Suspended sediment concentration (SSC) "spikes" or single SSC values significantly greater than those immediately before and after, do not accurately reflect SSC conditions. Spikes like those on January 5 and 19 in the example below primarily occur for 2 reasons.
First, barges, like the one shown here docked near the site, periodically navigate over the ADCP. Acoustic reflections off the bottom of the barge are interpreted as very high near-surface SSC. These erroneous near-surface values are incorporated into the profile average concentration resulting in the spike. The second cause of spikes occurs in winter months where ice passing over the ADCP can have the same effect as a barge. Near surface acoustic affects make the top ~2 meters of water column data unusable, so the ice needs to be exceptionally thick to be detected. A quick check in winter months to help distinguish between these possibilities is to look at a plot of instrument orientation for the time period in question. An example plot of instrument orientation is shown below which corresponds with the same period as the example SSC plot above.
Spikes in instrument orientation of more than 1 degree coincident with the SSC spike indicate a passing barge. The instrument orientation spike appears to indicate the ADCP moved, but in reality, the ADCP compass simply responded to the mass of iron passing above. In the examples above, SSC spikes on the 5th and late on the 19th were due to barges; identifying the cause of the early SSC spike on the 19th would require closer examination of profile data.
At this time we don't not have a way to automatically screen the data for spikes, however, the digital filtering of instantaneous load in the computation of net load effectively removes most of the spike signal.
|
U.S. Department of the Interior, U.S. Geological Survey
Maintainer: New York District URL: http://ny.water.usgs.gov/projects/poused/ssc_spikes.html Last update: 12:35:17 Monday 24 January 2005 Privacy Statement || Disclaimer |
|
