Using EEMS to Model the Effects of Sediment Dredging
Dredging refers to excavating sediments under a water body. The purpose for each dredging operation varies, but the primary purpose of dredging is navigation channel deepening. Other purposes include construction activities, removal of contaminated sediments, removal of sediment buildup within a reservoir, altering drainage paths to control excessive flow, etc. Dredging activity can cause significant impacts to the subject water body in both the long and short term, so modeling dredging impacts on hydrodynamics, water quality, salinity, and sediment transport is important.
Current mathematical models can simulate the “before” and “after” effects of dredging, but they cannot provide a continuous simulation during the dredging process, so until now, the effect of a dredging operation could not be evaluated in a single model run. To address this issue, DSI recently developed a feature to simulate the sediment dredging process.
In this post, we will present this newly added feature by demonstrating salinity intrusion in a channel during the dredging operation. Salinity changes directly affect the river habitat environment, so it is crucial to model its effect during the planning phase.
DSI developed a bathymetry field file for EFDC+ allowing users to specify temporal bathymetry changes during the model simulation. The bathymetry field file, like a pressure field file, gives the user control over each individual cell during the model run period. Users can adjust the bathymetry by raising or deepening at a specific time and location. This additional feature allows the modeler to create a long-term simulation that includes a dredging operation, whether it be a one-time event or a periodic maintenance event; this can now all be done in one single simulation.
In our demonstration, we built a simple straight river with an ongoing dredging operation; the characteristics of the channel and the dredging plan are described in Figure 2
The demo (refer to Figure 2) uses a standard sigma vertical layer scheme, has a tidal open boundary downstream (1m amplitude, 32 ppt) and a fixed flow boundary upstream (15 m/s, 0 ppt). The channel gets dredged in the middle at a rate of 1200 m3/day, and the entire operation lasts 220 days until the whole center channel is dredged through for a length of 60m, a width of 20m, and a depth of 1m.
Following the completion of dredging process, we see (Figure 3) that the salinity of channel in the upstream areas increased, illustrated by the isohaline lines (lines of same salinity). Through a time-lapse animation (Figure 4) we see the salinity wedge being pushed by the tidal motion at each cycle as dredging deepens the channel in the upstream direction. In this demonstration case, after the dredging process is completed through the full channel length, the 5 ppt isohaline is pushed 600 meters further inland than it was in the non-dredged channel at the peak of tide.
Figure 4. Animation showing plan view (top) and longitudinal profiles of No Dredging (middle) and Dredging (bottom) conditions.
Modelers can use this new capability to evaluate changes in other water quality indicators as well. In upcoming blog posts, we will demonstrate this capability in other real-world examples and provide detailed instructions on how to use this new feature. If you have any questions, please do not hesitate to contact us at [email protected]
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Water Resources Engineer
Nguyen Quang Trang
Water Resources Engineer