Detention Routing

Storage-indication routing

(2S/Δt + O)₂ = I₁ + I₂ + (2S/Δt − O)₁, solved step by step on one table of storage and outflow (FHWA HEC-22 4th ed. section 10.5). The page reports the volume balance and the change in peak outflow when the time step is halved.

Inflow

Paste the hydrographs from HEC-HMS, WinTR-55, SWMM or a spreadsheet, up to four, with a name and a source for each. Or build one from a drainage area, curve number and Tc with the NRCS dimensionless unit hydrograph (NEH 630 Chapter 16, peak rate factor 484) and an NRCS Type II or III storm or an Atlas 14 alternating-block storm.

Stage and storage

Elevation-area points give storage by the conic section or the average end area; or enter storage directly. The water surface may not rise above the top of the table: the page stops and says so, and never extends it.

Outlets

At one water surface the flows add. Orifice Q = C_d A (2gH)^1/2 with H to the center, or to the tailwater if higher. Sharp-crested and V-notch weirs, reduced by Villemonte under tailwater. A riser is the smaller of weir and orifice flow (HEC-22 Example 10.11). A barrel follows FHWA HDS-5, as in the Culvert calculator, and limits the devices that drain into it. Spillways are broad-crested. A rating that falls with rising stage is refused.

Target release

You enter a target for each inflow, for example the pre-development peak from the Pre/Post Development Comparison. The page sets the peak outflow beside it as a number.

The calculator is tested against the cases below every time its code changes.

Compared with USACE HEC-HMS 4.13 runs made by HERO E&C from the command line and read back from the result files: peak outflow and storage 2%, highest water 0.05 ft, time of peak within a step or two, drain time 5% or 15 minutes. The pipe barrel case allows 5%, 0.1 ft and 3%. Unit hydrographs allow peak 3%, two steps and volume 2%. Rating and storage tables are compared with FHWA HEC-22 Examples 10.10, 10.11 and 10.14 to the precision they print. Linear-reservoir and draining-orifice cases are compared with their exact solutions.

• A routing check that does not design the pond: you choose the pond and the outlets and read the result. It does not size them for you, and it does not state what your criteria require.
• Level-pool routing with one constant tailwater. No tailwater that changes in time, no backwater from the receiving channel, no pumps, gates, infiltration, seepage or evaporation. Retention and permanent-pool water budgets are not computed.
• An outlet below the bottom of the table is refused. Below an orifice crown the page applies the orifice equation to the wet area, a simple form that no reference program here uses: HEC-HMS has no such range, so that part of the drain time is not verified against an external case.
• Submerged broad-crested spillways are refused, because the sources used here publish no curve for them. A riser is modeled by its crest and open area: no vortex or anti-vortex devices, and no trash rack losses unless you lower the coefficients.
• The unit hydrograph is for one subarea with one CN and one Tc, peak rate factor 484, I_a = 0.2S. For several subareas or another peak rate factor, compute the hydrograph in a program and paste it.
• A pond of some size is a dam under Texas rules. The page mentions 30 TAC §299.1 when the table spans more than 6 ft; it does no dam safety or spillway design check. Whether the rule applies is for TCEQ and the engineer of record.

Which inflow should I paste?

The post-development hydrograph for each return period your reviewing agency asks for, down to near zero flow, with the time in minutes. Name each one and say where it came from, so the calculation sheet carries the source.

What do I do if the water goes above the table?

The page stops rather than guess. Add higher elevations to the table if the site allows, or add outlet capacity, and run it again.

Why does the peak outflow depend on the time step?

A step too long misses the peak of the inflow and the sharp change at a device crest. The page halves the step and reports the change; if it is over 1% it warns you. Use a finer hydrograph.

How is this different from the storage estimate in the comparison tool?

The TR-55 estimate in the Pre/Post Development Comparison is a preliminary ratio from average results. This page routes the hydrograph through your actual pond and outlets.

Can I attach the calculation sheet to a report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.