Cascading consequences of structural interventions in a tropical wandering gravel‐bed river

Abstract

AbstractRiverscapes are often modified and managed through ‘command and control’ structural engineering approaches. These structures not only alter flow and sediment transport patterns but also negatively affect riverscape biodiversity. These impacts are particularly acute for rivers that are characterised by high mobility and wide flow distribution during extreme events. We used hydrodynamic and geomorphological change assessment to appraise the impacts of existing flood structures and their potential for removal in a tropical wandering gravel‐bed river in the Philippines. Our approach informs a systematic decision‐making process to screen opportunities for channel reconnection. Two‐dimensional hydraulic modelling showed flood structures confine flood water within the active channel for more frequent flood events, but banks are overtopped during larger magnitude events (>50‐year recurrence interval). Incorporating modelling results, a multi‐criteria analysis was developed to appraise opportunities for river reconnection based on structure purpose, functionality, and local land‐use. We differentiate among four actions for each structure: removal, redesign, set back, or retention. One structure is recommended for removal because it cuts off an anabranch, reduces channel capacity, and increases main anabranch bed shear stress. Increased potential for bed and bank erosion due to the structure's presence had a cascading effect, resulting in further interventions downstream and on the opposite bank. We show how the use of system‐scale two‐dimensional hydraulic modelling alongside high‐resolution topographic mapping can support management of interventions to reconnect a river to its wider anabranch‐bar morphology. Such actions form part of proactive, adaptive, and nature‐based flood management plans with local and non‐local benefits.