12. Storm-sewer design panel

Tools → Storm Sewer Design runs StormSewer's own storm-sewer engine — Rational method, Manning, standard-step HGL, HEC-22 inlets, catalog sizing, design review — on the open SWMM model, and can write the sizes it recommends back into [XSECTIONS] as one undo step. It is the same engine the storm-sewer workspace and the stormsewer-cli use; the equations are in chapter 15 and the hand checks in VALIDATION.md.

The menu:

12.1 The design basis

The engine needs what a SWMM model does not have: an IDF curve, a return period, a minimum time of concentration, a junction loss coefficient, a minimum slope for flat pipes, and optionally a tailwater. These are a project of their own, shown at the top of the panel:

• IDF a, b, c — i = a / (t + b)^c, in/hr with t in minutes; the line under the fields restates it (i = 60.0/(t + 10.0)^0.80 in/hr, 10-yr).
• Return period (yr).
• Min Tc (min) — the floor on every inlet time.
• Junction K — the loss K·V²/2g at every structure.
• Min slope (ft/ft) — used for Manning capacity where the inverts give a slope of zero or less.
• Tailwater (ft) — a checkbox and a value; the starting HGL at the outfall when on.

The basis is seeded from the storm-sewer workspace's project the first time the panel opens; Copy from Storm Sewer workspace does it again. Editing the basis never touches that project. It is not saved in the .inp (there is no section for it); it lives with the session, and the design report prints it.

12.2 The mapping

Everything the panel computes depends on how a SWMM model becomes a storm-sewer network. The rules, and what each one assumes:

Units. FLOW_UNITS CFS, GPM or MGD mean feet, acres and in/hr, which is what the engine computes in. CMS, LPS and MLD are refused: the engine's results are U.S. customary only, and a silent conversion would still report feet and cfs for a metric model.

Nodes. [JUNCTIONS] become junctions, or inlets once a subcatchment or an [INLET_USAGE] row drains to them; [OUTFALLS] become outfalls; [STORAGE] and [DIVIDERS] become junctions with a note, because the engine routes peaks and does not store or divert. Invert is Elevation; rim is Elevation + MaxDepth, or the highest connecting crown when MaxDepth is zero — the same rule the profile uses.

Conduits. CIRCULAR, FORCE_MAIN and FILLED_CIRCULAR map to circular pipes by diameter; RECT_CLOSED to a box (Geom1 rise × Geom2 span); HORIZ_ELLIPSE and VERT_ELLIPSE to elliptical (rise × span); ARCH to arch. Every other shape — trapezoidal channels, custom, irregular, street — and every pump, orifice, weir and outlet is listed on the Skipped tab with the reason, never dropped silently. Length and Manning's n come from [CONDUITS]; pipe end inverts are the node invert plus the offset, honouring LINK_OFFSETS. A conduit with no [XSECTIONS] row is skipped.

Subcatchments. Each folds into the node it drains to, through other subcatchments when Outlet names one: its area in acres; a runoff coefficient from %Imperv by the straight line

C = 0.20 + 0.75 · (%Imperv / 100)

(0.20 fully pervious, 0.95 fully impervious — the usual Rational-table band for lawns and pavement), area-weighted when several drain to one node; and an inlet time by Kirpich over the overland flow length Area / Width at %Slope, the largest kept when several drain to one node, floored at Min Tc. The SWMM Area is authoritative; polygons are not re-measured. A subcatchment whose outlet chain never reaches a node is skipped with outlet "…" does not reach a node.

Inlets. [INLET_USAGE] rows mark their node as an inlet and carry the [INLETS] grate length and width (or curb length) and ON_SAG placement into the per-inlet HEC-22 overrides; a STREET cross-section on the conduit supplies the cross slope. Inlet count and clogging are noted, not carried.

Tailwater. A FIXED outfall's stage becomes the tailwater; other outfall types keep the basis's value.

Hydrology (IDF, return period, Min Tc, junction K, min slope) comes from the basis, since a SWMM model has none of it.

The Notes tab prints these assumptions as they applied to the open model.

12.3 The tabs

• Pipes — per mapped conduit: slope, ΣCA, Tc, intensity, design Q, just-full capacity, velocity, percent full, surcharge flag, HGL at both ends.
• Nodes — invert, rim, HGL, freeboard.
• Sizing — the smallest catalog RCP (8 to 72 in: 8, 10, 12, 15, 18, 21, 24, 27, 30, 33, 36, 42, 48, 54, 60, 66, 72) that carries the design flow within the criteria, beside the current size, with velocity and percent full at that size.
• Inlets — HEC-22 interception per inlet: gutter flow, spread, efficiency, intercepted and bypassed flow.
• Skipped — what did not map, and why.
• Findings — the design review (§12.5).
• Notes — the mapping's assumptions.

Click an id in any tab to select it on the map.

12.4 Auto-size

Auto-size… opens the Auto-size preview window: N conduit(s) change in [XSECTIONS]; one undo step. and a table of conduit, shape, design Q, before, after, note. Only recommendations with a solution and a size different from the current one are listed. Apply — Auto-size N conduits writes them:

• CIRCULAR and FORCE_MAIN get Geom1 = the recommended diameter;
• RECT_CLOSED gets Geom1 and Geom2 = the recommended diameter (a square box of that side, which carries at least what the circle does);
• ellipses get Geom1 = d and Geom2 = 1.5 d, as the storm-sewer workspace does;
• arches are left as they are and noted.

Ctrl+Z reverts the whole batch. The sizing rule is in §15.5.

12.5 Design review

The Findings tab checks the analysed network against the review criteria (defaults in brackets):

• velocity below the minimum [2 ft/s] — warning; above the maximum [10 ft/s] — warning;
• design flow above the percent-full limit [85 %] — warning; surcharge (design flow above open-channel capacity) — error;
• slope below the minimum [0.0005] — warning; adverse (uphill) — error;
• cover from rim to crown below the minimum [1 ft] at either end — warning;
• a pipe smaller than the pipe feeding it — warning;
• HGL freeboard below the minimum [0.5 ft] — warning.

These are the storm-sewer workspace's criteria; the SWMM design panel uses their defaults.

12.6 The design report

Report HTML… and Report PDF… write the storm-sewer design report for the mapped network: the basis, pipe and structure and inlet schedules, a plan schematic, a profile with HGL, and the review findings. It is the same report the storm-sewer workspace produces. It is about the mapped network; the Skipped list belongs beside it in any submittal.

12.7 Read this before you submit

The Rational pass and the SWMM run answer different questions. The Rational pass gives a peak flow for sizing under one intensity chosen by the pipe's own Tc; the SWMM run gives a hydrograph routed through storage, surcharge and backwater under a hyetograph. They will not agree on peaks, and the disagreement is informative (§16.13). Use the design panel to size and to check cover, velocity and freeboard; use the SWMM run to see what the sized system does.