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name: river-morphology-change description: > Extract shorelines, centerlines and channel widths from multi-temporal water body masks. Quantify shoreline migration, channel migration, change hotspots and migration zones. Use when analyzing river channel changes, identifying erosion/deposition areas, or generating river morphology reports.

River Morphology Change

Extracts shorelines, centerlines, and channel widths from multi-temporal water body masks. Quantifies shoreline migration, channel migration, and change hotspots.

Trigger

Use when the user wants to:

• Analyze river channel migration from multi-temporal imagery
• Identify erosion and deposition areas along river banks
• Compute channel width changes over time
• Detect change hotspots where migration is most severe
• Generate river morphology change reports
• Compare shorelines between different time periods

CLI Usage

# Basic analysis with default parameters
python scripts/river_morphology_change.py --output-dir ./rmc-output

# Custom transect spacing and hotspot percentile
python scripts/river_morphology_change.py \
  --transect-spacing 30.0 \
  --hotspot-percentile 85.0 \
  --output-dir ./rmc-output

# With custom water threshold
python scripts/river_morphology_change.py \
  --water-threshold 0.1 \
  --min-channel-width 15.0 \
  --output-dir ./rmc-output

Parameters

Parameter	Default	Description
--input-masks	None	Input water mask GeoTIFF files (ordered by time)
--transect-spacing	50.0	Distance between transects in map units
--min-channel-width	20.0	Minimum channel width for quality flagging
--shoreline-method	threshold	Shoreline extraction method: threshold, canny, manual
--water-threshold	0.0	Water extraction threshold (NDWI/MNDWI)
--hotspot-percentile	90.0	Percentile threshold for hotspot detection
--output-dir	./rmc-output	Output directory

Output

File	Description
shorelines.geojson	Shoreline features for each time period
centerlines.geojson	Centerline features for each time period
transects.geojson	Perpendicular transect lines along centerline
migration_rates.csv	Migration distances and rates per transect
change_hotspots.geojson	Hotspot features where migration exceeds threshold
request.json	Analysis request metadata
dataset-manifest.json	Dataset inventory and period information
output-manifest.json	Output file inventory and statistics
qa.json	Quality assurance checks

Quality Codes

Code	Name	Description
1	high	Clean intersection, single shoreline crossing
2	medium	Small migration or minor complexity
3	low	Multiple intersections, complex geometry
4	invalid	No intersection or processing error

Direction Codes

Code	Name	Description
-1	erosion	Shoreline retreats (water expands)
0	stable	No significant change
1	deposition	Shoreline advances (water contracts)

Key Algorithms

Water Body Extraction

Uses NDWI (Normalized Difference Water Index) or MNDWI (Modified NDWI) to extract water bodies from multispectral imagery:

• NDWI = (Green - NIR) / (Green + NIR)
• MNDWI = (Green - SWIR1) / (Green + SWIR1)

Centerline Extraction

Uses Voronoi-based medial axis extraction:

1. Sample points along water polygon boundary
2. Compute Voronoi diagram of boundary points
3. Extract Voronoi vertices inside polygon as centerline
4. Fallback to bounding box centerline if scipy unavailable

Transect Generation

Generates perpendicular transects along the centerline at regular intervals. Each transect is perpendicular to the local tangent direction.

Migration Rate Computation

Uses perpendicular intersection method:

1. For each transect, find intersection with both shorelines
2. Measure distance between intersection points
3. Determine direction (erosion/deposition) based on distance from center

Change Hotspot Detection

Identifies transects where migration distance exceeds the specified percentile threshold (default: 90th percentile).

Exit Codes

Code	Meaning
0	Success
2	Argument error
3	Dependency missing
6	Data validation failure
7	Processing failure

Limitations

• Water level changes may be misinterpreted as morphological change
• Narrow channels near pixel resolution limit have reduced accuracy
• Centerline topology handling is complex for braided/anastomosing channels
• Results are auxiliary analysis only; engineering decisions require manual review
• Same-season imagery recommended to minimize water level effects

References

• Pavelsky & Smith 2008, IEEE GRSL (RivWidth algorithm)
• Isikdogan et al. 2017, IGARSS (RivMap automatic river width extraction)
• Fisher et al. 2013, Nature Geoscience (global river delta assessment)

数据下载

本 skill 可自动从 Microsoft Planetary Computer 下载数据 (无需 API key):

python river_morphology_change.py --bbox 116,39,117,40 --date-range 2024-06-01,2024-06-30 --output-dir <tmp>

• --bbox W,S,E,N: WGS-84 边界框 (西, 南, 东, 北)
• --date-range START,END: 日期范围 (YYYY-MM-DD,YYYY-MM-DD)
• --aoi-file <path.geojson>: 替代 --bbox 的 GeoJSON 多边形
• --cache-dir <path>: 缓存目录 (默认 ~/.geoskill_cache)

当用户只给 --bbox + --date-range (没有 --image) 时，skill 自动下载数据。 当用户给 --image 时，走原文件路径 (向后兼容)。