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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual Report0.244 m (0.80 ft) (M&N 1999). Generally, the tidal range under the post-construction condition metthe target of the “full tidal range” objective of the project planning documents throughout themonitoring period. Muting of the low tides, however, has exceeded the predicted muting followingshoal development.This is partially attributed to the fact that preliminary engineering tidal predictions were based on atheoretical average spring tidal condition not the monthly maximum spring tide condition noted in themuting analysis (Table 2-3). Because of concerns for drain-fill hydraulics of the MTBs, however,these maximum ranges are of key interest as they pertain to potential for building water levels withinmuted basins.It was expected that the tidal range would gradually decrease and muting of the low tide wouldincrease over time. This was observed through the monitoring year. It is further expected that mutingand phase lag will become more severe due to effects of flood bar development in the FTB until theimplementation of the first dredging event.It is worth noting that the Basis of Design Report (M&N 2004) shows an initial planning goal ofachieving a tide range of nearly 2.75 meters (9.0 ft) during extreme tides. This objective, however,was impractical for the system given the short jetties and shallow shoreline condition at the inlet. Lowtides are truncated due to friction in the inlet and flood bar elevations with only moderate shoaldevelopment. As a result of the final design and constructed conditions match more closely to thepreliminary engineering models that target tide ranges of a maximum of 2.29 meters (7.5 feet). Thesite never met the tide range of 9.0 feet since being opened, but regularly met the 2.29 m (7.5 ft) tiderange over time, as shown in Table 2.3. Although the FTB is still close to meeting the target tidalrange of 2.29 m (7.5 ft) under shoaled (muted) conditions, the trend is toward a decreasing range andwhen considered along with other variables (shoal volume and area of low intertidal habitat lost)signals a need for maintenance dredging to occur.In the “Statements of Interest for the Environmental and Beach Profile Monitoring of the Bolsa ChicaLowlands Restoration Project”, a dredging trigger was proposed as follows: “a tidal muting of theaverage low tide elevations (Mean Low Water) in the order of 0.5 feet would indicate that the floodshoal maintenance dredging was warranted”. This should be revised since the Mean Low Water in theFTB is unlikely to ever be muted prior to failure of the MTB tide gates. The spring low tide would be amore appropriate parameter to gage the muting in the FTB. The drainage efficiency of the MTBstogether with the MTB function will be one of the triggers since the drainage efficiency can bemodified to a certain degree by limiting the intake water volume. The west MTB was connected to theFTB on March 5 th 2008. An appropriate dredging trigger related to tides should be developed aftermonitoring of the tides in the muted tidal basin in 2008.Tidal muting is dynamic and is generally comparable to what was expected during preliminaryengineering, with the following specific observations:Tidal muting is positively correlated with tidal range, with greater muting occurring duringspring tides and less muting occurring during neap tides;The trend of low-tide tidal muting is gradually increasing over time, with the greatest increasein muting occurring in the Spring and Fall of 2007; likely coincident with the major drainagerestriction due to the flood shoal and changes in shoal configuration;Merkel & Associates, Inc. 71