Federal Flood Control Channels in San Francisco Bay Region - A Baseline Study to Inform Management Options for Aging Infrastructure
This dissertation focused on flood control channels in urban areas built by the USACE between the 1950s and 1970s. These uniform-shaped earth or concrete lined channels were designed to "control" flooding and to make possible expanded floodplain developments. As many of these channels have been in service for over 50 years, it may be instructive to reassess them to examine how they were planned, designed, and maintained, and to identify the current conditions and issues. The assessment provides the basis to develop management strategies for this aging infrastructure. In this dissertation, I examined the flood control channels in the Federal scale and the local scale.
At the Federal scale, I reviewed the history and evolution of USACE policy, planning process, design criteria, and the 100-year flood standard. I also conducted a systematic evaluation of nine urban flood control channels built by USACE between the 1950s and 1970s in the San Francisco Bay Region. I found the commonality of problems in these flood control channels, driven by the USACE planning policies when these channels were built. The finding forms the justification to develop a channel safety program to manage all USACE flood control channels at the federal scale.
The policy review found that the planning process for these projects singly focused on national economic benefits, while environmental consequences were largely excluded. The land enhancement benefit in the cost benefit analysis encouraged floodplain and tidal marsh developments. However, habitat elimination resulting from the developments was not considered as a cost. The land enhancement benefit had significant impact on project justification. The San Francisco Bay Area case study found that six of the nine projects included the land enhancement benefit in the cost benefit analysis. If the land enhancement benefit were subtracted from the total project benefit, three of the six projects would not have been authorized due to low cost benefit ratio. This implies that if habitat protection had equal footing as flood risk reduction, as a planning criterion for these projects, many of the projects would not have been authorized in the form they were constructed.
The primary design criteria for these flood control channels were to maximize flow capacity within a minimal footprint. The hydraulic design assumed a clear water condition, ignoring sediment effects. However, oversized thalwegs invite in-channel sediment deposition, and sediment transport increases channel roughness. Both effects reduce channel capacity. The clear water assumption led to questionable channel designs. The San Francisco Bay Area case study shows that six of the nine flood control channels have channel thalweg elevation at the outfall lower than the adjacent bay bed, created sediment sumps. The study also found that eight of the nine flood control channels had concrete channel reaches designed under supercritical flow. Such design demands the channel to function as designed under the clear water condition.
The San Francisco Bay Area case study found that the maintenance guidelines were ambiguous, but sediment deposition created significant operation and maintenance burden to local sponsors. On average the current sediment maintenance cost is 5 times higher than the original design estimate. Due to sediment deposition and channel deterioration, the average channel capacity reduced by 31%, as compared to the original design capacity. None of the channels in the study have the capacity to convey the existing 100 year flow. Even under the original design condition, five of the nine channels cannot provide existing 100 year flow capacity, considering two of the five channels were designed for the Standard Project Flood. This result is a matter of concern, since even if the channels were maintained to the original design specification and under the clear water condition, these channels still cannot provide 100-year flood protection.
The Federal scale analysis identified common issues among these flood control channels. These channels provide a false sense of flood protection security to the communities they serve, and the resultant floodplain developments further increase flood risk. At the Federal scale, the findings form the justification basis to develop a channel safety program to manage all USACE flood control channels. The proposed program would be similar to the existing dam and levee safety programs. The proposed program provides a management framework for the entire USACE flood control channel portfolio, for periodic inspection and risk assessment, condition review and classification, and critical improvement prioritization for flood risk reduction.
At the local scale, I conducted a detail review on the planning process of the Corte Madera Creek flood control project, to validate the findings from the USACE planning policies review. I also developed a hydraulic analysis to evaluate how sediment management schemes affect channel capacity.
The case study at Corte Madera Creek further illustrated the sediment management issue found in the Federal scale analysis. The local sponsor devised a more efficient sediment removal plan for 100 year flood protection, however it still costs 14 times more than the original maintenance budget estimate. Furthermore, the proposed flood protection projects in the upper watershed were designed based on the flood control channel providing 5,600 cfs capacity, about 2,000 cfs lower than the original design capacity. The analysis shows that even if the channel were maintained in accordance with the USACE operation and maintenance manual, the channel still would not provide the capacity needed. This finding validated the fundamental project design issues found at the Federal scale analysis.
At the local scale, my analysis identified a need to update the USACE operation and maintenance manual, to specify sediment removal frequency and volume based on the required channel capacity. Local sponsors should develop a sediment deposition and channel capacity relationship, so the local sponsor can assess the need for sediment removal based on channel flow and stage monitoring data. The channel safety program provides the management tool to systematically prioritize and implement these updates to USACE projects.
USACE should update the engineering manual for flood control channel project planning and design. The revision should include methods to estimate channel roughness coefficient under various sedimentation and sediment transport conditions. To provide the technical basis to update the engineering manuals and project assessment methods, additional research is needed to (1) investigate how bedload sediment transport on smooth surface with no deposition affects channel roughness, and (2) improve existing risk analysis tools to quantify the stage discharge uncertainty.
In addition to managing the aging infrastructure to maintain its level of service, there is a need to develop intervention strategies to reinvent flood control channels to meet contemporary objectives in ecological restoration, floodplain management, recreation, and flood risk reduction. The intervention also needs to overcome the space limitation due to urban encroachments in the floodplain and often up to the channel banks.