Validation of Streaklines as Recorders of Synoptic Flow Direction in a Deltaic Setting

Abstract

Knowledge of the flow patterns within distributary systems is key for understanding deltaic hydro- and morpho-dynamics, yet synoptic measurements of flow fields remain virtually nonexistent. As a means of overcoming this problem, a small number of studies have used biogenic surface films as synoptic flow tracers, under the assumption that biofilm streaklines are tangent to the local flow direction. Here we rigorously test this assumption and show that, despite flow patterns that change severely in space and time (over a range >270°), streaklines are relatively accurate synoptic flow tracers for the Wax Lake Delta, in Louisiana. When the incoming discharge was greater than 2400 m3/s with stable or falling tides, the streakline-derived flow direction departed from near bed flow direction measurements of 22.8° (root mean square). When the discharge was greater than 2400 m3/s and the tides were rising greater than 0.03 m/hr, they were accurate within 28.0°. Under conditions of discharge less than 2400 m3/s and tidal change less than a positive 0.03 m/hr, they were accurate within 33.3°, while during low discharge and rising tides they were accurate within 58.9°. Accuracy varied with distance from the delta, with proximal sites having greater precision. Our results demonstrate that a streakline-derived flow direction can characterize the spatiotemporal variability in the flow directions, but that the accuracy is significantly influenced by the hydrodynamic conditions and location within the network.

Spatial Patterns of Deltaic Deposition/Erosion Revealed by Streaklines Extracted From Remotely‐Sensed Suspended Sediment Concentration

“…This is consistent with Cathcart et al. (2020), who found that under low river discharge, rising tide inhibited streaklines. Additionally, for the October 17 flight, diffuse regions of high SSC in island interiors could be a consequence of wave‐driven sediment resuspension.…”

“…Tidal cycle may play a role; the October 17 flight was during a period of rising tide, whereas the October 18 flight with visible streaklines took place near the end of a long period of falling tide (Figure S2 in Supporting Information S1). This is consistent with Cathcart et al (2020), who found that under low river discharge, rising tide inhibited streaklines. Additionally, for the October 17 flight, diffuse regions of high SSC in island interiors could be a consequence of wave-driven sediment resuspension.…”

Resilience of River Deltas in the Anthropocene

“…By interpreting the divergence of the flow direction field, the location of subaqueous channel tips can be estimated from streaklines (Shaw et al, ). The resolution produced by these new techniques is low compared to direct altimetry, and ideal hydrodynamic conditions are required (Cathcart et al, ). Even so, the shallow coastal zone on river deltas is vast and difficult to measure, so the systematic use of these techniques has the potential to significantly improve coastal DEMs.…”

Short-Term Ecogeomorphic Evolution of a Fluvial Delta from Hindcasting Intertidal Marsh-Top Elevations (HIME)

“…Due to its small size, relatively good historical record over the last several decades, and lack of substantial engineering [47], the WLD is an ideal place to study young delta dynamics at sub-decadal scales. The WLD has proven useful in previous geomorphological studies, such as on sediment diversion and marsh restoration potential [12,[51][52][53][54][55], (sub)aqueous sediment and channel dynamics and fluxes [56][57][58][59][60][61][62][63], river flood, hurricane, and cold front effects on marsh elevations [5,64,65], and delta sedimentation and lobe progradation [18,66]. The WLD has also yielded insights into delta hydrology, including on surface water residence and connectivity [67][68][69][70], surface water-groundwater interactions [31], and effects of river, floods, tides, wind, hurricanes, and cold fronts on water levels [4,5,31,50,64,65,71,72].…”