Heavy Rain Hazard Map: The Foundation of Flood Prevention

In short. A heavy rain hazard map shows, for defined rain scenarios, where water flows off at the surface, how deep it stands and how fast it flows. It is created from a hydraulic simulation based on a precise terrain model. It is the data foundation for all further prevention work, without it, hazard, exposure and measures cannot be seriously assessed. It is not protection in itself, but the prerequisite for it.

Why this matters for your municipality. During heavy rain, water does not flow where one assumes, but where the terrain directs it. A street becomes a stream, an underpass becomes a basin. The hazard map makes these flow paths visible before the event occurs. For you, this means you can base land-use planning, property protection and emergency planning on facts instead of experience.

What exactly does a heavy rain hazard map show?

The map shows, for one or more scenarios, the flood depth and in some cases the flow velocity. Three graduated scenarios are common: a rare event (roughly a 30-year recurrence interval), an exceptional event (roughly a 100-year recurrence interval), and an extreme event that far exceeds any sewer design capacity. Even the rare event already lies well above the sewer network's design capacity. Depending on the scenario, the affected areas shift considerably. Important: the map shows scenarios, not the next actual event. It is a planning tool, not a weather forecast.

How is the map technically created?

The basis is a high-resolution digital terrain model, usually derived from laser scanning data (LiDAR). A hydraulic model then simulates how a defined rainfall amount flows off at the surface, where it collects and continues along streets. Good models take kerbs, culverts, buildings and, in some cases, sewer capacity (coupled 1D/2D models) into account. Quality depends on the input data: a coarse terrain model produces a coarse map.

What do you do with the map in practice?

The map is the starting point for several processes. It feeds heavy rain risk management by overlaying flow paths with affected assets. It feeds into water-sensitive urban development, so that new development areas keep flow paths clear. And it serves the fire service as an operational basis. Communicating with residents is also valuable: anyone who knows their property lies on a flow path can take their own precautions.

The honest limit. A hazard map models, it does not measure. It is based on assumptions about soil permeability, sewer filling and blockage that only approximate reality. A blocked culvert or particularly dry soil shifts the actual picture. The map also becomes outdated: every new development changes flow paths. And the map protects no one on its own.

When KomAdapt is worthwhile. You commission a hazard map from a specialist engineering firm. The work begins afterwards: overlaying flow paths with asset data, assessing exposure, deriving and documenting measures. KomAdapt takes the map as an input and brings exposure analysis, prioritisation and reporting together in one model, instead of maintaining the overlay by hand in changing GIS exports.

_Who produces a heavy rain hazard map?_ Usually specialist water management engineering firms. The municipality supplies base data and defines the scenarios. Some states provide state-wide base data. _What does a heavy rain hazard map cost?_ Depending on area and level of detail; for smaller municipalities, often in the mid five-figure range. Part of this is fundable through the federal DAS funding guideline and state programmes; inter-municipal procurement reduces costs (last reviewed: August 2026). _Are heavy rain hazard maps available free of charge from the state?_ This varies by federal state. Some states provide state-wide indicative maps, sometimes coarser than a municipal detailed map. Check what your state agency offers. _How does it differ from the flood hazard map?_ Flood hazard maps under the EU Floods Directive relate to overflowing bodies of water and are legally required. Heavy rain hazard maps address surface runoff away from bodies of water and are not yet mandatory nationwide. _How often should the map be updated?_ Because every instance of sealing shifts flow paths, every five to ten years, sooner for larger construction projects or after an actual event.

Inbound links should be set from: heavy-rain-urban-water, heavy-rain-risk-management, climate-risk-assessment-municipalities.