Auto Design

Auto Design automatically sizes pipes in your network to meet hydraulic design criteria, giving you a starting point to refine. It selects the minimum pipe size from the active part family that satisfies capacity and velocity constraints, working from the outfall upstream through the network.

Scope: the Selected Alignment

Auto Design sizes the pipes on the alignment currently shown in the profile view. Pipes on other alignments keep their existing sizes and inverts, and act as fixed hydraulic context — the sized run ties into the live HGL at the connection structure. To size a different line, switch the profile view to that alignment and run Auto Design again. When the drawing has no alignment data, the entire network is sized in one pass.

Because Auto Design never modifies pipes outside the selected alignment, a size-rule conflict at the boundary — for example, the sized run coming out larger than the existing pipe it discharges into — is reported in the calculation messages instead of silently changing the other line.

Every Pipe Gets a Recommendation

Auto Design reviews every pipe in scope and renders a verdict on each one: confirm the existing size, upsize it, or downsize it — down to the minimum size in your settings when the flow warrants it. A pipe shown unchanged means Auto Design confirmed that size; pipes are never silently skipped or held. Dry pipes (no flow) are recommended at the minimum catalog size.

When no catalog size can satisfy every constraint at once (for example, a shallow crossing where the barrel the flow requires won't fit under the minimum cover), Auto Design still recommends the smallest size that carries the flow at the design grade — or the largest available size if none has the capacity — and lists each unmet constraint (cover, velocity, capacity, HGL clearance) in the calculation messages. The status bar reports how many recommendations carry unmet constraints so you know to review them.

Pipe Size Never Decreases Downstream

Each pipe is sized for its own accumulated flow, so pipes are naturally smaller toward the headwaters. Auto Design also enforces the standard rule that a pipe leaving a structure must be at least as large as every pipe entering it. When a constraint forces an upstream pipe up (for example, a flat run that needs a larger barrel to meet the minimum full-flow velocity), the upsize cascades downstream to the outfall, and each bump is reported in the calculation messages so you can see what drove it.

Workflow

1. Assign flows — Make sure you have flows assigned at each inlet or structure, using any of the available hydrology modes.
2. Set junction loss types — Assign appropriate junction loss types to each structure so the HGL calculation includes energy losses at junctions.
3. Calculate — Click Calculate (or press F5) to establish baseline hydraulic results with the current pipe sizes.
4. Run Auto Design — Click the Auto Design button. The suggested sizes appear in the results grid and profile view.
5. Review — Inspect the suggested sizes. Check for adequate cover, constructability, and any site-specific requirements the algorithm cannot account for.
6. Accept or reject — Click Apply to C3D to write the new sizes to Civil 3D, or Discard to revert to the original sizes.
7. Recalculate — After applying, run Calculate again to verify the final HGL with the accepted pipe sizes.

Sizing Storm

When multiple storms are configured, Auto Design uses whichever storm event is selected as the Sizing Storm in Project Settings > Hydrology. This is typically the design storm (e.g., the 10-year event). The check storm verifies performance after sizing, not to drive pipe selection.

Design Constraints

Auto Design respects the following constraints, configured in Project Settings > Calculation:

Constraint	Description
Minimum Pipe Diameter	No pipe will be sized smaller than this value. Most jurisdictions require a minimum of 18 inches for storm drain mains.
Maximum Full-Flow Velocity	Pipes are sized so the full-flow velocity does not exceed this limit. Typical values range from 10 to 20 ft/s depending on pipe material.
Minimum Structure Depth	Ensures structures are deep enough by constraining the minimum depth from rim to invert. Auto Design will not size a pipe so large that it causes a structure to be shallower than this limit.
HGL Cover	Minimum clearance (ft) from the hydraulic grade line to the ground surface or structure rim. When a violation is detected, Auto Design backtracks to upsize pipes downstream.

Mid-Run Pipe Size Changes (Splits)

Sometimes clearing an HGL cover violation means upsizing a long pipe run, even though only the last leg near the violation actually needs the larger size. Instead of always upsizing the whole run, Auto Design can propose breaking the run at a pipe-size-change null structure and upsizing only the downstream leg. Slopes are held between structures — the split doesn't change grades, just where the size steps up.

This behavior is controlled by Project Settings > Calculation:

Setting	Default	Description
Auto Design Size Changes	Prompt before adding	Never (upsize whole run) — legacy behavior; Auto Design always upsizes the entire run. Prompt before adding — Auto Design computes split proposals and shows a confirmation dialog before applying any of them. Add automatically — proposals are applied to the working model without a prompt.
Min Split Leg Length (ft)	20 ft	Shortest leg a split may produce. A run shorter than twice this value is never split — it's upsized whole instead.
Split Station Rounding (ft)	1 ft	The break station (measured from the downstream structure) is rounded up to the nearest multiple of this value. Rounding up only ever lengthens the larger downstream leg, which is always hydraulically conservative.
Match Crowns at Size Change	No	When enabled, the upstream leg's invert at the break is raised so the pipe crowns (soffits) line up instead of the inverts. This flattens the upstream leg's slope by the rise difference. On minimum-slope designs this can push the upstream leg below the minimum slope, in which case the split is rejected for that pipe — which is why this defaults off.

The Confirmation Dialog

When Auto Design Size Changes is set to Prompt before adding and a run qualifies for a split, Auto Design shows the Auto Design — Pipe Size Changes dialog before any split is added to the working model. Each row lists:

• Pipe and a Proposal summary — the break station, the size kept upstream, the larger size proposed downstream, and the whole-run upsize alternative for comparison.
• Break Sta (ft) — the break location, measured from the downstream structure. This is editable.
• Action — per row, choose Add size change (accept the split as proposed or as relocated), Upsize whole run (fall back to sizing the entire run at the downstream leg's size), or Skip pipe (leave it unchanged; its HGL violation stands).
• Issue — validation errors for an edited break station, described below.

You can move a proposed break station farther upstream (a larger station value) freely — that only lengthens the larger downstream leg, which is always safe. Moving it closer to the downstream structure than the hydraulic minimum, or to a station where cover at the size-change structure would be violated, is rejected with an explanation in the Issue column, and clicking OK re-opens the dialog until every row's station is valid.

Applying Splits to Civil 3D

Accepted splits are written to Civil 3D automatically the next time you click Apply to C3D — there's no separate step. Applying a split physically breaks the drawing pipe:

• Creates the size-change null structure at the break point.
• Creates a new pipe for the downstream leg, sized and connected between the new null structure and the original downstream structure.
• Shortens the original pipe to the upstream leg and reconnects it to the new null structure.
• Sets inverts and sizes on both legs to match the accepted (or relocated) proposal.

The new null structure is named <pipe>-SC and the new downstream-leg pipe <pipe>-DS, based on the original pipe's name. If those names are already in use in the drawing, Civil 3D's auto-generated name is kept instead and a notice appears in the Apply messages.

The Apply to C3D — Preview dialog lists every pending split — the null structure and new pipe it will create — alongside the other pending changes, before anything is written.

Part Family Sizes

Auto Design only selects sizes that exist in the pipe's Civil 3D part family. If your parts list has 18", 24", 30", and 36" RCP, Auto Design will choose from exactly those sizes. To make additional sizes available, update the parts list in your Civil 3D drawing before running Auto Design.