Hurricane Juan arrived (Table 4). Differences identified in the salinity data were 1) TBD marsh 

 had the lowest average salinity and the smallest standard deviation, 2) BID had the highest average 

 salinity (1.5 ppt higher than TBD), and 3) TB marsh station salinities are generally lower than BI 

 Bayou marsh salinities. This is the case even though salty tidal waters must move further into the 

 marsh system to enter the BI system than they do to enter the TB system. 



Hydroperiod 



The number of inundated days, number of flooding events, and number of days per event are 

 tabulated for each station (Table 5). Significant differences found are: 1) BIU has the longest 

 hydroperiod and the longest average period of flooding (13 days), 2) TBD has the greatest number 

 of flooding events and a hydroperiod approximately 75% as long as BIU, and 3) BID had the 

 shortest hydroperiod and least number of flooding events. These data show the effects of the 

 fixed-crest weir on marsh hydrology. Whereas the weir dampens the tidal peak in the marshes, 

 which prohibits inundation of downstream marshes found behind natural levees, the weir also 

 inhibits tidal egress, causing extended hydroperiod in the upstream marsh sites which are lower in 

 elevation than the downstream levees. 



Hurricanes Danny and Juan provided two natural experiments which documented the effects of 

 the weir on marsh hydrology. The period of inundation for both storms were extended from 21% 

 to 56% in the BI system over the TB system (Table 4). Because hurricane storm waters often 

 have elevated salinities (as in Hurricane Danny, which was a dry storm), the results of extending 

 the hydroperiod can result in severe impacts to marsh vegetation. 



DISCUSSION 



The need for installing weirs must be questioned based upon this study. If the two major 

 reasons for installing weirs are to stop overdrainage and prevent saltwater intrusion, weirs are not 

 doing their jobs. Semi-impoundment to produce better Scirpus olneyi (Ross and Chabreck 1972; 



Table 4. Post-storm periods of marsh inundation. 



a See Table 1 for site abbreviations. 



b Maximum number, may be up to 7 days shorter. 



c Minimum number of days; never dry before Hurricane Juan. 



138 



