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Sheet C-120
PAPPD PDD

Grading & Drainagepitfall

Detention vs retention ponds

One-line orientation

Detention and retention ponds both buffer stormwater, but one is normally dry and the other holds a permanent pool — and the two names get swapped constantly, which is exactly why it’s tested.

Key points

  • Detention pond:
    • Temporarily stores stormwater to prevent flooding and infrastructure overload.
    • Normally dry between storms.
    • Uses a restrictor pipe / restricted outlet to release outflow slowly and shave the peak.
    • Can be designed to look good even though it’s usually empty.
  • Retention pond:
    • A permanent body of water that holds stormwater and lets it percolate gradually into the ground.
    • Serves as a long-term water-management and landscape feature.
  • Memory hook: reTENtion holds waTER (permanent pool); DEtention = Dry, Delay (temporary, slow release).

Both basins fill in the storm — only detention ends up dry

SCHEMATIC

Both fill to the same storm peak; the outlet elevation decides where each one stops.

View diagram Hide diagram 2 panels

2 panels side by side — swipe to compare

Detention versus retention basins in section, showing where the water level falls to after a storm Two basins drawn in section, side by side, sharing one continuous top-of-bank grade line and one continuous dashed groundwater-table line that run across the whole width, with the earthwork shown in diagonal section hatch. Both basins are cut at the same side-slope angle, and a short dashed blue storm-peak level is drawn at the same elevation in both, because both fill during a storm. In each basin a blue arrow shows the water level falling after the storm, and how far that arrow gets is the difference. Left, detention: the arrow falls the full depth and lands on a paper-white bed that sits above the groundwater line, labelled empties over 12 to 48 hours; the concrete outlet structure has its restrictor opening at bed level, discharging into a buried outlet pipe drawn through the embankment with a blue metered-outflow arrow inside it, so the basin can drain completely dry. Right, retention: the matching arrow falls only a short way and stops dead at a permanent pool of water, marked with a water-level triangle; the outlet riser's crest sits exactly at that pool surface, which is why the level cannot fall below it. The groundwater line cuts through the pool's lower body and blue arrows show the pool percolating down into the saturated subgrade. Both basins share a matched, labelled emergency spillway notch and a blue inflow arrow. Each basin's water stops at its own outlet invert: detention's is at the bed, so it goes dry; retention's is at the pool surface, so the pool is permanent. GRADEGROUNDWATER TABLEDetentioninflowstorm peakempties12–48 hnormally dry bedoutlet: restrictor at bedmetered outflowemergency spillwayRetentioninflowstorm peakstops at the poolpermanent poolrisercrest = pool levelpercolates into subgradeemergency spillway
  • water in place — pool · flows · groundwater table
  • dashed — the temporary storm level

The names get swapped constantly: normally dry and releasing slowly through a restrictor is DETENTION; a permanent pool is RETENTION.

Mnemonic: reTENtion holds waTER (permanent pool); DEtention = Dry + Delays (dry between storms, slow metered release).

Confusions / comparison

Detention pondRetention pond
Between stormsDryWet — permanent pool
FunctionTemporary store, slow releaseHold + gradual percolation
Key componentRestrictor / restricted outletPermanent water level
Doubles as(mostly empty)Landscape / amenity feature
Manages mainlyPeak flow rateVolume + on-site amenity

→ Runoff strategies (this module): where ponds sit among shallow-site tools · Low-impact development (this module): infiltration vs release-controlled storage · Swales vs berms (this module): how water reaches the pond.

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LID & wetlandsRunoff strategiesSite slopesSwales vs berms