Flume experiments on macro-roughness elements: Hydraulic response and potential influences on flood risk and habitat diversity

Abstract

Macro-roughness elements (MREs) are widely used in river restoration to enhance flow heterogeneity and habitat diversity, but their impact on flow capacity and associated backwater rise (i.e., upstream increase in water level) remains insufficiently quantified. This limits the ability to balance ecological benefits against flood risk in river restoration applications. To address this gap, laboratory experiments in a fixed bed flume were conducted to investigate three simplified designs of MREs, namely, a boulder structure, an engineered logjam and a rootwad composite. Flow velocity, turbulent kinetic energy and backwater rise were measured under subcritical and supercritical flow conditions, for varying structure dimensions, solid volume fractions and configurations, including channel-spanning and partial-spanning setups (i.e., structures leaving a lateral gap with the channel wall). Results showed that backwater rise scales with Froude number and relative width of the structure, while it is insensitive to structure length and submergence depth. Backwater rise can be predicted by extending an existing semi-analytical model based on flow partitioning through and around the structures. Differences among MRE types had limited influence on backwater but strongly affected local flow patterns. In particular, porous structures with complex internal geometry (e.g., rootwads) enhanced flow diversion and gap velocities, despite their low solid volume fraction, whereas boulder arrays generated jets in the wake and higher turbulent kinetic energy. Flow heterogeneity increased under emergent and partial-spanning configurations, while higher Froude numbers and submerged conditions promoted more uniform flow. These findings provide guidance for selecting and configuring MREs across a range of flow conditions to optimize ecological benefits while minimizing flood hazards.