Flash Flood Runoff & Hydrograph Planner

Ground-tested hydrological modeling using SCS-CN runoff mechanics and dimensionless unit hydrographs. Predict discharge spikes, culvert bankfull breaches, and flash flood emergence from intense rain bands.

Presets:

Catchment Parameters

Total Rainfall Event 3.8 in 0.5" (Light)4.0" (Heavy)9.0" (Catastrophic) Storm Duration ($D$) 6.0 hrs 1 hr (Burst)6 hrs (Midwest front)24 hrs (Cyclonic) Soil Curve Number ($CN$) 84 (Clay/Dense) 50 (Sandy forest)75 (Pasture)98 (Paved) Drainage Basin Area 4.2 sq mi 0.2 sq mi (Local ditch)25 sq mi (Regional creek) Time of Concentration ($T_c$) 2.4 hrs 0.5 hr (Fast runoff)8.0 hrs (Gentle slope) Stream / Culvert Capacity 1,850 cfs 200 cfsBankfull threshold6,000 cfs

Retention ($S$) 1.90 in

Initial Loss ($I_a$) 0.38 in

Total Excess Runoff 2.28 in

Runoff Volume 510 ac-ft

Rain Hyetograph (in/hr) Stream Discharge (cfs) Bankfull Threshold

              Hover chart to inspect time steps
            

Calculated across 40 time-increments with synthetic SCS dimensionless curvilinear unit hydrograph.

Peak Discharge ($Q_p$) 2,142 cfs +15.8% above capacity

Time to Peak ($T_p$) 3.8 hrs From storm onset

Flooding Inundation Window 2.4 hrs Overtopping bank height

Evacuation Lead Time 1.6 hrs Before bank overflow

Hydrograph calibrated and ready.

SCS-CN Runoff Mechanics

The Soil Conservation Service (NRCS) method computes excess runoff ($Q$) by deriving initial abstraction ($I_a = 0.2S$) and potential retention $S = (1000/CN) - 10$. Saturated clay soils with $CN \ge 84$ resist infiltration, accelerating surface flow.

Dimensionless Unit Hydrographs

Peak discharge time relates directly to catchment time of concentration: $T_p = \frac{\Delta D}{2} + 0.6T_c$. Hyetograph rainfall increments are convolved with the synthetic catchment response curve to reveal true peak arrival timings.

Flash Flood Emergency Thresholds

Whenever discharge spikes beyond bankfull culvert capacity, water spreads laterally into low-lying roads and structures. Lead time quantifies the emergency response window before the bank crests.