Rainfall Overlay

Rainfall overlay, also known as the inundation module

What is the Rainfall Overlay?

The Rainfall Overlay is an overlay that displays the hazard and impact of heavy rainfall over the project area. Based on the elevation model, terrain roughness and representations of the sewer and water systems, several result types can be displayed.

A Rainfall Overlay Tutorial is available to start working with this overlay.

Result types

The rainfall overlay is a grid overlay showing results of heavy rainfall on the surface (flooding), sub-surface (groundwater), open water and sewer system. The following results are produced:

Result type	Unit	Description
BASE_GROUNDWATER_DISTANCE	m (below surface)	Initial groundwater level relative to the surface level (NL: ontwateringsdiepte).
BASE_TYPES	nominal value	Division of gridcells in water, land or sewer areas that are connected to the sewer.
EVAPORATED	m (mm)¹	Total evaporation over the simulation period
IMPACTED_BUILDINGS	nominal value	Constructions impacted by flood: the value from the function value indicator is assigned to a building if the water depth at a grid-cell surrounding the building exceeds the overlay attribute value IMPACT_FLOOD_TRESHOLD_M
SEWER_LAST_VALUE	m (mm)¹	The amount of water in the sewer storage at the end of simulation
SEWER_MAX_VALUE	m (mm)¹	Maximum amount of water in the sewer storage during the simulation
SURFACE_DURATION	s (min)¹	The amount of time the water depth in a cell exceeds the value set in the overlay attribute value SHOW_DURATION_FLOOD_LEVEL_M
SURFACE_FLOW	m3/m2	Total volume of water passed a grid-cell, scaled by the cell surface (grid cell-size^2)
SURFACE_LAST_VALUE	m (min)¹	Water depth at the end of simulation
SURFACE_MAX_VALUE	m (min)¹	Maximum water depth during the simulation
UNDERGROUND_FLOW	m³/m²	Totall infiltration amount from the surface to groundwater
UNDERGROUND_LAST_VALUE	m (mm)	The amount by which the groundwater table has risen above the initial groundwater level at the end of simulation
UNDERGROUND_MAX_VALUE	m (mm)	The maximum amount by which the groundwater table has risen above the initial groundwater during the simulation
WATER_STRESS	m (mm)	The Maximum water depth during the simulation. At water cells only a water depth is shown if the depth exceeds the overlay attribute value ALLOWED_WATER_INCREASE_M

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

Rainfall Overlay Calculations

Main article: Rainfall Overlay Calculations (Water overlay)

General Tab

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) m&sub3;/s respectively

Grid cell size

The Tygron Platform computes flow equations over a rectangular cartesian grid, the grid cell size can be set in the General Tab of the Rainfall Overlay. Note: if you choose a smaller grid cell size, both the amount of time steps (see Calculation Preference) and amount of computational cells increase.

Export GeoTiff & Water Balance

You can export the current result-type as GeoTiff and a waterbalance as Microsoft Excel file.

Debug Info

General 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)
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)
Max Courant avg: the maximum courant numbers reached during the simulation

Water balance

The water balance is made up of the following components:

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).

Overlay Keys

Keys referring to area attributes

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
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

Keys referring to structures (building attributes)

Attribute	Unit	Description
Weir Height	m + datum	weir crest level
Weir Width	m	the width of the crest of a weir
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

Overlay Attributes

Attribute	Unit	Description
ALLOWED_WATER_INCREASE_M	m	increment by which the water level on a water cell is allowed to increase before the depth is shown in the water stress result type
IMPACT FLOOD_THRESHOLD_M	m	maximum waterdepth around a building where after the building is assumed impacted
MIN_SLOPE		slope whereunder the pressure slope is assumed 0
SEWER_OVERFLOW_THRESHOLD		fraction of storage whereafter sewer overflows take place
SIMULATION_TIME_HOURS	hours	simulation time