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CALCULATORS / CIVIL, GEOTECHNICAL AND WATER SCREENING

Catchment runoff estimate calculator

Rational-method peak runoff for supplied catchment coefficient and uniform rain intensity.

civil-runoff.napkin / editable
✓ checks pass

SCENARIO COMPARISON

Compare the numbers.

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

ScenarioflowL/s
9.72222
7.77778

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.

drain flow budget9.72 L/s✓ passes

BASELINE INPUTS

CALCULATED

flow9.7222 L/s

Edits change the baseline source.

Calculated from your source · not saved
Baseline · Baseline
drain flow budget9.72 L/s ✓ passes
alternative · alternative
drain flow budget7.78 L/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.

flow
flow = area*rain*coefficient [L/s]

Original worked example

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

Given

area
1000 m^2
rain
50 mm/h
coefficient
0.7

Calculated

flow
9.72222 L/s

Assumptions and limits

Rational-method peak runoff for supplied catchment coefficient and uniform rain intensity.

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: Catchment runoff estimate. Inputs: catchment area; rainfall intensity; runoff coefficient. Outputs: peak runoff under selected rational-method assumptions. 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: catchment area, rainfall intensity, runoff coefficient
  • Outputs: peak runoff under selected rational-method assumptions
  • Modes: calculate, compare, sweep
  • Drawing: Load and site section diagram

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

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