Detention Pond Sizing: The Triangular Hydrograph Method
Most site development increases impervious cover, which increases peak runoff. Detention ponds exist to absorb that increase — temporarily storing runoff during a storm and releasing it slowly enough that the site's peak discharge doesn't exceed what the downstream drainage system, or the receiving waters, can handle. Sizing that pond correctly starts with a reasonable estimate of how much storage volume the job actually requires.
Why a Simplified Method Is Useful
Full detention design involves stage-storage-discharge routing: modeling the pond's actual geometry, its outlet structure's rating curve, and the inflow hydrograph shape through time. That level of detail is appropriate for final design, but it's overkill for early site planning, when the goal is simply to know roughly how much land area or excavation volume the pond will require. The triangular hydrograph method fills that gap — a closed-form estimate that needs only peak inflow, allowable outflow, and storm duration.
The Geometry Behind the Formula
The method assumes runoff arrives as a triangular pulse: rising linearly to a peak partway through the storm, then falling linearly back to zero. If the pond releases water at a constant allowable rate throughout that pulse, the volume that must be held back is the area of the triangle sitting above the outflow line — mathematically, one-half the base (storm duration) times the height (the difference between peak inflow and allowable outflow). It's a deliberately simple shape, but it captures the two things that matter most: how much bigger the peak inflow is than what's allowed out, and how long that peak condition persists.
Where the Inputs Come From
Peak inflow is typically the post-development Rational Method or TR-55 peak for the drainage area feeding the pond. Allowable outflow is usually set by the local jurisdiction as the pre-development peak rate — the idea being that the developed site shouldn't discharge faster than the undeveloped site did. Storm duration is less precise; many practitioners use a value near the site's time of concentration as a reasonable approximation of the critical duration, though a full analysis would check multiple storm durations to find the one that actually governs.
The Limits of a Preliminary Estimate
This method assumes a single triangular pulse and a constant-rate outlet, neither of which is exactly true for a real pond with a weir or orifice outlet whose discharge rate depends on water depth. It also doesn't check whether the resulting pond depth, side slopes, or emergency spillway are physically reasonable. Treat the result as a planning-level volume — a number to size the pond footprint against early, not a substitute for final routing once the outlet structure and grading are designed.
How much bigger is a triangular-hydrograph estimate compared to a fully routed pond design?
It varies by site, but the triangular method is generally conservative relative to a properly routed design, since real outflow through an orifice or weir increases with depth rather than staying constant. It's meant to bound the planning-level footprint, not replace final routing.
What storm duration should I actually use?
A commonly used approximation is the site's time of concentration, but the truly critical duration can be longer for some outlet configurations. A full design should check several durations and design for whichever produces the largest required volume.
Does this account for infiltration or storage losses within the pond itself?
No — this method treats the pond as impermeable, conveying inflow to outflow through storage alone. Where infiltration is a significant, reliable component of a pond's performance, a separate water-balance analysis should account for it explicitly.
Why does my jurisdiction require multiple design storms to be checked (2-year, 10-year, 100-year)?
A pond sized for only one storm frequency can fail to control smaller, more frequent storms or be overwhelmed by rarer, larger events. Most stormwater ordinances require detention performance to be verified across a range of storm frequencies.
Can this method be used for underground detention systems (chambers, vaults)?
The volume estimate applies the same way, but underground systems typically have a much more sharply defined stage-storage relationship than an open pond with sloped sides, so final sizing should move to routing sooner rather than relying on this planning-level estimate.