Water Overlay

What is the Water overlay

The water overlay is an implementation of a large number of hydrological formulas which together can simulate the flow of water across large areas with a high level of detail. Its calculations form a simulation of an arbitrary amount of time, during which water is allowed to move.

Water flow is affected by properties of the surface across which it flows, including the terrain height and the properties of the underlying terrain.

To perform the calculations, the project area is divided into a grid of cells. Each cell has a specific quantity of water, and specific hydrological parameters based on the data in the project. The total time which should be simulated is divided into discrete timesteps. Per timestep, every cell communicates with all adjacent cells to exchange water. By dividing the project area and simulation time into sufficiently small cells and steps the behavior becomes effective continuous.

The final results of the calculation can be inspected, as well as snapshots of the hydrological situation in the simulation, known as timeframes.

Variants

The water overlay can be added to a project as one of a number of variants. Each variant has a number of parameters tuned to best fit certain use-cases. The following preconfigured variants exist:

• Rainfall Overlay, provides insight into the water stress caused by (excessive) rainfall.
• Flooding Overlay, provides insight into water stress caused by breaches in levees or other sources causing excessive water inflow.
• Groundwater Overlay, provides insight into long-term processes of water flow both on the surface and underground.

Use cases

Main article: Use cases Water Overlay

This section is a stub.

The water overlay is complex and versatile, and can configured for a large number of different detailed use cases. Due to the complexity of the water overlay, if an exact understanding of the functioning of the water overlay is not required or desired, it may be preferable to follow the instructions to complete one or more specific use cases, as found on the water overlay's use cases page.

How to use the Water overlay

In general, when a water overlay is added to a project it will immediately be capable of calculating results. However, these will be based on default settings and will at best give a broad sense of water stress.

To use any variant of the overlay properly, it is recommended that you ensure the project meets a number of prerequisites. Next, it is recommended to prepare all data related to the hydrological model, which defines the functioning and flow of the water. Included in this preparation is a clear idea of the climate conditions and what kind of output is desired. After these preparations have been made, creation and configuration of the overlay can begin. When the configuration is completed, a recalculation of the overlay will yield more accurate and appropriate results.

After the overlay has calculated results, a number of means exist to analyse the results of the calculation performed.

Prerequisites

When creating your project, make sure it meets the following criteria:

• Your project has been loaded in with a high-resolution DEM. This can be configured during the new project wizard.
• Your project is large enough to account for edge effects.

Additional tips for preparation and use

There are a number of additional points of attention when creating a project with the intent of using this overlay:

• When creating a new project in the new project wizard, consider using the AHN3 dataset rather than the default AHN2. Where coverage is available, the AHN3 dataset will be more accurate. Where coverage is not available, the default AHN2 should be used.
• Additionally, when creating a new project, consider whether you want to use the IMWA dataset for hydrological constructions. Although this dataset is not complete, some information about constructions which serve as culverts other relevant objects can be loaded in from this datasource. If more complete or accurate data is available to be loaded in into the project after it is created, it may be desirable to disregard this source so that hydrological constructions are not doubly included.
• Water flow can often be dictated by small features in an area, such as small openings between buildings, and thin levees. To have these small features included properly in the calculations, the grid cell size will need to be set to an appropriate size. The default setting offered by the Tygron Platform will often need to be adjusted to allow for smaller features to be recognized without having their presence averaged out with their surroundings.
• The water overlay performs a complete simulation, which is a series of complex calculations across multiple layers. Depending on the configuration of the overlay, the calculation time can range from seconds to hours. If the overlay is to be used in a setting where response times need to be short, it may be preferable to configure the overlay for greater speed rather than excessive precision.

Configuration

See: Water Overlay Configuration

Results

See results.

Model connections

To gain insight into how this Water Overlay ties together several models, see Model connections.

Data

The calculations performed by the water overlay are influenced by many kinds of geographical information present in the project area. For any given location, terrain, constructions and other features can influence either the initial state of the simulation or how water flows in a given area.

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. See Hydological features for a list of supported features.

Hydraulic stuctures

See Hydraulic structures for the list of supported structures.

Hydrological attributes of constructions

Besides the constructions which directly influence the hydrologic model as a hydraulic structure, each construction may also have attributes which can contribute to the hydrological model in some way. These attributes are stored and used on a grid cell level. This is different from the hydraulic structure attributes, which are stored and used on an object level.

For a full list, see Additional hydrological attributes of constructions.

Hydrological attributes of terrain

Terrains in a project have a number of hydrological attributes which can influence the flow of water in a project. Because there is always both surface and underground terrain defined for the entirety of the project area, all cells are affected by all attributes of terrains, unless a construction is present with overwriting values.

For a full list, see Terrain attributes for the water overlay.

Settings

The water overlay features a number of overall settings which can be configured for the hydrological calculations and its results. These settings do not have a geographical or temporal element to them, and are fixed values relevant for the complete simulation.

Result type

Main article: Result types

Keys

The water model makes use of data present in geographical features in the 3D world. This data is used as input for the various calculations which together model the water flow. The water model recognizes what attributes of geographical features form the parameters required for the calculation through key definitions. Each key is the name of an attribute which the water model should use to determine a specific parameter.

Keys are set to default attribute names, which apply to hydrological features, hydrological constructions, and hydrological properties of terrain. Keys can be changed so that imported data can be interpreted without renaming attributes, or to allow different configurations to come forth from the same dataset.

Attributes

Main article: Water Model attributes

Calculations

The water overlay performs a large number of calculations to form a complete hydrological simulation. Depending on the desired viewpoint, both the overarching concepts as well as the implemented formulas can be reviewed for detailed insight into how the water overlay works.

Models

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

• Surface model
• Underground model
• Rain model
• Evaporation model
• Sewer model
• Storage model
• Chemical flow model
• Border model

Formulas

The precise calculations which govern the water overlay's simulation are many and varied, and based as much as possible on available expert knowledge.

Model related Formula's:

• Timestep formula
• Calculation preference formula
• Surface water level formula
• Groundwater level formula
• Surface flow formula
• Underground flow formula
• Surface evaporation formula
• Underground evaporation formula
• Surface infiltration formula
• Underground infiltration formula

Hydraulic structure related Formula's:

• Culvert formula
• Weir formula
• Breach growth formula
• Breach flow formula
• Pump formula
• Sewer Overflow formula
• Inlet formula

Computational structure

This section is a stub.

The formulas and concepts come together in a single computational structure which is repeated a large number of times until the total simulation duration has been reached.

Order of operations

During the calculation, multiple facets have to be calculated. In each timestep, each aspect of the calculation has to be performed. Although as timesteps become smaller exact order of operation becomes less important, the order of operations can lead to specific behavioral details in some edge cases.

Calculations are performed in the following order:

• Horizontal surface flow and horizontal underground flow
• Rain
• Building storage
• Sewer inflow
• Surface evaporation
• Groundwater evaporation (saturated zone)
• Groundwater evaporation (unsaturated zone)
• Underground infiltration
• Surface infiltration
• Exfiltration
• Hydrological constructions (culverts, weirs, pumps, in- and outlets, outlets)
• Hydrological areas (sewer overflow, breach in- and outflow)
• Chemical movement, based on the water flow and infiltration which has occurred

Warnings and recommendations

When the water overlay is used and calculations take place, there are some problems or points of attention the calculation can run into. Where possible, the water overlay will show appropriate warnings when running into any issues.

Configuration wizard warnings

While configuring the water overlay using the configuration wizard, each type of data loaded in or found in the project must meet certain requirements to be functional. For example, configured water levels may deviate greatly from the mean terrain height in the project, or certain hydrological constructions may not be shaped appropriately or intersect with required features. In these cases, the configuration wizard will show the warnings in the steps related to the specific type of data.

Inaccurate terrain

When the project is created, the advanced options allow for selecting a high-resolution height map to be loaded in. The default, low resolution height map can introduce artifacts in the calculation due to inaccuracy. This issue can currently only be resolved by reloading the project area.

Calculation halted

If the overlay is recalculated, but the (re)calculation is halted, the water overlay will not contain meaningful results. A warning will be displayed indicating that the calculation did not complete.

Large cell size

The water overlay performs its calculations based on a discretization of the project area. This means both that areas of water are considered a single block, and that obstacles and hydrological properties are averaged out over the extent of a cell. To best approach a realistic, continuous water flow and a realistic model of obstacles and values, it is sufficient to reduce the size of the cells the calculation uses.

Limited cycles

This section is a stub.

Calculation time impacts

This section is a stub.