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. For the complete list, see the result types section.

Also, the influence of subsidence can be included in the calculations.

Adding and removing the overlay

The rainfall overlay consists of several overlays or result types that show different analyses of the area after or during the extreme rainfall. The overlay can therefore 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 about overlays in general. These result types can be chosen via the Result type box or in the last step of the rain overlay wizard. Below they are listed and explained.

Result types

• BASE_GROUNDWATER_DISTANCE: this overlay shows the distance between the groundwater level (obtained from the water area level data/ GxG map (*) and the surface level. For now, the height of the ground level is including the buildings and other objects on the ground (the height to the surface). This will be changed to the terrain height in a following release.
• BASE_TYPES: from this overlay the division of the grid cells in water, land or sewer areas that are connected to the sewer are visible. Playing with the grid cell size, will make this division between areas/terrain types more or less accurate, which affects the calculation of the flooding.
• EVAPORATED: shows how much water is evaporated after the rainfall in the defined simulation time. For more information on how this layer is calculated, see the Rainfall overlay calculations page.
• 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
• SURFACE_DURATION: The total amount of time the surface has water on it
• SURFACE_FLOW: The total amount of water which has flowed across the surface
• 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 always included.
• UNDERGROUND_FLOW: The total amount of water which has flowed underground
• UNDERGROUND_LAST_VALUE: the amount of water which has flowed underground after the rain simulation is over.
• UNDERGROUND_MAX_VALUE: the largest amount of water that flowed underground at any time during the simulation
• 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", this depends on the threshold value which can be defined in the last step of the #Rain overlay wizard. If the amount of water exceeds this threshold value, the amount of water is visible on the water bodies.

Rain overlay wizard

The overlay itself can be configured to add additional data and adjust various values to change or enhance the way the calculations are made and make the model more accurate. To guide the user along these configurations, the rain overlay wizard provides several steps to add data about the water system and/or configure hydrological coefficients used in the calculations.

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 regarding the water system:

• Water level areas (peilgebieden)
• Hydrological constructions (stuwen)
• Ground water level (grondwaterstanden)
• Sewers (rioleringsgebieden)

Rainfall

The wizard exists of several steps. First the intensity and the length of the rain definition is set. The simulation time of the model compromises the length of the rainfall and the time it takes before it is dry. Also, the simulated rainfall will be equally distributed over the time. In the following steps, data about the water system can be provided.

Water level areas

Here you can add your own dataset of "level areas" with a set water level. This file is loaded in as a geojson file as areas. The following attributes are needed for the calculations.

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	This could also be the outlet of a gemaal

If you do not have any water level dataset, you can also generate a water level area. This will create one waterlevel area which covers the entire 3D world, with no OUTLET value and a WATER_LEVEL of -1000. In the wizard the attributes which contain values for the water level and outlet need to be set. If they have different names, these can be chosen in the select attribute table.

Hydrological constructions

For example weirs form connections between water level areas, where the water is transported from higher to lower water level areas. The weir (or other hydrological construction) dataset is also loaded in as a geojson file. The following attributes need to be present.

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.

This file is loaded in as constructions. For now, the underground construction drainage system can be used. 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.

Ground Water level data

This step in the rain overlay wizard provides the possibility to upload a GeoTiff file with ground water levels. By default the ground water level of the water level areas is used. If you choose for the option to upload a GeoTiff file, you can add you own GxG map with the distances between the ground level and the ground water level in meters. Or you can use one of the already loaded in GxG maps.

Sewers

The next step allows for the sewer areas to be uploaded. The file is loaded in as a geojson file as areas. The following attributes are needed:

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 constructions which are connected to the sewer in this particular sewer area, times this attribute.

If no sewers exist, the model has no water flowing into sewer containers for storage. Therefore, you can automatically generate these areas. For more information on how the generation of these areas is done or about the sewer system in general, see the Rainfall overlay calculations page.

Hydrological coefficients

In the next steps of the wizard, hydrological coefficients regarding the surface and the underground terrains, can be edited. For each of these coefficients, representative values are already entered in the forms.

• Water infiltration (m per day): the speed by which the water infiltrates the underground. The speed is also determined by the underground water infiltration factor. From these two values, the lowest value is used.
• Water manning: the Gauckler Manning coefficient, often denoted as n, is an empirically derived coefficient, which is dependent on many factors, including surface roughness and sinuosity. For more information about this formula see the Rainfall overlay calculations page.
• Water evaporation factor: this factor will be multiplied with the general reference evaporation.
• Reference Evaporation (mm per day): The Makking reference evaporation factor. This value ranges from 0.5 mm per day in the winter till 3 mm per day in the summer for the weather station ‘ De Bilt’ in the Netherlands.
• Water storage fraction: the percentage of underground volume that can be used for the storage of water. This number is determined by the difference between the ground water level and the surface height times the surface area.
• Vertical to horizontal infiltration factor: This factor will be multiplied with the vertical infiltration speed, to obtain the horizontal infiltration speed.

File:Visualisatie wateroverlast.JPG

Schematic visualization of the water flow over the water level areas and hydrological constructions.

Hydrological attributes of objects

Since constructions in the Engine have an effect on the flow of the water, for example if a building has a green roof, attributes concerning these values can be adjusted in the function values window. Representative values are already entered in the table.

Visualization of the water system

In the last step you can choose for a #Result types, as listed above. If you have provided the water level areas and the hydrological constructions to the Engine, along with the required attributes, a schematic visualization of the water flow from the various water level areas and the hydrological constructions is visible. The red spheres stand for water flowing to (an)other water level area. The green spheres stand for receiving water from (an)other water level area. The speed of the spheres is based on the WEIR_SPEED and the OUTLET values. If no spheres are visible, the water flows very gently between these water level areas. The pop-ups in the 3D world are panels which mark the middle of the water level area or the place of the hydrological constructions. In these panels, the provided attributes, such as the WATER_LEVEL_M or the WEIR_HEIGHT can also be edited. Play around with this to see how the water flow changes.

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.