The impact of inter‐flood duration on non‐cohesive sediment bed stability. Issue 14 (28th August 2019)

• Record Type: Journal Article
• Authors: Ockelford, Annie; Woodcock, Stephen; Haynes, Heather
• Is Part Of: Earth surface processes and landforms. Volume 44:Issue 14(2019)
• Page Start: 2861
• Page End: 2871
• Publication Date: 2019-08-28
• Subjects: inter‐flood duration -- entrainment threshold -- bedload flux -- grain size distribution; Geomorphology -- Periodicals; 551.4
• DOI: 10.1002/esp.4713
• Languages: English
• ISSNs: 0197-9337

Abstract

Abstract: Limited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter‐flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter‐flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter‐flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D 50 (4.8 mm); near‐uniform ( σ g = 1.13), unimodal ( σ g = 1.63) and bimodal ( σ g = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed ( τ* c50 ). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics. Results show that (i) increasing inter‐flood duration from 0 to 960 minutes increases the average threshold shear stress of the D 50 by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter‐flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter‐flood duration in both critical shear stress and bedload transport rate is non‐linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd. Abstract : The impact of sub‐threshold flow duration on three sediment beds has been quantified. Results show that increasing antecedent duration increases the average threshold shear stress of the D 50 by up to 18% and decreases bedload flux by up to 90%. The rate of response of both variables is non‐linear and inversely proportional to antecedent duration and there is a grade dependent response where the uniform bed is up to twice as responsive to the graded beds.