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

Routing starts from the pond's Initial WSE when one is set. Above the top of the storage table, storage and discharge are extended linearly and the solver warns that the pond overtopped.

When the pond takes its tailwater from a downstream element, each time step instead solves for the stage at which the tailwater-adjusted outflow satisfies the storage-indication balance, so the downstream water surface submerges this pond's outlets as the run proceeds.

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 is assembled as:
    • Primary outlet = the riser (its orifices, weirs, and top overflow) in series with the primary culvert — the lower of the two capacities governs at that stage. Riser openings themselves use Q = Cd × A × √(2gh) for orifices (h measured to the opening; a partly submerged opening uses its wetted area and the head to its centroid) and Q = Cw × L × H3/2 for weirs, with H = stage − crest elevation
    • plus secondary culvert flow (HDS-5 analysis)
    • plus spillway flow at higher stages
    • Exfiltration leaves the storage balance but is not routed downstream

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

A wall weir is capped at the orifice discharge for the same opening, so once the opening drowns out the orifice law governs.

Top Overflow

When water overtops the riser, effective weir length = riser perimeter − sum of weir widths, with Cw = 3.0. Above that, the riser throat controls: HydraLink takes the lesser of the weir discharge and the orifice discharge on the riser plan area.

Spillway Types

Type Equation Notes
Sharp-Crested Rectangular Q = Cw × (L − 0.1 × N × H) × H3/2 Francis formula; N = 0, 1, or 2 end contractions
Broad-Crested Rectangular Q = Cw × L × H3/2 Cw as entered (3.33 default)
V-Notch Q = Cw × (8/15) × √(2g) × tan(θ/2) × H5/2 Notch angle θ entered (90° default); Cw defaults to 0.58
Cipolletti Q = Cw × L × H3/2 Trapezoidal weir; Cw as entered (3.33 default)

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.