Pond Routing

Overview

Pond routing determines how an inflow hydrograph is attenuated as it passes through a detention or retention facility. HydraLink uses the Modified Puls (level pool) routing method for ponds, combined with a stage-storage-discharge relationship defined by the pond geometry and outlet structures.

Modified Puls Method

Theory

The Modified Puls method assumes level pool conditions — the water surface in the pond is horizontal at all times. This is valid for ponds where the inflow rate is small compared to the pond surface area.

Routing Procedure

  1. Compute the stage-storage-discharge table by evaluating all outlet structures at each elevation in the storage table
  2. Compute the storage-indication curve: SI = 2S/Δt + O at each stage
  3. For each time step:
    • SIj+1 = Ij + Ij+1 + (SIj − 2Oj)
    • Interpolate Oj+1 from the SI vs O relationship
    • Compute stage and storage from the stage-storage-discharge table
  4. Track peak stage, peak outflow, and timing
Pond stage-storage-discharge conceptual diagram

Stage-Storage-Discharge Computation

At each elevation in the storage table:

  • Storage S = cumulative volume from the lowest elevation
  • Discharge O = sum of all outlet flows:
    • Culvert flow (HDS-5 analysis)
    • Orifice flow: Q = Cd × A × √(2gh) for each orifice where h = stage − orifice center elevation
    • Weir flow: Q = Cw × L × H3/2 for each weir where H = stage − weir crest elevation
    • Riser top overflow when stage exceeds riser top
    • Spillway flow at higher stages

MRM Detention Sizing

For basins using the Modified Rational Method, the pond can perform a simplified detention sizing analysis without requiring a full stage-storage-discharge relationship.

See the Modified Rational Method methodology page for details.

Outlet Structure Hydraulics

Culvert (Primary and Secondary)

Full HDS-5 analysis. See the Culvert element page.

Riser Structure

The riser acts as a vertical pipe or box with multiple openings at different elevations.

Orifice Equations

  • Circular: A = π/4 × D²,   Q = Cd × A × √(2gh),   default Cd = 0.6
  • Rectangular: A = W × H, same discharge equation

Weir Equations

Q = Cw × L × H3/2    default Cw = 3.33

Top Overflow

When water overtops the riser, effective weir length = riser perimeter − sum of weir widths.

Spillway Types

Type Equation Notes
Sharp-Crested Rectangular Q = Cw × L × H3/2 Standard sharp-crested weir
Broad-Crested Rectangular Q = Cw × L × H3/2 Lower Cw than sharp-crested
V-Notch (90°) Q = Cv × H5/2 For low-flow measurement
Cipolletti Q = Cw × L × H3/2 Trapezoidal weir (1H:4V sides)

Design Considerations

  • The primary outlet (culvert or riser orifices) controls outflow for frequent storms
  • Weirs and spillways activate at higher stages for larger storms
  • The spillway should be sized to pass extreme events without overtopping the embankment
  • Total outflow is the sum of all outlet structure discharges at each stage

References

  • NRCS (2010). National Engineering Handbook, Part 630, Chapter 17 — Flood Routing.
  • FHWA (2001). Urban Drainage Design Manual, HEC-22.
  • Akan, A.O. (1993). Urban Stormwater Hydrology. Technomic Publishing.