Abstract Intensifying flooding throughout the western United States threatens human infrastructure, human life, and ecological integrity. Flash floods are particularly dangerous because they rise quickly and often unexpectedly. Floodplains that are hydrologically connected to river channels can act as buffers to attenuate peak flows and slow flood movement, providing natural resilience against flood risks. Comparatively low‐gradient reaches (1%–3% slopes) with wide floodplains (beads) have been identified as important for attenuating floods, yet the degree to which they can reduce peak flows is not well constrained. Additionally, the relative importance of attenuation mechanisms have not been discerned. We quantified flood attenuation provided by beads by utilizing two‐dimensional hydrodynamic models simulating flash floods in three river beads located in the Colorado Rocky Mountains, United States. We quantified the (a) magnitude of attenuation, (b) total accessible floodplain volume, (c) volume of floodwater stored in floodplain depressions, (d) variability of flow path travel times, (e) floodplain heterogeneity, and (f) relative importance of these mechanisms in flood attenuation. We found unprecedently high discharge attenuation with an average 13.8% reduction in peak flow per kilometer reach length and continued attenuation up to the 100‐year recurrence interval flood. For the studied sites the strongest correlations were between attenuation and storage in floodplain depressions. Flow path diversity metrics correlated best with attenuation for floods with a time‐to‐peak greater than 1 hr. Our findings also indicated that maintenance of high floodplain roughness and accessibility may be an effective strategy for bolstering attenuation of flash floods in mountain systems.
Christensen et al. (Thu,) studied this question.
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