In-situ geotechnical testing of surficial sediment layers in areas of active sediment dynamics can provide essential information about physical and geotechnical variations of sediment properties with regards to active sediment remobilization processes. For example, portable free fall penetrometers (PFFPs) can assist with the detection of mobile sediment layers. They are easy to deploy, and can provide a large spatial coverage in a time- and cost-effective manner. However, they often struggle to provide more detailed information about the properties of mobile sediment layers due to a lack of calibration and validation in existing data sets. Currently, existing sediment samplers often disturb, or ignore the uppermost sediment layers. Simultaneous sediment sampling and geotechnical profiling is needed to fill this gap, and to drive data interpretation forward. A field investigation of surficial sediments was conducted in the wetland waterways of coastal Louisiana in 2014. In-situ tests were conducted using PFFP, and disturbed sediment samples were collected in selected locations. The results allowed us to map changes in sediment strength and stratification, and correlate the geotechnical results to local site characteristics. However, the need for high quality sediment samples for calibration and validation was emphasized by the results. Three different sediment sampler add-on units targeting mobile layers were designed and manufactured based on lessons-learned from the literature. The designs were tested in the laboratory and in the field (Yakutat, Alaska and York River, Virginia) in 2017. The samples were analyzed to understand the influence of different sampler characteristics on collected sample quality, and, to define mobile layer sampler characteristics that enable simultaneous geotechnical testing and the collection of high quality samples. Following field survey campaigns in the York River, Virginia in 2016 allowed to assess surficial sediment layer characteristics and behavior based on a coupled analysis of geotechnical data from in-situ PFFP tests and the sedimentological data collected using box cores and the novel sediment sampler. In summary, novel strategies and instrumentation to carry out simultaneous sediment sampling and geotechnical profiling of seabed surface layers were tested, and new pathways for geotechnical data analysis for the investigation of mobile seabed layers were presented. / PHD / Coastal erosion and evolution, marine slope stability, river bank stability, maintenance of navigable water depth, or the stability of offshore structures are some of the modern challenges impacted by subaqueous sediment dynamics. Although, numerous researchers have investigated this issue for decades, some gaps in knowledge still prevail due to its interdisciplinary and complex nature. One of the most intriguing questions related to seabed soil behavior is the characterization of the sediment layers and textures at the seafloor surface being directly involved in sediment transport processes and local geomorphodynamics. These layers are often characterized by a most recent sediment deposition history, and a loose particle arrangements. Accordingly, these sediment layers show almost no resistance to accommodate loads (the sediment strength), and are highly erodible. The strength of surficial layers can be evaluated using portable free fall penetrometers (PFFPs) which are rapid and economic geotechnical site investigation tools designed to geomechanically test seabed surface layers. Nevertheless, there is a lack of data from areas of active sediment dynamics leading to gaps in understanding regarding sediment strength variations affected by active sediment transport processes. This research widens the use of PFFPs into wetland waterways (e.g. channels, lakes, and strait). Moreover, first attempts to quantify the influence of wave forces on sediment beds were also made and promising results were obtained which can open paths to new interdisciplinary. However, the PFFPs are challenged by a lack of physical sediment samples to groundtruth and verify the collected data. Thus, the sampling of such sediment layers is a currently missing part in the framework of in-situ investigations. This dissertation aimed to develop a novel field sampling technology in terms of an add-on unit that can be attached to portable dynamic penetrometers for deployment in areas of active sediment dynamics. Thus, the data to measure sediment strength can be collected simultaneously with physical seabed samples. Different sampler designs were tested and evaluated, and new pathways for joint geotechnical and sedimentological data analysis demonstrated. The results of this research can therefore contribute to the current understanding of seabed sediment behavior.