DEM for flood modeling

River runoff and flood models nowadays are based on digital elevation models (DEM). At Zesmill, we create DEMs in one of four ways:

• using aerial imaging and digital photogrammetrical technology
• from LIDAR cloud points
• with synthetic aperture radar (SAR) satellite data
• with stereo pairs of very high resolution satellite images.

Digital aerial imaging and photogrammetric processing is the most labor-intensive method, but it provides DEMs the most suitable for hydrologic modeling. LIDAR survey is the most high-cost approach; it is useful to create very detail DEMs for flood modeling in the urban and industrial areas. Both satellite methods are employed in the largest river basin models encompassing dozens of thousands of square kilometers.

LIDAR digital terrain models, in a nutshell, contain many man-made and surplus objects like buildings, roads, bridges and overpasses along with the vegetation. On the contrary, satellite-derived DEMs do not represent the surface topography in all its complexity; some streams and watershed lines should be matched. 

Thus, to be used in hydrological models, DEMs of all types must be adjusted and validated. This process is known as hydro-enforcement, or hydrologic-enforcement of topographic DEMs. Without it, river flood model would be functionally dammed.

Zesmill creates hydro-enforcement digital elevation models which are processed in the following way:

• the whole of drainage network is depicted and validated
• non-existent barriers, or “digital dams” (bridges, culverts) typical for LIDAR 3D models, are removed
• area sensitive to flooding under extreme rainfalls (“blue spots”) are detected and represented in a separate GIS layer
• ponds and permanent wetlands are outlined and represented in a separate GIS layer
• topographic wetness index (TWI) or soil-topographic wetness index (STI) are calculated and mapped
• quarries, pits and karst sinkholes are excluded from DEMs

Every Zesmill’s hydrologic-enforcement DEM represent the drainage area much better than original DEM. All terrain surface models can be used for numerical and neural-network models used for modeling floods, flash floods and storm surges. The results of hydrologic modeling represent in the form of flooding maps. The flooding maps depict what areas will flood within a given values of water level.