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Glossary

Direct Rainfall Modelling

DRM

A 2D hydraulic modelling technique that applies rainfall directly onto a terrain model to simulate surface water flood risk, capturing overland flow paths and ponding without relying on predefined catchment boundaries.

Direct rainfall modelling (DRM) is a hydraulic modelling approach where design rainfall is applied directly onto a high-resolution 2D terrain model, allowing the software to simulate how water flows across the ground surface, where it accumulates, and what depths and velocities result. Unlike traditional catchment-based approaches, DRM does not require pre-defined watercourse channels or catchment boundaries.

DRM is particularly valuable for assessing:

  • Surface water (pluvial) flood risk: The primary use case in the UK. DRM captures overland flow paths, ponding in topographic depressions, and interactions with buildings, walls, and other surface features.
  • Urban flood risk: In built-up areas where runoff interacts with roads, buildings, and below-ground drainage in complex ways
  • Combined fluvial-pluvial events: Where river flooding and surface water flooding occur simultaneously
  • Post-development impacts: Comparing pre-development and post-development scenarios to demonstrate that the proposed development does not increase flood risk

The modelling process involves:

  1. Building a high-resolution 2D terrain model from LiDAR data and site survey
  2. Incorporating building footprints, walls, and other obstructions as raised features
  3. Assigning surface roughness values by land use type
  4. Applying FEH design rainfall hyetographs for the required return periods
  5. Running the model with climate change allowances applied to rainfall intensity

TUFLOW with GPU acceleration is the industry standard for DRM in the UK, enabling rapid simulation of multiple storm events and durations. The Environment Agency and LLFAs accept DRM outputs as evidence of surface water flood risk.

Aegaea uses DRM extensively in our flood modelling and flood risk assessment work, particularly for urban sites and large-scale masterplan assessments.

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