What This Solves
Routes an inflow hydrograph through a detention basin or reservoir using the Modified Puls method to determine peak attenuation, lag time, and maximum storage.
Best Used When
- You need to demonstrate that a detention pond reduces the post-development peak flow to pre-development levels
- You have an inflow hydrograph and stage-storage-discharge relationships and need the outflow hydrograph
- You are refining pond and outlet sizing after preliminary estimates from pond-sizing calculations
Do NOT Use When
- You need to route a hydrograph through a river or channel reach (not a reservoir) — Use Muskingum Routing Calculator
- You only need preliminary storage volume estimation without full routing — Use Storage Volume Calculator
Key Assumptions
- The water surface in the reservoir is level (no significant velocity head or wind effects)
- The stage-storage and stage-discharge relationships are single-valued (no hysteresis)
- Inflow and outflow vary linearly within each time step
- No significant seepage, evaporation, or direct rainfall on the pond during the event
- The outlet structure operates as designed (no clogging or debris blockage)
Input Quality Notes
Accurate stage-storage and stage-discharge curves are essential. Use surveyed contours for storage and calibrated outlet equations for discharge. Time step should be small enough to capture the inflow peak.
Route Hydrograph Through Storage
For educational purposes only. Not a substitute for professional engineering judgment.
Modified Puls Method Overview
The Modified Puls (Storage-Indication) method routes flood hydrographs through reservoirs and detention basins. It assumes a level pool where outflow is a function of storage only.
Key Equations
- Storage-Indication: SI = 2S/dt + O
- Routing equation: (2S2/dt + O2) = (2S1/dt - O1) + I1 + I2
Where:
- S = Storage volume (acre-ft)
- O = Outflow discharge (cfs)
- I = Inflow discharge (cfs)
- dt = Routing time step (minutes)
Common Applications
Detention Basin Design
Size storage to meet peak flow reduction targets
Reservoir Flood Routing
Determine outflow hydrograph and peak water level
Dam Safety Analysis
Evaluate spillway capacity during design floods
Stormwater Management
Demonstrate compliance with post-development limits
Design Tips
Time step selection: Use a time step no larger than Tp/5 where Tp is the time to peak. Smaller time steps provide better accuracy but increase computation.
Storage-discharge curve: Ensure your data covers the full range of expected water levels. The method will extrapolate if values exceed the provided range.
Volume balance: Check that total inflow volume equals total outflow volume. Errors greater than 1% may indicate numerical instability.
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Last verified: February 2026