Level-pool (storage-indication) routing · Inflow → outflow attenuation · Peak reduction
This simulator applies level-pool (Modified Puls) routing to compute how a detention pond attenuates a stormwater inflow hydrograph, reducing the peak outflow rate — the core calculation required for on-site stormwater management compliance under local municipal separate storm sewer system (MS4) permits and state stormwater ordinances.
Level-pool routing assumes the pond water surface remains horizontal at all times (a reasonable assumption when the pond length-to-width ratio is small). The governing equation is the continuity equation: dV/dt = I(t) − Q(t), where I is inflow (cfs), Q is outflow (cfs), and V is storage volume (ft³). At each time step, outflow Q is computed from the storage-outflow relationship using the orifice equation Q = Cd · A · sqrt(2gh), where Cd = 0.6, A is the orifice area, g = 32.2 ft/s², and h is the pond stage (depth above the orifice invert).
Storage is modeled here as a constant-area prism: V = A_surface × h. The simulation integrates the continuity equation explicitly over the triangular inflow hydrograph (SCS shape with base time Tb = 2.67·Tp) to track stage and outflow over time. The peak outflow attenuation is reported as a percentage reduction from the inflow peak.
Detention pond design is governed by local municipal stormwater ordinances and state MS4 permit requirements, which typically require post-development peak flows to not exceed pre-development conditions for the 2-yr, 10-yr, and 100-yr design storms. ASCE Manual of Engineering Practice No. 36 (Water Quality Engineering) and the USWRC TR-55 / TR-20 manuals provide routing procedures. Many jurisdictions also require water quality volume detention per EPA stormwater Phase II rules, which mandate capturing the first 1–1.5 inches of runoff.
Orifice sizing drives the storage-outflow relationship: a smaller orifice stores more water at higher stage and provides greater peak attenuation. The pond must have sufficient storage volume (typically 0.5–2.0 ac-ft for a 5–50 acre watershed) to reduce the post-development 10-yr peak to the pre-development level. Emergency spillway design must pass the 100-yr storm without exceeding the maximum water surface elevation.
Sediment storage (typically 10–20% of the active pool) and maintenance access are critical design elements. Pond depth typically ranges 3–8 ft; shallow ponds with large surface areas provide better water quality treatment but require more land. Trash racks and anti-vortex plates prevent clogging of the outlet orifice.
Set the peak inflow (cfs) and time to peak (hr) to define the triangular inflow hydrograph. Adjust pond surface area (acres) to control storage volume and orifice diameter (inches) to control the release rate. The hydrograph chart shows inflow (teal) and attenuated outflow (orange) with the shaded area representing volume stored in the pond. Key outputs include peak outflow, percent peak reduction, maximum pond stage, and maximum storage used.
Most stormwater ordinances require post-development 10-yr peak flow to not exceed pre-development peak. Typical attenuation targets range from 20–60% depending on the degree of site impervious cover increase. Some jurisdictions also require matching the 2-yr peak to protect downstream channel stability.
A dry detention pond (dry bottom) empties completely between storm events and is used purely for peak flow attenuation. A wet pond (retention pond) maintains a permanent pool, which provides water quality treatment by settling sediment and nutrients. Many modern stormwater regulations require wet ponds or other water quality Best Management Practices (BMPs) in addition to peak flow control.
Reducing orifice diameter decreases the outflow rate for a given head, causing the pond stage to rise higher and store more volume before the peak outflow occurs. This results in greater peak attenuation but requires more storage capacity. If the orifice is too small, the pond may remain in flood stage long after the storm, reducing its effectiveness for back-to-back storm events.
The Modified Puls method transforms the continuity equation into a storage-indication form: 2S/dt + Q = (2S/dt − Q)_prev + (I_cur + I_prev). Given a storage-outflow relationship (stage-storage-discharge curve), the method iterates through time steps without requiring the stage as an intermediate variable, which improves numerical stability. This calculator uses the equivalent explicit form with a constant-area storage assumption.
Yes in final design. A sediment forebay capturing 10–20% of the permanent pool volume allows coarse sediment to settle before entering the main pond, extending the pond's service life. This volume is excluded from the active storage in routing calculations. VDCR and many state drainage manuals require a sediment forebay for ponds above a certain drainage area threshold (typically 10 acres or more).