CALCULATORS / CIVIL, GEOTECHNICAL AND WATER SCREENING

Open-channel flow estimate calculator

Manning uniform flow in a supplied rectangular channel; SI n carries time/length power.

civil-open-channel.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenario	aream^2	radiusm	flowm^3/s
	0.3	0.1875	0.207188
	0.24	0.171429	0.156138

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

flow capacity target0.21 m^3/s✓ passes

BASELINE INPUTS

width1 mdepth0.3 mslope0.001 roughness0.015

CALCULATED

area0.3 m^2

radius0.1875 m

flow0.20719 m^3/s

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# Open-channel flow estimate
# Scope: Manning uniform flow in a supplied rectangular channel; SI n carries time/length power.
# Status: illustrative worked example; engineering review pending.
# Inputs are example values. Checks are editable example requirements, not standards.

width = 1 m
depth = 0.3 m
slope = 0.001
roughness = 0.015 "Manning n numerical value in SI convention"

area := width*depth [m^2]
radius := area/(width+2*depth) [m]
flow := area*(radius/(1 m))^(2/3)*sqrt(slope)/roughness*(1 m/s) [m^3/s]

check flow capacity target: flow >= 0.15 m^3/s

scenario alternative: width = 0.8 m

Calculated from your source · not saved

Baseline · Baseline

flow capacity target0.21 m^3/s ✓ passes

alternative · alternative

flow capacity target0.16 m^3/s ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

area: area = width*depth [m^2]
radius: radius = area/(width+2*depth) [m]
flow: flow = area*(radius/(1 m))^(2/3)*sqrt(slope)/roughness*(1 m/s) [m^3/s]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

width: 1 m
depth: 0.3 m
slope: 0.001
roughness: 0.015

Calculated

area: 0.3 m^2
radius: 0.1875 m
flow: 0.207188 m^3/s

Assumptions and limits

Manning uniform flow in a supplied rectangular channel; SI n carries time/length power.

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: Open-channel flow estimate. Inputs: section geometry; slope; roughness model. Outputs: flow; normal depth; velocity. Model scope: Preliminary geometry and loads; site/code inputs supplied externally. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

Inputs: section geometry, slope, roughness model
Outputs: flow, normal depth, velocity
Modes: calculate, compare, sweep
Drawing: Load and site section diagram

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS