Parcels Integration

The Parcels hydrology mode reads drainage area directly from Civil 3D Parcel geometry. Runoff coefficient (C) and time of concentration (Tc) come from parcel User-Defined Properties (UDPs) or are entered manually. This mode lets your hydrologic inputs ride along with the surveyed parcel boundaries — when boundaries are revised, your basin areas update too.

Setup

  1. In Civil 3D, ensure your parcels are placed on a site you intend to drain (data-shortcut'd parcels are also supported).
  2. In HydraStorm, open Project Settings > Hydrology and set the mode to Parcels.
  3. Map UDP fields: choose which Civil 3D parcel UDP holds C, and which holds Tc. Parcels without these UDPs fall back to project-level defaults (or manual override per node).
  4. Use the Hydrology Table to assign parcels to inlet structures.

UDP Field Mapping

HydraStorm reads UDP values via Civil 3D's parcel extension dictionary. Common conventions:

  • RunoffCoefficient or C for the C value
  • Tc or TimeOfConcentration for Tc in minutes

If your firm uses non-standard names, choose them from the dropdown in Project Settings — HydraStorm enumerates all UDPs available on parcels in the current drawing.

Area Conversion

Civil 3D stores parcel area in square feet (drawing units). HydraStorm converts to acres automatically (1 acre = 43,560 ft²). All downstream calculations use acres, consistent with the Rational Method's standard form Q = CIA.

Caching

Parcel data is cached in your .hsp project file at save time. This means HydraStorm can compute even when the parent drawing is closed or when working from a data-shortcut that briefly loses access. The Refresh Parcels action re-reads from C3D and detects added, removed, or modified parcels.

Change Detection

When you reload a project after parcel edits in C3D, HydraStorm shows you a summary of changes:

  • Added — parcels that appeared since last save
  • Removed — parcels deleted from the drawing
  • Modified — parcels whose area or UDP values changed

Review the dialog and accept changes to update your assignments. Removed parcels drop their basin assignments.

Multi-Site and Data Shortcuts

HydraStorm reads parcels from all sites in the drawing plus any data-shortcut'd parcel objects. Identity is tracked via the persistent C3D handle, so a parcel that moves between sites stays linked to its assignment.

Parcels Without UDPs

If a parcel has no C UDP or no Tc UDP, HydraStorm uses the project-level Default C and Default Inlet Time (Tc). You can also override per-node in the Hydrology Table. The override breaks the link to the UDP for that node only.

Switching to Manual

Any project can be converted from Parcels to Manual at any time. The conversion copies current C/Tc/area values into the Manual editor, breaking the link to parcel geometry. Re-linking is allowed; HydraStorm prompts before overwriting your edits.

Tip: Use Parcels mode for built-out conditions where parcel boundaries are fixed; use Manual or HydraLink for early conceptual design when boundaries are still in flux.

For combining parcels into single inlets and weighted-C aggregation, see Basin Assignment.