Runoff Zones - [source code]
Tags an area of the site with how rough it is and how much rain it sheds. Manning's n here is the OVERLAND FLOW value, several times the channel value for the same material — at the millimetre depths of sheet flow the surface texture is the whole channel, and using a channel roughness makes runoff arrive far too fast. A Curve Number (1–100) models losses that grow through a storm as the ground wets up; leave it at 0 to use the flat runoff coefficient instead.
Input
| Name | Nickname | Description | Type |
|---|---|---|---|
| Areas | Closed curves outlining where this surface type applies, in plan. Curves are projected to the world XY plane, and a cell belongs to a zone when its CENTRE does. | Curve | |
| Surface | A preset surface type. Choosing one fills in the Curve Number and Manning's n below; override either by wiring it, or pick Custom and set both. | Text | |
| Curve Number | CN | SCS Curve Number, 1–100. Higher sheds more: 98 is sealed paving, 61 a healthy lawn on an average soil, 55 mature woodland. Unlike a flat coefficient it makes runoff GROW through a storm as the ground wets up, which is why it suits a design event. 0 disables it in favour of Runoff Coefficient. Wiring this overrides the preset. | Number |
| Runoff Coefficient | C | Fraction of rainfall that becomes runoff, 0–1, used only when Curve Number is 0. The cruder model, but the one most local guidance is written in. | Number |
| Manning's n | n | Overland-flow roughness (s/m^⅓). Wiring this overrides the preset. Typical: 0.012 smooth paving, 0.05 gravel, 0.24 short grass, 0.40 woodland litter. | Number |
| Initial Abstraction Ratio | Ia/S | Fraction of the soil's storage absorbed before ANY runoff starts. 0.05 is the default and the value re-analysis of the original gauge data supports for small, largely impervious urban catchments; the textbook TR-55 figure of 0.2 was fitted to agricultural watersheds and under-predicts urban runoff. Pass 0.2 to reproduce it. | Number |
Output
| Name | Nickname | Description | Type |
|---|---|---|---|
| Zones | Runoff zones for Stormwater Grid. | Generic Data | |
| Area | Total plan area covered (m²). | Number |