Rainfall Overlay

What is the rainfall overlay

The rainfall overlay is a grid overlay, which calculates where and how water would flow in situations where severe rainfall takes place. It does so by simulating water falling onto the 3D world over the course of thousands of steps. During each step, water can, among other things, land on the ground, flow over the surface, infiltrate into the ground, flow underground, and end up in surface water.

By repeating these simulation steps thousands of times, the influx and flows of water are accurately approximated.

What can the rainfall overlay be used for

The rainfall overlay is currently fit for simulating the flow and effects of severe rainfall on flat or mildly hilly areas, for map sizes of up to 5km. Larger maps, or calculations which involve an impulse of water such as levee breaches are not yet recommended.

The overlay can be configured to display various results of the simulated rainfall. For example it is possible to see the maximum amount of water certain locations had to endure, or the amount of water which has flowed across certain locations.

Output types

• WATER_STRESS: The maximum amount of excess water at any time during the simulation. Differs from SURFACE_MAX_VALUE because water stored on bodies of water are not immediately deemed "excess".
• SURFACE_LAST_VALUE: The amount of water remaining on the surface after the simulation is over
• SURFACE_MAX_VALUE: The largest amount of water that was on the surface at any time during the simulation. Differs from WATER_STRESS in that water stored on bodies of water is included.
• SURFACE_FLOW: The total amount of water which has flowed across the surface
• SURFACE_DURATION: The total amount of time the surface has water on it
• SEWER_LAST_VALUE: The amount of water remaining in the sewer after the simulation is over
• SEWER_MAX_VALUE: The largest amount of water that was in the sewer at any time during the simulation
• UNDERGROUND_FLOW: The total amount of water which has flowed underground

Adding and removing the overlay

The overlay can be added to the project multiple times, to present different outcomes or different scenarios. For information on adding and removing the overlay to and from the project, see the page on overlays in general.

Configuring the overlay

Accompanying the inherent complexity of the overlay is the ability to add additional data and adjust various values to change or enhance the way it calculates. The overlay itself contains a number of attributes which can be configured. Attributes in other parts of the project, such as the connected rain definition, buildings, terrains, and areas defining water level areas.

Data required

No data is explicitly required. The overlay is designed to provide initial calculations based on default values and minimal assumptions. However, the model can be refined with your own data. The following data can be prepared and uploaded in geojson format:

Water level areas

For defining "level areas" with a set water level. This file is loaded in as areas.

Attribute	Description	Example	Remark
NAME	The name of the water level area.	PG 256	This attribute is not loaded in as attribute, but can be used as name to identify the resulting area in the Engine later on.
WATER_LEVEL	The height of the water, in meters, measured from Amsterdam Ordnance Datum (mNAP).	1.6	For a water level area with infinite storage, this can be set to an extreme negative number (e.g. -9999). However, note that this would also place this area far below level areas with a proper height set.
OUTLET	The amount of water which disappears from this level area in cubic meters per second (m3/s).	0.007

If no water levels exist, a virtual water level area is assumed which covers the entire 3D world, with no OUTLET value and a WATER_LEVEL of -1000.

Sewers

For defining "sewers" which allow for water storage. This file is loaded in as areas.

Attribute	Description	Example	Remark
NAME	The name of the sewer.	Sewer North-East	This attribute is not loaded in as attribute, but can be used as name to identify the resulting area in the Engine later on.
SEWER_PUMP_SPEED	The speed at which water is pumped out of the sewer, in cubic meters per hour (m3/h).	1	All areas which are not plots of this kind should either not have PERCEEL as an attribute, or should have it set to 0.
SEWER_STORAGE	The amount of water which can be stored in this sewer, in meters (m).	0.007	The total amount of storage for this sewer is the surface area of the polygon representing this sewer, times this attribute.

If no sewers exist, the model has no water flowing into sewer containers for storage. It's possible to generate sewers automatically, using the " Create Sewer Areas" function in the editor: Template:Editor ribbon

Weirs

For defining connections between level areas. This file is loaded in as constructions.

Attribute	Description	Example	Remark
NAME	The name of the weir.	PG 256	This attribute is not loaded in as attribute, but can be used as name to identify the resulting contruction in the Engine later on.
WEIR_HEIGHT_M	The height of the weir, in meters, measured from Amsterdam Ordnance Datum (mNAP).	1.8	When using this model outside of the Netherlands, the height is in the same scale as the Terrain height of the project. When an extreme negative value is used, the construction acts like a culvert.
WEIR_SPEED	The speed at which water is moved from one level area to the other, in cubic meters per second (m3/s).	0.007	Once the water level exceeds the height of the weir, the water flows at this constant speed until the water level no longer exceeds the height of the weir.
WATEROUTLET	The speed at which this construction also functions as a water outlet, in cubic meters per second (m3/s).	0.005	This is in addition, and takes precedence, over the water pumped out at WEIR_SPEED.

If no weirs exist, there are no connections between water level areas and water is not transferred between them.

Weirs must overlap with at most 2 water level areas. If a weir overlaps with more that 2 water level areas, 2 areas are selected at random which the weir pumps between. If a weir overlaps with only 1 water level area, only its outlet function is processed. Weirs which do not overlap with any water level areas have no effect.

Warnings

* To calculate the correct flow use a grid size of max 2m.
* Use a hi-detail height map (1 point per 0.5m), select hi detail in the New Area Wizard under advanced options.