Theme for 2022:
Historic American River Floods and New Tools for Managing Future Extremes

Historic American River Floods in a Changing Watershed

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Abstract:

This presentation looks at historic American River floods in 1850s and 1860s by showing old lithographs, photographs, and maps to answer three questions:

1. Why did the City of Sacramento flood in 1850s and 1860s?
2. How has human activity changed the American River watershed and impacted precipitation runoff?
3. What was the January 1862 peak flow on the American River?

The early City of Sacramento was built on the low floodplain at the confluence of the Sacramento and American Rivers and subject to flooding. In response to this reality the physical environment was altered by building levees, changing the course of the American River, raising the level of streets, and filling low lands with the garbage dump (landfill).

The upper American River watershed was altered by gold mining activities including placer mining, hydraulic mining, river mining, tree cutting, and dam building. Photographs of impacts of river mining on three miles of the Middle Fork American River taken in 1858 and 1977-78 at the same location provides a visual record of these impacts. Folsom Dam plus 22 other dams have been built to reduce flooding, produce hydropower, and supply water for farms and cities.

Folsom Canyon is the 1862 name for the granite bedrock section of the American River starting below the Folsom Dam spillway and running past the Folsom State Prison for 2.5 miles downstream just beyond Rainbow Bridge. This is where the tailrace for Folsom Dam was excavated. Excavation work photographs are presented. Excavation planning and other historic documents provide data to help answer the question: What was the January 1862 peak flow on the American River? An answer is provided by Matt Weber's following presentation, where he discusses the HEC-RAS model and analysis of newly compiled data to estimate the 1862 peak flow.

Estimating the January 1862 American River Flood Peak with HEC-RAS Modeling

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Abstract:

The City of Sacramento sits within the floodplain of the Sacramento and American Rivers and has a long history of flooding. The City of Sacramento began flooding in 1850, the same year it was incorporated. A decade later, the winter of 1861-1862 yielded at least 3 events that flooded the city. These floods pre-date stage-discharge relationships within the American River watershed. Therefore, the magnitude of the 1862 peak discharge is highly uncertain. Better understanding the 1862 flood peak is important for informing future flood risk, as some prior estimates claim that the 1862 event represents the largest flood in recorded history.

This effort used newly compiled historical information to develop a one-dimensional HEC-RAS model to estimate the peak discharge in 1862. The area of focus was Folsom Canyon within the American River, a bedrock-dominated stretch of river below modern-day Folsom Dam.

Starting in 1951, Folsom Canyon was channelized and lowered by up to 100 ft as part of the tailrace channel construction for Folsom Dam. To approximate 1862 topography, a 1950 U.S. Bureau of Reclamation survey was digitized and incorporated into the HEC-RAS model. High water marks (HWMs) have been documented at the former Stockton and Coover Stone Stable within Folsom Canyon for the 1862, 1907, 1928, 1943, and 1950 flood peaks. The HEC-RAS model was calibrated to the HWMs for the 1907-1950 flood events and used to estimate the discharge that produced the 1862 HWM. Model results were compared to prior 1862 flood accounts and estimates.

Based on the 1D HEC-RAS model results, the 1862 peak flood event was approximately 260,000 cfs (+/- 15%, based on the uncertainty of historical USGS peak flood estimates). This finding places the 1862 event as similar in size to the 1964, 1986, and 1997 peak flood events on the American River.

History and Future of American River Flood Risk Management Projects

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Abstract:

Since 1989, SAFCA has been working with the State of California (State) and the U.S. Army Corps of Engineers to improve Folsom Dam and the state/federal levee systems downstream of the dam, which must safely contain the runoff from extreme storm events in the region. Over the next five years, SAFCA and its federal and state partners will complete the flood control system improvements needed to provide the Sacramento area with the 200-year urban level of flood protection mandated by the State Legislature.

Currently project activities include:

1. The Natomas levees

   The Natomas levees are being improved in two phases. SAFCA completed eighteen miles of improvements to the Natomas Cross Canal and the Sacramento River East Levee. The USACE Natomas Basin Project consists of levee improvements around the remainder of the 42-mile Natomas Basin perimeter.

2. The American River Common Feature (ARCF 2016) projects on the American and Sacramento Rivers

   The ARCF 2016 features include more bank protection along the American and Sacramento rivers, levee height and seepage improvements along Arcade Creek and changes to the Sacramento Weir and Bypass. On the Sacramento River East Levee downstream of the American River, ARCF 2016 is improving deficient sites with features like slurry cutoff walls and rock bank protection.

3. The Folsom Dam Raise

   The Folsom Dam Raise project will raise the height of the structures comprising Folsom Dam, including the main dam, wing dams, and dikes that contain Folsom Reservoir. Congress has authorized raising the height of the wing dams and dikes by 3.5 feet.

Nevertheless, the risk of catastrophic flooding, amplified by climate change and by the continued growth of the area's low lying urban core, will remain unacceptably high. This warrants a continued focus on flood risk reduction. Therefore, SAFCA is exploring the expansion of Forecast Informed Reservoir Operation (FIRO) to the upstream reservoirs on the American Basin and weir improvements on the Yolo Bypass.

Precipitation Forecasting: Past, Present, and Future

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Abstract:

Numerical weather prediction has advanced tremendously in the last couple decades. With higher resolution, better physical parameterizations and much more advanced ensembling methods, we have the same precipitation forecast skill now at 7 days lead time as we had at 3 days lead time in 2002. However, there remain challenges in predicting warm season precipitation and extreme rainfall.

This presentation will detail how numerical weather prediction has changed in the last 20 years and then discuss promising paths of the future which include high-resolution ensemble forecasting and advanced post-processing methods like machine learning. Lastly we will discuss how we can best use precipitation forecasts to ensure good decision-making in situations where there is significant weather-related uncertainty.

Streamflow Forecasting: Past, Present, & Future

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Abstract:

The California-Nevada River Forecast Center (CNRFC) has been providing streamflow forecasts in partnership with the California Department of Water Resources (CADWR) since the 1960s. Deterministic (or single value) streamflow forecasts have been around since its inception, and ensemble streamflow forecasts since the late 1970s.

Some of the earliest ensemble streamflow forecasts came from the joint federal-state partnership between the CNRFC and DWR. These forecasts were primarily used for long range water resource applications. In recent years, there have been significant improvements to the ensemble streamflow process focused on the near-term forecast horizon.

This presentation will discuss the past, present, and future of ensemble streamflow forecasting at the CNRFC.

Atmospheric Rivers: Past Explorations and Future Directions

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Abstract:

[Abstract not available]

Atmospheric River Reconnaissance: Filling Data Gaps to Improve Forecasts of Extreme Events

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Abstract:

Atmospheric River Reconnaissance (AR Recon) is an interagency, international collaborative project to collect unique gap-filling observations in the northeast Pacific to improve AR landfall forecasts and associated weather during the winter. These observations are now officially called for in the U.S. National Winter Season Operations Plan. Global modeling centers (e.g., U.S. National Centers for Environmental Prediction, U.S. Navy, European Centre for Medium-Range Weather Forecasts) that assimilate these data in real-time have developed a Research and Operations Partnership (RAOP) under the leadership of Scripps Institution of Oceanography's Center for Western Weather and Water Extremes to assess impacts and improve forecast skill.

Beginning in 2019, the group partnered with the Scripps Lagrangian Drifter Laboratory-based NOAA funded Global Drifter Program to deploy drifting ocean buoys with surface pressure sensors, in concert with dropsondes and data assimilation efforts, to support the project's forecast improvement objectives. In addition, data streams making use of airborne radio occultation, an innovative technique making use of both GPS and satellites, were made available in near real-time beginning in 2022.

This presentation will cover the accomplishments of the AR Recon RAOP, including a summary of targeting methods and data collection, plans for the coming years, and results to date of impacts on the forecasting of extreme events. It will also cover related enhancements of the onshore network ready to observe ARs and their associated precipitation and impacts after they make landfall on the U.S. West Coast.

Interplay of Geography and Atmospheric Conditions on California Precipitation

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Abstract:

We will present an overview of the large-scale and local-scale atmospheric conditions that are conducive to extreme precipitation events in California. In particular, we will discuss recurrent large-scale flow patterns and synoptic-dynamic processes that result in these events, as well as the influence of orographic processes as landfalling atmospheric rivers interact with the coastal topography and Sierra Nevada. Finally, we will present a detailed analysis of the atmospheric conditions supporting prolonged heavy precipitation and rapid runoff that culminated in the February 2017 Oroville Dam Crisis, during which the safe operation of Oroville Dam was threatened, and 188,000 people were forced to evacuate.

Evaluating the Feasibility for FIRO Improvements at Folsom Dam

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Abstract:

Reclamation water management is increasingly complicated by population growth, increasing environmental compliance demands, and accelerating climate change. These factors interact to intensify the requirements on water resources infrastructure and the scrutiny regarding how those resources are managed. Forecast Informed Reservoir Operations (FIRO) are an approach through which Reclamation can adapt to these factors. FIRO pilot studies have demonstrated the feasibility of utilizing improved meteorological/hydrological forecasts combined with better management techniques to simultaneously improve dam safety and water availability.

This presentation will summarize the current status of a FIRO pilot study at Folsom Reservoir to determine what, if any, water management alternatives are available to increase water availability, improve environmental compliance, and adapt to a changing climate. Folsom is of particular interest due to its buffering role in managing the Central Valley Project and its significant environmental demands, a demand aspect not present in previous FIRO pilots. Additionally, the basin is shifting from snowmelt driven inflows toward more rainfall under climate change which may increase pressure on its limited storage. The resulting Folsom FIRO management prototype will allow Reclamation to evaluate the potential benefit of improved forecasts, incorporation of quantitative uncertainty into management models, and dynamic trade-off of competing water objectives in a risk-informed framework.

Climate Change and Dam Flood Risk Across the West

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Abstract:

Reclamation operates facilities across the 17 western states that span a range of hydroclimates. The Safety of Dams Act, first implemented in 1978 in response to the failure of Teton Dam, authorized the Department of Interior to operate and maintain Federal Reclamation dams for safety of dams purposes. Reclamation uses risk guidance for decision-making at those facilities, with assessment of probabilistic flood risk being part of that process.

Flood risk has typically been determined using past flood data; however, climate change brings uncertainty to those estimates. Discussions surrounding climate change generally focus on increasing precipitation and temperature, but the associated hydrologic response to those changes is not linear due to complex interactions between changing climate and watershed conditions.

In order to better understand the changing potential flood risk at facilities across the west under climate chance scenarios, Reclamation is completing a number of different climate change research projects examining potential flooding impacts in different regions and hydroclimates. By completing a variety of projects focused on different scales (basin-specific to west-wide), different hydroclimatic regions, and different approaches, Reclamation is looking to gain insight into the complex interactions between climate change and hydrologic risk at our facilities.