FOR IMMEDIATE RELEASE
AUGUST 30, 2005
Study of flow through tidal inlets may lead to better understanding of factors influencing estuary health
COLLEGE STATION, TX — Texas’ bays and estuaries breathe.
Instead of air, the breath is the flow of water into and out of the bays from the open Gulf of Mexico through tidal inlets.
Inhale during flood tide: the waters of the open Gulf and their contents, including organisms like red drum larvae, which rely on Texas’ bays and estuaries as nursery grounds.
Exhale on ebb tide: freshwater and pollutants that flow into the bays from rivers and other sources.
Because the tide is cyclic, some water goes through many cycles before permanently leaving the estuary. And anything that disrupts or alters the flow can have an impact on the reproduction of native species and the dispersal of pollutants from sewage outflows and agricultural runoffs to oil spills.
Thus, knowing more about the basic hydrodynamic processes that govern the flow of water through these inlets is vital to understanding how human activity might change the flow in positive or detrimental ways.
Two researchers, Drs. Scott Socolofsky and Kuang-An Chang of Texas A&M University’s Department of Civil Engineering, are conducting laboratory experiments under a grant from the Texas Sea Grant College Program to study the patterns in the formation of eddies — currents created by the unsteady tidal pulsation and with a circular motion — as the water exits the gaps in barrier islands between bays and the Gulf.
“It’s important to understand mixing and exchange between the estuary and the open ocean,” Socolofsky said. “As the eddy forms, it captures a certain bulk of liquid, and it moves as a coherent structure, so if it moves out of the inlet and then far enough away that the tide doesn’t bring it back in, then that fluid has left the estuary and won’t come back. Or if it stays close enough to the inlet that it comes back in, then that fluid exchanges more slowly with the ocean.
“An example of a constituent that might move with the tide would be the red drum fish — they spawn offshore and then the larvae grow up in the estuary. They move with the current, so they are transported by exchange with the estuary, which can be governed by these vortices. So if we decide to build jetties or do something to the inlet, then we would want to know if we are going to change either the net exchange or the locations of the exchange.”
Although a number of studies have demonstrated the importance of the exchange process itself — and millions of dollars have been spent on its effects, from beach nourishment projects to research on the impact of pollution on coastal ecosystems — not as much attention has been paid to the underlying hydraulic mechanisms that drive the exchange itself.
In the new shallow wave basin of the Reta and Bill Haynes ’46 Coastal Engineering Laboratory at Texas A&M, Socolofsky and Chang built a simple model of a shallow bay with an outlet to the Gulf. Their “barrier island,” constructed of cement blocks, can be adjusted to increase or decrease the width of the outlet. The researchers can also change for other variables — the speed and volume of the outflow, the relative depths of the water — with the geometry simplified compared to conditions in the field.
“We do the experiments in the laboratory rather than in the field because we can control different water depths, different tidal periods, different openings, geometry variations and the bottom friction,” Chang said.
The researchers are using an overhead camera to record a full-field, two-dimensional snapshot of the positions of small, floating plastic pellets. Later experiments will use dye injected into the water. The camera images show the water’s movement and provide information needed to develop velocity vector maps — graphs that show how quickly the water is moving and in what direction.
They are verifying the results from the camera with readings from acoustic Doppler velocimeters (ADVs), devices that measure the water velocity. Chang said he has used the camera technique extensively for more than 10 years, but this is the first time he has conducted this type of experiment on such a large scale — usually the modeling is done on the scale of a few inches, while the wave basin they are using measures 75 feet by 125 feet.
“This is a relatively new technique,” Chang said of the larger field of view in their experiments. “That is one reason the Army Corps (of Engineers) is so interested.”
“They want to collaborate with us — they are interested in obtaining our data so they can use it to improve their numerical models,” Socolofsky added.
Socolofsky and Chang’s project is of particular interest to the U.S. Army Corps of Engineers’ Coastal Inlets Research Program because of its applications for ADCIRC, the Advanced Circulation model. ADCIRC is a system of computer programs for solving free surface circulation and transport problems in two and three dimensions. The Corps of Engineers (COE) was the original sponsor for development of ADCIRC in the late 1980s by a national team of university researchers and continues to sponsor and participate in its development, including the employment of its own ADCIRC modelers.
Socolofsky and Chang anticipate that the results from their research will help fine-tune the ADCIRC model for the state’s bays.
“This is important for Texas because there is now a complete ADCIRC model for the complete coast of Texas,” Socolofsky said. “ADCIRC is going to be important for modeling passes in Texas, and our data will do well to help calibrate and validate that.”
The researchers are completing the first year of the project, in which they used an idealized bay and inlet geometry. In the second year, they will explore more complicated versions of the system and add another variable in the form of a longshore current, a common phenomenon along the coast of Texas. They also will replicate conditions in a specific location — Port Aransas — in the second phase.
“We want to make sure the parameters — the water depth, the opening, the width, and the flow coming in, the velocity — are the same as Port Aransas, just scaled down to the lab and with simplified geometry,” Chang said.
With the greater complexity from the additional variables, the researchers hope to be able to provide the kind of information that will help keep human activity from disrupting the exchange processes at these tidal inlets.
“For example, building a jetty can change the eddies because the size of the inlet will control whether or not and to what extent they form,” Socolofsky said.
“Also, if you dredge the waterway you change the water depth,” Chang added. “We are trying to figure out the relationship between the water velocity, the water depth and the width of the inlet mouth, for example. If we dredge it, then that changes the ratio, it changes the parameters, and then we have to consider what kind of an effect we are getting.”
“We’d like to be able to use numerical models to run what-if scenarios. If a storm blows a hole through a barrier island, how will that affect the exchange with the estuary?” Socolofsky said. “Hopefully if we can understand the formation of these eddies better, then we can have a predictive tool that would be accurate for different morphology.
“The main goal of our project is to make field-scale models more accurate in their predictions for mixing, because the rate of exchange between the estuary and the open ocean controls the biology and ecology of the estuary.”
— 30 —
For more information:
Dr. Scott Socolofsky
Texas A&M University
Department of Civil Engineering
979-845-4517
E-MAIL: socolofs@tamu.edu
Dr. Kuang-An Chang
Texas A&M University
Department of Civil Engineering
979-845-4504
E-MAIL: kchang@tamu.edu
NR 05-10
Powell 8/5/05
The Texas Sea Grant College Program is a partnership of university,
government and industry, focusing on marine research, education
and advisory services. Visit our web site at http://texas-sea-grant.tamu.edu
|
