last run tab reads SWMM's report and lists:

The Last run tab with 1 warning: 1 node(s) flooded, Node 10. Water overflowed 1 manhole, 0.135 10^6 gal in all, worst at node 10 for 2.80 h; for a design storm a DPR normally shows no flooding. How to fix: check the pipes downstream for capacity, check rim levels, and upsize or regrade the limiting pipes or add relief or storage. A Node 10 button finds it. — Last run lists what went wrong in the most recent run, such as flooding.

Every ERROR and WARNING line SWMM wrote, with its meaning and fixes in plain language. Input errors name the section and line of the .inp file and quote the line. DrainPilot explains SWMM's run and input errors (101 to 235), its warnings (01 to 12) and its file errors.
Continuity errors above 1 % for flow routing, runoff and groundwater: a warning up to 5 %, an error above. See Running a simulation.
Flooded nodes, with the total volume and the worst node, as a warning: for a design storm a DPR normally shows no flooding.
Surcharged nodes, as a note: water rose above the crown of the highest pipe, which in a sewer designed to run part full means the pipes are short of capacity.
The elements that limit the time step (from SWMM's time step critical elements), as a note.
Links with oscillating flow, whose flow instability index is 10 or more, as a warning.
The share of routing steps that did not converge, when it is above 1 %, as a warning.

If the model has changed since that run, the footer says so. If the last run failed, the tab shows SWMM's error even when the report has no ERROR line.

Common fixes

Symptom	Usual cause	What to do
Error 145, no outfall	The last drain ends at a junction.	Add an outfall at the point of discharge and connect the last conduit to it.
Error 209 on subcatchments	A subcatchment has no rain gage or no outlet. New subcatchments have no outlet.	Set Rain Gage and Outlet in the Property Editor.
Warning 02, offsets above the manhole	Junction Max. Depth left at 0, or a diameter typed in millimetres.	Enter invert-to-rim depth in metres and diameters in metres.
Many flooded nodes	Rim levels too low, or pipes short of capacity.	Check Max. Depth; find pipes with Max/Full Flow above 1 in the Link Flow Summary of the status report; upsize or regrade them.
Large continuity error	Routing step too long, very short conduits.	Shorten the routing step, turn on the variable time step, merge short conduits.
Run was not started	A rainfall or other data file is not attached.	File > Attach Data Files...

An empty Problems window means SWMM raised nothing and the checks found nothing. It does not mean the levels, flows and rainfall in the model are right: check them against the survey and the design basis.