Special Sessions
Sponsored by the Geomorphology Specialty Group
Association of American Geographers 103rd Annual Meeting
San Francisco, California, April 17-21.

Flood Hazards in the Central Valley

Paper Session 5144
Saturday, 4/21/07, from 8:00 AM - 9:40 AM

Sponsorship(s):

Geomorphology Specialty Group
Water Resources Specialty Group
Hazards Specialty Group

Organizer(s):

L Allan James - University Of South Carolina
Susan L. Cutter - University of South Carolina

Chair(s):

Burrell E. Montz - Binghamton University

Abstract(s):

• 8:00 AM Author(s):
  *L Allan James - University Of South Carolina
  

  Abstract Title: Physiographic and Historic Underpinnings of Flood Hazards in the Sacramento Valley, California.

  
  

  This paper evaluates conditions underlying modern flood hazards in the Sacramento Valley, a broad alluvial plain with natural levees grading down to low backswamp basins. Broad areas of this so-called ‘Inland Sea’ were frequently inundated up through the Anglo-American settlement period. Hydraulic mining delivered massive volumes of sediment causing several meters of channel aggradation, and extensive leveeing in response further transformed channels. These changes did not alter the reality of low flood-prone basins along the lower rivers terminating in a low inland delta. In the early twentieth century, an innovative channel bypass system was implemented that routes flood waters through broad, ephemeral, channelized floodways. The NFIP further encouraged structural flood control in the Valley, but these developments along with levees re-engineered to protect against the 1%-chance flood fed the mis-perception that lowlands were safe from flooding. Rapid urbanization has accelerated recently with development moving aggressively onto flood-prone basin and deltaic lands, generating large residual flood risks and unprecedented social vulnerability. Densely populated areas such as Natomas in the American Basin are prone to deep catastrophic flooding if levees fail. Flood conveyance systems have proven difficult to maintain and levee decertifications may place large areas within the 100-year floodplain requiring mandatory purchase of flood insurance. Moreover, climate-change scenarios now predict increasing flood variability and larger spring floods. The growing risks raise doubts that structural flood control methods alone can provide adequate flood hazard mitigation. Sustainable planning measures are needed that include limits to development in flood prone lands.

  Keywords: flood, hazards, California

  
  
• 8:20 AM Author(s):
  *Ralph E. Klinger - Bureau of Reclamation
  John F. England, Jr. - Bureau of Reclamation
  

  Abstract Title: Late Holocene Paleoflood History of the American River Basin, Central California.

  
  

  Paleoflood data were developed from geomorphic, stratigraphic, and geochronologic evidence preserved in four study reaches of the American River basin: 1) Lower American River near Fair Oaks, 2) North Fork of the American River at Ponderosa Bridge, 3) South Fork American River near Lotus, and 4) South Fork American River near Kyburz. Fourteen stratigraphic sites, 38 radiocarbon ages and archaeological age correlations in these four reaches provide evidence for late Holocene paleofloods that are preserved at or above the peak stage of the largest historical floods. Age comparisons between the four sites established the regional nature of flooding and placed tighter constraints on the timing of individual paleofloods. Based on data at three of the sites, the American River basin experienced a paleoflood sometime between 1400-1600 years ago that had a magnitude significantly larger than any historical flood. Flood stratigraphy also records at least three additional paleofloods between about 150 and 650 years ago that were larger than historical floods, but smaller than the flood 1400-1600 years ago. At the Kyburz site, there is evidence for an additional paleoflood between about 650 and 1125 years ago that may have been equivalent in magnitude to the paleoflood 1400-1600 years ago. In the past 1600 years there have been at least four paleofloods with magnitudes about 1.3 times larger than the largest historical floods in the American River basin.

  Keywords: paleofloods, geomorphology, Quaternary stratigraphy

  
  
• 8:40 AM Author(s):
  *Michael B Singer - University of California Santa Barbara
  

  Abstract Title: Flood Risk in the Sacramento Valley: The Status of a Creaky, yet Functional, Flood Control System.

  
  

  The Sacramento River’s flood control system was conceived as a system of weirs and bypasses that would shunt floods out of the leveed main channel into engineered floodways that drain directly into the Sacramento Delta. The system was later augmented with several large dams that heavily influence hydrographs directly downstream, but that have had modest influence on flows in the downstream portions of the basin. The system still relies on the weir and bypass system to keep low-lying communities dry during floods. However, the Sacramento Valley bypass system exhibits widespread evidence of impairment by erosion and sedimentation. Episodic flooding in the basin delivers large volumes of sediment to flood bypasses that probably originates from legacy tailings fans from the hydraulic mining era. These deposits, in addition to decreasing flow capacity, promote colonization of vegetation, which increases roughness and thereby decreases flood conveyance. Deposits forming at the entrance to bypasses increase stage thresholds for floods entering the floodway, increasing flood risk in the main channel downstream of the entrance. These deposits promote lateral channel migration away from the bypass entrance and/or erosion of the bed in the vicinity of the entrance, both of which apply additional pressure to the flood control levees along the main channel downstream of the entrance. These factors and fallout from the Katrina disaster have precipitated bypass sediment removal and levee repair along main channels, but it is unclear how effective these measures will be in the coming decades, especially within the ontext of regional climate change.

  Keywords: floods, sedimentation, flood control

  
  
• 9:00 AM Author(s):
  *Roxane Fridirici - California State University, Sacramento
  

  Abstract Title: Floods of People: Migration and Opportunity Versus Flood Risk in San Joaquin County, California.

  
  

  Since the disastrous 1997 Central Valley floods, local San Joaquin County governments have approved 30,800 new houses in flood-prone areas, including land that was underwater in 1997. Some houses are in new subdivisions, tucked behind “super levees”, some are in older, expanding subdivisions, some are proposed for Sacramento-San Joaquin Delta islands below sea level. Migration for more affordable housing along the Interstate-5 corridor near Stockton, Lathrop, and Tracy is driven by San Francisco Bay prices. The opportunity is greater than the perceived flood hazard, supported by ignorance of the local environment. In San Joaquin Country, odds are greater that a home protected by a levee will flood rather than burn, yet only about 2% of San Joaquin single-family home owners have flood insurance. Comparison of the area flooded in 1997 and the new development which might now be affected is startling. Flooding is a reoccurring part of this region’s environment. While well-constructed and maintained levees will probably protect inhabitants within their immediate vicinity, they may create greater problems along less well-protected side-streams and sloughs, causing failures where prior floods were minor. When small levees fail, flooding may do an end-run around or undermine new levees. Finally, raised ground water levels during wet periods may result in unexpected flooding of some recently developed areas. Research suggests new inhabitants require education about flood hazard and disaster preparedness, while local governments must balance costs and problems when the levees break with the opportunity of tax revenues from lucrative developments.

  Keywords: Floods, hazards, California-Central Valley, housing, migration

  
  
• 9:20 AM Author(s):
  *Susan L. Cutter - University of South Carolina
  Christopher Burton - University of South Carolina
  

  Abstract Title: Social Vulnerability to Levee Failures in the Sacramento-San Joaquin Delta.

  
  

  In the aftermath of the levee failures in New Orleans, there is considerable public interest in where the next catastrophe might occur. Many have pointed to the Sacramento-San Joaquin Delta region as a likely worst case scenario given the concentration of levees and potential mechanisms for initiating breaches (seismic activity, design and maintenance, extreme runoff). This paper examines the spatial variability in the social vulnerability of residents to potential levee failures or breaches from this worst-case scenario. To determine the likely flood exposure, the levee system in the region was mapped with a distinction made between levees with US Army Corps of Engineers (USACOE) certification and those without. The flood inundation risk was mapped using existing digital Flood Insurance Rate Maps and making adjustments as warranted based on USACOE certification. Utilizing the existing methodological approach on social vulnerability metrics (Cutter et al. 2003), a social vulnerability index was computed at the tract level for San Joaquin, Sacramento, and Yolo counties in order to assess the relative vulnerability of residents. When integrated with the flood exposure data, there is a clustering of increased social vulnerability zones with high risk flood areas. While the spatial pattern is not uniform throughout the tri-county area, there are sufficient pockets of high levels of social vulnerability (largely based on race, class, and gender indicators) to warrant management concern about the disproportionate impact of catastrophic levee failures on these populations and the level of local, state, and federal preparedness to cope with such a catastrophe.

  Keywords: social vulnerability, levee failure, flood hazards, California

  
  

Session Description: Natural hydrologic, geomorphic, and climatic factors, together with anthropogenic changes to the flood conveyance system and potential changes in flood probabilities combine to form a serious physical flood hazard in the Central Valley. In addition, rapid settlement and questions about levee coordination suggest that public safety is at an unprecedented high risk in the Valley. This session evaluates the hydrologic and geomorphic systems that control flooding, growing levels of social vulnerability, and the implications of these two converging conditions.