Water & Drainage

When to use it

• Finding where water runs across a block after heavy rain, and which gullies carry the most.
• Locating low spots that will hold water, for drainage design or to avoid siting a building there.
• Planning culverts, swales and fence crossings.

How to use it

1. Select a property, draw an area or upload a boundary.
2. Run. Switch between the stream network and ponding views in the panel.
3. Export the channels as GeoJSON or KML to open in GIS or Google Earth.

How it works

Small pits in the LiDAR bare-earth model are filled so water can always find a way downhill. Each cell then passes its water to its steepest neighbour, and the number of upstream cells is counted for every cell. Cells that collect more than a threshold are drawn as channels and ranked by Strahler order, where two first-order streams meeting make a second-order stream.

Ponding compares the filled surface with the original ground: wherever the fill raised the ground, water would stand there, and the difference is the depth.

Reading the results

Thicker, higher-order lines carry more of the area's water. First-order lines are the small feeder gullies.

Ponding depth, area and volume describe the hollow when full. Real ponds drain into the soil and evaporate, so these are upper bounds.

Limits and warnings

• Culverts, pipes, stopbanks and open drains that are narrower than the sampling are not in the terrain, so water is routed over roads and through embankments.
• Channels are cut off at the edge of the area. Use Catchment to see where a channel's water comes from beyond the boundary.
• Where 1 m LiDAR has not been flown the tool falls back to the national 8 m elevation model, and the result says what share came from the fallback. The 8 m model was interpolated from 20 m map contours and is not fit for site-scale decisions.

Data and attribution

Results come from the LINZ national 1 m LiDAR bare-earth elevation model, read in your browser, with the national 8 m model filling any gaps. Tools that use the surface model (Sunlight, Viewshed, Object Heights) read the matching LiDAR surface tiles.

Elevation and cadastral data is sourced from the LINZ Data Service and licensed for re-use under the Creative Commons Attribution 4.0 International licence. Copyright in each LiDAR survey belongs to the body that commissioned it — a regional or district council, Waka Kotahi, NIWA and around thirty others — rather than to LINZ, which hosts and processes the national dataset. Every file exported from this site names the licensors whose surveys cover the area, and anything you publish from those files carries the same attribution requirement. See https://www.linz.govt.nz/products-services/data/licensing-and-using-data/attributing-elevation-or-aerial-imagery-data

Related

Neighbouring tools

• Catchment — Maps all the land upstream of a point you click, so you can see where the water arriving at a culvert, a gully or a stream crossing comes from.
• Wetness Map — Maps where the shape of the land tends to concentrate moisture, from how much ground drains to each point and how flat that point is.
• Landforms — Classifies every part of your area into one of ten landforms, such as ridge, spur, slope, hollow and valley, at a scale you choose.