The Modified Rational Method extends the Rational Method to estimate the required detention storage volume for a site. While the Rational Method computes only a peak flow, the MRM creates a simplified trapezoidal hydrograph and uses volumetric analysis to determine how much storage is needed to reduce the post-development peak flow to the pre-development (allowable) release rate.
Important: The MRM is a required volume computation only. It determines the minimum detention storage volume needed, but does not perform hydrograph routing or model the actual pond outflow behavior. The outfall structure (outlet pipe, orifice, weir, etc.) should be designed such that the design flow through the structure is less than or equal to the allowable release rate for each design storm event. Care should be taken not to over-detain, as the MRM does not verify that the pond releases enough flow to prevent overtopping. The engineer is responsible for designing the outfall structure to control releases appropriately.
The MRM computes the required detention storage volume by comparing the inflow volume (from rainfall runoff) against the allowable outflow volume across a range of storm durations. The critical duration, the duration that produces the maximum difference between inflow and outflow volume, determines the required storage. HydraLink supports two detention sizing approaches: the Standard Method and the Regional IDF Method.
The Standard Method is a general-purpose approach applicable to any region. It uses rainfall intensity-duration-frequency data (NOAA Atlas 14 or user-entered IDF data) and evaluates the full range of storm durations, starting at Tc. The critical duration is the one with the largest required storage.
For each duration:
Where C = runoff coefficient, i = rainfall intensity (in/hr) at duration td, A = drainage area (acres), and td = storm duration (minutes). The factor of 60 converts minutes to seconds.
If the 1.008 conversion factor is enabled in Project Settings, the exact acre-inch-per-hour to cfs conversion (43,560 / (12 × 3,600) ≈ 1.00833) is applied to Q. When disabled (default), the standard approximation 1 acre-in/hr ≈ 1 cfs is used.
Where Td = storm duration (min), Tc = time of concentration (min), Qallow = allowable release (cfs). The 0.5 factor assumes a triangular outflow hydrograph.
HydraLink supports the regional IDF table-based detention method used by both the NCTCOG iSWM (Integrated Stormwater Management) program in the Dallas-Fort Worth metroplex and the Atlanta Regional Commission Georgia Stormwater Management Manual. Both programs use the same underlying equations (Debo & Reese, 1995/2003) with region-specific rainfall coefficients.
Where a and b are county/region-specific rainfall coefficients and Qa is the allowable pre-development release rate.
P180 is the 3-hour rainfall depth and Ptd the depth at the critical duration; both are read from the rainfall table and can be overridden per return period on the basin panel.
| Program | Regions |
|---|---|
| NCTCOG iSWM (Texas) | Collin, Dallas, Denton, Ellis, Erath, Grayson, Hood, Hunt, Johnson, Kaufman, Navarro, Palo Pinto, Parker, Rockwall, Somervell, Tarrant, Wise |
| Atlanta Regional Commission (Georgia) | Albany, Atlanta, Athens, Augusta, Bainbridge, Brunswick, Columbus, Macon, Metro Chattanooga, Peachtree City, Rome, Roswell, Savannah, Toccoa, Valdosta, Vidalia |
Supported return periods for the regional method: 1, 2, 5, 10, 25, 50, and 100 years. Each county/city has unique a and b coefficients for each return period.
When multiple basins drain to a pond, each basin is assigned a role:
| Role | Description | Effect |
|---|---|---|
| Design Area | Post-development area detained by the pond | C and area included in composite inflow calculation |
| Target | Pre-development reference | Peak flow defines the allowable release rate for the pond |
| Bypass (default) | Site flow that does not enter the pond | Reduces the allowable release from the pond, increasing required detention |
| Pass-Through | Off-site flow that enters pond but is not detained | Outfall structure must be sized larger to convey pass-through flow in addition to the allowable detained release |
A basin belongs to one MRM element, whatever its role: a basin two elements hold is counted twice, in the storage both size or in the release both allow. Draining a basin into another element, or picking it in another element’s role lists, asks before it moves; Yes takes it out of the element that held it. A model where two elements already hold one basin is refused when you run the analysis, naming both.
A pond takes one MRM element. Give that element a design area covering everything the pond detains, rather than sizing parts of it with separate elements: two requirements sized separately do not add up to the pond’s. A pond that already has an MRM element is not offered to a second one, and a model that connects two to one pond is refused when you run the analysis until one is disconnected.
That MRM element is also the pond’s only inflow. Rational flows are peaks, not hydrographs, so the pond has nothing to route; it is sized from the element’s design area. Drain each basin that reaches the pond into the MRM element with its role, and connect the element to the pond. A Rational basin, junction, reach, culvert or channel cannot drain into the pond directly, and ponds in series need a unit hydrograph (SCS) network. A model that already has one is refused when you run the analysis, naming what drains into the pond.
| Output | Units | Description |
|---|---|---|
| Required Storage Volume | ft³ | Maximum required storage across all durations |
| Critical Storm Duration | minutes | Duration producing the maximum storage requirement |
| Allowable Release Rate | cfs | Combined target flow defining max permitted outflow |
| Inflow Volume (at critical duration) | ft³ | Total inflow volume at the critical duration |
| Outflow Volume (at critical duration) | ft³ | Total outflow volume at the critical duration |
Outfall Structure Design: The outfall structure (outlet pipe, orifice, weir, riser, etc.) must be designed so that the design flow through the structure is less than the allowable release rate for the design storm event. The MRM does not check that the pond releases enough flow to avoid overtopping, so care should be taken not to over-detain. The engineer should verify that the outfall structure provides adequate release capacity through all stages of the storm event.
The choice between MRM detention sizing and full hydrograph routing depends on the project requirements, local jurisdiction criteria, and the engineer’s judgment:
When using MRM for detention sizing, the engineer should independently design the outfall structure to meet the allowable release rate while also ensuring the pond can drain adequately between storm events. For complex sites with multiple outlets, full hydrograph routing provides a more complete picture of pond performance.