The Lower Susquehanna River Integrated Sediment and Nutrient Monitoring Program

In 2014 Exelon Generation Company, LLC (Exelon), in cooperation with the Maryland Department of Natural Resources (MDNR), Maryland Department of the Environment (MDE), University of Maryland Center for Environmental Science (UMCES), U.S. Geological Survey (USGS), U.S. Army Corps of Engineers (the Corps), and the U.S. Environmental Protection Agency (USEPA) Chesapeake Bay Program, initiated the multi-year Lower Susquehanna River Integrated Sediment and Nutrient Monitoring Program (Integrated Monitoring Program), with the following primary goals:

1. Determine the impact of storm events between 100,000 and 400,000 cfs on sediment and associated nutrient loads entering the Lower Susquehanna River from upstream sources (including Conowingo Pond), and
2. Determine the potential resulting impacts of storm events on Bay water quality from sediment and nutrients entering Conowingo Pond from upstream sources, scouring from sediment behind Conowingo Dam and passing through the Dam.

The original study was to target up to six storm events with flows equaling or exceeding 100,000 cfs. At the conclusion of all field efforts, the results of the Integrated Monitoring Program were to be used to update the suite of Chesapeake Bay (the Bay) Watershed and Water Quality models for use in the 2017 TMDL Midpoint Assessment. As of June 2016, two official sampling events have occurred, both of which had peak flows less than 182,000 cfs.^[1][2]

Supplemental Modeling Effort

In late 2015, due to a lack of storm events in the target flow range and the lack of available corresponding empirical data, Program partners began discussing alternative approaches that could be implemented in early 2016 to supplement the modeling efforts associated with the Midpoint Assessment.

From these conversations, Exelon developed a two phased-modeling approach that would enhance and complement the existing Phase 6 HSPF Watershed Model (HSPF) as well as the simulation of inputs to the Bay Water Quality and Sediment Transport Model (WQSTM). The two phased modeling approach includes developing:

1. An enhanced one-dimensional HEC-RAS model of Lake Clarke and Lake Aldred, similar in nature to the one developed by the USGS as part of the Lower Susquehanna River Watershed Assessment (LSRWA). Enhancements in the new model will include a longer calibration and verification period based on 2013 and 2015 bathymetry that were not previously available to the LSRWA modelers, modeling of suspended sediment load plus estimates of bed load, individual cohesive soil properties for each soil group, and true unsteady flow. The output from this model will result in improved sediment loads from Lake Clarke and Lake Aldred, which can then be used to re-parameterize HSPF and improve the sediment loads entering Conowingo Pond (the Pond); and
2. A coupled hydrodynamic, sediment transport, sediment nutrient flux and water quality mass balance model for Conowingo Pond – the Conowingo Pond Mass Balance Model (CPMBM). This model will allow for an improved evaluation of the extent to which changes in sediment storage and nutrient bioreactivity within the Pond affect sediment and nutrient delivery to the Bay. The output from this model combined with the results of the UMCES biogeochemical experiments being conducted as part of the Integrated Monitoring Program will allow for improved inputs to the WQSTM.

As part of this modeling effort, the Exelon team agreed to work closely with the Chesapeake Bay Program (CBP) Modeling Workgroup and Corps modelers throughout this process to ensure that all parties are in agreement on the methods used, deliverables provided, and implementation of results into the suite of CBP models (specifically Phase 6 CBP Watershed Model and WQSTM). In addition, Exelon agreed that all modeling efforts and reports will be subject to two independent reviews, including (1) interim review and guidance from an independent modeling evaluation group to be contracted and coordinated by the Chesapeake Research Consortium (CRC) and (2) additional review as part of a larger review of the CBP’s Phase 6 Watershed Model that is already scheduled for 2016 as part of the activities of the CBP’s Scientific and Technical Advisory Committee (STAC). The Evaluation/Review Team comprises four independent evaluators/reviewers. The Scope of Work (which includes this same background information as preamble) was agreed upon by all parties at the beginning of the review process. The evaluation group will review the modeling methods, results, and reports and provide feedback and comments as appropriate. Comments provided by the evaluators/reviewers will be taken under advisement by the Exelon team.

¹ Sampling Event No. 1 occurred April 6-14, 2015 with peak flows of 146,000 and 182,000 cfs. Sampling Event No. 2 occurred April 22-25, 2015 with a peak flow of 125,000 cfs.

² In addition, supplemental data were collected at Marietta, Holtwood, and Conowingo during a February 2016 high flow event (peak flows ~180,000 cfs). Although data were collected, this was not considered an official sampling event.