About direct rainfall modelling

In InfoWorks ICM, rainfall can be applied directly to a 2D mesh. Multiple rainfall profiles and infiltration models can be applied spatially across a network. There are also options to help streamline separating rainfall applied to the 2D mesh versus subcatchments in integrated models.

This example shows direct 2D rainfall on a mesh. Note the areas of the mesh that are wet above the theme threshold, and where the runoff is converging into flow paths.

The 2D infiltration model calculates infiltration properties in 2D elements. Areas in the network where infiltration is to be applied are represented either by an entire 2D zone or by infiltration zone (2D) objects that are set up.

If required, the volume infiltrated in the 2D surface can be connected to a node, to single or multiple links, or to a subcatchment in the 1D system by using a permeable zone (2D) object.

When applying direct rainfall on the mesh, several additional results fields are available for viewing in graphs and grids, such as the above graph, for effective infiltration for an element.

To apply direct rainfall to the elements, in the 2D zone Properties window, enable Apply rainfall etc directly to mesh elements.

Rainfall can also be applied ‘everywhere’ or ‘outside subcatchments’. The latter is useful in integrated modelling to avoid double counting runoff areas.

A specific rainfall profile and corresponding evaporation profile can be applied to the 2D elements per 2D zone. Any spatial rainfall profiles will override this value.

A blank infiltration surface will produce 100% runoff. The rainfall percentage can be adjusted if appropriate.

Infiltration surface (2D) objects:

Used to define the infiltration characteristics for specific areas, typically different surface types.

An infiltration model is selected for each infiltration surface, and parameters vary based on the infiltration model selected.

There are multiple infiltration types that can be applied to the 2D mesh:

• Fixed: A fixed percentage of the net rainfall that becomes runoff. Set as a value between 0 and 1. The effective infiltration rate is calculated as the rainfall that does not become runoff. Typically applied to impermeable surfaces, such as roads.
• Constant: Similar to the fixed model, but an additional constant infiltration loss coefficient is defined based on the maximum infiltration rate. Useful for modelling sustainable drainage structures, such as permeable paving.
• Deficit and constant loss: Models the surface as a single soil layer in which rainfall is initially stored and is subject to evaporative loss. When the soil layer reaches saturation capacity, infiltration may occur. Any excess rainfall goes to runoff.
• Horton: Infiltration on the surface is modelled directly using a variant of the Horton equation, which is a formula derived empirically from infiltrometer/small catchment studies. An initial infiltration rate, a limiting infiltration rate, and a decay factor are set. A separate recovery factor can also be set. Typically applied to pervious surfaces, such as fields and pastures.
• Green-Ampt: Infiltration on the surface is calculated as a function of the soil suction head, porosity, hydraulic conductivity, and time. When using this model for 2D simulations, it is possible to set the initial moisture deficit using the same objects as the Horton model. Moisture deficit values between 0 and 100 can be specified, where 0 represents a saturated initial condition, and 100 represents a dry initial condition.

Infiltration zone (2D) objects:

Used to represent areas of different surface types by a set of non-overlapping infiltration zones.

Typically, a separate infiltration zone is required for impermeable surfaces, rural areas, and drainage features.

Can be imported via the Open Data Import Centre or digitized directly on the GeoPlan.

Overrides the 2D zone infiltration surface properties.

When digitizing infiltration zones, take care to avoid self-intersecting polygons and hollow polygons. These issues are often generated when importing road polygons extracted directly from mapping. As such, it can help to simplify a model by using the 2D zone infiltration surface field and eliminating the need to import roads as a separate set of polygons. Set the 2D zone properties to the road infiltration surface and exclude them from the infiltration zone coverage.

When using the Deficit and constant loss, Horton, or Green-Ampt infiltration models, it is necessary to set the initial deficit, initial soil water content, or soil moisture deficit using a 2D initial condition zone. This is a separate polygon type, but is most likely a duplicate of the infiltration zones.

Permeable zone (2D) objects:

Used to link infiltrated volume from the 2D surface into the 1D system.

Infiltrated volume in the permeable zone can drain to a selected node, to single or multiple links, or to a subcatchment.

Drainage links for any infiltrated volume can be visually represented on the GeoPlan as arrows.

Can be imported via the Open Data Import Centre or digitized directly on the GeoPlan.

Parameters are edited in the permeable zone (2D) grid window of the polygons grid or the permeable zone (2D) property sheet.

Permeable zone (2D) objects can be used to model the flow through permeable surfaces that are part of a sustainable drainage structure, directed into the drainage system.

The example below is one method of representing a filter drain. The infiltration zone is linked to the permeable zone, which then drains infiltrated volume into the links.

Permeable zones can cover areas with different infiltration properties and therefore overlap several 2D infiltration zones.