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

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Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportThe June 2007 survey shows sedimentation along the inside bank of the tidal inlet channel, with thechannel being forced toward the outside bank. That pattern continued to evolve over time and is alsoreflected in the January 2008 survey. From mouth opening through January 2008, each survey showsthe flood bar growing progressively larger, corresponding to a period of time when low tides in thetidal basin were becoming increasingly truncated and tides were becoming more muted. The effects ofthe shoaling on tidal muting are analyzed in Section 2.2.Based on bathymetric surveys, the flood shoal volume anticipate to have been deposited during thefirst year, August 2006 through August 2007, was approximately 169,500 m 3 (222,000 y 3 ) (derived bytaking the 155,716 m 3 surveyed in June of 2007 and adding 6,779 m 3 X 2 months as an averageestimated deposition rate for the subsequent 6 month period). This method of annualizing the shoalvolume would be expected to result in an underestimate of shoaling due to the generally decliningshoaling rate. However, using this estimating approach, the actual shoaling rate is 34% higher than the126,200 m 3 (165,000 y 3 ) predicted as the first year shoal volume (M&N 1999).As expected, the subsequent sediment accretion rate shown in Figure 2-3 decreases gradually, whilethe flood shoal develops towards an equilibrium state. The second year shoaling rate from September2007 through August 2008 is roughly estimated at 75,600 m 3 (99,000 y 3 ) (derived by taking thedifference between the January 2008 survey volume and the estimated August 2007 volume mentionedabove and adding 6,779 m 3 X 7 months) for the remaining 7 months to annualize the shoaling rate forthe second year post opening. This rate is substantially lower than the 102,500 m 3 (134,000 y 3 /year)shoaling volume predicted by preliminary engineering modeling for the second year after the inlet wasto be connected to the ocean (M&N 1999). However, when taken as a whole, the model predicted twoyear accumulation volume of 230,000 m 3 (300,000 y 3 ) compares very favorable to the early periodmeasurements and rate-based escalation two-year volume of 245,100 m 3 (320,300 y 3 ) from the postconstructionmonitoring. The reasonably good alignment of modeling with observational data (7%deviation) is remarkable and illustrates the benefits of evaluation of change over the longer two-yeartime period. The variable seasonal influx of sand, and added complication of provision of local sourcesand in the pre-filled ebb bar and beach around the mouth is expected to have played a role in alteringearly infill rates. However, subsequent reduced rates of infill may illustrate more rapid achievement orrelative stability following the initial system loading. The lack of temporal precision and high varianceassociated with early system dynamics is to be expected with limited predictive methods.The flood shoal volume, area of shoaling, and shoaling rate all have occurred similar to processespredicted during the project design. The notable difference between predicted shoaling and thatactually observed has been the bypass of much of the maintenance basin by the shoal formation andthus a greater penetration into the FTB than would be expected at this point in shoal formation. Inretrospect, this bypass should have been predictable given past observations of shoal development insystems such as Batiquitos Lagoon, Agua Hedionda Lagoon, San Elijo Lagoon, and the TijuanaEstuary.The manner in which the basin performs as expected or different from expected is a factor indetermining the necessity for shoal dredging and the establishment of triggers. This is addressed inSection 2.4.Merkel & Associates, Inc. 65
Bolsa Chica Lowlands Restoration Monitoring2007 Annual Report2.2. TIDAL MONITORINGIntroductionTidal monitoring is fundamental to understanding the factors influencing physical and biologicalstructure of the Bolsa Chica Lowlands. As a non-estuarine system with very minor surface freshwaterinput, oceanic tides combined with winds are the principal extrinsic forces driving the hydrodynamicswithin the wetlands. Conversely, as these factors act to sculpt the physical and biologicalenvironments, feedback loops associated with alteration of basin bedform and shoreline conditionsinfluence tidal conditions within the system. Ultimately, roughness associated with vegetationdevelopment will also play a factor, however presently; this is an inconsequential variable in assessingsystem conditions.At the present time, accretion and erosion of sand within the flood shoal of the FTB is the singularmost important factor resulting in tidal lag and muting. While is anticipated that the future opening ofthe muted tidal basins will influence the shape of tidal curves in the FTB, it is expected that theprincipal factor influencing performance of the entire system will be the tidal drain and fill parametersbetween the ocean and the FTB. Tides in the FTB can, however, have a profound effect on the tidalconditions, and even the operational regime of the muted tidal basins.The restoration and opening of the Bolsa Chica Lowlands to the Pacific Ocean allows for the trappingof nearshore littoral sands to be drawn into the FTB, forming a flood shoal that restricts and retardstidal flows at the entrance of the FTB (refer to Section 2.1). Tidal monitoring provides a means oftracking the tidal lag and muting to provide information regarding the functionality of the system andthe need for maintenance dredging.The tidal monitoring program also offers key insights into intertidal mudflat and vegetative habitatdevelopment within intertidal elevation ranges. Because tidal muting and loss of drainage affectinundation frequency within the intertidal zone that further effects oxidation-reduction potential (ORP)of sediments changes in tidal hydroperiods and ranges can have substantial consequences on mudflatsand marshland development.MethodologyTidal monitoring began in the FTB on December 21, 2006 and was continuous throughout 2007 withdata collected at 6-minute intervals. The data were collected with a RBR Instruments TGR2050pressure gauge. The TGR2050 has a depth accuracy of ±5 mm and a resolution of ±0.1 mm. A secondTGR2050 pressure gauge was deployed nearby, on shore and used to correct the submerged pressuregauge for atmospheric pressure.The pressure data obtained from the submerged and atmospheric pressure gauges were used tocalculate water depth at the sensor with the following formula:Depth = (P w – P atm ) / ( * 0.980665);where depth is the water depth in meters at the pressure gauge, P w is the pressure in deciBars read atthe in-water pressure gauge, P atm is the local atmospheric pressure in deciBars, is the density ofseawater measured at the study site (1.027 g/cm 3 ), and 0.980665 is a gravitational constant (RBR2007).Merkel & Associates, Inc. 66
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