Basin assignment maps drainage basins (HydraLink basins or Civil 3D parcels) 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.
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. HydraStorm flags any unassigned basin as a warning so you don't lose drainage area by accident.
One inlet, on the other hand, can capture many basins. The aggregation rules below apply.
When N basins drain to one inlet, HydraStorm aggregates parameters as follows:
Total area is the sum of contributing basin areas.
C is area-weighted across contributing basins. This is the standard rational-method aggregation rule and gives the area-correct combined runoff response.
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.
Open the Hydrology Table. Each row corresponds to a basin or parcel. The Inlet column lets you pick the structure that captures it.
When using HydraLink mode, basin-to-inlet mapping comes from name matching first (exact, then normalized, then fuzzy / Levenshtein), then spatial proximity (configurable radius in Project Settings, default 5 ft), then manual mapping. The HydraLink Integration page describes the full pipeline.
HydraLink junctions can be near inlets and represent already-accumulated basin parameters. When this happens, HydraStorm uses the junction's accumulated values directly and marks the contributing basins as consumed so they aren't double-counted.
Assignments are persisted in your .hsp project file. They survive C3D drawing closes and parcel reloads. If a basin is removed (parcel deleted, HydraLink basin renamed beyond fuzzy match), HydraStorm flags the orphaned assignment so you can clean it up.
Tip: When a single inlet captures many basins of varying surface types, the weighted C is essentially a composite C — equivalent to manually computing a composite C for the impervious-pervious split.