Theory¶

The 1:15 slope criterion¶

The Norwegian Water Resources and Energy Directorate (NVE) established in Kvikkleireveileder 1/2019 that a conservative limit for landslide retrogression in quick-clay is a slope of 1:15 (vertical:horizontal).

This means: for every 1 meter of height difference, the landslide can propagate up to 15 meters horizontally.

The same conceptual model is also used for the two-phase retrogression workflow: an initial slope around 1:4 can be used to evaluate whether erosion may trigger an initial landslide, followed by retrogressive propagation using the more conservative 1:15 criterion.

Methods¶

Terrain Criteria¶

What it does: For each pixel in the DEM, checks if the slope from that pixel back to any source point exceeds 1:15 (or a custom threshold).

How it works:

1. Load a DEM (from Høydedata or a local raster)
2. For each pixel, compute the slope to the nearest source point
3. Classify pixels by slope: 1:20, 1:15, 1:10, 1:5, 1:3
4. Polygonize the result into a GeoDataFrame

Best for: Quick overview of potentially affected areas. Fast, memory-efficient.

Limitations: Doesn't model actual propagation — just geometric reach.

Retrogression¶

What it does: Starting from an initial release area, expands outward step-by-step, only into pixels where the slope criterion is met.

How it works:

1. Rasterize the source geometry onto the DEM
2. Create a buffer ring around the current release
3. For each pixel in the buffer, check if the slope from the source to that pixel exceeds the threshold
4. Add qualifying pixels to the release area
5. Repeat until no more pixels qualify (or max_length reached)

Best for: Continuous, realistic release area shapes. Honors terrain features that might block propagation.

Variants:

• run_retrogression() — Single-phase with one slope threshold
• run_retrogression_with_initial_landslide() — Two phases: first a steep initial failure criterion (e.g. 1:4) to assess whether erosion can trigger an initial slide, then retrogressive propagation with 1:15

Profile Retrogression¶

What it does: Analyzes retrogression along cross-section profiles perpendicular to a source line.

How it works:

1. Generate terrain profiles perpendicular to the source line
2. For each profile, find where the 1:15 slope criterion is met
3. Connect the retrogression points across profiles
4. Build a release area polygon from the connected points

Best for: Detailed analysis along a known escarpment or riverbank. Gives you control over the analysis direction.

Limitations: Requires pre-defined profile lines.

Parameters¶

All methods share key parameters: