Pond Element

The Pond element models detention and retention facilities that store and attenuate stormwater runoff. Ponds use elevation-area tables to define their storage geometry and can route inflow hydrographs using the Modified Puls method or size detention using the Modified Rational Method (MRM).

When to Use

• Detention pond design and analysis
• Retention basin modeling
• Any facility that temporarily stores runoff and releases it at a controlled rate
• MRM detention sizing for small watershed studies

Storage Modes

Two storage modes are available.

Above-Ground (Elevation-Area Table)

Define the pond geometry using an elevation-area table. HydraLink computes the volume between each elevation increment using one of two methods:

• Conic Method (default): V = h/3 × (A1 + A2 + √(A1 × A2)), accurate for ponds with sloping sides.
• Average End Area Method: V = h/2 × (A1 + A2). Simpler, but slightly overestimates volume.

Example Elevation-Area Table

Elevation (ft)	Area (ft²)
100.0	0
101.0	500
102.0	1,200
103.0	2,100
104.0	3,200
105.0	4,500

Rows can be added, inserted, or deleted in the elevation-area editor, imported from CSV, or pasted straight from a spreadsheet.

Underground Chamber System

Instead of an elevation-area table, an underground system can be laid out from a library of manufactured chambers. Pick a product, lay the chambers out in rows, and optionally count the surrounding stone backfill (and the stone base beneath it) as storage using a void ratio. HydraLink builds the stage-storage curve from the layout, and routing and MRM sizing then use it exactly as they use an above-ground table.

Outlet Structures

Ponds use a combination of outlet structures to control the outflow at different stages. The primary outlet is the riser and the primary culvert in series — when both are enabled, the lower of the two capacities governs at each stage. Total outflow is that primary discharge plus the secondary culvert and the spillway. Exfiltration is tracked separately and is not routed downstream.

1. Primary Culvert

A culvert barrel at the base of the pond. Analyzed using the same HDS-5 methodology as the standalone Culvert element. Configure barrel shape, dimensions, entrance type, length, slope, and Manning's n. An optional restrictor plate can be added over the barrel entrance to further throttle outflow, with the more restrictive of the plate and the barrel governing.

2. Primary Riser

A vertical riser structure with multiple outlet openings at different elevations:

Orifices

Circular or rectangular openings; up to five can be placed on the riser.

• Circular: A = π/4 × D²
• Rectangular: A = W × H

Weirs

Openings along the riser wall, up to five, each either rectangular or V-notch.

Defined by crest elevation, width, and weir coefficient.

As the head rises the opening drowns out: HydraLink caps each wall weir at the orifice discharge for the same opening, so the weir law never reports more than the opening can pass.

Top Overflow

When water overtops the riser, the riser perimeter acts as a weir. Effective width = riser perimeter minus the sum of all weir widths. At high head the riser throat controls instead, and HydraLink takes the lesser of the weir and orifice discharge on the riser plan area.

3. Secondary Culvert (Optional)

An additional culvert for higher-stage discharge, configured the same as the primary culvert. Like the primary culvert, it can take an optional restrictor plate over the barrel entrance, with the more restrictive of the plate and the barrel governing.

4. Spillway

Emergency overflow weir with four types:

• Sharp-Crested Rectangular (0, 1, or 2 end contractions)
• Broad-Crested Rectangular
• V-Notch (notch angle entered, 90° by default)
• Cipolletti (trapezoidal)

5. Exfiltration / Infiltration

Models water leaving the pond through bottom and side infiltration into the surrounding soil using Darcy’s law:

Where K = saturated hydraulic conductivity (in/hr), A = infiltration area, and SF = safety factor.

• Soil Type: select from 11 Green-Ampt soil texture presets (Sand through Clay) to auto-fill the hydraulic conductivity, or choose Custom to enter a value directly.
• Safety Factor is a reduction factor (0–1, default 0.5) that accounts for long-term clogging and compaction.
• Bottom Infiltration uses the pond-bottom area from the elevation-area table, or a custom area override.
• Side-Wall Infiltration is optional. It estimates infiltration through the pond side walls from contour perimeters.

6. User-Defined Stage-Discharge Curve

For non-standard outlet configurations, you can enter a custom elevation-discharge table directly. This overrides the computed stage-discharge from other outlet structures.

• Enter elevation (ft) and discharge (cfs) pairs manually or import from CSV.
• Useful for modeling proprietary outlet devices, vortex valves, or complex multi-stage structures that cannot be represented by the built-in outlet types.

Modified Puls Routing

The primary routing method for ponds. At each time step:

1. Compute the storage indication value: SIj+1 = Ij + Ij+1 + (SIj − 2Oj)
2. Interpolate outflow O and storage S from the stage-storage-discharge relationship.
3. Track stage, storage, and outflow over the full hydrograph.

The stage-storage-discharge table is computed internally by evaluating all outlet structures at each elevation in the storage table. Routing starts from the Initial WSE when one is set. Above the top of the table, storage and discharge are extended linearly and the run log reports that the pond overtopped. See the Pond Routing methodology page for full details.

On the pond's Tailwater tab, Use Downstream Element WSE as Tailwater lets a downstream pond or channel submerge this pond's outlets: at each step the routing solves for the stage at which the tailwater-adjusted outflow balances the storage indication.

MRM Detention Sizing

When upstream basins use the Modified Rational Method, the pond can perform simplified detention sizing. The detention method, the basin roles and the allowable release are configured on the MRM basin; the pond reports required versus provided storage and checks its outlet against the allowable rate.

Standard Method

Every storm duration from Tc to 24 hours — or to the longest duration in the rainfall table, whichever comes first — is evaluated in 1-minute steps. If the required storage is still rising at the last duration, the run log says so rather than extrapolating. For each duration, HydraLink computes:

Required storage = V_in − V_out. Reports the critical duration that maximizes required storage. The intensity i is used at two-decimal precision, matching published IDF tables.

Regional IDF Table Method

Uses county- or city-specific coefficients (a, b factors) from the NCTCOG iSWM program (17 DFW-area Texas counties) or the Atlanta Regional Commission Georgia Stormwater Management Manual (16 Georgia cities).

Basin Roles

Basin roles control how each upstream basin participates in detention sizing:

Design Area

Post-development area detained by the pond. Its C and area contribute to the composite inflow calculation.

Target

Pre-development reference; its peak flow defines the allowable release rate from the pond.

Bypass

Flow from the site that does not enter the detention pond. Bypass flow reduces the allowable release from the pond, increasing required detention.

Pass-Through

Off-site flow that enters and exits the pond but is not detained. The outfall structure must be sized to convey pass-through flow in addition to the allowable detained release.

Input Parameters Summary

Parameter	Units	Description
Storage Mode	—	Above-Ground (Elev-Area Table) or Underground Chamber System
Elevation-Area Table	ft, ft²	Pond geometry (above-ground mode)
Volume Method	—	Conic or Average End Area
Initial WSE	ft	Starting water surface elevation; leave blank to start empty, or enter a permanent pool
Tailwater Condition	—	Per outlet culvert: Free Outfall or Specified Elevation
Tailwater Elevation	ft	Downstream water surface (if specified)
Use Downstream Element WSE	—	Take tailwater from the downstream pond or channel instead (Tailwater tab)
Primary Culvert	various	Barrel shape, dimensions, entrance type, etc.
Riser	various	Orifice and weir definitions
Secondary Culvert	various	Optional additional barrel
Spillway	various	Emergency overflow weir type and geometry
Exfiltration	various	Soil type, hydraulic conductivity, safety factor, bottom/side infiltration
User-Defined Stage-Discharge	ft, cfs	Custom elevation-discharge table for non-standard outlets

Results

Output	Units	Description
Peak Inflow	cfs	Maximum inflow rate
Peak Outflow	cfs	Maximum outflow rate
Peak Stage	ft	Maximum water surface elevation
Max Storage	cu ft	Maximum stored volume
Peak Time	h:mm	Time of peak outflow
Exfiltrated Volume	acre-ft	Volume lost to infiltration (when exfiltration is enabled)
Stage-Storage-Discharge Table	ft, acre-ft, cfs	Computed relationship at all stages
Outflow Hydrograph	cfs vs. time	Full routed outflow
Stage Hydrograph	ft vs. time	Water surface elevation over time

Tips & Best Practices

• Always verify the stage-storage-discharge table before running by inspecting the computed outflow at each elevation to ensure outlet structures are behaving as expected.
• Use the Conic volume method for natural ponds with irregular grading. Average End Area is acceptable for rectangular facilities.
• Place the spillway crest at or below the top of the storage table to ensure emergency overflow is modeled.
• For MRM sizing, ensure all upstream basins have their roles properly assigned.
• When comparing Modified Puls routing to MRM sizing, the Puls method is more accurate but requires a fully defined outlet structure. MRM provides a quick volumetric estimate.
• Ensure the elevation-area table starts at the invert (lowest point) of the pond.