DISCUSSION 



Question: You stated that your 

 measurements show that about 50 per- 

 cent of the sediment delivered to 

 Atchafalaya Bay vicinity is being re- 

 tained in delta building and related 

 processes, and that another 50 per- 

 cent is escaping. Is that basically 

 correct? 



Answer: The only thing we have 

 to base that estimate on are some 

 hydro surveys done in the bay and 

 some cross-sectional sediment range 

 measurements taken inside the basin 

 from which we computed sediment bud- 

 gets for the Atchafalaya Basin. What 

 leaves the system was determined by 

 looking at sequential hydrographic 

 surveys. Looking at one in 1967, one 

 in 1972, and one in 1977, we have 

 tried to quantify the amount of de- 

 position in the bay. And then we 

 just simply subtract the amount of 

 deposition from what was coming into 

 the system and, from that, we deter- 

 mined that less than 50 percent of 

 the material is being retained in the 

 bay. But that's not exactly the 

 whole story because the type of mate- 

 rial that is being retained in the 

 bay is the coarse-grained material or 

 the sandy material, whereas the fine 

 silts and the clay particles as seen 

 on the landsat imagery, are easily 

 escaping the bay with tidal processes. 

 It's a complex problem. 



Question : I have a second part 

 to my question. My name is Sherwood 

 Gagliano. Is the escape of this 

 other 50 percent or less due largely 

 to the maintenance of the navigation 

 channel through the Achafalaya, and, 

 if so, has the sediment transport in 

 that channel been measured? 



Answer: We're currently in a 



cooperative program with USGS mea- 

 suring some of the major distributa- 

 ries in the delta. However, it's al- 

 most impossible, as your group has 

 found, to measure out around Eugene 

 Island at the break in the reef, 

 where the navigational channel cuts 

 through, to determine just how much 

 is escaping by that route. But I 

 understand in talking to Johannas 

 that your measurements indicate that 

 perhaps as much as 20 percent is es- 

 caping at least below the subaerial 

 portion of the delta. Whether or not 

 it's going into the bay or the reef, 

 no one knows. There is some indica- 

 tion from our studies that there is a 

 good possibility of a marine delta 

 forming seaward of the shell reef. We 

 have some side scan sonar data and 

 sediment data and there's been some 

 sand sampled out there. It could be 

 that the dredging of the navigation 

 channel and the confinement of flow 

 is carrying some of the sediment out 

 beyond the reef. But you know, this 

 is all kind of speculation right now. 

 We haven't really confirmed that. 

 We're working on that problem right 

 now. 



Speaker: John Weber "Planning Prob- 

 lems Associated with Freshwater In- 

 troduction into Louisiana Coastal 

 Areas" 



Question: (Dan Taylor, Fish and 

 Wildlife Service). Have your studies 

 progressed far enough to determine 

 what you will tie the fish and wild- 

 life benefits to. That is, what phy- 

 sical parameters, like the movement 

 of isohalines, or reduction in marsh 

 deterioration, and, if so, what do 

 you plan to tie those benefits to? 



Answer : I'd like to pass that 

 question on to the next speaker. He's 

 more familiar--are you talking about 

 the Mississippi-Louisiana Estuarine 

 Study, or both of them? 



365 



