Mapping of Floodplain Retention and Active Flow Area in 1D Models for Large and Regional-Scale Hydrodynamic Modeling

One-dimensional (1D) models based on the Saint-Venant set of equations (1D–SVE) are frequently used for large- and regional-scale hydrodynamic modeling due to their low implementation costs and small computation time compared with more advanced two-dimensional (2D) or three-dimensional (3D) models. A simplified description of the flow transformation, which is a weakness of the most commonly used methods to solve 1D-SVE, and the lack of accurate river topography can make it difficult to achieve reliable flow transformation results for a long section of a river. Mapping of floodplain retention and determining the active flow area are issues that significantly affect the results obtained from 1D numerical models. The methods described in this study attempt to reduce the retention mapping error and facilitate the determination of the active flow area in models based on 1D-SVE. On the basis of high-resolution digital elevation models (DEMs) a volume–water level relationship was introduced into the continuity equation. As a result, the error of water volume retention mapping in the applied hydrodynamic model based on 1D-SVE was decreased by 33% for the case study section of the Warta River in Poland. It was also shown that the proposed methodology for determining the active flow area in 1D models can be used for both wide natural floodplains and narrower embanked floodplains. It allows mapping the active flow area by introducing an additional model-specific parameter $C_T$ (slip velocity in Pasche’s method).