What Terrain, Grading & Drainage does

Bring terrain from an upload, any location on Earth, the Rhino document or the demo site. A survey opens on Surface: the clean 3D ground with its contours, spot heights and boundary, ready for Rhino or Revit, with drainage one click away. Flow mode reads where water runs and where it collects; Rainfall mode puts a design storm on the site and balances runoff against what ponds and what leaves; Grading mode lets you sculpt the ground and re-runs everything live, with cut and fill volumes as you go.

Input:

.asc / .xyz / .obj / .3dm / location / Rhino selection

Modes:

Surface · Flow · Rainfall · Grading

Output:

Flow lines, ponding sites, cut/fill, DXF / SVG / .3dm, PDF report (Pro)

How it works

Standard raster terrain hydrology, the same chain GIS packages use, with a water balance on top. Depressions are filled first with a tiny gradient so every cell has a way out, then flow is routed cell to cell and the filled-minus-original difference tells you where water can actually sit.

• Priority-Flood depression filling with an ε gradient (Barnes et al. 2014), so every cell drains to the edge; NaN / NODATA cells are treated as boundaries.
• D8 flow direction: each cell drains to the steepest of its eight neighbours. Flow accumulation and contributing area (m²) follow from that.
• Depression inventory from filled − original: each ponding site gets a spill level, a storage volume and an area.
• Rainfall mode is a water balance: SCS Curve Number runoff depth (USDA TR-55) per surface preset, routed by D8, then filled and spilled through the depressions in downstream order. The mass balance is shown: runoff = ponded + outflow.
• Grading re-runs the whole chain on the edited terrain; cut / fill = Σ|Δz| · cell².
• Contours are drawn from the original grid; slopes come from the filled grid — drawings show the ground you have, routing shows the ground water sees.

How to use it

1. Bring terrain. Drop a .asc, .xyz, .obj or .3dm; load a location from real elevation tiles; pick the Rhino selection; or start with the demo.
2. Pick a mode. Surface gives the clean 3D ground; Flow reads the land; Rainfall tests a storm; Grading regrades.
3. Read the legend. Amber = ridges, cyan = valleys. Toggle contours, accumulation and ponding sites on top.
4. Test a storm. Set storm depth and surface type; read runoff, ponded and outflow against each other.
5. Grade and re-check. Sculpt the ground; cut/fill volumes and flow update live.
6. Export or bake. Download the 3D ground for Rhino or Revit, or open More formats for a DXF plan, SVG drawing, GIS layers, CSV data or the PDF report (Pro).

Best used for

• Finding where water wants to go across a site before fixing levels.
• Locating the low points where a storm will actually pond, and how much they hold.
• Testing whether a proposed regrade moves a ponding problem or solves it.
• Putting a number on earthworks while you are still sketching levels.
• Reading a site catchment structure — ridges, valleys, basins — from a DEM you already have.

Not a substitute for

• Drainage design, pipe or culvert sizing.
• Flood-risk assessment or any regulatory submission.
• Anything that needs a hydraulic model — roughness, time-stepping, infiltration dynamics.

Known limits

• D8 forces all flow from a cell into one of eight directions, so it exaggerates channelisation on gentle, broad slopes.
• The result is only as good as the DEM: resolution, vertical accuracy and vintage all propagate straight through, whether uploaded or fetched from elevation tiles.
• No time-stepping: the storm is one runoff depth, not a hydrograph. Infiltration is only the Curve Number abstraction, not soil dynamics.
• Pipes, culverts, drains and anything buried are invisible; the terrain is treated as the only path.
• Filling adds an ε gradient so water always leaves; a ponding site is where the filled grid rose above the original, not a surveyed pond.
• A design-stage reading, not a hydraulic model or a regulatory flood study.

Read the full Terrain, Grading & Drainage guide →