An urban geomorphic assessment of the Berryessa and Upper Penitencia Creek watersheds in San Jose, California
Date
2009
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Abstract
A paired watershed study for two adjacent urbanized watersheds in San Jose, California was conducted to investigate vastly different channel morphologic response to urbanization and valley subsidence. The urbanized portion of Berryessa Creek (15.5 km2) exhibits system-wide channel instability, meanwhile Upper Penitencia Creek (61.3 km2) has remained stable despite similar urban build out trends. Currently, there is a paucity of field measurements documenting channel response to urbanization and subsidence in the academic literature. Detailed geomorphic field surveys were undertaken to establish 90 permanent cross sections over 9.2 km of urban channel. These surveys were used for sediment transport modeling in this study and will provide a permanent monitoring network. Historic data sources were utilized to establish a baseline context and chronicle change in the watersheds. The historic data sources, field data, and numerical modeling were used to investigate the relative effects of hydrologic alteration, valley subsidence, and river infrastructure on water yield, sediment yield, and channel stability. Drainage area capture by the urban storm sewer network, a component of urbanization that has not previously been addressed in the scientific literature, and engineered river infrastructure elements are the primary causes of system-wide channel instability in the urbanized valley portion of Berryessa Creek. Hydrologic and sediment transport modeling indicates that drainage area capture and urban land use change has increased water yield 48% and sediment yield up to 61% from 76 to 121 tonnes/yr-km2. These hydrologic changes have transformed historically depositional reaches into incised reaches leading to system wide instability. An on-line sedimentation basin and a 1.85 m grade-control structure have reduced downstream sediment yield by 15% from 88 to 76 tonnes/yr-km 2 and increased channel incision rates by capturing coarse bed material in transport. Models indicate that measured valley subsidence of 0.23 m results in upstream incision, however sediment yield is not affected and the morphologic response to subsidence is likely obscured by current instability processes dominant in the system. In the current hydrologic regime of Upper Penitencia Creek, flow diversion and basin reduction by the storm sewer network offset increased runoff produced by the urban landscape and channel stability is not adversely affected by the hydrologic alteration. Water yield is increased by 7%, however sediment yield is reduced by 4% from 41.7 to 39.8 tonnes/yr-km2 at the outlet. River infrastructure in the form of a system of small grade-control structures aids in the stability of the upstream reaches. Valley subsidence of 1.1 m is predicted to cause incision that would progress 1800 m upstream of the zone of maximum subsidence. Modeling results were verified by reach-scale instability observed upstream of the subsidence zone. The reach scale instability is a result of increase stream power resulting from valley subsidence and channel realignment.
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California
sediment transport
urban hydrology
urbanization
watersheds
hydrologic sciences
civil engineering