Reservoir Delta Progradation Simulator

Grain Size (settling class)

Inflow Conditions

Sediment load 500,000 t/yr

Simulation speed 15 yr/s

Live Readout

Years simulated

0 yr

Delta front from inlet

0 m

Live storage remaining

100.0%

How it works

When a sediment-laden river enters the still water of a reservoir, its velocity — and the turbulence keeping grains in suspension — drops sharply. Coarse grains settle almost immediately near the inlet; fine grains stay suspended and travel far into the pool before settling. Repeated over years this builds a classic Gilbert-type delta with three beds:

Topset  — near-flat, at reservoir water level (river's own bed load)
Foreset — steep face at the sediment's angle of repose, advancing lakeward
Bottomset — thin drape of the finest wash load far from the inlet

This model deposits incoming sediment volume with an exponential decay away from the inlet (shorter decay length = coarser grains, settling closer in), then relaxes each new increment downslope wherever it exceeds the grain's angle of repose — the same rule that builds any granular slope:

V(t) = (sediment load × Δt) / bulk density        [deposited volume]
w(x)  = exp(−x / L_settle)                        [where it lands]
slope > tanθ_repose  →  material avalanches downslope until slope ≤ tanθ_repose

Grain size — coarse gravel has a short settling length and a steep repose angle (~32°), building a tall delta close to the inlet. Fine silt travels far (long settling length) at a gentle angle (~12°), spreading a low wedge most of the way to the dam.
Sediment load — the mass of sediment entering per year; higher load fills storage faster.
Storage remaining — the pool volume between the current bed and the water surface, divided by the original (unsedimented) volume. This is exactly the quantity dam operators track: reservoirs worldwide lose roughly 0.5–1% of their capacity per year to sedimentation on average, and some silt-heavy systems (parts of the Himalayan and Andean river basins) lose several percent a year.

Reservoir water Deposited sediment bed Original (pre-dam) bed

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