Development and field evaluation of a biphasic portable streamer trap (BPST) for phase-resolved measurement of bedload and suspended load on the Southern Caspian coast

Introduction
Coastal sediment transport, particularly longshore sediment transport (LST), governs shoreline evolution and port sedimentation. Accurate prediction requires separate quantification of bedload and suspended load—a capability absent in traditional single-chamber streamer traps, which merge both phases into one sample. This technological gap prevents independent calibration of two-phase numerical models, introducing significant uncertainty into coastal management decisions. This study addresses the gap by developing and field-testing a novel Biphasic Portable Streamer Trap (BPST) equipped with an internal horizontal baffle that physically separates bedload from suspended load at the point of capture. The BPST provides, for the first time, phase-resolved field data essential for advancing sediment transport modeling and enabling evidence-based Integrated Coastal Zone Management (ICZM).
Materials and Methods
The BPST was developed by modifying the full-depth streamer trap design with a stainless-steel horizontal baffle installed 150 mm above the base plate, creating independent lower (bedload) and upper (suspended load) compartments. The 150-mm height was determined from bedload layer thickness theory. A 100-µm polyester mesh covers the frame. An extended-handle configuration with a boat-based mooring system was developed for deployments deeper than 1.0 m. Field tests were conducted over one year (December 2022–December 2023) on the sandy southern Caspian coast near Nowshahr Port, Iran. The bimodal wave climate comprises calm conditions (Hs < 0.5 m, 85% of the year) and storm events (Hs > 0.5 m, 15%). Twenty tests were performed at depths of 0.3–1.3 m. Samples from both compartments were separately collected, desalinated, oven-dried, and weighed to 0.1 g precision. Wave, current, and wind data were obtained from a nearshore ADCP and ERA5 reanalysis.
Results and Discussion
The BPST successfully separated the two transport phases across all 20 field tests. The mean total captured sediment was 1665 g per deployment, comprising 1543 g bedload (92.7%) and 122 g suspended load (7.3%). Decisive proof of selective performance was the simultaneous recording of substantial bedload (up to 4890 g) with zero suspended load during calm conditions, confirming complete physical separation. During storm events, suspended load increased significantly, reaching up to 560 g, demonstrating that high-energy waves drive sediment suspension. The extended-handle configuration performed stably in water depths up to 1.3 m, overcoming traditional depth limitations. Analysis of phase-resolved data against synchronous wave measurements confirmed that suspended sediment mobilization occurs primarily during storms, while bedload dominates the annual transport budget. These phase-resolved datasets provide the missing empirical foundation for independently calibrating bedload and suspended load components in two-phase sediment transport models.
Conclusion
This study successfully developed and validated the Biphasic Portable Streamer Trap as a reliable instrument for phase-resolved measurement of coastal sediment transport. The internal baffle design proved effective in physically separating bedload and suspended load, verified by the selective capture of bedload during calm conditions. On the studied coast, bedload accounts for approximately 93% of total annual LST, with suspended load mobilized primarily during storm events. The BPST bridges the critical gap between two-phase numerical models and field observations by delivering the first phase-resolved datasets for this environment. Minor limitations include the fixed 150-mm baffle height, which may be exceeded by saltating grains in extreme plunging breakers. The BPST offers a robust, cost-effective monitoring platform for Iranian coastal waters and supports evidence-based ICZM. Future work should deploy BPST arrays across broader spatiotemporal scales to construct regional phase-resolved sediment transport atlases and systematically integrate these data into numerical model calibration routines.