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

Bolsa Chica Lowlands Restoration Monitoring
2008 Annual Report
II. PHYSICAL MONITORING PROGRAM
The physical monitoring program focuses on large-scale morphological changes of the system and
tidal response to these changes. Principally this monitoring includes evaluation of inlet shoaling,
coastal beach response to inlet conditions and sand loss to the inlet shoal, and tidal reaction to shoal
development. The physical monitoring program is intended to monitor changes in relation to
management needs, and to adaptively evaluate and recommend adjustment of maintenance and
management triggers where appropriate to ensure health of the system and protection of coastal beach
resources.
2.1. INLET FLOOD SHOAL
Introduction
A newly constructed inlet to a tidally influenced system will typically interrupt longshore sediment
transport and divert sediment both offshore (creating an ebb bar) and towards the tidal basin (creating a
flood shoal). As the ebb bar forms, it affects the wave and current regime. This, in turn, causes the
shoreline planform to evolve toward a new dynamic equilibrium condition. Similarly, the flood and
ebb tidal currents moving through the inlet will build and shape a flood shoal in the interior of the tide
basin. The configurations and sizes of the bars depend on the tidal prism of the basin, cross-sectional
area of the tidal inlet, length of the jetties, tidal range, sediment characteristics, and longshore sediment
transport rate. While complete equilibrium is rarely achieved, rates of change within the ebb bar and
flood shoal typically diminish as the conditions around a new inlet stabilize. To limit early adverse
impacts of ebb bar development on the shoreline processes, approximately 929,326 m 3 (1,214,579 y 3 )
of sand was placed as pre-fill to form the ebb bar at Bolsa Chica prior to opening of the inlet. This fill
was placed to avoid the potential for the full ebb bar developing from available beach sand engaged in
longshore drift; thus robbing the littoral cell of mobile sand supply.
As beach sand migrates longshore, it is made available for capture by flood tides entering the Bolsa
Chica FTB. Sand is moved into the system where it settles into a flood shoal. A portion of this sand is
moved back to the beach with the ebbing tide while a portion of the sand remains trapped in the shoal
deposits. As the flood shoal matures, it will begin to restrict ebbing tidal flow through the inlet. Tidal
flow restriction will diminish or mute the full tidal range in the system relative to the tidal range that
would exist without the flood shoal. Therefore, a monitoring, maintenance, and maintenance dredging
plan was incorporated into the Bolsa Chica Lowlands Restoration Project and is being implemented as
an essential component to the long-term health and viability of the system.
The oversized inlet of Bolsa Chica was sized to accommodate the tidal prism of the FTB, the three
MTBs, and the FFTB. From the time of opening and throughout 2007, only the FTB supplied tidal
prism through the inlet. In February 2008, the West MTB was opened adding additional muted prism
to the FTB. Other basins remained closed through the remainder of 2008. As a result of the oversizing
of the inlet, tidal velocities through the inlet are too low to keep the channel between the jetties fully
open and sedimentation has occurred in the inlet channel as would be expected. As additional tidal
prism is added to the system, the inlet mouth will increase in cross-sectional area as it responds to the
higher tidal velocity required to feed the system during tidal exchanges.
The preliminary engineering studies (M&N 1999) done for the project predicted a flood shoal volume
of 126,200 m 3 (165,000 y 3 ) at the end of the first year and a shoaling rate of 102,500 m 3 /year (134,000
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