Storm Sync Time Area converts rainfall excess over each catchment into a runoff hydrograph using travel-time bands. It is available in Storm Design analysis alongside the Rational methods. This guide covers the rainfall, catchment-band, and loss-model setup.
Choose the rainfall pattern and durations #
In Storm Design → Design Guideline → Malaysia++ → Temporal Pattern, check the rainfall-pattern source and region for the project. The duration list follows the selected pattern. Only supported durations are analysed; a duration can remain ticked when unavailable, but it is omitted until the selected pattern supports it.
In Storm Design → Storm Analysis, select Analysis and Catchment Hydrology as the inflow source, then choose Time Area. The duration checklist offers All and Clear and shows the selected count. Flow proportioning assigns catchment runoff to the drain’s beginning node, end node, or splits it equally between them. Hydraulic routing controls remain under Hydraulic Routing.
Configure catchment bands and rainfall losses #
In Catchment Spread Input, open Time Area Settings for the selected catchment. Each band has a start and end travel time, total area, and impervious area; the remainder is pervious. New catchments can be populated with default bands using the catchment’s own overland-flow time and total area. The default splits the area equally among its bands and initially treats the area as impervious. It is an editable starting point, not a survey-derived land-cover map. Review the travel-time extent and area split against the catchment data.
Editing takes place on a draft: Apply saves the band and loss edits as one change, while Cancel discards that draft.
Choose one of the three loss models. Initial loss and continuing loss are calculated separately for pervious and impervious surfaces, and applied loss in each rainfall interval cannot exceed that interval’s rainfall.
| Model | How it works | Defaults and editable inputs |
|---|---|---|
| Initial loss + constant continuing rate (IL-CL) | Applies an initial abstraction, then a constant continuing-loss rate over time. | Default initial abstraction is 10 mm pervious and 1.5 mm impervious. Default pervious continuing loss is 0.5 mm/h with the Clay soil preset. Soil presets are Sandy (10–25 mm/h), Loam (3–10 mm/h), and Clay (0.5–3 mm/h); the rate remains editable. Impervious continuing loss defaults to 0 mm/h. |
| Initial loss + decaying continuing rate (IL-DCL) | Applies an initial abstraction, then a rate that decreases linearly with elapsed storm time to a floor. | Enter separate pervious and impervious initial losses, starting rates, decay rates, and floor rates. The model generalizes the decaying-loss example in MSMA2 Appendix 2.F2; the manual does not prescribe a general decay curve. |
| Initial loss + proportional loss (IL-PL) | Reproduces the legacy PondCAD proportional-loss calculation for comparison. | Defaults are 10 mm initial loss and 20% proportional loss for pervious area; 1.5 mm and 0% for impervious area. All four values are editable. |
For IL-CL and IL-DCL, the editor shows a potential continuing-loss schedule for the selected surface and preview interval. The schedule shows the model rate without rainfall; during analysis, applied loss in each interval cannot exceed the rainfall available. IL-PL explains that its numeric loss schedule needs rainfall amounts, which are supplied by the selected storm.
Run the analysis #
Before dispatch, Time Area preflight checks the selected rainfall region, MSMA2 station and IDF data, storm events and durations, catchment bands and area totals, drain assignment, and active loss settings. If a check fails, analysis stops with an error that identifies the missing or invalid input and, where applicable, the catchment to correct.
After a successful run, use the Storm Sync Time Area Results guide to review graphs, the Catchment Hydrology report, isochrone strips, and EPA SWMM export.
For the two model-specific numerical reference checks, see the comparison drafts in the Publishing repo: MSMA2 Appendix 2.F2 and legacy PondCAD. These comparisons use different loss models because the references specify different loss assumptions.
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