Avulsion cycles and their stratigraphic signature on an experimental backwater‐controlled delta

Abstract

River deltas grow in large part through repeated cycles of lobe construction and channel avulsion. Understanding avulsion cycles is important for coastal restoration and ecology, land management, and flood hazard mitigation. Emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics; however, our knowledge of backwater‐controlled avulsion cycles is limited. Here we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges, shallow bed slopes, and subcritical flows. The experimental avulsion cycles consisted of an initial phase of avulsion setup, an avulsion trigger, selection of a new flow path, and abandonment of the parent channel. Avulsions were triggered during the largest floods (78% of avulsions) after the channel was filled by a fraction (0.3 ± 0.13) of its characteristic flow depth at the avulsion site, which occurred in the upstream part of the backwater zone. The new flow path following avulsion was consistently one of the shortest paths to the shoreline, and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel. Experimental synthetic stratigraphy indicates that bed thicknesses were maximum at the avulsion sites, consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks. Finally, we discuss the implications of our findings within the context of sustainable management of deltas, their stratigraphic record, and predicting avulsions on deltas.

Origin of a Preferential Avulsion Node on Lowland River Deltas

“…In their models, a wedge of sediment migrated downstream on a riverbed with an initially uniform slope, and eventually aggradation exceeded an imposed avulsion threshold. Thus, in contrast to experiments (Ganti et al, ) and earlier models (Chatanantavet et al, ), these studies suggest that backwater‐scaled avulsions can be produced in models with constant water discharge. However, these models invoked a potentially unrealistic riverbed of uniform slope as an initial condition and measured the potential for avulsion in terms of sediment accumulation thickness relative to the initial topography.…”

“…We used an avulsion criterion given by a critical thickness of aggradation, which we refer to as superelevation ( ∆η ): in which H c is the bankfull channel depth and H * is the avulsion threshold, a dimensionless number that is of order unity (Ganti et al, ; Jerolmack & Mohrig, ), which we set to H * = 0.5 consistent with field and experimental observations (Ganti et al, ; Mohrig et al, ). The critical superelevation ∆η may represent the local floodplain (or levee) elevation relative to the distant floodplain or inactive lobes, or the bed aggradation thickness since the last avulsion (Figure c; Ganti et al, ; Hajek & Wolinsky, ; Mohrig et al, ).…”

“…Chatanantavet et al () did not simulate avulsions and lobe switching, and the river mouth in their study was unrealistically fixed, which prevented riverbed aggradation due to progradation. Nonetheless, subsequent flume experiments that included progradation and natural lobe switching and reactivation supported their hypothesis by showing persistent avulsions about a preferential node at L A ∼ 0.5 L b , coinciding with a peak in channel‐bed aggradation for an experiment with variable flows (Ganti et al, ). In contrast, a comparable constant discharge experiment did not produce a persistent node, indicating that flow variability is the dominant mechanism to produce backwater‐scaled avulsions.…”

Modeling Deltaic Lobe‐Building Cycles and Channel Avulsions for the Yellow River Delta, China

“…High water discharge events (i.e., floods) cause a downstream acceleration of flow velocity by hydrodynamic drawdown, which erodes the channel bed near the river mouth (Lamb et al, ). The net effect of the two conditions is to produce a preferred region of net bed aggradation (Chadwick et al, ; Chatanantavet & Lamb, ; Chatanantavet et al, ; Ganti et al, , ), which raises river stage and, in time, superelevates the water surface above the floodplain. This produces a gravitational instability favoring an avulsion (Bryant et al, ; Edmonds et al, ; Mohrig et al, ; Slingerland & Smith, ; Smith et al, ).…”

“…This produces a gravitational instability favoring an avulsion (Bryant et al, ; Edmonds et al, ; Mohrig et al, ; Slingerland & Smith, ; Smith et al, ). Indeed, numerous studies have demonstrated that the avulsion length ( L A )—the distance from contemporaneous coastline to avulsion location—scales with the backwater length ( ; Chatanantavet et al, ; Ganti et al, , , ; Zheng et al, ; Jerolmack & Swenson, ).…”