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Alteration of the groundwater table due to construction of a floodplain bypass at Upper Pine Creek, Concord, California

  • Author(s): Williams, John L, III
  • et al.
Abstract

The Upper Pine Creek Flood Control Project in Concord, California, includes the construction of a floodplain bypass to the southwest of the natural stream path between stations 140+06 and 168+10 (station numbers increase in an upstream direction). This project increases the capacity of Pine Creek from around 1000 to 3250 cubic feet per second (cfs) in the project reach. Construction of the floodplain bypass required excavation to an elevation below that of the pre-project groundwater table. Such excavation creates a local depression in the water table and threatens to drop the local water table below the bed of the existing creek, effectively killing the natural channel.

Analysis of contour maps of the pre- and post-project groundwater table reveals that the water table has dropped by up to 7 feet in the area between the natural channel and the floodplain bypass. Additionally, the slope of the water table has been altered to a direction that seems to favor flow along the path of the floodplain bypass. A diversion pipe directs flows of up to 100 cfs into the natural channel. This, in conjunction with natural bounding of the aquifer by the Concord Fault to the southwest and by topographically driven flow from the slope of Lime Ridge from the east, effectively mitigate expected groundwater table elevation problems and maintain the natural creek as an active and dynamic channel system.

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