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

Bolsa Chica Lowlands Restoration Monitoring2007 Annual ReportII. PHYSICAL MONITORING PROGRAMThe physical monitoring program focuses on large-scale morphological changes of the system andtidal response to these changes. Principally this monitoring includes evaluation of inlet shoaling,coastal beach response to inlet conditions and sand loss to the flood bar, and tidal reaction to shoaldevelopment. The physical monitoring program is intended to monitor changes in relation toestablished management triggers, and to adaptively evaluate and recommend adjustment of triggerswhere appropriate to ensure health of the system and protection of coastal beach resources.2.1. INLET FLOOD SHOALIntroductionA newly constructed inlet to a tidally influenced system will typically interrupt longshore sedimenttransport and divert sediment both offshore (creating an ebb bar) and towards the tidal basin (creating aflood shoal). As the ebb bar forms, it affects the wave and current regime. This, in turn, causes theshoreline planform to evolve toward a new dynamic equilibrium condition. Similarly, the flood andebb tidal currents moving through the inlet will build and shape a flood shoal in the interior of the tidebasin. The configurations and sizes of the bars depend on the tidal prism of the basin, cross-sectionalarea of the tidal inlet, length of the jetties, tidal range, and longshore sediment transport rate. Whilecomplete equilibrium is rarely achieved, rates of change within the ebb bar and flood shoal typicallydiminish as the conditions around a new inlet stabilize. To limit early adverse impacts of ebb bardevelopment on the shoreline processes, approximately 929,326 m 3 (1,214,579 y 3 ) of sand was placedas pre-fill to form the ebb bar at Bolsa Chica. This fill was placed to avoid the potential of the full ebbbar developing from beach sand longshore drift and thus robbing the littoral cell of mobile sandsupply.As beach sand migrates longshore on the coast, it is made available for capture by flood tides intoBolsa Chica. Sand is moved into the system where it settles into a flood shoal. A portion of this sandis moved back to the beach with the ebbing tide while a portion of the sand remains trapped in theshoal 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 rangethat would exist without the flood shoal. Therefore, a monitoring, maintenance, and maintenancedredging plan was incorporated into the Bolsa Chica Lowlands Restoration Project and is beingimplemented 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 threeMTBs, and the FFTB. From the time of opening and throughout 2007, only the FTB supplied tidalprism through the inlet. As a result, tidal velocities through the inlet are too low to keep the channelbetween the jetties fully open and sedimentation is expected and has been observed in the inletchannel. As additional tidal prism is added to the system with the opening of the MTBs, the inletmouth will increase in cross-sectional area as it responds to the higher tidal velocity required to feedthe system during tidal exchanges.The preliminary engineering studies (M&N 1999) done for the project predicted a flood shoal volumeof 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 (134,000y 3 /year) for the second year after the inlet was to be connected to the ocean, for a total ofapproximately 230,000 m 3 (300,000 y 3 ) over the first two years. The predicted flood shoal location isMerkel & Associates, Inc. 59