Which calculation to run. 'rational_method' computes peak runoff Q = CiA; 'mannings_flow' computes open-channel flow via Manning's equation; 'both' runs both. If omitted, defaults to 'both'.

Contributing drainage area in acres. The Rational Method is valid for areas up to ~200 acres; larger watersheds need unit hydrograph methods. Only required when calc_mode is 'rational_method' or 'both'.

Design rainfall intensity in inches per hour for the selected return period and time of concentration. Obtain from local IDF curves or NOAA Atlas 14. Only required when calc_mode is 'rational_method' or 'both'.

Rational method runoff coefficient C (dimensionless). Typical: 0.30 impervious lawn, 0.70-0.95 paved/roof. If omitted, looked up from the reference table using land_use. Provide either this or land_use for rational_method.

Land use category for runoff coefficient lookup (e.g., 'commercial', 'residential_suburban', 'industrial'). Only required when runoff_coefficient is not provided and calc_mode is 'rational_method' or 'both'.

Cross-section geometry of the open channel or conduit. Only required when calc_mode is 'mannings_flow' or 'both'.

Channel lining material for Manning's n lookup (e.g., 'concrete_trowel_finish', 'corrugated_metal', 'earth_clean'). Only required when mannings_n is not provided and calc_mode is 'mannings_flow' or 'both'.

Manning's roughness coefficient (dimensionless). Typical: 0.013 concrete, 0.024 corrugated metal, 0.030 natural earth. If omitted, looked up from the reference table using channel_material. Provide either this or channel_material for mannings_flow.

Channel longitudinal slope in ft/ft (dimensionless). For example, a 1% slope = 0.01. Only required when calc_mode is 'mannings_flow' or 'both'.

Flow depth in feet measured from the channel invert. For circular pipes this is the water depth, not the pipe diameter. Only required when calc_mode is 'mannings_flow' or 'both'.

Channel bottom width in feet. Not used for circular. Only required when channel_shape is 'rectangular' or 'trapezoidal'.

Inside diameter of circular pipe in feet. Common sizes: 1-10 ft. Only required when channel_shape is 'circular'.

Trapezoidal channel side slope expressed as horizontal:vertical (z:1). For example, z=2 means 2H:1V. Only required when channel_shape is 'trapezoidal'.