Flood extent maps (FEM) and flood hazard maps (FHM) serve as legal instruments for spatial planning, decision-making, strategic flood risk planning, and public awareness, supporting sustainable and safe land use along the river corridor. This study aims to improve existing FEM and create FHM for the Sanica river, prone to frequent flooding. The existing FEM were developed using a 1D HEC-RAS model under steady-flow conditions, applying a single uniform Manning roughness coefficient along the entire river reach. The study presents the first application of an unsteady 2D HEC-RAS model along Sanica river, integrating LiDAR-based topography and updated hydrological data to derive FEM and FEH for common return periods. The final 2D hydraulic model was selected through calibration of seven variants of the Manning roughness coefficient, three lumped and four distributed, with the optimal configuration identified based on three goodness-of-fit measures. The comparison of 1D and 2D FEM shows close agreement in morphologically confined canyon reaches, while significant differences occur in river sections with floodplain inundation and dominant 2D flow. These results indicate that model dimensionality can be selected based on reach-scale morphology, enabling the use of 1D models in canyon sections to reduce computational time, while applying 2D models only where complex flow dynamics are present within the Sanica river study area.

<p>Regional analysis is often used for flood quantile estimation in ungauged catchments. The regionalization procedure has two phases: the formation of homogeneous regions and flood quantile estimation. The presented research results consider the first phase of the regional analysis for 41 catchments in Serbia. The catchment similarity attributes are catchment area and catchment mean elevation. The number of formed regions and the number of stations within the regions are determined by maximising the mean silhouette width of the region. Regions were first obtained by cluster analysis and then adjusted to comprise catchments with a positive silhouette width. For the three formed regions, homogeneity was checked by the Gini index - GI.</p>

Flood quantile estimation in ungauged basins is often performed using regional analysis. A regionalization procedure consists of two phases: the definition of homogeneous regions among gauged basins, i.e., clusters of stations, and information transfer to the ungauged sites. Due to its simplicity and widespread use, a combination of hierarchical clustering by Ward’s algorithm and the index-flood method is applied in this research. While hierarchical clustering is very efficient, its shortcomings are the lack of flexibility in the definition of clusters/regions and the inability to transfer objects/stations from one cluster center to another. To overcome this, using silhouette width for induced clustering of stations in flood studies is proposed in this paper. A regionalization procedure is conducted on 53 gauging stations under a continental climate in the West Balkans. In the induced clustering, a negative silhouette width is used as an indicator for the relocation of station(s) to another cluster. The estimates of mean annual flood and 100-year flood quantiles assessed by the original and induced clustering are compared. A jackknife procedure is applied for mean annual flood estimation and 100-year flood quantiles. Both the Hosking–Wallis and Anderson–Darling bootstrap tests provide better results regarding the homogeneity of the defined regions for the induced clustering compared to the original one. The goodness-of-fit measures indicate improved clustering results by the proposed intervention, reflecting flood quantile estimation at the stations with significant overestimation by the original clustering.

Some usual hydrological methods and the hydrological model EBA4SUB are used in the paper to determine the design peak discharge for various return periods for catchments in Bosnia and Herzegovina. The aim of the paper is to test for the first time the EBA4SUB model in the selected catchments. The results obtained by the EBA4SUB model compare well with other related methods. The advantages of the model lie in the fact that it takes into account physical processes taking place in the catchment, influencing formation of surface runoff.