the bay remain in the vicinity of Millstone after 

 twenty tidal cycles (i.e., about 10 days). Their 

 model also showed that, although larvae leaving 

 the river were subject to entrainment losses, a 

 large fraction of them would have been flushed 

 from the area by tidal action in the absence of 

 power plant entrainment. Because it seems rea- 

 sonable to expect lower larval survival in the bay 

 (and even more so in open waters of Long Island 

 Sound) than in the protected nursery areas in 

 Niantic River, absolute entrainment numbers do 

 not represent a fair estimate of additional loss due 

 to the operation of MNPS. In this context, the 

 modeling work carried by Dr. Saila's research 

 team at URI represented a first attempt to estimate 

 the actual larval losses attributed to entrainment 

 at MNPS. Since then, new data on the early life 

 history of winter flounder resulting from our own 

 studies (NUSCO 1987) suggested that vertical 

 movements of older larvae in response to tidal 

 and diel cycles may invalidate some of the as- 

 sumptions made in the early URI models. In 

 order to address this problem NU has contracted 

 for the development of a new larval dispersal and 

 entrainment model with the Department of Civil 

 Engineering at the Massachusetts Institute of 

 Technology (MIT). 



This new MIT model wiU use the hydrody- 

 namics TEA (tidal circulation) and LEA 

 (advective transport) submodels as a framework 

 for larval dispersal and entrainment simulation. 

 These two model components were recently used 

 to describe the dynamics of the thermal plume at 

 MNPS (Adams and Cosier 1987). Although TEA 

 and LEA are nominally similar to the correspond- 

 ing models used by Hess et al. (1975) and Saila 

 (1976), there are important differences as well. 

 Eirst, both TEA and LEA use irregular, triangular, 

 grid elements, rather than regular, square, grid 

 elements. The former configuration allows for 

 easy grid refmements in critical regions, such as 

 the Niantic River and the plant intake area. Sec- 

 ondly, both submodels take advantage of newly 

 developed computational methods which provide 

 better accuracy and higher speed for extended and 

 more detailed simulations. In addition, the area 

 covered by TEA and LEA in the previous appli- 



cation mentioned above will be extended to in- 

 clude the northern reaches of the Niantic River 

 and to the south, in Niantic Bay, to include a 

 larger portion of the local area in Long Island 

 Sound. 



The larval dispersal component of the MIT 

 model will be able to simulate continuous pro- 

 duction of newly hatched larvae that matches the 

 actual length of a typical spawning season, and 

 will track larval ages in days. Larval behavior 

 will be simulated by reducing advection (corre- 

 sponding to larvae moving to the bottom) as a 

 function of tide phase and time of day and ac- 

 cording to larval age. Although four larval stages 

 will be simulated separately, simulation results 

 can be presented in terms of total larval population 

 by integrating over larval stages. The model will 

 be run to simulate various scenarios where each 

 will be of seasonal duration involving a repeating 

 average tide. Among contemplated simulations 

 are comparisons between runs with and without 

 larval "behavior", between runs with and without 

 power plant operation, and between runs employ- 

 ing different distributions of larval hatching in 

 space and time. Drifting of "foreign" larvae into 

 the simulation area through the open boundaries 

 will also be simulated to assess the effect of larval 

 sources other than the Niantic River. 



Important inputs required by the hydrodynamic 

 components of the MIT model are the tidal 

 boundary conditions and dispersion coefficients 

 which influence flushing from the rivers and dis- 

 persion away from the power plant intakes. The 

 tidal boundary conditions will be established by 

 comparing measured and simulated tidal currents, 

 larval dispersion coefficients will be validated by 

 comparing measured and simulated larval flushing 

 rates from the Niantic River and by comparing 

 measured and computed intake dye concentrations 

 resulting from instantaneous dye releases in 

 Niamtic Bay. Additional inputs related to larval 

 dispersion will be the empirically estimated 

 (NUSCO 1987) vertical distribution of larvae at 

 various tides and times of the day. Daily larval 

 mortality rates, needed to simulate naturally oc- 

 curring larval concentrations in the river and bay 



Winter Elounder Studies 



209 



