2007 Annual Monitoring Report (pdf 16MB) - Bolsa Chica Lowlands ...

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportAnalysis of Closure Risk TriggerThe Closure Risk trigger is not specifically defined or quantified, but is presented as an opinion of riskdue to the shoaling pattern of the inlet. A review of the bathymetric changes at the inlet shows that themajor cross-sectional flow area varies by season and is presently located within a very narrow thalwegon the southern side of the inlet (Figure 2-2). Based on the quantity of material passing through theinlet and the apparent narrowing of the thalweg, it is prudent to recommend dredging to prevent aclosure of the inlet in a large storm event, especially since there is no mechanism for re-opening theinlet if it closes.Of greatest concern relative to an episodic closure event are large accumulations of sand within theinlet channel, high and over-steepened channel banks, and a sinuous course of flow between high sandbars or adjacent to the armored shoreline. In such cases as these, a major storm event combined with aweak neap tide series could lead to either full or partial closure of the mouth. This could then result inloss of drainage and rising water levels within the MTBs.A conditions necessary to cause a catastrophic closure event have not yet materialized, however,response to such an occurrence should be included in emergency planning contingencies.Muted Tidal Basin FunctionThe trigger statement of “A tidal muting of the average low tide elevations (Mean Low Water) on theorder of 0.5 feet would indicate that the flood shoal maintenance dredging was warranted” needs to bemodified since the Mean Low Water in the FTB is unlikely to be ever be muted before the MTBdrainage requirements fail. The spring low tide should be used as the parameter to gage the muting inthe FTB. The drainage efficiency of the MTBs together with the MTB function should be one of thetriggers considered for maintenance since the drainage efficiency can be modified to a certain degreeby limiting the intake water volume. The west MTB was connected to the FTB on March 5, 2008 andthus no data are yet available to begin to explore the relationship between the FTB tidal environmentand that of the MTBs. The dredging trigger related to tidal muting will likely be driven almost entirelyby the drainage performance of the MTBs and thus these parameters will be set and adjusted over timeas the MTBs are sequentially brought on line.3.3 DREDGE TRIGGERS - CONCLUSIONS AND RECOMMENDATIONSFlood shoaling has occurred within the FTB at a two-year rate that is nearly precisely as predicted witha greater rate of shoaling in Year 1 and a lesser rate of shoaling in Year 2 than expected. The shoalgeometry is also generally as expected. The scheduled maintenance dredging is expected to removethe volume predicted at the frequency planned. For this reason, there is no reason not proceed with themaintenance dredging as scheduled.In reviewing the established dredging triggers, it is clear that some of the triggers may never be metexcept under extreme circumstances, while more significant triggers may exist that have not as yetbeen quantified. Chronic beach erosion triggers are not likely to be met because of the ongoingreplenishment at Surfside-Sunset and the program’s effect on long-term beach growth trends.Similarly, acute erosion triggers are not likely to be met due to the generally broad beach profiles attrigger point transects. This is not to say that beaches would not benefit from replenishment with floodshoal sand bypass. Rather, it acknowledges that beach erosion is not likely to occur to the extent thatwould trigger an obligatory maintenance-dredging event for replenishment purposes.Merkel & Associates, Inc. 97