Downstream Sediment Delineation

The problem

A regional geochemical baseline study needed to know exactly which portions of the drainage network contribute material to each downstream sampling site — across 4,200 km² of semi-arid catchment. Manual delineation at that scale is slow, inconsistent and impossible to iterate.

This project developed an end-to-end automated workflow to delineate downstream sediment pathways and accumulation zones. A 10 m resolution DEM derived from Copernicus GLO-30 was the primary input; hydrological conditioning, flow direction and accumulation rasters were generated with ArcPy's Spatial Analyst toolbox, with custom Python scripts automating basin delineation and outlet snapping across all 38 identified pour points.

Methodology

1. DEM pre-processing — Pit-filled and conditioned using the Agree method to enforce known drainage lines from 1:25,000 hydrography.
2. Flow direction & accumulation — D8 flow direction raster computed; accumulation thresholds calibrated against mapped channels to extract the stream network.
3. Catchment delineation — Watershed function called iteratively in Python for all 38 sample-site pour points; outputs merged and attributed.
4. Sediment source ranking — Slope, upstream area and lithology rasters combined in a weighted index to rank source contribution by sub-catchment.
5. Validation — Delineated boundaries verified against field GPS tracks and historical stream gauge records; mean boundary error <180 m.

Tools & stack

Outcomes

• Delivered 38 attributed sub-catchment polygons and a ranked sediment source raster to the client within a 6-week schedule.
• Automated workflow reduced manual delineation time from ~3 days to under 2 hours per catchment iteration.
• Outputs directly informed 14 follow-up soil and rock-chip sampling sites, reducing field programme cost by an estimated 18%.
• Methodology documented and handed over as a reusable Python toolbox for the client's internal GIS team.