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

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