Discharge

Comprehensive Discharge Calculator — Common Indian Methods

Multiple quick calculators: open-channel (Manning / Chezy), weirs & notches, orifices, Parshall/Flumes (power-law), pipes (Darcy–Weisbach & Hazen–Williams), Rational & SCS runoff, rating curves. Enter consistent units (SI) — notes below.

Continuity — Q = A × V

Use when you know cross-sectional area (A, m²) and mean velocity (V, m/s).

Cross-sectional area A (m²) Mean velocity V (m/s)

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Manning & Chezy (Open-channel)

Manning: Q = (1/n) A R^(2/3) S^(1/2). Chezy: Q = C A sqrt(R S).

Wetted area A (m²) Hydraulic radius R (m) Slope S (m/m) Manning's n Chezy C (optional, leave blank to compute from Manning)

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Weirs & Notches

Rectangular sharp-crested weir: Q = (2/3) C_d L sqrt(2g) H^(3/2). V-notch (triangular): Q = K_theta · H^(5/2) (K may be provided or computed).

Choose type Rectangular sharp-crested Triangular (V-notch)

Length L (m) Head over crest H (m) Discharge coefficient C_d (typ. 0.6–0.67)

Notch angle θ (degrees, e.g., 90) Head above vertex H (m) Discharge coefficient K (optional). If left blank, default coefficient for 90° ≈ 0.247 will be used.

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Orifice (Free discharge)

Free orifice: Q = C_d A sqrt(2 g H). For submerged orifices, effective head reduces — select below.

Orifice area A (m²) Head above orifice centre H (m) Discharge coefficient C_d (typ. 0.6–0.8) Submerged? FreePartially/Submerged

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Flumes / Parshall (Power-law)

Many field flumes follow Q = K × H^n. For Parshall flumes, K & n are tabulated per throat size. Use calibrated K and n when available.

Water depth at designated point H (m) Coefficient K (m^(3−n)/s) — enter if known Exponent n (typical 1.0 to 2.5) — enter if known

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Pipes — Darcy–Weisbach & Hazen–Williams

Darcy–Weisbach (general): hf = f (L/D) (V^2/2g). For head-driven flow between two heads ΔH, we can solve Q. Hazen–Williams is empirical for water in pipes.

Pipe diameter D (m) Length L (m) Upstream head H1 (m) Downstream head H2 (m) Darcy friction factor f (enter or leave blank to estimate using Swamee–Jain with roughness) Roughness height ε (mm) — used only if f not provided

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Rational Method (Peak runoff)

Qp = C × i × A. Qp in m³/s when: i in m/s, A in m². Often i is intensity for duration equal to Tc (m/s). More commonly in practice: Q (lps) = 0.278 × C × i(mm/hr) × A(ha).

Runoff coefficient C (0–1) Rainfall intensity i (mm/hr) Catchment area A (hectares)

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SCS-CN Method (Runoff depth)

Standard curve-number method: S = (25400/CN) - 254 (when using mm). Initial abstraction Ia commonly = 0.2 S. Direct runoff depth Q = (P - Ia)^2 / (P - Ia + S) for P > Ia.

Rainfall depth P (mm) Curve Number CN (30–100)

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Rating Curve (Stage–Discharge)

Enter a polynomial or power fit: Q = a0 + a1·H + a2·H² ... OR Q = K·H^n. Use measured gauging data to fit coefficients externally, then compute Q for any stage H.

Choose form Power-law Q=K·H^nPolynomial Q=a0+a1H+a2H²...

K n Stage H (m)

Enter coefficients separated by commas (a0,a1,a2,...) Stage H (m)

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Comparative Table — Methods, Use & Remarks

Method	Typical Application (India)	Inputs	Accuracy / Notes	Remarks
Continuity	General channel/sectional measurement	A, V	High if mean velocity & area measured accurately	Simple; use acoustic/area-velocity gauging for field
Manning	Open channels, irrigation canals, rivers (design)	A, R, S, n	Good for uniform flow; sensitivity to n	n should be chosen per channel lining; empirical
Chezy	Open channels (alternative)	A, R, S, C	Equivalent to Manning if C derived	Useful when C known from tables
Weirs / Notches	Irrigation outlets, temporary gauging weirs	L (or θ), H, Cd / K	Good for small flows; crest and upstream conditions matter	Sharp-crested weir formulas need velocity head correction for upstream approach
Orifice	Outlet pipes, sluices	A, H, Cd	Good for free-flowing orifices; submerged needs correction	Coefficient depends on shape and edge (sharp/rifled)
Parshall / Flume (power-law)	Field flow measurement at irrigation canals, wastewater	H, K, n (calibrated)	Very good when calibrated; robust for varying heads	Parshall tables commonly used in India — use throat-size table
Pipes (Darcy–Weisbach)	Pressurized pipe systems — water supply, irrigation	D, L, ΔH, f or ε	General and accurate when f estimated correctly	Use Moody chart or Swamee–Jain formula to estimate f
Hazen–Williams	Urban water pipes (empirical)	D, L, C_HW, ΔH	Good for water at normal temperatures; not universal	Common in practice but empirical — not for hydrocarbons
Rational	Stormwater / urban peak runoff (small catchments)	C, i, A	Reasonable for small (<50 ha) urban catchments	i should correspond to Tc; widely used in Indian municipal design
SCS-CN	Hydrological runoff depth & volume	P, CN	Good for event-based runoff estimation; needs proper CN	CN depends on soil, land use; common in watershed studies
Rating Curve	River gauging stations	Stage H, fit coefficients	Reflects channel characteristics; accurate within observation range	Careful extrapolation beyond gauged range can be wrong

Legend: A — area (m²), V — velocity (m/s), R — hydraulic radius (m), S — channel slope (m/m), H — head (m), L — length of weir (m).

Notes: This tool provides quick engineering calculations. Always check units and local standards (IRRIGATION / IWRS / State Irrigation Dept tables) and calibrate field instruments. For high-stakes designs, use detailed standards and site measurements.