Use of a Continuous Water-Quality Monitor to Examine Sediment and Nutrient Transport in the Lower Connecticut River during Tropical Storm Irene 2011.

Abstract:

On August 28-September 5, 2011 tropical storm Irene delivered between 3 and 8 inches of rainfall in the Connecticut River watershed, with higher amounts occurring in the upper part of the watershed in Vermont and New Hampshire. Peak streamflow for tributaries in the upper watershed reached an annual exceedance probability of greater than 1%. The peak streamflow for the Connecticut River at Thompsonville, CT was 3,570 meters cubed per second (M³/s), an annual exceedance probability of 14%. This extreme event eroded and transported large amounts of sediments and nutrients to the main stem of the Connecticut River.  In addition to streamflow gaging station, a continuous water quality monitor was operated at the USGS streamgaging station at Middle Haddam, Connecticut. The station monitored continuous turbidity in formazin nephelometric units (FNUs) and used an automatic sampler to collect samples for sediments and nutrients through the duration of the storm event.

The monitor tracked changes in the turbidity of the Connecticut River through the storm that was not coincident with the changes in streamflow. During the event the turbidity had two pronounced peaks; the turbidity rose rapidly to over 400 FNUs and then receded to under 300 FNUs and then rose again to 600 FNUs while the streamflow rose steadily from 142 M³/s to a single peak of 340 M³/s, and then receded to 142 M³/s. The changes in the turbidity corresponded well with the changes in suspended sediment and phosphorus concentrations measured in samples collected during the event. The high frequency measurements from the continuous turbidity data provided useful insight into what would have otherwise been a confusing pattern in concentration data from this extreme event.