Threshold-based interpretation of morphological pressures in hydromorphological river assessment: a comparison of River-MImAS, SYRAH-CE and a landscape-hydrological model in the context of the EU Water Framework Directive

Abstract

Abstract: The paper provides a comparative analysis of the morphological component embedded in three hydromorphological assessment approaches for rivers—River-MImAS (United Kingdom), SYRAH-CE (France), and the landscape–hydrological model of the river basin geosystem by Yushchenko Y.S. (Ukraine)—within the shared framework of the EU Water Framework Directive (WFD) requirements for supporting hydromorphological quality elements and their role in achieving and maintaining the target ecological status of river water bodies. The aim is to compare how each method “translates” morphological alterations of the channel and floodplain into a formalized management judgement, with a specific focus on the choice of spatial assessment unit, the set of morphological indicators/descriptors, the way mapping/GIS support is organized, and the presence of threshold boundaries used for interpretation. The analysis shows that River-MImAS offers the most rigid formalization of cumulative engineering pressures through the concepts of system “capacity” and the share of capacity used, which enables its application as a threshold-based screening tool for project proposals on short reaches and supports upscaling conclusions to the water-body level. In contrast, SYRAH-CE is designed for national-scale GIS auditing and risk mapping, where segmentation of the river network into homogeneous sections is driven by natural controlling variables (stream order, slope, substrate, valley/floodplain attributes), and outputs are expressed as probabilistic categories of hydromorphological degradation risk. The landscape–hydrological approach, in turn, most explicitly preserves the morphodynamical meaning of disturbances through multi-temporal reconstruction and mapping of the young river landscape and homogeneous channel–floodplain reaches (ODRZ), supported by morphometric indicators and thresholds of significant change (including those related to anthropogenic channel incision and transformation of the multi-year channel-forming belt). Synthesizing the differences and “best-use domains” of these approaches substantiates the value of their combined use in basin management practice: risk screening to rank the network and select priority reaches, detailed landscape–morphological diagnostics to identify mechanisms and feasible restoration boundaries, and capacity/threshold checks to evaluate the acceptability of specific engineering scenarios and their cumulative effects.).