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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportData from the vessel based bathymetric survey were post-processed to correct for tidal elevations at thetime of the survey, and the boat and shore data was used to develop bathymetric contour plots for thebasin.For each survey, detailed contour plots were aligned with plots of the same area from the postconstruction,pre-opening, contour data collected on August 20, 2006 that was provided by Moffatt &Nichol.ResultsThe contour plots for the pre-opening survey and the three post-opening surveys are presented inFigure 2-2. The volume of sand accreted within the inlet survey area in comparison to the pre-openingconditions is presented in the following table (Table 2-1).Table 2-1. Increase in inlet sediment volume in comparison to pre-opening conditions.Sediment AccretionSurvey date cubic meters cubic yardsJanuary 2007 + 90,311 + 118,122June 2007 + 155,716 + 203,669January 2008 + 197,692 + 258,571The contour plots of each survey were also compared to detect areas of erosion and accretion betweensurveys. These comparisons are presented graphically in Figure 2-3. The large accretion of sandobserved on the southeast shore of the inlet during the January 2007 survey was eroded to some degreein the following months, with most of the incoming sand accreting on the western shore of the inlet, asrecorded in the June 2007 and January 2008 surveys. The shoals appear to be highly transitory, withdaily and weekly fluctuations in the distribution of the sand. The deepest portion of the inlet channelhas shifted to run up the eastern shoreline.Flood shoal development within Bolsa Chica exhibits patterns typical of coastal wetland systems.Shoals develop as individual depositional fans along the primary flow alignment. As sediments aredeposited, the resistance to flow along the channel increases and shoals continue to build until suchtime as the flows break out of the main channel and define a new primary channel. As a result of thecontinued process, the flood shoal builds as a series of teardrop shaped lobes running into the basin.These are subsequently modified by wave and current erosion as water moves past and across thedeposited fan. The importance of this shoaling process is that it creates a regular depositional patternthrough a process of unpredictable events. The shoaling by highly settleable sands follows a pathwayalong the principal coarse of flow with little lateral spread in footprint. As a result, quiescent watersthat are outside of the higher velocity effective flow path may not receive sediment deposition andmore linear shoals may develop in alignment with flow patterns. Terminal and lateral slopes of theflood shoal deposit are typically at or near the angle of repose for the clean sands (approximately 4:1).Table 2-2 tracks the accretion of sand over time and shows a large input of sand between the basinopening on August 24, 2006 and the first survey on January 19, 2007. The total rate of volume changefrom the basin opening to the January 2008 survey, roughly 17 months later, was 392 m 3 /day. It isimportant to note, however, that this average rate does not represent the actual accretion per day, asdeposition and erosion occurred throughout the period at an uneven rate.Merkel & Associates, Inc. 61