Mapping of tidal channels from digital elevation data (AHN) using topographic position index

The Geological Survey of the Netherlands aims at building a 3D geological property model of the upper 30 meters of the Dutch subsurface (‘GeoTOP model’). This 3D model provides a basis for answering subsurface related questions on, amongst others, groundwater extraction and infrastructural issues. Modelling is carried out per province using the digital core-database DINO (www.dinoloket.nl) containing several hundreds of thousands of core-descriptions and a context of geological maps that were created during the last few decades. An important component of the modelling is mapping Holocene fluvial and estuarine channel belts. Besides from published maps, detailed information on the position of the channel belts is extracted from the digital elevation model of the Netherlands (AHN). The AHN provides accurate information about the natural height differences in the shallow subsurface. Mapping of tidal channels is also revolutionised using the AHN. In the province of Zeeland the subsurface is formed by Holocene deposits of tidal channel, tidal flat and lagoonal sediments that alternate with peat beds and coastal shore face and dune deposits. The most upper part consists of deposits of the Walcheren member. The Walcheren member comprises shell rich tidal channels and flat deposits which were formed during catastrophic floods. Channels with a sandy fill of the Walcheren member are still visible in the subsurface today because of the compaction of the surrounding peat and clay. So far, mapping of channel belts using AHN have been restricted to manual digitizing. In the province of Zeeland, a semi-automatic method using the Topographic Position Index (TPI) has been developed to extract tidal channel belts in an efficient way. This method adds to the quality and consistence of the tidal channels of the Walcheren member.