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
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.
- Main article: Use cases Water Overlay
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.
Configuration
See: Water Overlay Configuration
Results
See results.
Input Data and Settings
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.
Furthermore, 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 simulation as a whole.
- Hydological 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 structures
- 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.
- Model settings
- For a full list, see: Water Model attributes
Calculation Models
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.
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
Model connections
To gain insight into how this Water Overlay ties together several models, see Model connections.
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
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
Troubleshoot, 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. See Troubleshoot, warnings and recommendations;