J.B. Shaw and D. Mohrig

Size: px

Start display at page:

Download "J.B. Shaw and D. Mohrig"

Meghan Harmon
5 years ago
Views:

1 GSA DATA REPOSITORY J.B. Shaw and D. Mohrig Supplementary Material Methods Bathymetric surveys were conducted on 26 June- 4 July, 2010 (Fig. 2A), 7 March, 2011 (Fig. 2B), August, 2011 (Figs. 1 and 2C) and 2-3 February, 2012 (Fig. 2D) using a 6.6 m wide swath array of four depth-sounders mounted on the R/V Fearman. Each transducer (210 khz) was controlled by a Hydrobox TM profiler (Syquest Inc.), with a vertical resolution of 0.01 m and a minimum survey depth of 0.3 m. The depth sounders were oriented in space using two Precise Point Positioning (PPP) GPS systems (Trimble NetRS mounted with Zephyr antennae) and a tilt-meter to measure pitch, yaw, and roll (MicroStrain 3DM-GX1). Depth-sounder data were cleaned of spurious points and concatenated with GPS, tilt-meter, and tide gauge from Amerada Pass ( N W, 4.2 km East of the Eastern boundary of Fig. 1, NOAA, 2012a) to produce bathymetric data referenced to WGS84 horizontal datum and Mean Lower Low Water (MLLW) vertical datum. Bathymetric surfaces were created by manually interpreting 0.2 m isobaths between survey lines and interpolating a 5 m gridded digital elevation model (DEM) from the isobaths using the Topo To Raster function in ArcMap 9.3 with a non-enforced drainage criterion, a minimum depth 0.2 m below the deepest isobath, and a maximum height of 0 m MLLW. Comparison of the bathymetric surfaces to surveyed lines yields a ~0.1 m standard deviation, which can be primarily attributed to error associated with waves and sub-grid scale bed fluctuations such as bedforms. Ship tracks for the three survey periods are shown in supplementary Figure 1. A 1935 bathymetric survey of Atchafalaya Bay (NOAA, 2012b) corrected for changes in relative sea level (see Shaw et al., submitted, for details) was used to determine the depth of the cohesive, muddy, bedrock substratum found in Atchafalaya Bay. The bay bottom is flat, with a depth of -2.3 m MLLW. Areas deeper than this level have been verified via grab sample as bedrock, and areas shallower than this level have been verified as sandy. Bathymetric surfaces are compared through measurements of the channel network. The channel network boundary was mapped along the minimum curvature (maximum concavity) of a channel bank where the gradient is oriented toward the channel, i.e., where a channel bank rolls over into the non-channelized overbank region. Channel extension (and bifurcate shoal migration) was calculated by comparing the most downstream (upstream) points of the channel network boundary. Channel area was defined as the area within the CNB and downstream of an upstream boundary held constant between surveys. Average channel depth was measured from MLLW within the same area. Channel bifurcation asymmetry was calculated by dividing the cross sectional area of the larger bifurcate by the cross sectional area of the smaller bifurcate measured at the tip of the bifurcate shoal (gray lines, Figs. 2A-D). The cross sectional area is 1

2 Distributary Channel Evolution measured as the area between Mean Sea Level, the bed, and the channel network boundary. These measurements are tabulated in Supplementary Table SM1. Records of water (Qw) and suspended sand (Qsand) discharge entering the Wax Lake Delta (Fig. 2F,G) were collected from the Calumet Gauge on the Wax Lake Outlet by the US Geological Survey (USGS , N W, 17.9 river kilometers upstream of the northern boundary of Fig. 1). Suspended sand discharge was integrated over time to produce a sand flux to the delta during high and low flow periods (Assumed sediment density ρ = 2650 kg m-3. Grain Size distributions were measured with a Mastersizer Laser Particle Size Analyzer (Malvern Instruments, Malvern, UK) to compare with shear velocity measurements collected over a tidal cycle and to characterize the fining of sediment distributions beyond the channel network. Grain size distributions gathered from grab samples of the bed are illustrated in Supplementary Figure 2. 2

3 Supplementary Figure DR1. Bathymetric data points showing ship tracks (in black) over the four bathymetric surfaces used in this study. 3

Supplementary Figure DR2, Grain-size trends from grab samples collected basinward of Channel II on Gadwall Pass in February, 2012.

4 Supplementary Figure DR2, Grain-size trends from grab samples collected basinward of Channel II on Gadwall Pass in February, (A) Map of the study area, with channel network boundary in shown with a dashed line and grain size measurements marked as circles. (B) Tenth, 4

5 fiftieth, and ninetieth percentiles of grain size distributions plotted against distance from the tip of Channel II at Gadwall Pass. Error bars show one standard deviation of percentiles across 4-6 analyses. Between 0 and 1.5 km, grain size trends are well characterized by exponential functions. The median grain size becomes finer than sand (<62.5 μm, dashed line) at 0.6 km beyond the channel tip. However, all samples contain at least 24% sand (>62.5 μm). Beyond 1.5 km, grain sizes are roughly uniform. (C) Bathymetry of the same transect, referenced to Mean Lower Low Water (MLLW). The black line indicates the elevation of bedrock bottom of Atchafalaya Bay. References NOAA, 2012a, Tides and Currents: Lawma, Amerada Pass, LA, a%20pass,%20la&type=historic+tide+data (Accessed August 2012). NOAA, 2012b, U.S. Estuarine Bathymetric Data Sets Home Page: U.S. Estuarine Bathymetric Data Sets, (Accessed August 2012). 5

Observed bed elevation changes in the data may arise as a result of any of several causes:

Observed bed elevation changes in the data may arise as a result of any of several causes: 10 July 2014 TECHNICAL MEMORANDUM: INTRODUCTION NEWARK BAY STUDY AREA, NEW JERSEY BATHYMETRIC COMPARISON AND ANALYSIS Periodic single- and multi-beam sonar bathymetric surveys within Newark Bay (Bay) in