2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

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Aug06 to Jan07 Jan07 to Jun07 assumed no change assumed no change Sediment Assessment Polygon Jun07 to Jan08 Jan08 to Jul08 Jul08 to Dec08 Erosion/Accretion (m) -2.6 - -2.4 -0.7 - -0.6 -2.3 - -2.2 -0.5 - -0.4 -2.1 - -2 -0.3 - -0.2 -1.9 - -1.8 -0.1 - 0 -1.7 - -1.6 0.1 - 0.2 -1.5 - -1.4 0.3 - 0.4 -1.3 - -1.2 0.5 - 0.6 -1.1 - -1 0.7 - 0.8 -0.9 - -0.8 0.9 - 1 1.1 - 1.2 1.3 - 1.4 1.5 - 1.6 1.7 - 1.8 1.9 - 2 2.1 - 2.2 2.3 - 2.4 Survey Range Time Erosion Accretion Net Influx Influx Rate day m 3 m 3 m 3 m 3 /day Aug06 - Jan07 148 -1,157 60,638 59,481 402 Jan07 - Jun07 159 -11,412 55,543 44,131 278 Jun07 - Jan08 197 -10,612 46,381 35,769 182 Jan08 - Jul08 179 -9,828 32,480 22,652 127 Jul08 - Dec08 167 -16,742 40,523 23,781 142 Full Tidal Basin accretion and erosion comparisons between surveys Bolsa Chica Lowlands Restoration Project Figure 2-3 Merkel & Associates, Inc.
Bolsa Chica Lowlands Restoration Monitoring 2008 Annual Report Figure 2-4 presents the net accretion rate graphically as net sediment accretion per month within the assessment area shown in Figure 2-1. Again, the averages do not represent the actual accretion per day or month, as field observations noted that deposition and erosion occurred over time at an uneven rate. 15,000 Net Accretion Rate (m 3 /month) 10,000 5,000 12,139 m 3 /mo 8,327 m 3 /mo 5,419 m 3 /mo 3,775 m 3 /mo 4,247 m 3 /mo 0 Opening through Jan. 2007 Jan. 2007 through June 2007 June 2007 through Jan. 2008 Jan. 2008 through July 2008 July 2008 through Dec. 2008 Figure 2-4. Net sediment accretion rate per month A grain size analysis of the sand accreted in the FTB inlet was conducted in June 2008 in anticipation of maintenance dredging activities scheduled for 2009. The shoal sediment was documented to be 99% sand with minor components of fine gravel (shell hash) and silt/clay fractions (M&A 2008b). Discussion Flood shoal development within Bolsa Chica exhibited patterns typical of coastal wetland systems. Shoals develop as individual depositional fans along the primary flow alignment. As sediments are deposited, the resistance to flow along the channel increases and shoals continue to build until such time as the flows break out of the main channel and define a new primary channel. As a result of the continued 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 the deposited fan. The importance of this shoaling process is that it creates a regular depositional pattern through a process defined by unpredictable events. The shoaling by highly settleable sands follows a pathway along the principal coarse of flow with little lateral spread in footprint. As a result, quiescent waters that are outside of the higher velocity effective flow path may not receive sediment deposition and more linear shoals may develop in alignment with flow patterns. Terminal and lateral slopes of the flood shoal deposit are typically at or near the angle of repose for the clean sands (approximately 30-35 degrees). The bathymetric assessment of the FTB inlet and numerous site visits indicated that the tidal inlet morphology and sediment depositional areas (inlet thalweg and flood shoal patterns) are highly Merkel & Associates, Inc. 104
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Total Future Full Tidal Basin FFTB
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2008 Annual Monitoring Report (pdf 10.9MB) - Bolsa Chica ...

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