Assessing heavy rainfall during design: how BIM integrates flood retention checks

RM Rudolf Müller Events GmbH

A path becomes wider, a paved area larger, a swale smaller. Such changes are routine for landscape architects. For a flood retention assessment, however, they can mean measuring areas again and rerunning calculations. Using a real project in Dresden, Nic Züchner investigated what happens when design, heavy rainfall data and drainage calculations are brought together. After a design change, his BIM model shows whether the available retention capacity is still sufficient.

Images of flooded streets and waterlogged basements following extreme rainfall dominated the summer's news. Climate change is causing a significant increase in extreme weather events, challenging designers to make cities more resilient and sustainable. Yet while building and infrastructure construction are advancing their use of Building Information Modeling (BIM), landscape architecture is still far from fully integrated into digital processes. Only 15 per cent of landscape architecture practices in Germany use BIM regularly, compared with 29 per cent of architecture practices.

External spaces must be able to absorb, retain or safely direct water away. A flood retention assessment establishes whether rainwater from an intense event can be retained on a site without causing damage. In practice, this assessment is often separate from the design. Areas are laid out, changed and subdivided, while calculations are performed in spreadsheets, separate software or by specialist engineering firms. When the design changes, area measurements, surface types and catchments must be checked and transferred again. If the design and the data used for calculation fall out of step, a small change can quickly become a coordination problem.

This is where Nic Züchner's work begins. In his master's thesis at TU Dresden, the architect investigated how complex climate data from the German Weather Service (DWD) can be integrated directly into digital design to support precise assessments of rainwater management from an early stage. His focus is flood retention assessment in landscape architecture and how a BIM-based process can reliably and automatically update it as the design changes.

In 2026, buildingSMART Germany named the thesis 'BIM in Landscape Architecture' a BIM Champion in the 'Student and Trainee Work' category. The study does not present a finished standard workflow ready for widespread use. It does, however, demonstrate in concrete terms how flood retention assessment can become more closely integrated with design.

A small design change with a big impact

The problem begins with an everyday question: what happens when an area changes in the design? A path might become wider, a material might change or the boundary of a plaza might shift. On the drawing, this is a routine revision. For drainage design, it means more: a larger or more impermeable surface can generate more runoff. The areas used in the flood retention calculation therefore change.

There is also the interface between landscape architecture and water management to consider. Flood retention assessments are often assigned to specialist designers or engineering firms. Everyone involved must therefore work with the same areas, catchments and assumptions. Different design revisions, unclear boundaries or subsequent material changes can leave landscape architects and drainage engineers calculating with different areas.

Züchner's starting question was therefore: why manually extract data from a digital design, transfer it and recalculate it when the model could establish these relationships itself?

A real project reveals the weak point

Züchner based his investigation on a real reference project: the external spaces of a student residence at Strehlener Straße 20 in Dresden. STORCH.LANDSCHAFTSARCHITEKTUR had already designed the site of almost 5,200 square metres in Vectorworks. The design had reached service phase 3, when the initial concept becomes a coordinated, technically developed BIM design with reliable quantities, costs and discipline-specific models. A drainage design and a conventional flood retention assessment were also available.

These documents already exposed the data problem. Relevant areas had been recorded and assigned runoff coefficients for the original assessment, but there was no clear subdivision into catchments. This made it difficult to trace which areas actually drained into which drainage facilities. Parts of the area breakdown also no longer matched the site plan under review.

The example clearly shows what can happen when drawings and calculations do not share the same data: the current design and the figures used to assess it can drift apart.

Nic Züchner

Turning site areas into calculation inputs

Züchner transferred the project into a discipline-specific BIM model and added information to the relevant areas. An area was no longer simply an outline on a drawing. The model recorded its catchment, proposed surface type, size and applicable runoff coefficient.

Vectorworks reads the area directly from the geometry. Using the stored surface type, the system assigns the corresponding runoff coefficient and calculates the proportion of rainfall that becomes runoff. Each area can also be assigned to the channel, swale or infiltration trench into which it drains.

In conventional or less connected workflows, these values often end up in separate spreadsheets, drawing appendices or calculation sheets. In the experimental model, they are derived directly from the same geometry used for the design. Changing the area therefore also changes the inputs to the calculation.

Heavy rainfall becomes part of the model too

A flood retention assessment needs more than site areas alone. Designers also need to know which rainfall event to use for the location. Drainage design draws on the German Weather Service's regional heavy rainfall data. KOSTRA-DWD provides the required rainfall values for different locations, durations and statistical return periods.

Züchner first prepared these data in QGIS, an open-source geographic information system, then integrated them into Vectorworks with their geographic references. Preparation is time-consuming because different rainfall durations and datasets must be combined. Once complete, however, the required rainfall data are available in a structured form within the model.

The discipline-specific model thus connects three sets of information that often remain separate in practice: the current design, the properties of its surfaces and the heavy rainfall data relevant to the site. Repeated manual entry and transfers between software packages are not eliminated entirely, but are substantially reduced.

Change, update, recalculate

The practical difference becomes clear when the design changes. If a catchment grows or shrinks, the designer adjusts the geometry. If a paved area changes to a different surface type, the stored property is updated. The length or size of an infiltration system can also be adjusted.

The user then updates the associated calculation worksheet. The model recalculates dependent values, including effective runoff areas, governing rainfall values, drainage system dimensions and the required retention volume. The designer can then assess whether the system remains adequate or whether drainage, grading or landscape design needs to change.

The prototype has an important limitation: calculations do not yet run fully automatically in the background. The user must open the worksheet and actively update it. Even so, a substantial part of the previous workflow is removed. Area measurements no longer need to be read from the drawing and transferred to a separate calculation, and rainfall data already integrated into the model do not need to be entered again for every design option.

When the minimum size is not enough

The infiltration trench provides a particularly clear example. This is an underground storage system that collects rainwater and allows it to infiltrate into the ground. For one catchment, the model initially calculated a required minimum length of 7.28 metres, giving a storage volume of 29.80 cubic metres.

At first glance, the system might therefore have appeared adequately sized. However, Züchner's model also linked the infiltration trench to the above-ground retention space, where excess water can be stored temporarily without causing damage during an exceptional rainfall event. This showed that, at the initially calculated dimensions, the required retention volume would exceed the available capacity by 15 per cent.

Züchner therefore increased the trench length to 8.50 metres. For a heavy rainfall event with a statistical return period of 30 years, the model then calculated a required retention volume of 36.17 cubic metres. Around 38.80 cubic metres were actually available. The available retention capacity would be about 93 per cent used, but would be sufficient in this scenario.

The calculation answers a specific design question: does the combination of underground storage and above-ground open space provide enough capacity for the heavy rainfall event being assessed?

From a retention check to a tool for comparing options

This also changes the role of flood retention assessment. A conventional workflow often produces a calculated volume. According to the expert interviews conducted for the thesis, the potentially flooded area on level ground is often only estimated. Subsequent design changes can then require a new calculation.

Züchner's model connects the assessment more closely to the design. The site was divided into several catchments and assigned to different drainage systems: an infiltration trench, a swale and several channels. If the size or type of an area, or the dimensions of a drainage system, change, the associated values are adjusted when the calculation worksheet is updated.

Designers can therefore compare options without separating the design from the heavy rainfall assessment each time. What happens if a surface becomes more impermeable? How does retention capacity change if a swale is made smaller? Is a raised kerb enough to hold water temporarily on an area? These questions can be addressed earlier in the design process and linked to the model data.

This is the study's real value. It goes beyond digitising the flood retention assessment and brings it closer to the point at which design decisions are made.

Nic Züchner

Room tag1 / 7

Nic Züchner

Catchment areas2 / 7

Nic Züchner

IFC model view3 / 7

Nic Züchner

Flood-risk assessment data form4 / 7

buildingSMART Deutschland

BIM Champion 2026 award ceremony5 / 7

André Terpe, TU Dresden

Kurt Beyer Award 20266 / 7

Nic Züchner

Nic Züchner7 / 7

Why the study is not yet a production-ready workflow

The work remains a feasibility study. It cannot demonstrate that the BIM-based assessment produces exactly the same result as the existing conventional assessment because their starting conditions were too different. The original flood retention assessment considered only the infiltration trench. Its connected areas were not divided into the same clearly defined catchments that Züchner created for his investigation.

Züchner also asked the urban hydrology engineering firm ifs to check the new calculations. This external review confirmed the calculation approach in principle and supported its practical feasibility. Expert review nevertheless remains essential. A BIM model replaces neither water management expertise nor checks that the results are plausible.

The digital tools are not yet fully equipped for this use case either. To model the infiltration trench, Züchner used a straight-road tool because it made length, width and height available as readable values. For a swale, he had to create a custom solid, and some values were transferred manually.

Open data exchange also has limitations. IFC lacks standardised property sets for some information needed in flood retention assessments, so the study had to define custom property sets. During validation of the IFC export in Solibri, a very small infiltration rate was also rounded to zero. Such details determine whether information can be exchanged reliably between discipline-specific models, checking software and other project participants.

What the study means for landscape architecture

BIM is less established in landscape architecture than in building design. The thesis reports that 15 per cent of landscape architecture practices in Germany use BIM regularly, compared with 29 per cent of architecture practices. At the same time, pressure is growing to design external spaces for climate resilience and account for heavy rainfall risks earlier.

Züchner's work shows what a specific application could look like. It stays close to a real project, uses existing design software and highlights where software developers and standards need to improve. Widespread use would require dedicated tools for catchments, swales, infiltration trenches and retention volumes, alongside more consistent data structures for rainwater management and flood retention assessment.

The most important contribution is not a single tool, however. The study shows that climate adaptation need not be treated as a check added after design. When design, surface information and heavy rainfall data share a common basis, flood retention assessment can become part of the design process itself. Instead of only confirming at the end whether the calculations work, it can show which option is more resilient while the design is still evolving.