Tide gates form physical and ecological obstacles to river herring (Alosa spp.) spawning migrations

River herring (Alosa spp.) are anadromous fish that enter North American Atlantic coastal rivers and lakes each spring to spawn. Anthropogenic structures such as dams and tide gates serve as physical obstacles that limit river herring access to spawning habitat. This study examined the physical and ecological components affecting herring passage through a tide gate by applying a time-to-event analysis framework to multiple movement behaviors derived from telemetry data. Herring had higher passage success early in the season (78%) than later on (16%). Key behaviors that govern passage varied with diel period, tide, and flow direction through the gates. Furthermore, these behaviors shifted as the season progressed, consistent with the hypothesis that predator avoidance may be driving passage failure late in the spawning season.

Impacts of Tide Gate Modulation on Ammonia Transport in a Semi-closed Estuary during the Dry Season—A Case Study at the Lianjiang River in South China

Recovery of tide-receiving is considered to improve the water quality in the Lianjiang River, a severely polluted and tide-influenced river connected to the South China Sea. A tide-receiving scenario, i.e., keeping the tide gate open, is compared with the other scenario representing the non-tide-receiving condition, i.e., blocking the tide flow during the flood phase, by numerical simulations based on the EFDC (Environmental Fluid Dynamics Code) model. The impacts of tide receiving were evaluated by the variation in the concentration of ammonia and its exporting fluxes, mainly in the downstream part of the river. With more water mass coming into the river, in the tide-receiving scenario, the averaged concentration of ammonia reduced by 20–40%, with the most significant decrease of 0.64 g m−3. However, the exporting flux of ammonia has decreased in the tide-receiving scenario, as the consequence of the back–forth oscillation of tidal current. In the tide-receiving scenario, the time series of ammonia concentration approximately followed the tidal oscillation, with increased concentration during the ebb tide and reduction in the flood tide. In the non-tide-receiving scenario, the ammonia concentration decreases when the tide gate is open which results in further intrusion of seawater. This was followed by an increase in ammonia concentration again after the currents shift seaward and water mass with higher concentration from the upstream part is transported downstream. Given the identical ammonia input and river runoff, the ammonia concentration stays lower in the tide-receiving scenario, except for short periods after the tide gate opening and neap tides in the downstream part which lasts for around half a day. This study highlights the importance of hydrodynamic condition, specifically tidal oscillation, in the semi-diurnal and fortnight cycles, for the transportation of waterborne materials. Furthermore, the operation of the tide gate was additionally discussed based on potential varied practical conditions and evaluation criteria.

ASSESSMENT OF DAMAGE AND MITIGATION STRATEGIES FOR THE MISQUAMICUT, RI COASTAL COMMUNITY FROM A 100 YEAR STORM EVENT AND SEA LEVEL RISE

Misquamicut is a coastal community located in Westerly, Rhode Island with a high density of structures that is at risk from inundation, wave, and wind damage from large storm events. The area has suffered significant damage from storms in the past such as the Hurricane of 1938, Hurricane Carol in 1954, and most recently, Superstorm Sandy in 2012. This area is also highly susceptible to sea level rise (SLR), which is predicted to be as high as 7 feet by the year 2100 (NOAA, 2017). A tool called the Coastal Environmental Risk Index (CERI) was used to estimate damage to structures located in Misquamicut due to a 100 year storm event with and without 7 feet of sea level rise. Using CERI, four mitigation strategies were evaluated to improve resiliency of the community: basement window plugs, dune restoration and reinforcement, a tide gate, and elevation of structures.