Sediment Dynamics & Environments in the lower Connecticut River
What: Sediment dynamics in the lower Connecticut River.
When: mid-June to mid-July
Who: Faculty – Suzzane O’Connell (Wesleyan) and 3 students.
Where: Wesleyan University, Middletown, CT and the Connecticut River Basin
Project Description and Goals
There are three potential study areas. Depending upon student and faculty sponsor interest each area could accommodate up to three students and no more than two areas will be investigated.
Multiple data sets can be collected from each of these areas to better understand the relationship between flow dynamics and sediment transport. This will include Sediment Trend Analysis (STA), current flow (Acoustic Doppler Current Meter), bottom and sub-bottom imaging (side scan sonar and subbottom profiler) and water elevation. Data will be collected from each study area twice (near the beginning and end of the and all data sets will be taken within a few days.
Study areas are given in order of priority, with priority 1 definitely being studied and then there will be a choice between area 2 or 3. Between Rocky Hill and Hartford (Fig. 1), extensive sand-wave fields were observed in the straight reaches between the meanders. The estuary, the second area, was extensively studied 20-30 years ago by Horne and Patton (1989, Fig. 2). They concluded that even under low flow conditions, sediment was being transported through the CT River estuary to Long Island Sound during ebb flow and that the bed of the estuary is in equilibrium with its sediment supply. And third, impact of tributaries to the Connecticut River on sediment transport, especially fine-grained, potentially contaminated sediment (Fig. 3).
Introduction
The CT River extends from the Canadian border in Quebec, 650 km to Long Island Sound. (LIS). It is the longest river in New England and the main source of fresh water (70%) to LIS. Almost 400 towns and cities, with a population of approximately 2.3 million lie in its 29,000 km 2 (7.2 million acre) watershed. In the northern reaches, were it forms the border between Vermont and New Hampshire, forests dominate the landscape. Most of the CT and MA watershed are urban (Springfield MA and Hartford, CT) or suburban. Roughly 35,000 acres, mostly in Vermont, are preserved from development by the Nature Conservancy, local land trusts and as part of the Silvio Conte National Fish and Wildlife refuge.
Flow rates for the Connecticut River vary seasonally, with highest flows in the late spring and early fall. In any year, water flow velocities between Middletown and Hartford vary between < 0.1 to about 1.6 m/s. Flume and field (Rubin and McCulloch, 1980) suggest that sand waves are likely to be active when water flow is > 1m/s. As this study (s) will take place in the early summer with low to moderate flow velocities, bedload transport is not expected to be active or very active.
Study Approach
In the study areas, the same suite of data will be collected to provide a comprehensive understanding of the sediment dynamics. The extent to which each data set is analyzed will depend upon the interest and background of the participants and their faculty sponsors.
Sediment size analysis will be done using Sediment Trend Analysis (STA). STA was initially developed to examine coastal sediment transport (McLaren and Bowles, 1985). It has also been applied to rivers (Manley and Singer, 2007), areas of heavy contamination (McLaren and Beveridge, 2006) and tidal zones (Héquette et al., 2008). The STA method requires high density of sampling within a clearly defined spatial grid. It uses relative changes in grain size distribution to identify areas of erosion, deposition and dynamic equilibrium. Several computer programs are available to help to analysis the data (e.g. Gao, S., 1996, Le Roux et al., 2002, Comiti et al, 2009).
Sampling will be coordinated with riverbed imagining and flow velocity measurements, allowing the acoustic image to be correlated with the grain size. The river bed and subsurface will be imaged with a Datasonics 1000 side-scan sonar and subbottom profiler. Within the same time window, river velocity will be measured over a full tidal cycle with an Acoustic Doppler Current Meter (ADCP). Each study area will be sampled twice during the one-month field season, at different parts of the tidal cycle.
Analysis of trace elements and other contaminant concentrations should also prove fruitful. The Connecticut River watershed is densely populated from Middletown, CT to north of Springfield, MA. Both airborne (e.g. N (Fig. 3) and Hg) and sediment transported contaminants have been measured by the state (http://ct.usgs.gov) and as part of the National Water Quality Assessment (NAWQA). Tributarites such as the Matttabassett and Park Rivers traverse through populated and formerly industrial areas potentially transporting contaminated sediments. In addition, the River is the recipient of most towns treated wastewater and several power plants are situated along its bank. In contrast, the area south of Middletown is much less developed and some of the tributaries such as the Eight-mile River flow through relatively pristine, undeveloped land.
With this data set a student could model flow dynamics, acoustic backscatter patterns (e.g. Nit et al., ), map areas of transport, erosion and non-deposition and measure the chemistry of the sediment (e.g. trace elements and contaminants).
Student Projects
There are three potential study areas, all within less than an hour’s drive from Wesleyan. The first area is the most unique and will be explored. Although analysis of the second and third site will provide useful data about sediment dynamics, it is highly unlikely that both can be surveyed in the one-month field season. Which area gets studied will depend upon the level of interest of the participants.
Project 1: Sediment wave dynamics
In 2002, on the straight reaches between Hartford and Rocky Hill (Figure 1) high resolution bathymetry, imaged asymmetric sediment waves with wavelengths from 3 to 40 meters and heights of 0.1 to 1 m. Acoustic backscatter (strength of the returned echo) was mostly strong indicating coarse sediment. However, there were some pockets of low backscatter, which is likely to be fine silt and clay.
Grab samples taken at a later time recovered sand and fine gravel (up to about 5 mm). The samples were not taken from identifiable parts of the bedforms and too much time had elapsed to relate the sample to the bedform.
In this study knowing the sample location within the bedform is critical. This will be done by placing a series of stakes across the sand waves, including the stakes as part of the STA sampling grid, and monitoring the sand-wave movement by noting the elevation changes along the stakes on at least the two comprehensive data gathering sessions. This will require scuba equipment and will nicely link sediment size with its location in the sand wave.
Trace element analysis of the fine-grained samples may show differences as the distance from Hartford, it’s waste water treatment facility, and power plant increase.
Project 2: Connecticut River Estuary
The Connecticut River provides about 70% of the freshwater inflow to Long Island Sound. This estuary is remarkably undeveloped for such a densely populated area. In 1994 it was designated a “Wetland of International Importance” in accordance with the Ramsar Convention. There are only 21 such designated wetlands in the United States (http://195.143.117.139/key_sitelist.htm).
Horne and Patton (1989) published an extensive study of bedload transport in the Connecticut River estuary based on analysis of station data collected in the early 1980’s. Their work showed that sediment deposited in the lower river is being transported to LIS (Figures 1 and 2).
With new technology and using STA we will be better able to describe and model sediment dynamics. In addition, at the CT DEP Marine Headquarters, south of the I-91 bridge, temperate and salinity are being recorded near the surface and close river bed, providing important information about flow between the CT River and Long Island Sound.
Project 3: Sediment dynamics at the confluence of tributaries and the main stem of the Connecticut River
There are many tributaries and types of tributaries to choose from. For example, the Farmington River, the largest tributary, flows through rural and suburban areas. Other rivers flow through Brownfield’s areas (e.g. Matttabassett) or small pristine landscapes (e.g. Eight-mile River). Choosing two different types of rivers (size, watershed features, type of sediment) would provide comparison data to address such questions as: where are the tributary fines being deposited, how far are the coarser grains being transported, how much of the fine sediment (potentially contaminated) gets transported into and through the main stem.
Logistics
Facilities and Plan
Wesleyan University in Middletown, CT will be the base station for students. The Connecticut River and two of its tributaries flow through Middletown, so students will have easy access to the River. Wesleyan has a 24-ft pontoon boat from which equipment is easily deployed and a van (Suburban) for towing the boat and transporting students. Students will have access to a Datasonics 1000 side-scan sonar and subbottom profiler, an Acoustic Doppler Current Meter, grab sampler and several types of coring equipment. A settling tube is available at SUNY Stony Brook, for coarse-grained (> 2mm) size analysis. We do not have a laser sizer, but have access to one at Southern CT State University in New Haven.
Projects will be discussed with students and their professor prior to arrival. During the first two weeks, approximately every other day, depending upon the weather, will be a field day. The third week will be spent compiling and analyzing data. Additional data will be collected the fourth week to complete the tidal cycle.
Students will be housed at Wesleyan. All housing has kitchen access, the university dining facility is open and many inexpensive eating places are within walking distance. Large supermarkets can be accessed via public transportation.
Health and Safety
A faculty member and boat operator will always be with the students while they are in the field. The pontoon boat meets US Coast Guard requirements and is equipped with life preservers and a first aid kit. An experienced diver will accompany us on dives.
Recommended course background
Sedimentology, knowledge of GIS, modeling and/or scuba certification would be helpful but not required.
References
- Comiti, F., D. Cadol, and E. Wohl, 2009. Flow regimes, bed morphology, and flow resistance in self‐formed step‐pool channels, Water Resour Res, 45, W04424.
- Gao, S., 1996, A fortran program for grain-sized trend analysis to define net sediment transport pathways, Computers and Geosciences, v.22: 49-452.
- Héquette, A., Y. Hemdane and E. J. Anthony, 2008, Determination of Sediment Transport Paths in Macrotidal Shoreface Environments: A Comparison of Grain-Size Trend Analysis with Near-Bed Current Measurements, J. Coastal Res., v.24: 695-707.
- Horne, G., and Patton, P., 1989. Bedload-sediment transport through the Connecticut River estuary, GSAB, v101, p.805-819.
- Le Roux, J. P., R. D. O’Brian, F. Rios, and M. Cisternas, 2002, Analysis of Sediment Transport Paths using Grain-Size Parameters, Comp. and Geosciences, v.28:717-721.
- Manley, P. L. and J. K. Singer, 2007, Assessment of sedimentation processes determined from side-scan sonar surveys in the Buffalo River, NY-USA, environmental Geology, v.55: 1587-1599.
- McLaren, P. and P. Beveridge, 2006, Sediment Trend Analysis of the Hylebos Waterway: Implications for Liability Allocations, Integ. Env. Asses. And Man., v.2: 262-272.
- McLaren, P. and D. Bowles, 1985, The effects of sediment transport on grain-size distributions, Journal of Sedimentary Petrology, v.55: 457-470
- Rubin, D.M. and McCulloch, D.S., 1980. Singel and Superimposed Bedforms: A Synthesis of San Francisco Bay and Flume Observations, Sedimentary Geology, v. 26, 207-231.
