TTU Researchers Study Possible Effects to Streams from Bridge and Culvert ConstructioYou have probably driven over a bridge or culvert crossing a stream and never noticed it, but somewhere beneath these structures lie communities of insects, fish and other animals that make their homes in the streams below.
Researchers with Tennessee Tech University's Center for the Management, Utilization and Protection of Water Resources, in cooperation with the Tennessee Department of Transportation, are studying the possible effects of bridge and culvert construction on aquatic life and stream habitat.
"We wanted to determine if the construction caused impacts, and if so, what were they?" said Brad Cook, TTU associate professor of biology. "We also wanted to determine how long the impacts lasted if there were any. And finally, did the stream recover after the construction?
"TDOT can use the information we find in different ways, and it's up to them what to do with it," Cook said. "They want to know if there’s something they can do to reduce the impact of the construction projects, and they want sound data to rely on."
Possible effects of construction projects on aquatic life include the loss of fish spawning sites, changes in growth, or fish species leaving the habitat and not returning. The effects could be different for bottom-dwelling animals like crayfish, snails and aquatic insects -- known as benthic macroinvertebrates -- since they are not as mobile as fish and cannot relocate as quickly when impacts occur. The project, which is being conducted in three phases, includes the study of potential impacts on fish and macroinvertebrates and the sediment deposition in the stream.
"We are finding some impacts to the streams," Cook said. "From the habitat, we can tell that something besides the topography has changed. Overall, though, the fish seem to be recovering, but there are several species that we’re still monitoring."
During construction of the bridges and culverts, fish tend to leave their habitat, but they do return later, according to TTU biology professor Dan Combs.
TTU researchers on the project include Cook, Combs, research specialist Ginger Ensor, and biology graduate students. The biology graduate students, who in the current phase of the project include Susan Malone and Rob Nichols, perform much of the sampling and some of the lab work.
During the first phase, lasting from 1997 to 1999, 42 bridges and culverts that had already been constructed were studied. These included bridges and culverts in central and east Tennessee that were less than 5 years old, 5 to 10 years old, and more than 10 years old.
In the second phase, which will be completed in December, sampling to assess the short-term impacts on stream ecology was conducted before, during and after construction. This phase includes streams in Rutherford, Wilson, McMinn, Bradley, Sevier and Greene Counties. The third phase of the project, which will begin in January, will study the impacts of bridge and culvert construction in west Tennessee. During this phase, Hayden Mattingly, TTU assistant professor of biology, and three graduate students will join the project team.
Various water quality and habitat characteristics, which can affect species' abilities to survive, are measured at each site every six to eight weeks. The stream is divided into four 100 meter sampling areas, including undisturbed control sites 100 meters and 250 meters upstream of the structure and "disturbed" sites at the structure and 150 meters downstream of the structure. Water quality characteristics measured include dissolved oxygen content, temperature, conductivity, turbidity, which is a measure of particles suspended in the water, and pH, which is a measure of how acidic or basic the water is.
Electrofishing, which involves sending electric shocks through the stream to bring fish to the surface of the water to be identified, is done at each site as well as taking macroinvertebrate samples by dragging the streambed with a kicknet. Researchers also note the water and sediment depths and pebble sizes at each site.
Collected organisms are identified later in the laboratory. Finding a similar diversity of macroinvertebrates and the same types of species upstream and downstream from the structure indicates healthy stream conditions with little or no adverse impacts from construction. Differences in species diversity and other water quality characteristics upstream and downstream from the structure can indicate that the construction project impacted the stream.
"We look at the conditions upstream from the construction, at the construction and downstream from the construction to see if the stream changes," said Ensor, who analyzes stream samples in the laboratory and compares data to see if major changes in water quality have occurred over time. "So far, all of our data indicates that conditions revert back to pre-construction levels fairly quickly."
Construction impacts seem to vary between bridges and culverts, however. According to Cook, the negative effects seem to be greater in the streams where culverts are built. Unlike bridges, culverts have a bottom surface that is constructed on the streambed. Scouring, or erosion, occurs at culverts, resulting in increased accumulation of sediments. Some "intolerant" macroinvertebrate species can be smothered while other "tolerant" species can withstand the habitat changes. This can result in a shift of the species composition in the stream habitat.
"There are short-term effects to the stream," Cook said, "but it has been shown that there's not a long-term effect from bridge and culvert construction."