Application guide: Local water stress test for extreme rainfall

This Application Guide describes how the Tygron Platform can be used to perform a local water stress test for extreme rainfall. It focuses on detailed analyses of neighbourhoods, urban districts, redevelopment areas and new developments.

The guide is intended for study areas in which local terrain, buildings, roads, sewer systems, surface water and water-storage measures can be represented in sufficient detail. It is not intended as a workflow for regional or upper-regional stress tests, where rainfall patterns, catchment-scale processes and the interaction between multiple water systems require a different modelling approach.

This guide bridges policy questions and Tygron functionality. It does not replace the technical documentation for the Rainfall Overlay, Water Overlay or Water Module.

Purpose

The purpose of this guide is to support users who want to answer the following question:

Where does waterlogging occur during extreme rainfall, and which areas, objects or routes are vulnerable?

Typical follow-up questions are:

Which streets or Neighborhoods are most vulnerable to waterlogging?
Which buildings are affected at a chosen water-depth threshold?
Which roads become difficult or unsafe to use?
Which vulnerable functions, such as schools, healthcare facilities or public buildings, are affected?
Which measures reduce waterlogging most effectively?
How do current and future design scenarios compare?
What data is needed for an extreme rainfall analysis?
How can results be prepared for a risk dialogue or decision-making process?
When is specialist hydraulic, sewer or groundwater software needed?

Scope: local and regional stress tests

This Application Guide describes a workflow for local water stress tests for extreme rainfall, such as analyses of neighbourhoods, urban districts, redevelopment areas and new developments.

Regional and upper-regional stress tests are related applications, but require a different workflow. These analyses cover larger and often interconnected water systems and may require spatially varying rainfall, catchment-scale runoff, interactions between urban and rural areas, and the combined functioning of regional surface-water systems.

The assumptions, rainfall scenarios and modelling choices described in this guide should therefore not automatically be applied to regional or upper-regional stress tests.

Target audience

This guide is intended for:

policy officers for climate adaptation;
municipalities;
water authorities;
provinces;
consultants;
GIS specialists;
urban planners;
hydrologists and modelers;
project leaders for public space;
stakeholders preparing risk dialogues or climate adaptation plans.

When to use Tygron

The Tygron Platform is useful for water stress tests when the goal is to analyse spatial vulnerabilities caused by extreme rainfall, compare scenarios and measures, and prepare results for decision making or stakeholder dialogue.

The Rainfall Overlay is the main Tygron component for this application. It is a Water Overlay connected to the Water Module. Users can start from the Demo Rainfall Project, follow How to work with the Demo Rainfall Project, or configure their own model using How to manually configure a Water Overlay and How to configure the Water Overlays.

Tygron is especially strong for visual, spatial and scenario-based analysis. It helps users understand where water accumulates, which areas are vulnerable, and how measures affect the situation.

Tygron is less suitable as a full replacement for specialist hydraulic, sewer or groundwater modelling software when detailed calibration, design calculations or formal engineering verification are required. In many projects, Tygron works best alongside specialist software.

Suitable use cases

This Application Guide is suitable for:

neighbourhood-level and district-level analyses of waterlogging;
water stress tests for existing urban areas;
redevelopment and new-development projects;
detailed analyses of water depth, duration and flow patterns;
assessing the impact of extreme rainfall on buildings, roads and vulnerable functions;
analysing the interaction between terrain, sewer systems, surface water and local storage measures;
comparing local rainfall scenarios and adaptation measures.

Less suitable use cases

Tygron is not suitable when the question mainly concerns:

multi-layered groundwater modelling;
hydraulic structure design;
static reporting without spatial analysis;

Example projects

The examples below show how the Tygron Platform has been used for local water stress tests in existing urban areas, redevelopment projects and new developments. The external project pages provide additional information about the project context, the analysis and the organisations involved.

Kronenburg-Uilenstede, Amstelveen

For the redevelopment of Kronenburg-Uilenstede, the proposed urban design was analysed in Tygron to assess the local water system and the effects of extreme rainfall. The model was also used to test design choices and adaptation measures, including floor levels, water storage and the routing of excess surface water.

Read the project description on the TAUW website: https://www.tauw.nl/projecten/watersysteemanalyse-kronenburg-uilenstede-amstelveen.html?utm_source=chatgpt.com

Stationsgebied Hilversum

For the redevelopment of the station area in Hilversum, Buro Regen&Water used Tygron to simulate rainfall events of 20 mm and 70 mm per hour. The analysis identified locations where waterlogging could occur and assessed the consequences for infrastructure, accessibility and the future spatial design.

Read the project description on the Buro Regen&Water website: https://www.buro-regen-en-water.nl/projecten/project-stationsgebiedhilversum

Ibisplantsoen, Amsterdam

For the redevelopment of the public space around Ibisplantsoen in Amsterdam, the preliminary design was assessed against a water-storage requirement of 70 mm of rainfall. QGIS and Tygron were used to simulate extreme rainfall and to test measures such as above-ground and underground storage, rain gardens and shell-based water buffers.

Read the project description on the Buro Regen&Water website: https://www.buro-regen-en-water.nl/projecten/project-IbisplantsoenTO

Vogeldorp, Amsterdam

For the existing residential neighbourhood of Vogeldorp in Amsterdam, Tygron simulations and AHN4 elevation data were used to create a map of rainfall-related bottlenecks for 276 homes. The study examined the sewer system, surface runoff and locations in the public space where waterlogging could occur.

Read the project description on the Buro Regen&Water website: https://buro-regen-en-water.nl/projecten/project-vogeldorp-modellering-openbare-ruimte.html

Schilderswijk and Noordpolder, Terneuzen

For the existing residential area of Schilderswijk and Noordpolder in Terneuzen, BOOT assessed a green-blue design in a Tygron digital twin according to the Zeeland climate label. Based on the simulation results, measures such as surface drainage channels towards water-storage facilities were added to the design. The plan is intended to prevent water from surrounding areas from flowing into the low-lying neighbourhood.

Read the project description on the BOOT website: https://www.buroboot.nl/project/klimaatrobuust-maken-van-de-schilderswijk-in-terneuzen/

De Nieuwe Stad, Amersfoort

For the transformation of De Nieuwe Stad in Amersfoort, BOOT created a water-on-street map for extreme rainfall. Tygron was used to compare the current and future situation and to assess measures including green roofs, infiltration crates, green strips and water-storage buffers.

Read the project description on the BOOT website: https://www.buroboot.nl/project/klimaatrobuuste-ontwikkeling-van-de-nieuwe-stad/

Analysis framework

A water stress test for extreme rainfall can contain several sub-analyses. These sub-analyses should not be treated as completely separate workflows when they are actually extensions of the same underlying analysis.

For this Application Guide, the recommended structure is to work with a limited set of main analyses and describe possible extensions underneath each main analysis. This prevents overlap between related topics such as impacted buildings, vulnerable functions, damage indication and critical infrastructure.

The main analyses are:

Water depth and duration on the surface during extreme rainfall.
Accessibility of critical routes for vehicles, such as cars, and bikes, but also pedestrians
Impact on buildings.
The availability after a rainfall event of water storage measures, such as wadis and blue-green measures.

1. Water depth and duration on the surface during extreme rainfall.

Core question:

Where does water occur on the surface during extreme rainfall, how deep does it become, and how long does it remain there?

This is the central analysis for a water stress test. It shows where water accumulates, which locations are most exposed and how severe the calculated water depth is. From this analyses the other analyses can be derived:

The extensions include:

Maximum water depth during the simulation (result type Surface_max_value_result_type_(Water_Overlay));
The amount of time the water depth on the surface exceeds a given value Duration_min_level_m_(Water_Overlay) (result type Surface_duration_result_type_(Water_Overlay));
Water-depth during the rainfall event (result type Surface_last_value_result_type_(Water_Overlay)).

2. Accessibility and critical routes

Core question:

Which roads, routes or destinations become less accessible because of water on the surface?

This analysis combines water-depth and duration results with road data, critical routes or destination areas. It should include clear assumptions about which water-depth threshold is used for passability and for which road user this threshold applies.

3. Impact on buildings

Core question:

Which buildings, objects and functions are affected by water on the surface?

This analysis combines water-depth results with buildings, vulnerable functions and critical infrastructure. It is broader than a simple count of affected buildings. It can include regular buildings, critical buildings and vulnerable objects that may not be represented as standard buildings in the source data.

4. The availability after a rainfall event of water storage measures, such as wadis and blue-green measures

Core question:

How do wadis, storage facilities and blue-green measures function during and after the rainfall event?

This analysis focuses on the behaviour and reusability of local water-storage and infiltration facilities. It can be used both for existing facilities and for proposed climate adaptation measures.

Scenarios

A local water stress test should include multiple rainfall scenarios. The scenarios below are based on the Dutch DPRA standards for water stress tests and represent rainfall events with different intensities and durations.

The scenarios are used to identify where water accumulates, which buildings and roads may be affected and at which rainfall intensity parts of the local water system become limiting.

Rainfall scenario	Duration	Application in a local water stress test
70 mm	1 hour	Used to analyse waterlogging during a severe short-duration rainfall event, with a focus on surface runoff, sewer-surface interaction, buildings and accessibility.
90 mm	1 hour	Used to assess the same local system under a higher level of stress and to identify additional locations and functions that become affected.
155 mm	2 hours	Used to assess a very extreme and longer-lasting rainfall event, in which the interaction between surface runoff, sewerage and surface water becomes increasingly important.
200 mm	48 hours	Used to assess prolonged waterlogging under conditions in which the soil and surrounding surface-water system are also heavily loaded.

The DPRA provides the policy context, methodological background and further guidance for the use of these scenarios:

https://klimaatadaptatienederland.nl/stresstest/bijsluiter/bepalen-dreigingen/wateroverlast/

The detailed rainfall standards are described in:

https://klimaatadaptatienederland.nl/publish/pages/236276/geactualiseerde-notitie-standaarden-stresstest-wateroverlast-november-2024.pdf

Preferred data for detailed water stress analysis

This checklist can be used as a data intake specification when discussing data availability with a municipality, water authority or other data provider. It also helps connect commonly used Dutch data sources and project assumptions to the corresponding Tygron concepts.

With the setup of a project in Tygron there is always a datasource which can be used to do the analysis. This source is titled below as the Current starting point. There is also a preferred data source which gives a more detailed result of the analyses. There is also the issue of mapping the right attribute of the datasource to an attribute used by Tygron. For the default data this is already set up. But when a beter data sources is used the mapping is import. This attribute is given under the tile Required attributes

Elevation model

Why it matters: The elevation model strongly determines flow directions, local ponding, water depths, and the locations where waterlogging occurs. For most analyses, this is the most important data source.
Default Tygron data: The latest available AHN dataset.
Preferred data: Elevation measurements with a higher resolution than the AHN, preferably 0.5 m × 0.5 m or better.
Required attributes or values:
- An elevation value for each raster cell in m relative to datum.
File format: GeoTIFF.
- Tygron term: Elevation GeoTIFF.

Surface water level

Why it matters: The surface water level determines the available storage capacity in open water, as well as the transportation of water through the model.
Default Tygron data: By default, the water level is set to 0.
Preferred data: Water level areas provided by the relevant water authority, usually this is the waterboard.
Required attributes or values:
- The design water level WATER_LEVEL for each area in m relative to datum.
- To ensures that WATER_LEVEL is interpreted as an absolute elevation, the attribute WATER_RELATIVE is 0.
Preferred file format: Polygon geometry in GeoJSON or GeoPackage format.
- Tygron term: Water area (Water Overlay).

Culverts

Why it matters: culverts determine hydraulic connectivity under roads, embankments and other barriers between waterways. Missing or incorrect culverts can strongly affect local ponding and flow paths.
Default Tygron data: IMWA.
Preferred data: municipal and water authority dataset with culvert information. Depending on the waterboard the HyDamo dataset is in principal a better source.
Required attributes or values:
- Culvert location;
- Culvert diameter CULVERT_DIAMETER, or height CULVERT_RECTANGULAR_HEIGHT and width CULVERT_DIAMETER for rectangular culverts;
- bottom height of the culvert CULVERT_THRESHOLD;
- Manning value CULVERT_N.
Preferred file format: spatial dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Culvert (Water Overlay).

Weirs

Why it matters: weirs control overflow between water areas and can influence water levels, storage and discharge during and after rainfall.
Default Tygron data: IMWA.
Preferred data: Preferred data: municipal and water authority dataset with weir information. Depending on the waterboard the HyDamo dataset is in principal a better source.
Required attributes or values:
- weir location;
- overflow height in m relative to the applicable datum WEIR_HEIGHT;
- overflow width WEIR_WIDTH;
- weir orientation as a top down geo angle (0-360°) WEIR_ANGLE;
- weir coefficient WEIR_COEFFICIENT;
- weir shape exponent WEIR_N.
Preferred file format: spatial dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Weir (Water Overlay).

Sewer areas and sewer outflow

Why it matters: sewer storage determines how much rainfall can temporarily be stored below ground. Sewer outflow represents water that leaves the model through the sewer system. The storage in mm multiplied by the sewer area surface gives the available storage volume in m³.
Default Tygron data: none, but a sewer can be generated based on the default urbanization factor of a neighborhood.
Preferred data: municipal dataset with sewer area information. There is a national dataset called GWSW which is in development and it depends on the municipality how usauble this dataset is.
Required attributes or values:
- sewer storage for stormwater, in m, per sewer area SEWER_STORAGE;
- sewer outflow, in m³/s, from the sewer system SEWER_PUMP_SPEED;
- geometry of each sewer area.
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Sewer area (Water Overlay).

Sewer overflows

Why it matters: sewer overflows determine where stored sewer water can return to the surface or connect to the water system. This can be important during rainfall events.
Default Tygron data:: none.
Preferred data: municipal dataset with sewer overflow information.
Required attributes or values:
- location where stormwater overflows from the sewer system. The area cannot overlap with another sewer overflow area.;
- the geometry of each sewer overflow must be located within the associated sewer area.
- overflow height, in m relative to the applicable datum SEWER_OVERFLOW.;
- overflow threshold, in m relative to the applicable datum SEWER_OVERFLOW_THRESHOLD;
- maximum overflow discharge, in m³/s SEWER_OVERFLOW_SPEED.
Preferred file format: spatial dataset, preferably GeoJSON or GeoPackage.
Tygron term: Sewer overflow (Water Overlay).

Surface infiltration

Why it matters: surface infiltration determines how much rainfall can infiltrate into the ground before contributing to surface runoff or waterlogging.
Default Tygron data: Surface water can infiltrate into the unsaturated ground layer. It infiltrates at a speed defined by:
- by the paved surfaces. The default dataset used is the BGT and the BAG;
- the surface terrain's GROUND_INFILTRATION_MD attribute. The default dataset used is the BRO;
- the ground terrain's GROUND_INFILTRATION_MD attribute. The default dataset used the BRO soil map;
Preferred data: soil investigations with infiltration measurements.
Required attributes or values:
- permeability of paved surfaces, in m/day GROUND_INFILTRATION_MD.
- permeability of the ground cover, in m/day Terrain_ground_infiltration_md_(Water_Overlay);
- permeability of the soil, in m/day Terrain_ground_infiltration_md_(Water_Overlay);
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Surface infiltration.

Ground percolation

Why it matters: percolation influences how much water can move from the surface and unsaturated zone to saturated zone. The initial saturation assumption can strongly affect the available storage for infiltrating rainwater.
Default Tygron data: Infiltrated water can percolate through the unsaturated ground layer. It uses by default the setting Infiltration only (in the rainfall overlay the attribute GROUND_WATER is set to 2). Therefore the groundwater level does not need to be sed and is NOT used in the calculation. Instead the maximum amount of water that can infiltrate is used MAX_INFILTRATION_M with a default of 1 meter of water storage [m3/m2].
- It flows at a speed defined by he ground terrain's GROUND_INFILTRATION_MD attribute. The default dataset used the BRO soil map;
- The default saturation of the unsaturated zone is set to 0%.
- The total amount of infiltration is set and NOT derived from the groundwater level.
Preferred data: soil permeability and soil saturation of the unsaturated zone.
Required attributes or values:
- permeability of the soil, in m/day GROUND_INFILTRATION_MD.;
- degree of soil saturation as a fraction WATER_STORAGE_PERCENTAGE;
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Ground infiltration.

Water storage constructions

Why it matters: water storage constructions can temporarily store rainfall and reduce local waterlogging. They can be relevant for both existing infrastructure and future adaptation measures.
Current starting point: none.
Preferred data: dataset with water storage constructions, such as a building with roof for storing water.
Required attributes or values:
- location;
- storage capacity, in m³/m² WATER_STORAGE.
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
- Tygron term: Water storage m2 (Water Overlay).

Wadis

Why it matters: wadis can store and infiltrate water, but may also have overflow or drainage structures. Depending on whether the water must remain inside the project area or may leave the project, an overflow or intake can be represented using a pump, inlet or outlet-like modelling approach.
Default Tygron data: The latest available AHN dataset.
Preferred data: specific information about the wadi or the infiltration construction

Required attributes or values:
- Wadi:
  - wadi location;
  - height map of the wadi in m relative to datum;
  - the surface terrain infiltration in m/day GROUND_INFILTRATION_MD ;
  - the underground terrain infiltration in m/day GROUND_INFILTRATION_MD;
  - construction related to the wadi:
    - Culvert
      - Culvert location;
      - Culvert diameter CULVERT_DIAMETER, or height CULVERT_RECTANGULAR_HEIGHT and width CULVERT_DIAMETER for rectangular culverts;
      - bottom height of the culvert CULVERT_THRESHOLD;
      - Manning value CULVERT_N.
    - Drainage
      - drainage location;
      - The maximum pump flow capacity in m/sec DRAINAGE_Q;
      - The height of the drainage in m relative to datum DRAINAGE_DATUM;
      - The threshold of the water end-point of the drainage in m relative to datum DRAINAGE_OVERFLOW_THRESHOLD;
    - Outlet
      - Outlet location;
      - The maximum amount of water flowing into the model through this Outlet in m3/dag. A negative value means the building functions as an outlet, and water is removed from the hydrological model INLET_Q;
      - The maximum amount of water which can flow in or out through this Building in m3 INLET_CAPACITY;
      - If a lower threshold is set drain, 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 LOWER_THRESHOLD;
      - If an upper threshold is set, water will only flow out of the model through this inlet until the water level at the point of this Inlet is equal to or less than the threshold UPPER_THRESHOLD.
    - Pump
      - Pump location
      - The flow rate at which water is pumped from the lower end-point to the higher end-point in m3/sec PUMP_Q;
      - The threshold in m relative to datum of the lower end-point of the pump. Water flows from the lower end-point to the higher end-point until the water level at the lower end-point has reached the threshold. If not selected, the Lower Threshold will be ignored as a mechanic LOWER_THRESHOLD;
      - The threshold in m relative to datum of the higher end-point of the pump. Water flows from the lower end-point to the higher end-point until the water level at the higher end-point has reached the threshold. If not selected, the Upper Threshold will be ignored as a mechanic UPPER_THRESHOLD;
      - The top down orientation geo angle of the pump (0-360°) PUMP_ANGLE;
      - The maximum amount of water in m3 which can flow in or out through this Pump INLET_CAPACITY;
Preferred file format: spatial dataset, preferably GeoJSON or GeoPackage. For the height of the wadi a GeoTiff.
- Tygron terms: none

Buildings with a critical function

Why it matters: building data is needed to translate water-depth results into vulnerable functions and critical infrastructure impact.
Default Tygron data:

BAG. Critical functions may be identified using schools, healthcare facilities and other vulnerable functions. A generic threshold height may be assumed, for example 0.1 m above ground level.

Preferred data: building dataset with critical functions and threshold information.
Required attributes or values:
- building location;
- whether the building is critical infrastructure;
- building function or vulnerability category;
- threshold height for impact assessment.
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
Tygron terms:
- Critical infrastructure (Function Value)

Threshold for buildings to be impacted

Why it matters: A building is considered to be impacted by water when the surface water level adjacent to the building reaches a certain height.
Default Tygron data: The settings for the result type impacted buildings result are by default 0,1 m.
Preferred data: a building dataset with threshold information. For example:
- Shops = 0,001 m
- Buildings constructed before 2022 = 0.05 m
- Buildings constructed from 2022 onwards = 0,02 m
Required attributes or values:
- building location;
- building threshold height IMPACT_FLOOD_THRESHOLD_M;
Preferred file format: polygon dataset, preferably GeoJSON or GeoPackage.
Tygron terms:
- Impacted buildings

Priority roads and accessibility

Why it matters: road data and accessibility thresholds are needed to analyse blocked roads, critical routes, emergency access and reachability during extreme rainfall.
Default Tygron data: none.
Preferred data: dataset with road categories giving information on the critical roads.
Required attributes or values:
- road location;
- road category or priority class;
- water-depth threshold for accessibility.
Preferred file format: line or polygon dataset, preferably GeoJSON or GeoPackage.

Data quality notes

When using this checklist, the following data quality aspects should be checked explicitly:

whether all spatial datasets use the correct coordinate reference system;
whether polygons, lines and points have valid geometry;
whether areas, such as sewer and water level areas, do not overlap;
whether each sewer area has a maximum of one sewer overflow;
whether culverts, weirs, pumps and inlets have plausible orientation and height attributes;
whether all heights use the same vertical reference, for example m NAP;
whether units are in the same unit system;
whether default assumptions are clearly separated from measured or supplied data;
whether missing data has been replaced by assumptions, proxies or simplified representations;
whether local experts have reviewed the data before results are interpreted.

Hydrological initial conditions

The hydrological initial conditions describe the state of the water system at the start of the simulation. These conditions influence how much rainfall can initially be stored, infiltrated or discharged before water begins to accumulate on the surface.

For a local water stress test, the municipality and the water authority should jointly determine the initial conditions. The selected values should represent a plausible unfavourable situation and should be documented together with the model results.

The following initial conditions require an explicit choice:

the initial water levels in watercourses and other surface-water bodies;
the initial saturation and available storage of the unsaturated zone;
the initial groundwater level.

Initial surface-water levels

Initial water levels in watercourses are relevant for all rainfall scenarios.

The water level determines how much storage is available in the surface-water system and whether water from the urban area can be discharged into surrounding watercourses. A high initial water level reduces the available storage and may limit discharge from sewer outlets, overflows, drainage systems and surface-flow routes.

The initial water levels should be determined in consultation with the water authority. As a conservative starting point, use a relatively high but realistic water level:

for a controlled water-level area, use the summer level or another agreed high target level;
for an uncontrolled or freely draining area, use a water level associated with a high groundwater for example a level consistent with the Mean Highest Groundwater Level (GHG).

Unsaturated-zone storage

For the short-duration scenarios of 70 mm, 90 mm and 155 mm, the initial saturation of the unsaturated zone is not important in a local urban water stress test. These scenarios are dominated by high rainfall intensity, runoff over paved surfaces and interaction between the surface, sewer system and surface water.

The storage condition of the unsaturated zone should therefore be assessed explicitly for the 200 mm in 48 hours scenario. During this prolonged rainfall event, infiltration and the progressive filling of the soil can have a substantial effect on runoff and waterlogging.

For the 200 mm scenario, this Application Guide recommends the following default assumptions:

initial saturation of the unsaturated zone: 80%;
total thickness of the unsaturated zone: the difference between ground level and the initial groundwater level;

The geometrical thickness of the unsaturated zone is calculated as:

Unsaturated-zone thickness = ground level − initial groundwater level

This thickness is not itself the volume of water that can be stored. The actual water-storage capacity also depends on soil properties, particularly porosity and the relationship between water content and pressure head. When detailed soil parameters are available, these should be used to calculate the effective storage capacity.

Initial groundwater level

The groundwater level determines the thickness of the unsaturated zone and therefore influences the amount of water that can potentially be stored in the soil above the groundwater table.

Groundwater flow itself is not considered relevant for this analysis. The groundwater level therefore only needs to be represented as an initial boundary for calculating the thickness and available storage capacity of the unsaturated zone.

Required agreement and documentation

Before running the scenarios, the municipality and the water authority should agree on:

Initial condition	Applicable scenarios	Recommended starting point	Purpose in the analysis
Initial surface-water levels	70, 90, 155 and 200 mm	Use a relatively high but realistic water level, determined in consultation with the water authority. For controlled water-level areas, use the summer level or another agreed high target level. For uncontrolled or freely draining areas, use a water level associated with high groundwater conditions, for example a level consistent with the Mean Highest Groundwater Level (GHG).	Determines the available storage in watercourses and the ability of the urban area to discharge water to the surrounding surface-water system.
Initial saturation of the unsaturated zone	200 mm in 48 hours	Use an initial saturation of 80%.	Determines how much additional rainfall can be stored in the soil during the prolonged rainfall event.
Initial groundwater level	200 mm in 48 hours	Use a high but realistic groundwater level, preferably based on the GHG or another locally representative high groundwater level. Calculate the thickness of the unsaturated zone as ground level minus the initial groundwater level.	Defines the lower boundary and thickness of the unsaturated zone. Groundwater flow itself does not need to be modelled for this analysis.

The selected values should be recorded for every model run. This is essential because different initial water levels or groundwater conditions can produce different water depths and durations, even when the same rainfall event is applied.

More information about initial conditions in Dutch water stress tests is available in:

Updated memorandum on standards for water stress tests, November 2024:

https://klimaatadaptatienederland.nl/publish/pages/236276/geactualiseerde-notitie-standaarden-stresstest-wateroverlast-november-2024.pdf

Workflow

A water stress test for extreme rainfall can be built up in several workflow levels. The core workflow is the rainfall simulation itself. Additional workflows support data preparation, impact analysis, result processing and reusable templates.

Workflow

Workflow for analysing water depth and duration on the surface during extreme rainfall

This workflow describes how to configure and perform the analysis:

Where does water occur on the surface during extreme rainfall, how deep does it become, and how long does it remain there?

The workflow focuses on calculating and assessing water depth and water duration. Follow-up analyses, such as the impact on buildings, the accessibility of roads and the functioning of water-storage measures, are described in separate workflows.

The workflow consists of the following main steps:

Start the project.
Add and prepare the required data.
Create and configure the scenarios.
Calculate the scenarios.
Review the calculated water depth and duration.
Document and share the model and results.

The workflow is iterative. The first calculations should be used to check the data and model configuration. The calculation settings can then be refined step by step.

1. Start the project

Create a project in the Tygron Platform.

When creating the project, carefully select the project area. The project area should be large enough to include the relevant rainfall-runoff processes, flow routes and receiving water system. Water should not leave the project area through an artificial boundary close to the area of interest.

At the same time, an unnecessarily large project area increases the number of grid cells and can increase the calculation time.

Optionally, a previously prepared template can be used as the starting point for the project.

Relevant How-to's

3. Create and configure the scenarios

Create the scenarios that will be calculated for the water-depth and water-duration analysis.

A scenario represents a complete calculation situation. It includes:

the rainfall event;
the simulation period;
the initial hydrological conditions;
the active data and measures;
the configuration of the Rainfall Overlay;
the result types and timeframes that will be stored.

A rainfall event is therefore one component of a scenario. The same rainfall event can be reused in multiple scenarios, for example to compare different initial conditions or spatial situations.

Where possible, use one common baseline configuration for comparable scenarios. Only change the settings or objects that are explicitly part of the scenario definition.

3.1 Add the Rainfall Overlay

Add a Rainfall Overlay to the project.

The Rainfall Overlay calculates the movement and storage of water during and after the rainfall event. It also contains the configuration of the weather event, simulation time, hydrological processes, timeframes and result types.

Relevant How-to's

3.2 Configure the rainfall event and simulation time

Create or select the rainfall event that will be used in the scenario.

Configure:

the total rainfall amount;
the duration of the rainfall;
the distribution of rainfall over time;
the dry period before or after the rainfall;
the total simulation time.

The simulation should continue after the rainfall has stopped when the duration of water on the surface is part of the analysis. The required period depends on the drainage characteristics of the study area and the purpose of the analysis.

Relevant How-to's

3.3 Configure the timeframes

Configure the timeframes at which intermediate calculation results are stored.

Each timeframe represents a snapshot of the model results at a specific moment during the simulation. These snapshots can be used to inspect the development of water depth and to determine how long a selected water-depth threshold is exceeded.

For an initial model check, use a limited and clearly distributed set of timeframes. This makes it easier to inspect the development of the rainfall event without producing an unnecessarily detailed series of intermediate results.

After the model configuration has been checked, additional or custom timeframes can be added around relevant moments, such as:

the start of the rainfall event;
the period of maximum rainfall intensity;
the end of the rainfall event;
the expected moment of maximum water depth;
one or more moments during the drainage period;
the end of the simulation.

The number of timeframes does not affect the calculation time of the Water Overlay. It does affect the number of intermediate results that can be viewed and processed.

By default, the first timeframe represents the state after the first calculation interval. Add a timeframe at time zero when the initial conditions must also be available as a result.

Relevant How-to's

3.11 Configure the result types

Configure the result types required to analyse water depth and duration.

At minimum, include the results required to determine:

the maximum water depth reached during the simulation;
the water depth at selected timeframes;
the duration for which a selected water-depth threshold is exceeded.

Additional result types can be temporarily enabled when they are needed to understand or check the model behaviour, such as surface flow or water balance results.

Avoid adding result types that are not required for the analysis. This keeps the project and the presentation of results understandable.

Relevant How-to's

4. Calculate the scenarios

Calculate one representative scenario before calculating the complete set of scenarios.

The first calculation is intended primarily to check:

whether the data is interpreted correctly;
whether the water system is connected correctly;
whether the rainfall event and simulation time are configured correctly;
whether water follows plausible flow paths;
whether the calculation completes successfully.

For the first calculation, it is recommended to use a grid cell size of approximately 5 m by 5 m. A relatively coarse grid makes it possible to identify major data and configuration errors without immediately using the calculation time required for a highly detailed model.

When the model calculates successfully and the main flow patterns appear plausible, reduce the grid cell size step by step. Recalculate and assess whether the additional spatial detail materially changes the results.

A smaller grid cell size results in more grid cells and generally increases the calculation time. The final grid cell size should therefore be detailed enough to represent the relevant terrain features and flow routes, without being unnecessarily fine.

The grid cell size is a shared project setting for grid calculations. Changing it affects all applicable grid overlays in the project.

Relevant How-to's

5. Review the calculated water depth and duration

Review the results of the representative scenario before calculating and assessing the complete set of scenarios.

Check at least:

where water first appears on the surface;
the direction in which water flows;
where water accumulates;
the maximum calculated water depths;
how the water depth develops during the rainfall event;
how long water remains after the rainfall has stopped;
whether important terrain barriers and passages are represented;
whether surface water, culverts, weirs and sewers respond plausibly;
whether unexpected results can be explained by the data or model configuration;
whether water enters or leaves the project through unintended project boundaries.

Compare results calculated with different grid cell sizes. A finer grid may reveal narrow flow routes, kerbs, embankments or local depressions that are not represented by a coarser grid. However, differences between grid sizes should be understood before the finest calculation is accepted as the final result.

Where possible, compare the results with observations, previous studies, historical rainfall events and local expert knowledge.

When errors or implausible behaviour are identified, correct the relevant data or configuration and repeat the calculation.

Core workflow: configure and run a Rainfall Overlay

Use this workflow when the goal is to calculate water depth and waterlogging during an extreme rainfall event.

Define the policy question and study area.
Choose the rainfall event and simulation duration.
Add a Rainfall Overlay.
Use the Rainfall Overlay tutorial and Water Overlay Wizard to configure the water system.
Check the elevation model, terrain roughness, water areas, sewer districts and hydraulic structures.
Set relevant simulation settings, timeframes and result type.
Run the calculation.
Inspect maximum water depth and other relevant result types.
Validate the results with known problem locations, expert judgement and system checks.
Export maps or use results in follow-up analyses.

Example workflow: start from the Demo Rainfall Project

Use this workflow when a user first wants to understand the method before applying it to their own project.

Open the Demo Rainfall Project.
Follow How to work with the Demo Rainfall Project.
Inspect the configured Rainfall Overlay.
Review the input data, such as water level areas, weirs, culverts and sewer areas.
Inspect the resulting water-depth maps.
Use the project as a learning reference before configuring a new project.

Data workflow: add or prepare external spatial data

Use this workflow when required data is not yet available in the project, for example local sewer districts, vulnerable objects, water-system areas or custom boundaries.

Check which data is missing for the analysis.
Prepare data as WFS, GeoJSON, GeoPackage or another supported geodata route.
Add the data to the project.
Map the imported data to the correct Tygron object type or attribute.
Check whether the data is spatially correct.
Use the data in the Rainfall Overlay, Combo Overlay or impact analysis.

Relevant implementation links:

Impact workflow: calculate affected buildings or vulnerable functions

Use this workflow when water-depth results need to be translated into impact.

Examples:

buildings affected by more than 0.10 m water depth;
public buildings affected by a lower threshold;
road segments affected by more than 0.10 m water depth;
vulnerable objects intersecting waterlogging zones;
neighbourhoods with a high fraction of built area affected by inundation.

Steps:

Select the relevant water-depth result.
Select the relevant object category, such as buildings, roads or vulnerable locations.
Define thresholds.
Combine water-depth results with object attributes.
Calculate statistics or classify objects.
Check map output against statistics.
Export results for reporting.

Relevant implementation links:

Result-processing workflow: combine multiple calculations into one result

Use this workflow when results from several overlays, scenarios, areas or timeframes must be combined.

Examples:

combining multiple limited-area calculations;
accumulating results across timeframes;
combining water depth with building vulnerability;
preparing one output map for reporting;
creating a reusable result layer for a template.

Relevant implementation links:

Advanced workflow: connect overlays using prequels

Use this workflow when the result of one overlay needs to be used as input for another overlay.

Examples:

using a processed result as input for a Combo Overlay;
chaining rainfall results into a follow-up analysis;
building reusable calculation chains for templates.

Relevant implementation links:

This is an advanced reusable workflow, not a first-step workflow for new users.

Practical How-to routes

The following How-to pages can be used to translate common user questions into practical Tygron workflows. They are grouped by the type of task a user is trying to perform.