Water Overlay

What are the Water Overlays

With the Water Overlays the following overlays are meant:

• Rainfall Overlay
• Flooding Overlay
• Groundwater Overlay

Each of these overlays have some common configuration steps which will be described on this page.

Tips on creating a new project when working with the Water Overlays

When creating a new project in the new project wizard, take into account the Advanced options. In this menu for example the AHN3 dataset can be selected, which, if available for your project area, provides the most recent heightdata available as Open Data. Also, the IMWA dataset is already checked, this means that if available in your project area, Water Level areas and/or Culverts are already imported.

Configuration wizard

Each of the Water Overlays has a configuration wizard which helps the user with configuring the overlay. The Configuration Wizard can be found in the General tab of the overlay.



For the Flooding Overlay the wizard has an extra step, the Rainfall and Groundwater Overlay wizard are basically the same. The settings in the wizard can always be changed after finishing the wizard by opening the wizard again and changing a value and recalculating the overlay.
Step 1 In the first step the simulation time is set, based on either a rainfall or without a rainfall. Also the evaporation reference factor (over time) can be set.

Step 2 In the second step the elements of the water system can be imported. In some sub-steps the there are four options: to do nothing and proceed, to import the data with the Geo data wizard, or when you already have the data imported to select it based on an attribute of the data or to let the Tygron Platform automatically generate some data. The following elements can be imported:

• the breach areas (only for the Flooding overlay)
• water level areas; when automatically generating data one water level area covering the whole project area is created. The water level can be adjusted afterwards.
• ground water data
• sewer areas; can also be automatically generated based on the urbanization of the project area
• inundated areas; these are areas that are already inundated at the start of the flooding
• constructions; for the constructions keep in mind that when the data is point data, it has to overlap two water level areas. Therefore make sure the point data is on the border of two water level areas
  • Weirs
  • Culverts
  • Pumps
  • Sewer overflows; if chosen to automatically generate the sewer areas, the sewer overflows can also be automatically generated

Step 3 Adjust, if needed, the hydrological coefficents used for the calculations for the surface terrain, the underground terrain and the function values (for example the amount of water storage for a certain type of building).
Step 4 The water system can be made visable with panels and a network visualization.
Step 5 In step 5 you have to choose the result types (the different results which can be exported) you want to see after the calculation is done. Some overlays require a threshold value. Read below for the different result types to choose from and the threshold values. One of the overlays can be selected as the first overlay, which then will be visible as the parent Overlay. You can always adjust the result types you want to see later and calculate the Overlay again. Also you can set the number of timeframes in the slider. The timeframes are the intermediate results of the calculation and are not the same as the timesteps. Each timeframe contains numerous timesteps, based on the grid cell size and the speed of the flooding (see for more information the Courant number). The more timeframes you choose, the more insight you will get into the calculation. Each of the timeframe results can be exported.
Step 6 In the last step, some additional result types can be chosen to be visualized. Thes overlays contain input data that is used for the calculation, for example the manning value.

Model connections

The hydrological model can be linked to other models, which adds and defines more data to the simulation.

Weather

Weather in the Tygron Platform is modeled in the form of a weather definition.

Weather defines a number of environmental circumstances the hydrological model is subject to. It also defines the (total) simulation time.

Weather is a required connection. There is always exactly one weather connected to a water overlay, and by default if no weather exists an appropriate weather effect is created and connected automatically.

Rain and simulation time

Rain is a consistent addition of water to the hydrological model over a specified period of time. At the end of the rainfall's duration, the specified amount of rain will have fallen in each location in the project. The simulation can calculate both periods of rain as well as dry periods.

The total simulation time is composed of both the periods of rain, and the dry periods. It is possible to set up a simple, linear rainfall situation, in which a period of consistent rain is followed by a dry period. More complicated, custom configurations can be loaded in as well.

During a period of rain, the rainfall is constant. In each timestep an equal amount of water will fall, such that by the end of the period of rain that exact of rain will have fallen.

Linear configuration
When configuring a simple rainfall situation, it is possible to enter the properties for rain and simulation time by adjusting the linear properties. When using this method, the simulation will be composed of one period of rain, followed by one dry period.

Property	Unit	Description
Rain for	minutes	How long rain should last at the start of the simulation.
Total rainfall	mm	How much rain should fall in the specified period.
Dry after rain (days, hours, minutes)	days, hours, minutes	How long the simulation continues after the rain has fallen.

Custom configuration
If a use-case requires a more complex sequence of rain than a single period of rain followed by a single dry period, it is possible to prepare a comma-separated values file with a sequence of periods and values.

Rain and simulation CSV specification
Data per line	Additional rainfall until specified moment
Criteria	Time should always be greater than or equal to previous time Rain should never be negative
	First entry	Second entry
	Time (s)	Rain (m)
Line 1	Time when first period ends	Total rain during first period
Line 2	Time when second period ends	Total rain during second period

The last time value also indicates the end of the simulation.

Evaporation

Evaporation is the consistent removal of water from the hydrological model over a specified period of time. As long as evaporation takes places at a certain rate, water both on the surface and underground can be subject to removal from the hydrological model. The evaporation rate defined by the weather is the base amount of evaporation for the evaporation model.

Linear configuration
When configuring a simple evaporation situation, it is possible to enter this property directly by adjusting the linear property. When using this method, the simulation will use a single rate of evaporation for the duration of the simulation.

Property	Unit	Description
Surface evaporation	mm/day	The speed at which water evaporates during the simulation

Custom configuration
If a use-case requires a more complex pattern of evaporation than a single evaporation rate, it is possible to prepare a comma-separated values file with a sequence of periods and values.

Evaporation CSV specification
Data per line	Rate of evaporation until specified moment
Criteria	Time should always be greater than or equal to previous time Evaporation should never be negative
	First entry	Second entry
	Time (s)	Evaporation (m)
Line 1	Time when first period ends	Amount of evaporation during first period
Line 2	Time when second period ends	Amount of evaporation during second period

Ground water GeoTIFF

Ground water in the Tygron Platform is modeled in the form of a GeoTIFF.

The hydrological model can simulate the underground environment as well. To enhance the level of detail of the underground environment, it is possible to connect a groundwater GeoTIFF to the water model. The ground water GeoTIFF will dictate the underground water levels relative to datum at the start of the simulation, influencing how much more water can be stored underground and how much water can flow from the underground.

Ground water is only a relevant connection when the underground model is active. If the ground water model is not active, a connection with a ground water model is not relevant, regardless of whether it's present or not.

Ground water is an optional connection. If no ground water is connected to the water model, the ground water level relative to datum is equal to the water level as defined by the water level areas.

Subsidence

Subsidence in the Tygron Platform is modeled in the form of a subsidence overlay.

The hydrological model is greatly influenced by the height of the terrain. In virtually all cases water flows from higher places to lower places. The water model can be connected to a subsidence calculation which affects the terrain height. This allows the model to take into account a period of subsidence which changes the terrain, and calculate the impact, effects, and flow in the future.

When a subsidence calculation is connected to the hydrological calculation, the outcome of the subsidence calculation affects the terrain height used by the hydrological calculations. The effect does not apply the other way around; output from the water model is not used as input or effect for the subsidence model.

Subsidence is an optional connection. If no subsidence model is connected to the water model, no subsidence is applied to the model prior to the calculations. Other effects on the terrain height, such as breaches, still apply.

Hydrological features

The water system can be enhanced with a number of hydrological features, which can be loaded in as areas. These hydrological features form special properties or modifications on the hydrological system.

Water level area

A water level area represents real-world water level areas. Within a water level area, the heights of all water terrains are set to a specified level.

Attribute	Unit	Description	Default (when attribute is not present)
WATER_LEVEL	m + datum	The water level for all water terrains in this water level area	n/a

If no water level area is present in the project, the water level on water terrains is assumed to be extremely low. This allows water to flow into the open water areas at all times.

Sewer area

A sewer area is part of the definition of a system of sewers in the specified area. Sewer storage is present in the hydrological model wherever the sewer area intersects with a sewered construction.

Attribute	Unit	Description	Default (when attribute is not present)
SEWER_STORAGE	m	The maximum height the water can reach in this sewer. This value, multiplied by the surface area of the sewered constructions the sewer area intersects with, forms the total amount of water this sewer can store.	n/a
SEWER_PUMP_SPEED	m3/s	The amount of water removed from the sewer by removing it from the hydrological model entirely.	0

Sewers don't have default storage amount, but when generating them automatically in the configuration wizard, suggested values are 0,007m for older sewers and 0,04 for newer sewers.

Breach

A breach is a modification to the terrain height, with an optional in- or outflow of water for the hydrological model. This can be used to represent calamitous situations, such as a breach in a levee. Breaches can also be used to easily simulate a terrain height increase, effectively creating a levee.

A breach can either be defined solely as a terrain height modification using its SURFACE_OVERRIDE attribute, or as an in- or outlet by adding an INLET_Q attribute as well. If the breach is only defined as a terrain height change, only water that is already created or defined in some other way in the hydrological model can flow through and from it. If the breach is also given an inlet speed, water will automatically be created or removed uniformly on the breach.

Attribute	Unit	Description	Default (when attribute is not present)
SURFACE_OVERRIDE	m + datum	The new terrain height at the location of the breach.	n/a
INLET_Q	m3/sec	The maximum amount of water flowing into the model through this breach. A negative value means the breach functions as an outlet, and water is removed from the hydrological model.	0
INLET_CAPACITY	m3	The maximum, total amount of water which can flow in or out through this breach. Water flowing back in the other direction replenishes the capacity.	Infinite
LOWER_THRESHOLD	m + datum	If a lower threshold is set, water will only flow into the model through this breach until the average water level on this breach is equal to or greater than the threshold. If the threshold is not set, the amount of water flowing in is not limited in this fashion.	None
UPPER_THRESHOLD	m + datum	If an upper threshold is set, water will only flow out the model through this breach until the average water level at the point of this breach is equal to or lower than the threshold. If the threshold is not set, the amount of water flowing out is not limited in this fashion.	None

Note that all inlet attributes function as flow limits. If multiple are defined, water can flow in or out up until any of those limits are reached. If none are defined, no water flows in or out.

Also note that a breach shares attribute names with the inlets, and that changing the attribute keys for breaches also affects the keys for inlets.

Inundation

An inundation is an initial placement of a quantity of water. This differs from the water level areas in that an inundation level allows you to place water anywhere on the surface.

Attribute	Unit	Description	Default (when attribute is not present)
INUNDATION_LEVEL	m + datum	The height of the water.	n/a

Hydrological constructions

The water system can be enhanced with a number of hydrological constructions. These are constructions which effect water flow in specific cells, according to the parameters and rules of the constructions used. The effects of these constructions can be adjusted by setting the appropriate attributes.

Hydrological constructions can be either line-based or point-based:

• Line-based constructions
  Line-based constructions form a direct connection between two exact cells, allowing water to flow from one point to another. The flow is dictated by the construction's formula. The endpoints of a line-based construction, the exact cells which are connected by the construction, are computed based on the orientation and size of their polygon. Essentially, the furthest ends of the polygon are used as end-points. Because the cells are considered adjacent, any calculated flow through line-based hydrological constructions is instantaneous.
• Point-based constructions
  Point-based constructions add or remove water in one or more computational layers, based on their formula's. The centerpoint of a point-based construction, the exact cell where the effect takes place, is is the geometric center of the construction's polygon.

Note that the more complex the polygon is, the more difficult it is for the Tygron Platform to resolve it to a simple line or center point.

When the calculation of the water overlay completes, the total amount of water which has flowed through a specific construction is stored in an attribute in that construction. By default, this attribute is OBJECT_OUTPUT_FLOW, and the flow is expressed in m3. If multiple water overlays exist in the project simultaneously, the attribute name is appended with a number so that each overlay (as they are added to the project) has a unique attribute it writes its results to.

Hydrological constructions can only function as a single hydrological construction. If a single construction has attributes related to multiple hydrological constructions, the resulting behavior is undefined.

Culvert

Culverts are effectively tunnels or pipes directly connecting two bodies of water, and allow water to flow in either direction. Culverts can also be used to model tunnels on land, creating a hole which water can flow through when it is flowing over land. The throughput of a culvert is limited by its dimensions.

A culvert is a line-based construction.

Attribute	Unit	Description	Default
CULVERT_WIDTH	m	The diameter of the culvert. For throughput calculations, the culvert is assumed to have a spherical cross-section.	1
CULVERT_HEIGHT	m + datum	The height of the culvert. (When set to a level lower than the terrain for either endpoint of it, the culvert's height is equal to the height of the (highest) terrain under either endpoint.)	0
CULVERT_N	manning value	The manning value of the culvert's material, which influences the flow speed.	0,014

Weir

Weirs are effectively small dams in the water, and allow water to flow from a water body with a higher water level to a lower water level. Any water exceeding the height of the weir can flow over it, increasing the throughput as the water level increases. Strictly, water can flow over the weir in either direction.

A weir is a line-based construction.

Attribute	Unit	Description	Default
WEIR_HEIGHT	m + datum	The height of the weir.	n/a
WEIR_WIDTH	m	The width of the weir.	5
WEIR_COEFFICIENT	coefficient	The flow coefficient related to the shape of the weir	1,1

Pump

Pumps are constructions which can move water against its natural flow. Specifically, it moves water from the lower end of the pump to the higher end of the pump. The terrain height is used to determine the low end and the high end of the pump.

A pump is a line-based construction.

Attribute	Unit	Description	Default
PUMP_SPEED	m3/s	The speed at which water is pumped from the lower water level to the higher water level.	n/a

If a pump is placed such that both end-points are at locations with equal terrain height, the pump will be inactive and no water will flow through it.

Sewer overflow

Sewer overflows are points where water is moved from the sewer area to the above-ground water system. A sewer overflow will allow water to flow through if the water in the sewer exceeds the SEWER_OVERFLOW_THRESHOLD, and the water in the connected sewer exceeds the height of the terrain at the location of the sewer overflow.

A sewer overflow is a point-based construction, and must intersect with a sewer area.

Attribute	Unit	Description	Default
SEWER_OVERFLOW	m + datum	The height of the bottom of the sewer, relative to the average terrain height of the connected sewer. Starting from this height, the water level in the sewer must exceed the height of the terrain at the location of the overflow in order for water to flow out.	n/a
SEWER_OVERFLOW_SPEED	m3/s	The maximum speed at which water can flow out from the sewer through this overflow.	10

Inlet

Inlets are points where water is either added to or removed from the hydrological model. It will add or remove water at a defined maximum rate, with optional thresholds for the amount of water to add or remove.

An inlet is a point-based construction.

Attribute	Unit	Description	Default
INLET_Q	m3/sec	The maximum amount of water flowing into the model through this inlet. A negative value means the construction functions as an outlet, and water is removed from the hydrological model.	n/a
INLET_CAPACITY	m3	The maximum amount of water which can flow in or out through this construction. Water flowing back in the other direction replenishes the capacity.	Infinite
LOWER_THRESHOLD	m + datum	If a lower threshold is set, water will only flow into the model through this inlet until the water level at the point of this inlet is equal to or greater than the threshold. If the threshold is not set, the amount of water flowing in is not limited in this fashion.	None
UPPER_THRESHOLD	m + datum	If an upper threshold is set, water will only flow out the model through this outlet until the water level at the point of this inlet is equal to or lower than the threshold. If the threshold is not set, the amount of water flowing out is not limited in this fashion.	None

Note that all inlet attributes function as flow limits. If multiple are defined, water can flow in or out up until any of those limits are reached. If none are defined, no water flows in or out.

Also note that an inlet shares attribute names with the breaches, and that changing the attribute keys for inlets also affects the keys for breaches.

Miscellaneous hydrological properties of constructions

Besides the constructions which directly influence the main water flow in the hydrological model, all constructions have properties which may interact with the hydrological model in some way.

The effects of these constructions can be adjusted by setting the appropriate attributes. In some cases, these are attributes which relate to function values. For these attributes, either can be adjusted to the same effect. Note that attributes which are connected to a function can be redefined, like the attribute names for hydrological constructions and hydrological features can be redefined.

In contrast with hydrological constructions and their properties, all constructions can have any or all of the following miscellaneous effects on the hydrological model.

Sewered constructions

Sewered constructions are constructions under which a sewer exists, and through which water can flow into the sewer. When a sewered connection overlaps with a sewer area, that overlap forms an actual sewer, with the storage capacity of the SEWER_STORAGE attribute of the sewer area. Any surface water entering the cell of a sewered construction is directly moved to the sewer (unless the sewer is filled to capacity).

Attribute	Unit	Function value	Description
SEWERED	boolean	Connected to sewer	Whether this construction is connected to the sewer.

Water storage constructions

Constructions capable of water storage can store some surface water without allowing it to flow back into the rest of the model. Water stored in constructions can not flow out or evaporate away.

Attribute	Unit	Function value	Description
WATER_STORAGE	m³/m²	Water storage (m³/m²)	How much water this construction can store.

Porous constructions

Some constructions are porous or open, and can allow water to infiltrate into the underground unsaturated zone.

The speed at which water can infiltrate is dependent on both the infiltration properties of the constructions as well as on the underlying surface terrain. Of the infiltration values of the construction and the surface terrain, the lowest value is used. If either has an infiltration value of 0, water cannot infiltrate into the underground unsaturated zone.

Attribute	Unit	Function value	Description
GROUND_INFILTRATION_MD	m/day	Ground infiltration per day (m)	The speed at which water can flow vertically from the surface to the underground unsaturated zone.

Crops and foliage

Crops and foliage can draw water from the underground, allowing it to evaporate.

Attribute	Unit	Function value	Description
ROOT_DEPTH_M	m	Depth of plant roots (m)	The depth of the roots of this construction, relative to the terrain height at the location of this construction. Water can be drawn from the underground and evaporated if the roots can reach it.
WATER_EVAPORATION_FACTOR	factor	Water evaporation	How fast this construction can evaporate water from the underground. The weather's evaporation speed is multiplied by this factor to determine the rate of evaporation.

Note that when a construction is present in any given location, the values for evaporation will overrule any values set by terrain in the same location. To model underground evaporation without a construction, set these attributes on the applicable terrain type instead.

Critical structures

Some constructions may be considered critical, meaning the consequences of water stress are greater for these structures than for others. Examples include hospitals and (elementary) schools. Critical constructions will receive additional highlighting by the IMPACTED_BUILDINGS result type when the building is impacted by the amount of water defined by IMPACT_FLOOD_THRESHOLD_M.

By using different values for differing (kinds of) constructions, it is possible to have impacted structures highlight with different values as well. This makes it possible to differentiate in greater detail between the kinds of impacted structures.

Attribute	Unit	Function value	Description
CRITICAL_INFRASTRUCTURE	nominal integer	Critical infrastructure	Whether this construction is deemed a critical construction. 0 means the construction is never deemed impacted.

Chemical emitters/decomposer

Chemical emitters are constructions which produce specific chemicals. The net amount of chemicals a single construction creates is spread out across it's surface. After the chemicals are created, any water flowing through the same location will carry a part of the chemicals with it.

Structures which are defined to create a negative amount of chemicals function as a scrubber, removing the specified quantity of chemicals from the hydrological model.

In situations where water is absent, chemicals cannot move between cells.

Attribute	Unit	Description	Default
CHLORIDE	x/m²	The amount of chloride created per second per m² in this location.	0
NITROGEN	x/m²	The amount of nitrogen created per second per m² in this location.	0
PHOSPHORUS	x/m²	The amount of phosphorus created per second per m² in this location.	0

Chemical emitters's attributes do not take the form of function values, and must be added manually or as part of loading in geodata.

Hydrological properties of terrain

Terrains in a project have a number of hydrological properties which can influence the flow of water in a project. The following attributes of terrains have effects on the hydrological model:

Water

Water terrains are processed by the water model in a specific manner before the simulation is started. For each water terrain in the 3D world, the bottom of the water body is treated as a land surface in the same fashion as dry land. Water is then placed on it on the surface layer, up to the level defined by the overlapping water area's WATER_LEVEL attribute. Terrains not marked as water terrain are not initiated with water.

Terrains marked as water are subject to an additional check for the WATER_STRESS result type. If the amount of water on a water terrain has not increased by more than ALLOWED_WATER_INCREASE_M relative to the water level area's water level, that terrain will not count as stressed for that result type. The amount of water on that location must be at least ALLOWED_WATER_INCREASE_M more than the water level area's water level.

Attribute	Unit	Terrain type	Description
WATER	boolean	Surface	Whether the specified terrain is a water terrain.

Evaporation

Terrains can be configured to draw water from the underground and evaporate it.

Attribute	Unit	Terrain type	Description
ROOT_DEPTH_M	m	Surface	The depth of the roots of this surface terrain, relative to the surface. Water can be drawn from the underground and evaporated if the roots can reach it.
WATER_EVAPORATION_FACTOR	factor	Surface	How fast this terrain can evaporate water from the underground. The weather's evaporation speed is multiplied by this factor to determine the rate of evaporation.

Note that when a construction is present in any given location, the values for evaporation of the construction will overrule any values set by terrain in the same location. This is also true if the construction has its evaporation values set to 0; they will overrule the terrain's values and thus not allow evaporation of underground water to occur.

Also note that the groundwater level reduction is inversely proportional to the WATER_STORAGE_PERCENTAGE, as the contribution of a given volume of water to the groundwater level increases as the capacity for water storage in the underground layer decreases.

Infiltration and storage

Based on the properties of the terrain, water may infiltrate into the underground water system.

The speed at which water can infiltrate from the surface to the underground unsaturated zone is dependent on both the infiltration properties of the surface terrain, as well as any construction in that location, if present. Of the infiltration values of the construction and the surface terrain, the lowest value is used. If either has an infiltration value of 0, water cannot infiltrate into the underground unsaturated zone.

Attribute	Unit	Terrain type	Description
GROUND_INFILTRATION_MD	m/day	Surface	The speed at which water can flow vertically from the surface to the underground unsaturated zone.
GROUND_INFILTRATION_MD	m/day	Underground	The speed at which water can flow vertically from the underground unsaturated zone to the underground saturated zone, and horizontally through across the saturated zone.
WATER_STORAGE_PERCENTAGE	percentage	Underground	The percentage of the underground volume which can be filled with water. A lower percentage means the underground will be able to store less water, and the saturated zone will rise higher with the same amount of water in the underground layer.

Result types

The water model performs complex calculations, and multiple types of results can be provided. In principle, each overlay can be configured to display a single result type.

Result types can differ in the kind of data they display, the layer (surface or underground) of which they display that information, and how that data is recorded. Different result types can monitor data in the following ways:

• Start: The data is determined at the start of the simulation, and does not change afterwards.
• Last: The data is the latest value determined at the timestep the data is recorded. The values can increase and decrease between different timesteps. This mode is primarily used for monitoring progression.
• Maximum: The data is the highest value determined up until the timestep the data is recorded. The values can only increase or stay the same, but will never decrease. This mode is primarily used to gain insight into impact; the most severe situation any point had to endure.
• Total: The result of a running tally, counting the relevant data up until the timestep the data is recorded. The value can only increase or stay the same, but will never decrease. This mode is primarily used to gain insight into quantities rather than duration.

The following results types are available:

Result type	Unit	Display mode	Description
BASE_TYPES	Nominal value	Start	Categorization of the individual cells based on how they are processed by the water model, displaying which cells are considered to be specific features. 0: Cell on the edge of the project area 1: Water level area 2: Land 3: Sewer 4: Breach 5: Hydrological construction
CHLORIDE	x/m²	Last	The amount of chloride present. The value is the sum of the quantities on the surface, and the underground.
DIRECTION	Degrees	Last	The direction in which water is flowing.
EVAPORATED	m (mm)¹	Total	The amount of water that has evaporated. The value is the sum of the quantities evaporated from the surface and the underground.
GPU OVERVIEW	nominal value	Maximum	Shows which GPU cluster calculated which part of the overlay.
IMPACTED_BUILDINGS	nominal value	Maximum	Constructions impacted by excess water. Constructions are considered impacted when the construction itself or an adjacent cell contains more water on the surface than configured in IMPACT_FLOOD_TRESHOLD_M. 0: Construction is not impacted 1...N: The (critical) construction is impacted, and has a critical function value set to this value.
LAST SPEED	m/s	Last	The speed of water flow in any given location.
MAX SPEED	m/s	Maximum	The speed of water flow in any given location.
NITROGEN	x/m²	Last	The amount of nitrogen present. The value is the sum of the quantities on the surface, and the underground.
PHOSPHORUS	x/m²	Last	The amount of phosphorus present. The value is the sum of the quantities on the surface, and the underground.
SEWER_LAST_VALUE	m (mm)¹	Last	The amount of water stored in the sewer.
SEWER_MAX_VALUE	m (mm)¹	Maximum	The amount of water stored in the sewer.
SURFACE_DURATION	s (min)¹	Total	The amount of time the water depth on the surface exceeds SHOW_DURATION_FLOOD_LEVEL_M.
SURFACE_FLOW	m³/m²		Description wil be added
SURFACE_LAST_VALUE	m (mm)¹	Last	The amount of water on the surface.
SURFACE_MAX_VALUE	m (mm)¹	Maximum	The amount of water on the surface.
UNDERGROUND_FLOW	m³/m²		Description wil be added
UNDERGROUND_LAST_STORAGE	m (mm)¹	Last	The (effective) amount of water in the underground unsaturated zone.
UNDERGROUND_LAST_VALUE	m (mm)¹	Last	The distance between the surface and the groundwater level.
UNDERGROUND_MAX_STORAGE	m (mm)¹	Maximum	The (effective) amount of water in the underground unsaturated zone.
UNDERGROUND_MAX_VALUE	m (mm)¹	Maximum	The distance between the surface and the groundwater level.
UNDERGROUND WATERTABLE	m + datum	Last	The groundwater level, relative to datum.
WATER_STRESS	m (mm)¹	Maximum	The amount of water on the surface, similar to SURFACE_MAX_VALUE. However, for water terrains, the water level must rise by at least ALLOWED_WATER_INCREASE_M. Otherwise, the value in those locations is 0.

¹ the units between () are as displayed in the 3D client. If exported to GeoTiff the SI-convention is used: meters (m) and seconds (s).

Result child overlays

Each overlay can only display a single result type. When using a water overlay, it is conceivable that multiple result types are relevant to a project's use case. It's possible to duplicate the overlay, and set the copy of the overlay to a different result type, but this is not recommended. Downsides of this approach are that the simulation has to run in full multiple times, causing a severe increase in calculation time, and that when changes to the overlay's configuration have to be made those changes need to be made to all water overlays.

It is possible to add result child overlays overlays to a water overlay, which can display different results coming forth from the same calculation. The advantages of using result child overlays are that for any given water overlay, the calculation of the overlay only occurs once, rather than multiple times equal to the amount of desired result types. Additionally, the configuration for the calculation is only defined in a single overlay, which makes it easier to make sure all results come forth from the exact same simulation.

Result child overlays do not recalculate if either they or their parent is set to inactive.

If a calculation overlay is removed, all result child overlays which are children of that overlay are removed as well. Separate overlays set as child overlays (such as input overlays) of the overlay will not be removed.

It is only possible to add result child overlays via the configuration wizard.

Calculations

Models

Multiple models are implemented which in conjuction form the water model in its entirety.

Surface model

The water model's primary function is the simulation of the flow of water on the surface of the terrain. The surface model includes the flow of water across the surface of the terrain, including over water terrains, including the flow through hydrological constructions and water that is created or removed by hydrological constructions.

The surface is defined by the terrain height in the project. The terrain height is further influenced by the height of constructions present in the project (bounded by DESIGN_FLOOD_ELEVATION_M) and by the SURFACE_OVERRIDE of breaches.

The surface water level is initialized based on hydrological features present in the project. For all water terrains, water is placed on the surface of the world. The amount of water placed is such that the resulting water level in that location is equal to the WATER_LEVEL attribute of the Water level area in that location. If there is no water level area in that location, the water level is assumed to be so low that no water is created. Besides the water level areas, Inundation is added to the model. Water is placed in all locations where inundation is defined (regardless of the terrain type in that location, in contrast to the water level areas), such that the resulting height of the water inundating the land is equal to the inundation area's INUNDATION_LEVEL attribute.

After the surface is initialized with water, all water on the surface will flow in accordance with the same rules. It does not matter whether the water was created when the model was initialized, and whether that water was due to a water terrain or due to inundation, or whether the water came in from another source.

On the surface, water can flow from one cell to an adjacent cell based on the relative heights of the water, the slope of the terrain, and the manning value of the terrain or construction in that location.

In addition to water flowing between geographically adjacent cells, water can also flow through hydrological constructions. When a line-based hydrological construction exists in the project area, the 2 cells indicated by the endpoints of the line are considered adjacent as well. Flow between those cells is not dictated by the same parameters as the regular surface flow. Instead, water can flow between the 2 indicated cells based on the construction's underlying formula.

Water can also be added to or removed from the water model by line-based hydrological constructions. Based on the construction's underlying formula water can be added or removed to the cell indicated by the construction. Only that single cell will receive or lose the calculated amount of water.

Water can also be added to or removed from the surface by other properties of constructions, based on the construction's polygons. When water is added to the surface by a construction's polygon, the water is spread out equally over the entire construction. When water is removed from the surface via a polygon-based construction, the removal of water is calculated per individual cell.

Underground model

The water model includes an underground model which dictates the movement of water in the soil. The underground model includes the flow of water from the surface into the underground via infiltration, the flow of water from one underground location to another, and the exfiltration of water from the soil back onto the surface layer.

The underground model can be explicitly activated or deactivated by setting the GROUND_WATER attribute of the water overlay to the appropriate value. If the underground model is deactivated, no water can move from or to the underground in any form, including underground evaporation.

The underground is bounded vertically by the surface of the terrain at the top, and an assumed impenetrable layer at the bottom. The distance between the surface and the impenetrable layer, and thus the effective height of the underground, is equal to GROUND_BOTTOM_DISTANCE_M. In other words, the impenetrable underground layer is assumed to be a set distance below the surface. The distance is uniform across the entire project area, and follows the profile of the surface.

The underground is composed of 2 layers: the unsaturated zone and the saturated zone. The saturated zone is the region of the underground where the soil is saturated with water. This water is assumed to work as a continuous volume of water able to flow horizontally. The unsaturated zone is the region of the underground above the saturated zone. The edge between the unsaturated and saturated zone can be considered the groundwater level.

The groundwater level, and thus the height of the saturated zone, is determined both by the amount of water in the saturated zone, and the underground terrain's WATER_STORAGE_PERCENTAGE. The lower the water storage percentage of the soil, the greater the volume of soil that is saturated by the same amount of water, and thus the higher the the groundwater level will become.

The underground water level is initialized with the values of the Ground water GeoTIFF connected to the water model. If no ground water data is connected, the ground water level relative to datum is equal to the surface water level relative to datum, as defined by the WATER_LEVEL attribute of the Water level area in that location.

When water infiltrates from the surface, it infiltrates at a speed dictated by the surface terrain's GROUND_INFILTRATION_MD attribute, or (if present) by the construction's GROUND_INFILTRATION_MD, whichever value is lower. If a highly porous constructions is present above a non-porous surface terrain, or the other way around, the lower infiltration value of either will cause the water to infiltrate slower despite the porous properties of the other.

Surface water infiltrates into the underground unsaturated layer. Water in the unsaturated layer is assumed to be spread equally across the entire unsaturated volume. Water then flows from the unsaturated zone into the saturated zone at the speed dictated by the underground terrain's GROUND_INFILTRATION_MD. For a given timestep, the distance the water travels is determined. The amount of water that flows from the unsaturated zone to the saturated zone is equal to the amount of water in a section of the unsaturated zone the height of which is equal to that distance. After water has been added to the saturated zone, the groundwater level (and thus the height of the saturated zone) is redetermined. The water in the unsaturated zone is redistributed uniformly across the (remaining) unsaturated zone.

Water stored in the underground saturated zone can flow horizontally from one underground cell to another, if the groundwater level relative to datum is higher than the neighboring cell's ground water level, relative to datum. The amount of water which can flow from one cell to another is dictated by the underground terrain's GROUND_INFILTRATION_MD.

Water stored in the underground saturated zone can also exfiltrate out of the underground and back onto the surface, if the groundwater level relative to datum exceeds the neighboring cell's surface water level relative to datum. The amount of water which can flow from the underground of one cell onto the surface of an adjacent cell is dictated by the underground terrain's GROUND_INFILTRATION_MD.

Rain model

Rain is implemented in the water model.

Rain can be implicitly activated and deactivated by defining an appropriate period of rainfall. If a period with no rainfall is defined, that period is simulated but no rain is simulated.

Currently, during a period in the simulation where rainfall is defined, water is uniformly added to the surface of all cells in the project area. During any single defined period of rain, the amount of rain is consistent over time. At the end of the defined period of rain, exactly the defined amount of rain will have fallen on each cell.

Evaporation model

Water can evaporate from the hydrological model over time. Multiple forms of evaporation are implemented.

All forms of evaporation can be implicitly activated and deactivated by setting the weather's evaporation rate. If the evaporation factor is set to 0, no evaporation will take place in any form.

The weather's evaporation rate is defined as a period during which a certain rate of evaporation will take place. Multiple periods of evaporation can be defined, and at any specific moment during the simulation an exact evaporation rate is defined by the weather.

For all forms of evaporation, the weather's evaporation rate is used as a base for determining the exact rate of evaporation for that form of evaporation.

Surface evaporation model

Water can evaporate from the surface, based on the weather's evaporation factor and the overlay's SURFACE_WATER_EVAPORATION_FACTOR. These values compute to a net rate of evaporation which is applied to the surface of all cells. Only water on the surface of cells is affected by this evaporation.

Cells without water on the surface are not affected by evaporation.

Underground evaporation model

Water can evaporate from the underground if the cell has either a construction which allows for underground evaporation, or a surface terrain type which allows for underground evaporation and is unobstructed by a construction. In other words: if a construction is present the construction's properties are used, otherwise the terrain's properties are used.

Underground evaporation can be implicitly activated or deactivated by setting the relevant properties of all terrain types and constructions to appropriate evaporation values. If the relevant properties are set to 0, no underground evaporation will take place. Underground evaporation is also explicitly deactivated when the underground_model is deactivated.

Water can evaporate from the underground via crops and foliage. It can draw water from the underground unsaturated and saturated zones, if their roots reach deep enough and the terrain or construction have a configured evaporation factor. Water is drawn directly from the underground and evaporated, removing it from the hydrological model entirely.

The rate of evaporation is determined by the weather's evaporation factor, and either the construction's WATER_EVAPORATION_FACTOR or the surface terrain's WATER_EVAPORATION_FACTOR.

Evaporation can only take place if the roots of the terrain or construction can reach underground water. The depth the roots can reach is defined by either the construction's ROOT_DEPTH_M or the surface terrain's ROOT_DEPTH_M.

Water can be evaporated both from the saturated and the unsaturated zones of the underground. The amount of water that can be taken from the saturated and the unsaturated zones is limited by the amount of water in either zone in reach of the roots.

Storage model

Water on the surface model can be stored in water storage constructions.

Water storage in constructions can be implicitly activated or deactivated by ensuring that all constructions in the project area have appropriate water storage properties. If there are no constructions in the project area with water storage capacity, no water storage will take place.

When water flows onto a cell with a construction capable of storing water, that water will be stored in the construction until the construction's water storage capacity has been reached. Water cannot leave that storage, either through flow back into the hydrological model or by being removed from it altogether. When the storage is filled, no additional water cannot flow into that storage for the remainder of the simulation.

Model border

The outer edge of cells of the water model are excluded from calculations. No water can flow from or to there.

Formulas

Timestep formula

An adaptive timestep is implemented according to Kurganov and Petrova (2007). At every timestep, the courant-number is kept smaller than 0.25 for every active computation cell.

Especially at low depths, choosing the appropriate timestep is critical to avoid numerical instability. Therefore the following principles are used to determine the right time step:

• the timestep is choosen so that all computation cells follow one of the following criteria.
• if a cells waterdepth is below the flooding threshold, 5 * 10^-3 (m) there is no flow assumed between that cell and it neigboring cell.
• if the cells waterdepth is above above the flooding threshold, the maximum timestep is assumed to be 100 * the waterdepth at the cell.
• if the waterdepth increases, the timestep is assumed to be not larger than the formula above.

If the numerical flux decreases, larger timesteps are allowed than set by Kurganov and Petrova, depending on the configured calculation.

Calculation preference formula

The calculation preference influences the calculation of individual timesteps.

Δt = Δx /u_max

where:

• Δt = computational timestep
• Δx = grid cell size
• u_max = max velocity, assumed 2.5 (SPEED), 5 (AVERAGE) and 10 (ACCURACY) m/s respectively

Surface flow formula

Surface flow is calculated using the 2D Saint Venant equations.

2D Saint Venant

The base equations describe the conservation of mass and momentum in both the x and y direction.

The following processes are described in these equations:

• friction
• bed slope
• water pressure
• convection (changes in bathemetry over space)
• inertia (increase or decrease of velocity over time)

Numerical scheme for the 2D Saint Venant equations

Source: Horváth et al. (2014)

Source: Horváth et al. (2014)

The explicit second-order semi-discrete central-upwind scheme for the 2D Saint Venant Equations is implemented. A reconstruction of cell bottom, water level and velocity at the interfaces between computational cells as proposed by Lax and Wendroff (Rezzolla, 2011). The Tygron Platform's water model relies on the scheme described in Kurganov and Petrova (2007). The reconstruction method is taken from Bolderman et all (2014) and ensures numerical stability at the wetting and drying front of a flood wave.

A clear explanation on the numerical approach can be found at Horváth et al. (2014), but in general it follows these steps:

1. The elevation value of the cell (denoted as B in included figures) is equal to the elevation value at the center of the cell. At the same time, it is equal to the average value of the elevation values at the cell interface midpoints.
2. The slopes of the conserved variables (denoted as U in included figures), continuity and momentum in x and y direction, are reconstructed.
3. Values of conserved variables at the cell interface midpoints are compared with the left-sided and right sided values at cell centers.
4. At partially dry cells, the slope is modified to both avoid negative depths and numerical instability.
5. (Numerical) fluxes are computed at each cell interface to determine the values of the conserved variable at the cell centers for the next time-step.

Surface infiltration formula

Surface infiltration capacity is calculated per cell.

I = min(Icon, Isurf)

where:

• I = The cell's infiltration capacity (m/day)
• Icon = The GROUND_INFILTRATION_MD of a construction on a specific cell (if present).
• Isurf = The GROUND_INFILTRATION_MD of the surface terrain type. This value should be interpreted as the vertical conductivity (Kv) of the sub-soil.

Underground infiltration formula

Underground infiltration (from the unsaturated zone to the saturated zone) is calculated per cell.

Δw = (Δt * Iund * Wunsat) / (GBDM - (Wsat/WSP))

where:

• Δw = The underground infiltration which takes place.
• Δt = Computational timestep.
• Wsat = The amount of water stored in the saturated zone.
• Wunsat = The amount of water stored in the saturated zone.
• Iund = The GROUND_INFILTRATION_MD of the underground terrain type.
• GBDM = GROUND_BOTTOM_DISTANCE_M (effectively available height in the underground model).
• WSP = WATER_STORAGE_PERCENTAGE.

General Tab

The General Tab can be found at the panel on the right side, when selecting a Water Overlay. In this tab several settings can be adjusted.

Calculation Preference

Here you can manipulate the computation time step by choosing between the option SPEED, AVERAGE and ACCURACY. The computational timesteps will be set according to the Courant criterion:

Δt = Δx /u_max

where:

• Δt = computational timestep
• Δx = grid cell size
• u_max = max velocity, assumed 2.5 (SPEED), 5 (AVERAGE) and 10 (ACCURACY) m3/s respectively

Grid cell size

The Tygron Platform computes flow equations over a rectangular cartesian grid, the grid cell size can be set by clicking on Change Grid. After the grid cell size is changed, the Overlay is immediately being recalculated. Note: if you choose a smaller grid cell size, both the amount of time steps (see Calculation Preference) and amount of computational cells increase.

Refresh Grid

By Refreshing the grid, the Overlay is recalculated. This may take some time, depending on the grid cell size, the size of your project and the calcualtion preference. Below some use cases when to use Refresh Grid:

• when you have changed a setting in the Configuration Wizard, but not clicked on the Finish button of the wizard.
• when you change the Result type or calculcation preference in the General tab
• when you change the legend in the Legend tab
• when you change the keys in the Keys tab
• when you change the attrbutes of the Overlay in the Attributes tab

Export Grid File

You can export the current result type as either a GeoTiff (binary file/bitmap image can be opened in a GIS or in an image viewer) or ASCII (text file, can also be opened in a GIS but is also readable in a text editor) file. Also, it is possible to export the difference result type of the current and maquette situation.

Save Overlay result

With this option you can create a duplicate inactive copy of the current overlay. This is noticeable in the Overlays panel on the left side of the screen. The duplicated overlay will have (inactive copy) behind the overlay name and is greyed out. This copy is not being recalculated when changing settings in the original duplicated overlay. When checking the checkbox: active in simulation, the copy becomes active again and can be recalculated. Also the overlay name is again in black and not grayed out anymore.

Show Water Balance

The Water Balance panel shows the in- and outflow of water in m3. The Water Balance panel is also visible when clicking on the Debug info (see below). The water balance is made up of the following components (depending on what is in the project):

• Land surface: amount of water on the land surface after simulation (m3)
• Water surface: amount of water stored on water cells after simulation (m3)
• Building storage: amount of water stored in building storage (m3)
• Sewer storage: amount of water stored in sewers after simulation (m3)
• Underground storage: amount of water stored in the sub-soil after simulation (m3)
• Evaporated: amount of water evaporated after simulation (m3)
• Outlet: amount of water extracted from the model during the simulation via outlets of water areas or sewer pumps (m3).

Warnings

When the grid cell size is too large in combination with the project size, a warning pops up. This means that calculated results are not accurate. The solution to this is to reduce the grid cell size.

Debug Info

The debug info contains the following information:

• Rain: the amount of rainfall (mm) in the duration of the rainfall event (hours)
• Total Simulation: the simulation period (hours)
• Cells: The dimensions of the simulation: the amount of computational cells and the amount of time steps (cycles)
• Water areas:
• Weir flow: the amount of water flown over weirs during simulation (m3)
• Sewer overflow: the amount of water flown over sewer overflows during simulation (m3)
• GPU time: the computation time on GPU (seconds)
• Volume: total volume of the in- and outflow of water in the model.

If you click on the Debug Info the Water Balance panel pops up.

Keys tab

The tab Keys refers to the settings for the water system (areas and constructions) that are adjusted by following the configuration wizard in the General tab. It is therefore generally not needed to change the keys in this tab, as most of the keys are adjusted via the configuration wizard. However, below an explanation of the Keys for areas and buildings.

Attribute	Unit	Description
Water Level	m + datum	initial water level, relative to datum, for all water cells in a water area
Outlet	m3/s	water abstraction for all water cells in a water area
Outlet level	m + datum	initial water level, relative to datum, for all water cells in an outlet area (breach for example)
Outlet capacity	m3	maximum amount of water that can be used for the outlet
Sewer Storage	m	available storage in sewers at the start of simulation in a sewer area
Sewer Pump Speed	m3/s	sewer water abstraction for all cells in a sewer area
Breach Floor	m + datum	Modified terrain height, to simulate a breach in a levee.
Inundation level	m + datum	Initial water height, to simulate water present at the start of the simulation.

Keys referring to constructions (building attributes)

Attribute	Unit	Description
Weir Height	m + datum	weir crest level
Weir Output	m3/s	The amount of water that flows through a weir/culvert
Weir Coefficient		linear weir coefficient for accounting discharge & contraction losses
Culvert Speed	m3/s	the discharge capacity of a culvert
Pump Speed	m3/s	the capacity of a pump
Sewer Overflow Height	m + datum	the height of a crest of a sewer overflow
Sewer Overflow Speed	m3/s	the discharge capacity of a sewer overflow
Chloride	x/m2	Amount* of chloride which is created per second, per m² of construction with this attribute.
Nitrogen	x/m2	Amount* of nitrogen which is created per second, per m² of construction with this attribute.
Phosphorus	x/m2	Amount* of phosphorus which is created per second, per m² of construction with this attribute.

* The unit of the substances is left incomplete, as the project creator is free to set their own unit for quantities of substances.

Attributes tab

This tab shows the attributes of result types that can be adjusted. These attributes can also be adjusted in the configuration wizard in step 5 in the General tab. It is therefore generally not needed to change the attributes here. However, below an overview of the Overlay Attributes.

Attribute	Unit	Description
ALLOWED_WATER_INCREASE_M	m	The amount by which the water level on a water terrain must increase before it is considered stressed by water. This is used to compute the WATER_STRESS result type.
DESIGN_FLOOD_ELEVATION_M	m	Constructions in the 3D world are assumed to have at most this height compared to the surface of the terrain. Greater values can create a more accurate model but will impact performance.
GROUND_BOTTOM_DISTANCE_M	m	Assumed distance under the terrain surface where the soil becomes impenetrable for water. The groundwater level cannot go below this depth, relative to the surface. The maximum amount of water that can be stored underground is equal to this attribute multiplied by the local terrain's WATER_STORAGE_PERCENTAGE.
GROUND_WATER	boolean	Whether underground water flow is simulated during the calculation. If this is deactivated, surface infiltration, underground infiltration, and underground evaporation do not occur. Water flowing in- and out of the sewer are still simulated when sewers are present.
IMPACT_FLOOD_THRESHOLD_M	m	The amount of water a construction must experience before it is considered impacted by water. Water must reach this height either on one of the cells the construction is on, or on one of the cells adjacent to it. This is used to compute the IMPACTED BUILDINGS result type.
MAX_SPEED_MS	m/s	Maximum speed at which water is allowed to flow. This effects the preservation of impulse in water, and as a result the length of computational timesteps. Impulse is more accurately preserved as the maximum speed increases, but will reduce the time per step of the calculation, increasing the total time for the calculation to complete.
MIN_SLOPE	ratio	The minimum slope required to account for the effect of gravity on the speed of the water. If the slope of the terrain is less than the minimum slope, the effect of gravity on the speed of the water is assumed to be 0. The ratio is the height over distance.
SEWER_OVERFLOW_THRESHOLD	fraction	How much of a sewer's storage must be filled with water before the sewer's overflows are allowed to overflow water.
SHOW_DURATION_FLOOD_LEVEL_M	m	The amount of water which must be present in a specific location before the duration of surface water can be recorded. This is used to compute the SURFACE_DURATION result type.
SUPERGRID		This attribute name is reserved for future functionality. Currently, this marks an experimental feature which is currently under development and may result in unexpected behavior when activated.
SURFACE_WATER_EVAPORATION_FACTOR	factor	The factor by which the weather's evaporation factor is multiplied to compute the amount of evaporation which takes place on the surface.
TIMEFRAMES	amount	The number of intermediate results recorded during the calculation. Each timeframe becomes a snapshot of data which can be viewed and analysed. The total simulation time is divided by this value, and at each interval of that period of time a snapshot of the results so far is made. Note that the first timeframe does not contain the starting conditions of the simulation, but the state of the simulation after the first period of time has passed.