Aftermath of rebuilding low-head dams in river basins: hydraulic study of two rivers located in the northern and western hydrological regions of Estonia

Localized and distributed energy losses arising from the extreme flow conditions at the lowland rivers' barriers are hydraulically estimated. To perform a standard hydraulic analysis of rapidly varying flows at two Estonian rivers’ sites, specific-energy and momentum-equation functions for the channels with non-rectangular cross-sections are introduced. The quadratic-type channel hydraulic framework is used for the low-head dam downstream-side initial and sequent heights due to the formation of hydraulic jump, where two controls along the river course are required for the free-surface flow, changing between super- and sub-critical flow conditions. It is shown that the localized energy loss of pre-rebuilt weirs in both rivers’ sites is considerably larger than the post-rebuilt situations. It is likely that the non-vertical side banks and stoney bottom cause wavy forms on the free surface. Changes in water depth of the two rivers’ sites are related to the transformed overflow patterns of the modified hydraulic structures resulting from those pre- and post-rebuilt situations. Some flood-related aspects are addressed for planning the scope of river basin restoration within the hydraulic modelling of flows at the low-head dams and rapids, where rapidly varying flow process conditions are explained with extreme flow events in marine-estuarine rivers.