This is a 10-week summer research experience for undergraduate students (junior or senior) to discover exciting research experiences related to coastal processes and sediment transport. Undergraduate students will work with Prof. Hsu and his research group at Center for Applied Coastal Research, Civil and Environmental Engineering, University of Delaware (UD). Research activities may include designing laboratory experiment, literature survey, numerical model simulation and data analysis/interpretation. Students will also have the chance to participate in the outreach programs during the summer at UD to introduce coastal sciences to local high school students. These summer outreach programs are organized by College of Engineering and College of Marine and Earth Studies.
Dr. Tian-Jian Hsu (Tom), Assistant Professor
For more details on an overview of coastal sediment, research activities and more web resources related to coastal sediment, please check the following information:
We are interested in studying the supply, transport and deposition of sediment in the coastal ocean, which includes estuaries, nearshore regime and continental shelf. For example, we consider non-cohesive sand transport processes that shape the beach and shoreline. In this case, water waves, breaking waves and wave-induced currents (e.g., undertow) are the major driving forces of sand transport. Often times, we need to study the fate of terrestrial sediment (river-borne sediment) in the coastal ocean that usually involves fine cohesive sediment transport processes (mud). In this case, we study sediment-laden river plume and initial sediment deposition at the river mouth (see Figure 1), the subsequent re-suspension processes by tide, wave and coastal currents, and the final deposition. In many conditions, it turns out the interactions between sand and mud need be explicitly considered!
Here are some examples of coastal sediment that are critical to our lives, our environments and the societal needs:
Shoreline evolution and coastal geomorphology
The coastal zone is of vital importance to our Nation’s economic, security, and quality of life. Making good use of the coastal environment while preserving the shoreline has become a challenge task. Shoreline can be eroded and accreted through longshore drift and in the long run, natural accretion and erosion processes are often in equilibrium (see Figure 2). However, human intervention, such as construction of a harbor or building a seawall, often destroys such equilibrium and causing beach erosion. Understand how sediments (mostly sand) are transported by nearshore waves and currents and how shoreline is shaped is therefore critical. A commonly used engineering approach to mitigate beach erosion is beach nourishment. Its success has been (and will be) greatly due to our improved understanding on the hydrodynamic and sediment transport processes in the nearshore environment.
Sandbars located at the nearshore surf zone provide a natural barrier to protect the shoreline from the wave impact (see Figure 3). Sandbar often moves onshore and offshore in respond to the nearshore wave condition and it affects shoreline evolution by contributing to part of the littoral drift. The nearshore wave condition is in direct response to the offshore wind field. Therefore, one often observes sandbar moving offshore (or completely washed out) during stormy weather where the offshore wave height is large, while sandbar slowly moves onshore (or re-appear) during calm weather (in-between two storms). In the past, the US Army Corps of Engineers field research facility at Duck, NC has been used as a natural laboratory to study surf zone processes and nearshore sandbar migration.
Seabed properties in determining surface wave propagation and acoustic wave transmission
Seabed properties can change the characteristics of surface wave propagation through dissipating surface wave energy due to bottom friction. Such bottom friction is often considered as some sort of equivalent “roughness”. Additionally, these seabed characteristics or “roughness” can further alter underwater sound wave propagation when acoustic waves approach and interact with the seabed. Seabed properties are primarily determined by bedforms and mud contend (sand-mud ratio), which are highly dynamic due to sediment transport processes.
When seabed are covered by ripples or bedforms, bottom friction felt by the surface waves become much larger than that during flat bed condition. In addition, the presence of bedforms can also significantly change the scattering and transmission loss of the acoustic sound wave. If one wishes to use sound wave (e.g., sonar) to detect underwater objects, information on these seabed properties is crucial. The challenge here is that the ripple dimension, or whether ripple shall present or not are part of the complicated sediment transport processes driven by surface waves and currents. For example, when wave motions are very weak, there is no sediment transport. Then, as wave motion becomes moderate, bedforms appear and migrate. However, as wave motion becomes rather energetic, such as during storm, bedforms can be completely washed out. Therefore, bedforms are highly dynamic seabed feature and are difficult to predict. Scientists are using sophisticated sensors to monitor bedform evolution and sediment suspension in the continental shelf to understand bedform dynamics (Figure 4). If a ship sends out sound waves to detect underwater object, it is important that the sonar operator knows if there are bedforms present at the seabed and their averaged height and length. Hence, one of the major tasks in sediment transport is to study and predict bedform dynamics.
Similarly, seabed properties can be very different if noticeable amount of mud presents. Unlike sandy seabed, muddy seabed sometimes can be very soft (see Figure 5). One can imagine that soft mud behaves like a very sticky fluid, such as honey. When surface wave propagates over muddy seabed, the wave height can be significantly damped over a very short distance. Moreover, it is not difficult to imagine if sound wave approaches the soft muddy seabed, it may behave very differently as compared to sandy seabed (e.g., transmission loss is larger). Again, the challenge is that mud is highly dynamic in the coastal ocean. For the purpose of illustration here, we can consider sand really belongs to the coastal ocean while mud is simply a “traveler”. Mud is delivered by the river from the land. Mud may be first deposited in estuary or nearshore regime near the river mouth (see Figure 1). But they can be easily re-suspended by waves, currents and tides and delivered further offshore. Hence, mud at the continental shelf is often highly ephemeral. Understanding the dynamic of mud transport and deposition and to predict when, where and how much mud appears at the seabed is a fascinating but challenge task in coastal sediment transport.
There are many past and ongoing multi-institution research programs supported by Federal agencies (such as Office of Naval Research, National Science Foundation, National Oceanic and Atmospheric Administration, US Army Corps of Engineers, etc) to understand the dynamics of wave propagation and mud transport in the coastal environment, to study the fate of terrestrial sediment in the coastal ocean, and to develop open-source numerical models for surface dynamics, coastal hydrodynamics and sediment transport.
Figure 5: A soft mud bed at a tidal flat in Willapa Bay, WA.
Coral reefs in many parts of the world are diminishing. Because our current understanding on the main processes controlling the health of coral reef is limited, effective mitigation plan for coral reefs are not yet available. An article published in American Scientist (by Eric Wolanski, Robert Richmond, Laurence McCook and Hugh Sweatman) discusses scientists’ new findings to better understand the key processes determining the health of coral reef. Turns out mud delivered to the coastal regime during river flooding can be a major threat to coral reef. As such river flooding events (often driven by tropical cyclones or typhoons) are natural disturbance and short-lived, human influence on these river-borne sediments (e.g., increased nutrient concentration) often greatly enhance the impact and makes natural recovery less likely. In this case, our capability in predicting the fate of terrestrial fine sediment in the coastal ocean is the key to the further development of an ecosystem-based tool for land and coastal resource managers.
Fine sediments delivered by the rivers form floc aggregates, which can easily contain toxic materials, such as PCBs and DDT. For example, at Palos Verdes shelf contaminated sediments are deposited there via Los Angeles County Sanitation outfalls due to some chemical plant manufacturing DDT between late 40s and early 80s. In the present days, it often becomes a challenging environmental problem as how shall we deal with those contaminated sediment deposits? To answer this, coastal sediment transport researchers often need to evaluate the probability that these contaminated sediment deposits can be resuspended (or exposed) by waves and currents. Check out a related article on how scientists at USGS are trying to solve this problem at Palos Verdes shelf.
Yes, sediment transport plays a critical role in assessing global warming! The carbon flux delivered by the rivers to the ocean is a critical factor in the global carbon budget balance. Particulate Organic Carbon (POC) and also nutrients are carried by river-borne sediments and delivered to the ocean. Hence, to understand carbon sequestration, it is critical to understand the fate of terrestrial sediment in the coastal ocean (i.e., source to sink). A recently study (by Goldsmith, Garey, Lyons, Kao, Lee and Chen) shows that significant amount of carbon is supplied to the ocean by river flooding events of small mountainous river triggered by typhoon (See Figure 6). The study site is the Jhoushui (or spell as Choshui) River in the west coast of Taiwan (see Figure 6). According to a pioneering study by Milliman and Syvitski (1994, J. Geology, v. 100, p. 525-544), small mountainous rivers are major source of terrestrial sediment in the ocean. And in terms of sediment yield (averaged sediment concentration in the river), Jhoushui River is among one of the highest in the world. In summary, study the fate of terrestrial sediment in the coastal ocean can help us understand carbon sequestration and other dissolved fluxes into the ocean.
Sediments often deposit in estuary where fresh water meet saline seawater. This location is called Estuary Turbidity Maximum (ETM). As ETM and the related sediment dynamics can greatly affect the morphology, ecosystem, and water quality of an estuary, one of the major concerns of these deposited sediments is to keep waterway navigable. Hence dredging is often carried out to remove those unwanted sediment. As dredging is a rather expensive and labor-intensive engineering solution, better understanding ETM and sediment deposition for a given estuary system can help waterway manager to come up with the most effective approach to keep waterway navigable while maintain a healthy estuarine ecosystem. Interested readers are referred to the following web document for more information on ETM dynamics, dredging and related environmental problems at Ems estuary.
This is a 10-week summer research experience (June to August) for undergraduate students (junior or senior) to discover exciting research experiences related to coastal processes and sediment transport. Undergraduate students will work with Prof. Hsu and his research group at Center for Applied Coastal Research, Civil and Environmental Engineering, University of Delaware (UD). Students from underrepresented groups are especially encouraged to apply and in this case the opportunity is also open to students from greater Philadelphia/Delaware area through the RISE program at UD and the Greater Philadelphia Louis Stokes Alliance for Minority Participation (AMP) effort. Undergraduate students at UD should contact Prof. Hsu directly for more information. The research opportunity can be extended to spring and/or fall semester. Each summer student stipend is $3000 with an additional $500 provided as housing allowance. This compensation is consistent with existing UD Undergraduate Research Program.
For the summer of 2009, the effort will focus on developing hands-on laboratory experiments for coastal sediment transport with an aim toward developing several exciting and educational demonstrations for high school and middle school students. Topics may include, sediment-laden density currents, bedforms evolutions under wave and currents and settling of floc aggregates. Undergraduate student may be also involved in interpreting measurement data from the experiments and simulated data from numerical models in order to understand coastal sediment transport processes.
Taiwan Coastal Sediment Transport Study (Jhoushui River system)
Taiwan Source-to-Sink Sediment Study (Gaoping River-Shelf-Canyon System)