Theme for 2015:
Navigating Central Valley Flood Risks with Climate Change

Changing Flood Risks in the Central Valley

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Atmospheric Rivers in California: Climate Change Projections and Potential for Earlier Warnings

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Abstract:

The global water cycle is expected to intensify under current climate change projections, which will lead to more extreme precipitation and flood events. In California, many of the extreme hydrological events are caused by atmospheric rivers (ARs), narrow regions of high water vapor transport across the mid-latitudes. This presentation will cover the characteristics of ARs, discuss climate change impacts on ARs according to the latest Coupled Model Intercomparison Project, Phase 5 (CMIP5) simulations, and will introduce a method that may yield earlier warning of upcoming extreme hydrological events. The results show that in the future, water vapor transport, a key variable used to identify ARs, is expected to intensify, which is in turn expected to cause more extreme events. Potential for earlier warning of future extreme events based on forecasts of water vapor transport instead of precipitation will also be presented, an approach that may aid hydrometeorological applications.

Sensitivity of San Joaquin River Basin Floods to Temperature Increases

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The San Joaquin River watershed extends almost thirty-two thousand square miles from its headwaters in the Sierra Nevada Mountains to its outlet in the Sacramento–San Joaquin Delta. It is a snow-melt dominated watershed with its average hydrograph peak in May aligned with the spring melt. However, major winter storms can cause system flooding in the winter. A key element to the size of the flood is the freezing elevation — the point where rain transitions to snow. As climate continues to evolve over the 21st century, warmer temperatures will lead to higher freezing elevations. This will have significant impact on the size of floods in the San Joaquin Basin.

In this talk, the watershed and its elevation distribution is described along with key elements of winter versus spring flooding. Then using results from recent studies, the temperature sensitivity of the watershed is illustrated both for current events and potential future conditions. Because of the area elevation distribution of the major tributaries, the San Joaquin Basin can generate significantly larger floods simply by having more land area contribute to storm runoff due to a higher freezing elevation. The consequences of such events as they relate to both winter and spring melt flood management are then discussed.

Integrating Climate Change in Regional Floodplain Management Planning

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Floodplain Managers must consider many sources of uncertainty in their planning efforts, including: hydrologic, hydraulic, and gEODechnical. In addition, to these well-established sources of uncertainty, are the effects of climate change. There is not a scientific consensus on the specific, anticipated effects of climate change, but these effects could be any or all of the following: rise in sea level, shift in the form precipitation from snow to more rainfall, change in intensity and/or duration of precipitation events. Each of these effects would produce its own unique set of additional challenges for floodplain managers.

Efforts have been undertaken at the Federal and State levels of government to develop programs that require consideration of climate change in floodplain management, e.g., the recently issued executive order establishing a Federal flood risk management standard and the State's Urban Levee Design Criteria (ULDC). At the regional level, these programs have been applied, along with other considerations, to plan improvements to the flood management system that are robust and resilient to climate change and the other sources of uncertainty.

Trading Ecological Problems for Climate-Resilient Solutions

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California is facing an unfamiliar and uncertain future as a result of climate change. One of the key characteristics of this future is that precipitation patterns will change, bringing more extreme highs and lows, worse floods and longer droughts. It will also be a future with far less of our annual runoff and water supply stored as snowpack. At the same time, most inland native fish species are at risk, declining, or extinct. Many other species of birds, plants, insects and reptiles that depend on the lush riparian habitat that was once characteristic of the state's river corridors are also at risk, declining or gone. The dual imperatives to address climate change and ecological losses are each massive challenges, but they are challenges that can, in some cases, benefit from the same solutions. Indeed, by looking to ecological needs first, we have an opportunity to identify previously under-appreciated solutions that may make sense to pursue only because of, or especially because of, those dual benefits. In this presentation, strategies will be identified that address the needs of California's native species while providing climate resilience co-benefits. Examples will be drawn from the Central Valley to illustrate these concepts.

Analyzing Options for Responding to Central Valley's Future Flood Risks

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Abstract:

Central Valley has historically remained vulnerable to flood hazards, and the flood risk trend is projected to increase under the impact of stressors like climate change, aging infrastructure, development in floodprone zone, and population growth. Climate change in particular is projected to bring in more frequent and severer weather events and thus more extreme flood flows which will put unprecedented stress on the Central Valley's flood management system. As such, an ongoing Basin-Wide Feasibility Study (BWFS) seeks to analyze options for better long-term flood management planning with consideration of climate change.

The BWFS is a study of enhancement of the system element of the Central Valley Flood Protection Plan (CVFPP). The 2012 CVFPP is guiding California's participation in managing flood risk along the Sacramento River and San Joaquin River systems, with plan updates every five years. The objective of the BWFS is to evaluate options to improve flood risk management by enhancing capacity, reliability, and redundancy and meeting the supporting goal of creating a resilient flood management system. BWFS is evaluating structural management actions to meet the CVFPP goals and objectives. The major structural management actions that are being evaluated to improve the capacity, reliability, and resiliency of the flood management system include three categories: Raising levees, increase in flood storage and expansion of bypasses. Balanced, reasonable and cost-efficient approach from the above three categories of management actions will be put into consideration in formulating a State Preferred Plan as State's longterm flood management plan addressing Central Valley's future flood risks.

Addressing Hydrologic Variability in Complex Water Management Systems

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Hydrologic variability is one of several key considerations in managing complex water management systems, such as California's Central Valley State Plan of Flood Control. While the need for sizing channels, levees, reservoirs and other physical features to address the peak runoff volumes associated with extreme precipitation events is well understood by the flood management community, these same facilities also are used to address needs for other hydrologic extremes (such as base flow conditions).

Australia's Millennium Drought ended with several significant floods in the Murray-Darling river basin and Texas's most recent flooding (May 2015) also represent a dramatic end to an extended drought. Recognizing the role that hydrologic variability plays in defining both floods and droughts, it is equally important to recognize that the tools and approaches used to study extreme hydrologic event need to be as diverse as the range of flows being managed. This presentation will share experiences learned in the Murray-Darling river basin in addressing hydrologic variability as a prelude to a panel discussion focused on the importance of addressing hydrologic variability in California's Central Valley. This discussion will be used to help inform the current (2017) and future Central Valley Flood Protection Plan updates.

Panel Discussion: Science Informing 2017 Central Valley Flood Protection Plan: What is Needed? What is Missing?

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