Rainfall Overlay

Calculation principles

The basic principle of the calculations is that all water is stored in a grid of containers, representing possible water storage for the data of a given location. The size and location of the containers is based on the size of the grid, and the type of container. Water can flow from some containers to other containers. In the Containers section, more information can be found regarding each type of container.

Most containers are part of a water level area. A water level area is an area which is treated as an area with a single water level set for the surface water. In principle, water never flows between water level areas. However, weirs can be defined which do allow for water to be transferred between water level areas.

Containers

Here there should be an overview of all the calculations steps within a water level area (peilgebied). BOB: Flowchart?

Rainfall

Water is created in the form of rainfall. The amount of water created per calculation step is based on the amount of calculation steps for the simulation, the duration of the rain, and the amount of rains which falls during that time. Water is created for each grid cell. Water is never "stored" as rainfall. Directly after being created by rainfall, water can be placed in one of the follow containers:

Container	Condition
Sewer	If there is a SEWER area present, which is defined by the presence of a non-zero SEWER_STORAGE attribute, and there is a building present with the SEWERED attribute ("Connected to Sewer") set to 1, and the sewer container is not yet full, the water is moved directly into the sewer.
Building storage	If there is a building present with the WATER_STORAGE_M attribute ("Water Storage (m3/m2) )") set to a value greater than 0, and the building container is not yet full, the water is moved directly into the building container.
Surface water	If there is surface water present (i.e. a terrain with the WATER attribute set to 1), the water is moved into the surface water container.
Surface container	If there is no building connected to an empty sewer, no building with water storage, and no water terrain, the water is placed in a surface container.

Surface containers

Water in a surface container is resting on the surface of the world. Each grid cell has its own surface container.

The storage capacity of surface containers is effectively infinite.

For some calculations, the height of this container is relevant. The height is defined as the terrain height, plus the height of the water in the container.

During a calculation step, the water can move to one of the following containers:

Container	Condition
Underground container	If the underground container is not filled to maximum capacity, an amount of water is transferred from the surface container to the underground container, based on the formula for vertical infiltration described below. Note: after calculating vertical infiltration, the flow calculations continue, both for the surface containers and the underground containers.
Surface water or surface container	Water may flow into the surface water container or into other underground containers, based on the flow-by-height principle and the Manning formula (for surface runoff) described below.

Underground containers

Water in underground containers is water which has infiltrated into the ground, and is now resting on, (or is part of) the ground water. Each grid cell has its own underground container.

The storage capacity of underground containers is based on the WATER_STORAGE_PERCENTAGE of the terrain in that location, the ground water level and the height of the surface.

For some calculations, the height of this container is relevant. The height is defined as the ground water level height, plus the height due to the amount of water in the container. This also takes into account the WATER_STORAGE_PERCENTAGE. They lower this percentage, the faster the container's height increases.

During the calculation step, water can move to the following containers.

Container	Condition
Surface water or underground container	Water may flow into the surface water container or into other underground containers, based on the flow-by-height principle and the formula for horizontal infiltration described below.

Surface water

Water in surface water containers is water which has been stored on existing bodies of water. These bodies are often intended to house some amount of water in them in cases of rainfall. There is a single surface water container for the entire water level area.

The storage capacity of surface water containers is effectively infinite.

For some calculations, the height of this container is relevant. The height is defined as the ground water level height, plus the height due to the amount of water in the container. This also takes into account the WATER_STORAGE_PERCENTAGE. The lower this percentage, the faster the container's height increases.

During the calculation step, water can move to the following containers.

Container	Condition
Surface container	Water may flow into surface containers, based on the flow-by-height principle and the Manning formula (for surface runoff) described below.

Sewer

Water in the sewer system is stored in underground pipes or other water systems. The sewer is defined using areas, and each area forms its own sewer, and thus its own container. There can be multiple sewers in the same level area, and sewers may extend beyond level areas.

The storage capacity of the sewers is based on the SEWER_STORAGE attribute of the area in that location.

Water may disappear from the model entirely by being pumped out of the sewer. This happens at a speed defined by the SEWER_PUMP_SPEED attribute of the area.

Water in the sewer does not move into other containers.

Building storage

Water in building storage is stored on roofs and in gutters. In principle, all constructions can also store water, but may have a capacity of 0, rendering their containers inaccessible.

The storage capacity of buildings is based on their WATER_STORAGE_M attribute ("Water Storage (m3/m2) )").

Water never leaves building storage. It does not disappear from the model, nor does it move to a different container.

Between level areas

Water can only flow between level areas via the surface water containers, and only when there is a weir connecting the two areas. The threshold for when water starts flowing between level areas

Weirs

A weir is a structure which connects two level areas. It exists in the Engine as a construction. It is a structure which can modelled as a small wall between the bodies of water, in this case the surface water containers of the relevant level areas. A weir has a height, defined by the attribute WEIR_HEIGHT_M. When the calculated height of the surface water container of either level area exceeds the height of the weir, water will be moved from the level area with the higher water level to the level area with the lower water level. The speed at which water is transferred is defined by the attribute WEIR_SPEED. This speed is constant, and does not vary with the amount of water in excess of the weir's height.

Formulas

Flow by height

When a flow-by-height calculation is performed, the Engine takes into consideration for each container whether any adjacent containers have a height lower than or equal to the current container's height. All containers which meet this requirement are candidates for receiving water flow. The candidate container for which the flow would be greatest is the single container which actually receives water from the current container. All candidate containers, whether they receive water from the current container or not, can still receive water from other containers.

BOB: Diagram van hoogteberekening tussen 2 containers

To prevent issues with water "slushing" around in rare edge-cases, the direction of flow for surface containers on extremely flat land is "fixed" after the first few calculations. This means that some containers will only ever exchange water in 1 direction.

Data required

The model can be refined with your own data. The following data can be prepared and uploaded in geojson format:

"Water level areas" file, for defining "level areas" with a set water level. This file is loaded in as areas.

Attribute	Description	Example	Remark
NAAM	The name of the 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.
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.

"Weirs" file, for defining artificial connections between level areas. This file is loaded in as constructions.

Attribute	Description	Example	Remark
NAAM	The name of the construction.	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.