MY SIMULATOR

Beaver Watershed

Three dams, one storm: how a pond cascade slows floods and holds water for the dry weeks.

INTERACTIVE 3D

Time 0:00Inflow 0 m³/sOutflow 0 m³/sFPS 0

STORED WATER0 m³

PEAK CUT0 %

DAMS INTACT3 / 3

TIME☀🌧

Beaver Watershed: Dam Cascade, Storm Hydrographs & Dry-Season Storage

Three beaver dams in a row on one stream. Rain makes a flood wave (NRCS curve-number runoff), each pond stores some of it (level-pool routing), the dams leak and spill, and a hard enough storm breaks them one after another. Switch to a dry spell to see what the ponds give back.

Scenario

Time speed

Preset

Storm, catchment and dams

Live numbers

Hydrograph (inflow dashed, outflow below dam C)

Pond levels above the bed at each dam

Total stored volume

Missions

Independent numerical checks

Also run outside the browser: node src/check.js (mass-balance closure, steady state vs bisection, stage-storage vs numerical integral of the valley, SCS example, hydrograph volume, weir value).

Educational model, not a flood forecast. One lumped pond per dam, a single catchment, a fixed reach lag and a simple failure rule. Constants marked [assumption] or [verify against source] are illustrative. Vegetation, animals and the valley are schematic; vertical scale is true, animals are drawn a little large.

Controls

Drag / one finger: orbit; wheel / pinch: zoom; Hero, Section and Map buttons (keys H, S, T) jump the camera.
The TIME slider scrubs the whole run (it is precomputed, so it is exact in both directions); Start plays it.
Sliders re-run the model at once. Crest sliders raise or lower each dam (the 3D dam grows or shrinks).
Scenario switches between a 24 h storm and a 30 day dry spell. Keys: Space play/pause, R reset.

The science

Rain to runoff. NRCS curve number: S = 25400/CN − 254 (mm), Q = (P − 0.2S)² / (P + 0.8S) for P > 0.2S. The triangular unit hydrograph has peak q_p = 0.208 A Q / T_p (m³/s, A in km², Q in mm, T_p in h), T_p = D/2 + 0.6 T_c, recession lasting 1.67 T_p; its area equals Q A exactly. [verify against source: the peak-rate factor 484 and the 0.2 initial abstraction are textbook defaults]

Pond storage. The valley has a flat 3 m channel, floodplain sides 1:12 and bed slope 0.01. A pond of depth h at its dam therefore holds V = w₀h²/(2S₀) + z_sh³/(3S₀), with surface area dV/dh. Each pond obeys dV/dt = Q_in − Q_seep − Q_weir − (E + f) A(h), integrated with Heun's method (10 s step).

Dam. A stick-and-mud dam leaks through its body (Dupuit-Darcy: Q = K W h² / 2T, K = 0.004 m/s [assumption]) and spills over its crest as a broad-crested weir Q = C_w W e^3/2, C_w = 1.6 [assumption], e = h − crest. Flow from one pond reaches the next after a 120 s reach lag [assumption].

Failure. If the overtopping depth stays above the strength limit for 10 minutes the dam breaches: a 10 m gap opens down to 15 % of the crest in 20 minutes. The released flood arrives at the next pond, which may fail in turn (the domino). In the dry spell beavers repair a breach after 2 days and raise crests at the chosen cm/day up to 1.4 m.

Dry spell. Inflow decays exponentially (15 day time constant [assumption]); ponds lose water by evaporation and infiltration (recharge of the water table) and release it through the dam body. Whether the downstream flow is helped depends on seepage versus these losses.

Honest limits. Beaver ponds hold a small fraction of a big flood: the peak is cut a lot for small storms and ponds with freeboard, hardly at all when they are already full. Not modelled: sediment, ice, groundwater feedback, channel routing beyond a fixed lag, backwater between ponds, 2D flow.