Catchment Runoff & Hydrograph Simulator
Explore how antecedent soil saturation, severe convective downpours, and watershed land cover transform rainfall into rapid stream surge and culvert bankfull overtopping.
How Rain Becomes a Flash Flood
Flash flooding occurs when the rate of precipitation vastly outpaces a watershed's infiltration capacity and natural channel storage. In the American Midwest, intense mesoscale convective systems (MCS) or stalled frontal boundaries drop multi-inch rainfall accumulations across short timeframes.
Ia = 0.2 × S
Q = (P - Ia)² / (P - Ia + S) [for P > Ia]
The NRCS Curve Number (CN) represents the relationship between soil type, land use, and surface runoff. Lower numbers represent porous forest soils with deep absorption, while urban asphalt approaches CN 98, generating near-instantaneous sheet flow.
Antecedent Moisture Condition (AMC) & Midwestern Soils
The primary driver of severe Midwestern river and creek surges is often not the initial storm alone, but repeat rounds of rain. When consecutive squall lines strike a region:
- AMC I (Dry): Extended dry periods leave soil pores open, maximizing initial abstraction ($I_a$). Peak streamflow is substantially buffered.
- AMC II (Normal): Standard hydrologic design baseline.
- AMC III (Saturated): Previous rain saturates the upper soil layers. Soil suction drops toward zero; effective Curve Number surges, causing virtually 80-90% of incoming rain to transform into direct surface runoff.