The Pulse, the Sink, and the Shortfall

deltaic_subsidence_Page_1

[subsidence in the California Delta; once exposed to the air through reclamation, the peat soils of the delta have subsided to more than 20 feet below sea level, which in turn places even greater stress on the levees; darker green indicates more subsidence; map produced from 2008 lidar data from the CA Department of Water Resources]

This piece by Tim Maly, Brett Milligan, and Rob Holmes in the recent issue of Ground Up (published by the University of Berkeley) gives a history of anthropogenic sediment in the California Bay Delta, beginning with “the pulse” that resulted from the massive excavation and earthmoving efforts touched off by the California Gold Rush. This land-building dynamic from the hills was augmented and shaped by reclamation efforts in the delta and ongoing urbanization around its edges, which aimed to protect newly arable land from floods and to create navigable channels for barge traffic.

This history can be understood through three overlapping periods– the Pulse (1850-1915), Reclamation (1850-1950), and the Shortfall (1950-now). Engineering, land management efforts, and policy that aimed to create hydroelectric power and minimize erosion in the second half of the 20th century created an inverse dynamic– too little sediment reaching the delta. This has resulted in a situation where deltaic farmlands and towns are sediment-starved and subsiding as the levees that protect them strain against rising sea levels. Meanwhile navigational channels on the seaside of the levees require that 3 to 6 million cubic yards of sediment per year be removed.

The article finishes by arguing that this situation calls for radical sedimentary intervention. Rather than struggling to maintain a platted-and-leveed landscape in a state of equilibrium, it might be possible to allow key areas to shift and change through deposition, erosion, dredging. The goal is a more productive delta landscape of dynamic ecosystems, cities, farms, and economies.

bay-dredging

[current dredging within the California Bay-Delta]

 



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