Basin Assignment

Basin assignment maps HydraLink basins to the inlet structures that capture them. HydraStorm enforces a one-inlet-per-basin rule to prevent double-counting and aggregates parameters using engineering-standard formulas when multiple basins drain to a single inlet.

One Inlet Per Basin

Every basin maps to at most one inlet. This constraint prevents the most common rational-method mistake: counting the same drainage area at multiple structures, which inflates downstream flows.

The opposite mistake (drainage area that reaches no structure at all) is quieter, because an unassigned basin simply contributes nothing and no number looks wrong. HydraStorm names it after every calculation, and before you even open this dialog: a count badges the Tables button, this menu item turns amber with an inline count, and the dialog's own header shows the same count live so you can watch it fall to zero as you assign. On the Basin Parameters basis the count covers more than the basins already offered here — it also names any SCS or other non-rational basin in the linked model, because this basis can only ever carry C·A·Tc and such a basin's runoff has nowhere to go no matter what you assign in this dialog. See Basins That Reach No Structure for the full accounting, including the Fixed Flows basis and the calculation warning's wording.

One inlet, on the other hand, can capture many basins. The aggregation rules below apply.

Multi-Basin Aggregation

When N basins drain to one inlet, HydraStorm aggregates parameters as follows:

Drainage Area

Total area is the sum of contributing basin areas.

Runoff Coefficient (Weighted C)

C is area-weighted across contributing basins. This is the standard rational-method aggregation rule and gives the area-correct combined runoff response.

Time of Concentration (Maximum Tc)

The longest contributing Tc controls: this is the time after which the entire combined area is contributing flow simultaneously. Using a shorter Tc would overstate the design intensity.

Assigning Basins

There are two ways to assign basins. The Basin Assignment dialog is the visual one and is usually faster; the Hydrology Table is the row-by-row one.

The Basin Assignment Dialog

Open Tables > Basin Assignment. The command is available in HydraLink mode only. Manual mode has no basin objects to assign, and Peak Flow Only mode has no basins at all.

The dialog has three panes:

• Left — sources. Your basins, or (with the toggle) the HydraLink junctions that collect them. Each row shows a status glyph, the name, and its area, C and Tc.
• Middle — the selected run's structures, listed upstream at the top and downstream at the bottom, the same order as the results grid and the profile view. Each row shows the structure and the pipe leaving it.
• Right — the runs in your network, with laterals nested under their trunk line. Pick a run here to fill the middle pane.

Drag a basin from the left onto a structure in the middle. If dragging is awkward in a long list, double-click a basin to assign it to the highlighted structure, or select several and use Assign to Selected Structure. Remove a basin with the × on its chip, or select it on the left and press Del.

Status is shown by shape and color, so it reads without relying on either alone:

• ○  not assigned anywhere
• ◎  suggested by the import, not yet accepted; it carries no flow until you accept it
• ●  assigned to some other structure
• ◉  connected to the structure you have selected

The header shows a running count of unassigned basins. Nothing is committed until you click OK, and Cancel discards the whole session's changes. One OK is a single undo step: Ctrl+Z restores both the assignments and the structure hydrology they rebuilt.

The Dialog Drives the Drawing

While the dialog is open, whatever you select in it lights up in the drawing, and only what you select. The basin boundaries themselves come from HydraLink, so this works whether or not you have Basin Map in Drawing switched on in the View menu; closing the dialog hands the drawing back to that toggle.

• Select a basin on the left and its boundary lights up in yellow, and the viewport frames it.
• Select a junction and every boundary it collects lights up together, so you can see the whole area behind that flow number at once.
• Select a structure in the middle pane and the drawing outlines everything draining to it: its own basins plus every basin on every structure upstream. That is the total accumulated catchment, the same ΣA behind the Q = CIA calculation at that structure, not just the chips on its row.

A selection is drawn in full: its boundary in its structure's color, its name, its area, C and Tc, and the line back to the structure it feeds. Everything you have not selected stays off the screen, so the drawing is labelled where you are looking and nowhere else. If you want the surrounding basins outlined underneath for context, switch on Basin Map in Drawing in the View menu: the map then draws as usual and your selection simply brightens inside it. When you close the dialog the viewport returns to wherever you were before you opened it, so a browsing session never leaves you lost in the drawing.

If a basin has no boundary in the HydraLink data it is drawn as a marker at its location instead. If it has neither, the status line at the bottom of the dialog says so rather than leaving the click silent.

The Plan Preview

Above the runs list, a plan view shows the real Civil 3D drawing around the structure in question: surface, curbs, labels and all. Where the yellow highlight out in model space tells you which area a basin covers, the preview tells you which structure that area is about to drain to, without looking away from the dialog.

• Select a structure in the middle pane and the preview frames it.
• Select a basin or junction and the preview frames the structure it drains to. The preview is a structure instrument, so it never draws basin outlines; that is the drawing highlight's job.
• Drop a basin on a different structure and the preview follows it there, so a reassignment is confirmed visually before you commit it.

It opens at 1" = 10', close enough that the structure sits alone in the frame, which is what makes its type readable, and holds that scale as you move around, so structures stay comparable; Fit zooms out to take in the connected runs. The preview only reflects your selection: clicking it never changes anything, and it never disturbs your Civil 3D viewport.

What Arrives Already Assigned

Most links are made for you when you link the HydraLink file. The Use checkbox in the flow assignment grid (Project Settings → Hydrology) governs the whole element, not just its flow: leave it checked and that basin's area, C and Tc land on the matched structure along with its peak flow. Clear it and both come off again. You do not confirm the same match twice.

Three kinds of link still arrive as a suggestion for you to review here:

• Basins with no row in the flow assignment grid: nothing there said yes or no to them.
• Basins the flow grid matched to a different structure than the basin matcher chose. Accepting both would put the flow on one structure and the same drainage area on another, counting the watershed twice, so HydraStorm leaves the call to you.
• Anything the import cannot match to a structure at all.

Once you have moved, removed or accepted a basin here, it is yours: re-linking or re-saving the HydraLink file refreshes its area, C and Tc, but never moves it to another structure and never reverses your decision, including a basin you deliberately removed, which will not come back.

A basin marked at a specific point in HydraLink's Civil 3D palette (rather than left at its computed centroid) arrives already assigned, not suggested — a picked point is treated as deliberate placement, so it lands on its matched structure with no review step. Marking also lets a second basin co-claim the same structure instead of being pushed to the next-nearest one, though only onto a structure another basin already holds — never a junction or a pond. See Shared Inlets on the HydraLink Integration page for the full rule.

The Downstream-Most Structure

The last structure on a run is greyed and will not accept a drop. On an outfall there is no pipe leaving it, so flow entered there has nothing to carry it. On a lateral, the last structure is the tie-in to the parent run: assign to it from that run instead, where it does have an outgoing pipe. Either way the row stays visible so the run still reads as complete.

Assigning a Whole Junction

Switch the left pane to Junctions and you can drop a junction rather than picking its basins one at a time. A junction is a shortcut, not a thing that gets stored: dropping it assigns every basin in its full upstream tree, including basins feeding junctions further upstream, because that is what the junction's flow number represents.

The link is derived, never stored, which makes breaking it predictable. If a junction collects basins A1, A2 and A3 and you later move A1 to a different structure, then A1 goes where you put it, A2 and A3 stay exactly where they were, and the junction simply reads as unassigned again — a junction is either fully on one structure or it is not; there is no partial state. Nothing is silently rearranged behind the assignment you just made.

Reassigning

Because a basin drains to one structure only, dropping an already-assigned basin somewhere else moves it. HydraStorm asks first, naming both the old and the new structure, and a junction drop that would displace several basins asks once and lists them all. Assigning a basin that is not yet assigned (the common case) never prompts.

Through the Inlet, or Piped Directly

Each assigned basin carries a flag for how its flow reaches the structure, shown as a small line glyph on the basin's chip:

• Solid (━) — through the inlet opening. The flow arrives as gutter flow and is subject to the inlet's interception capacity; whatever the inlet cannot capture bypasses to the next one. This is the default.
• Dashed (╍) — piped directly to the structure. A roof drain, area drain or yard drain stub that enters the structure below the grate. This flow never bypasses: it enters the system here regardless of what the inlet can intercept.

Click the glyph to switch a basin between the two. New assignments follow the checkbox at the bottom of the left pane, so you can set the default before dropping a group. This flag feeds the inlet analysis directly: it is the same gutter-versus-direct split the inlet module already uses.

The Hydrology Table

Open the Hydrology Table for row-by-row assignment. There each row is a structure, the reverse of the Basin Assignment dialog's basin-first view: an Assigned Basins column holds a checklist of every basin currently contributing to that structure, and you check or clear entries to assign or remove them. This is the better tool when you know exactly which structure you want and would rather not hunt for a basin on a run.

HydraLink Basin Mapping

When using HydraLink mode, basin-to-inlet mapping comes from name matching first (exact, then ignoring common prefixes, then close matches), then location within a search radius you set in Project Settings (default 5 ft), then manual mapping. The HydraLink Integration page describes each step.

HydraLink Junction Intelligence

HydraLink junctions can be near inlets. When the import matches a junction to a structure, HydraStorm does not store the junction itself — instead each of its member basins is suggested at that structure individually, labelled via the junction, and you accept them like any other suggestion. The result at the structure is identical to assigning the basins one at a time: weighted C, summed area, and the largest Tc. Because every basin has exactly one assignment, a basin can never be counted once on its own and again through a junction.

Persistence

Assignments are persisted in your .hsp project file. They survive C3D drawing closes and file reloads. If an accepted basin's assigned structure disappears — deleted in Civil 3D, or deleted-and-recreated with a new identity at the same location and the re-key offer was skipped — HydraStorm reports it as an orphaned assignment in the Network Issues dialog. That basin's area, C, and Tc contribute nothing until you re-assign it, so it is a Warning, not a silent drop: an understated ΣCA with no message is exactly what this row exists to prevent.

When Civil 3D recreates a structure (a common Auto Design / drafting side-effect), the Refresh from CAD toolbar button opens a review dialog offering to re-key it — checked by default — which carries assignments and hydrology data over to the new handle. Accept it and nothing is orphaned; uncheck it and the basins land in Network Issues instead. See Refresh from Civil 3D.