North Marin Water District*

Climate change is increasing the atmospheric rivers' strength and frequency which impacts communities across Marin County. Failure to include and recognize these growing threats underestimates current dam safety risks and possible preventive strategies. Response = Disagree partially with the finding Statement: We agree that climate change is having an effect on the strength and frequency of weather events, including larger storm events that are referred to as atmospheric rivers. NMWD staff have been actively participating in webinars and email updates from the Center for Western Weather and Water Extremes as well as the California-Nevada Drought Early Warning System for the last few years. In addition, as a member of the Sonoma County Water Agency's Technical Advisory Committee, NMWD is very familiar with atmospheric river forecasting and Forecast Informed Reservoir Operations (FIRO). However, to our knowledge, there is no published guidance on how to incorporate the current and evolving science on atmospheric rivers (or other weather events impacted by climate change) into the appropriate engineering analysis of the dam performance and safety or hazard mitigation actions. The responsibility to develop engineering criteria or guidance lies with state and federal agencies tasked with dam safety and water resources planning and engineering, and these agencies have the expertise and funding for those efforts. See response to F2 regarding what NMWD is currently doing regarding dam safety.

MMWD and NMWD are in full compliance with both state DOSD (sic.), as well as all federal regulations. However, dam safety analysis and reporting would be enhanced by including current data on probable maximum precipitation (basis for risk analysis) numbers. Response = Disagree partially with the finding Statement: It is not clear what “current data” in general or specific to the Novato Creek watershed above Stafford Dam is available and would be included in a probable maximum precipitation (PMP) analysis. Stafford Dam was raised and modified in 1985 based on a probable maximum flood (PMF) developed from a PMP calculated from data in the Hydrometeorological Report No. 36 (HMR 36) as well as HMR 49. NMWD acknowledges that these HMR reports were updated in 1999 and superseded with reports HMR 58 and HMR 59, however they would also not have "current data" as the Report suggests, especially atmospheric river data from current scientific research. ATTACHMENT 2 Page | 2 As part of an ongoing project development for the Stafford Dam Adjustable Spillway Gate (ASG) project1 consideration of a new PMP analysis is being evaluating by NMWD engineering staff in coordination with NMWD's dam consultant and ongoing coordination with the Marin County Flood Control & Water Conservation District, including how best to modify or adjust available hydrometeorological data. One concern NMWD has is if the methodology chosen to evaluate extreme rainfall events (with climate change impacts) and subsequent runoff is later superseded by or not in compliance with forthcoming official guidelines or regulations from the state then the costly effort has to be repeated. There is currently no scientific consensus on how to best incorporate climate change into PMP values2. It is a topic of active research at the federal and state levels, and to reinforce an earlier point made above (see F1 statement) regarding which entities are best equipped to integrate scientific research into engineering practice, the National Academies of Sciences, Engineering and Medicine's ad hoc committee project "Modernizing Probable Maximum Precipitation Estimation" and subsequent studies planned by the National Oceanic and Atmospheric Administration (NOAA) illustrate the more appropriate efforts to address the concern addressed in this finding. At the state level the California Extreme Precipitation Symposium (CEPSYM) (https://cepsym.org/) is an annual meeting of scientific and technical presentations meant to increase our knowledge and understanding of extreme precipitation events. The goals of CEPSYM are to improve flood risk management planning and increase warning time for large floods, including impacts from atmospheric rivers. The website "CalAdapt" (https://cal- adapt.org/about/) compiles Climate Change projections for California, however, their "Extreme Precipitation" projections are limited to 100-year storms and smaller, which are too small for dam design.

MMWD and NMWD hazard mitigation plans fail to incorporate the latest scientific studies on climate change. They use DOSD and FEMA climate models that were last updated in 2012. This eleven-year gap may lead to an underestimation of current and future risks. Response = Disagree partially with the finding Statement: NMWD is a participant in the Marin County Multi-jurisdictional Hazard Mitigation Plan (MCMHMP). Marin County is currently leading the effort to update the adopted 2018 MCMHMP for 2023. Based on NMWD's participation in the 2023 update, it is anticipated that climate change impacts will be addressed in that updated plan. See explanation to R2 below for more information on this topic.

FEMA and National Flood Insurance maps may not have entirely incorporated the most recent dam inundation maps and are not available on the MMWD and NMWD websites. Response = Disagree wholly with the finding 1 See approved CIP, of the FY23-24 Budget: https://nmwd.com/wp-content/uploads/2023/06/Budget- Final-FY-23.24.pdf 2 US Army Corps of Engineers ECB 2018-14 Guidance for Incorporating Climate Change Impacts to Inland Hydrology in Civil Works Studies, Designs, and Projects, Rev 2 August 19, 2022. Page | 3 Statement: The FEMA Flood Insurance Rate Maps (FIRM) and the companion Flood Insurance Study (FIS) for Marin County don't include dam inundation mapping. These flood risk products are instead based on flooding from various hydrologic scenarios and used for flood insurance purposes. The Grand Jury needs to inquire with FEMA directly about the mapping criteria and subsequent information included in those products. The NMWD website does provide information on the dam and spillway inundation maps: https://nmwd.com/your-water/novato-water/ (see "dam Inundation Mapping" link). The website links directly to the Department of Water Resources (DWR) Division of Safety of Dams' (DSOD) inundation mapping portal. NMWD feels this is the most appropriate public user interface for dam inundation mapping and is the repository of the most current "approved" mapping for Stafford Dam. The website also provides a link to the County of Marin's Marin Map website which hosts the latest FEMA flood insurance rate maps (FIRMs): https://nmwd.com/your-water/novato-water/ (see "FEMA Flood Maps" link). NMWD doesn't have a local or regional flood control mission or purpose and Stafford Dam only provides a minor flood control function. In 1985, NMWD and the Marin County Flood Control & Water Conservation District (MCFC&WCD) entered into an agreement as part of joint project to modify the Stafford Dam spillway to delay the passage of flows downstream. The project, however, doesn't provide any flood control storage in the lake. Stafford Dam's primary function is water storage for domestic water supply.

The advancement of dam safety is greatly enhanced with the expertise of scientific institutions. They use a range of tools and practices such as FIRO, flyovers, weather balloons, radar along the coast, and collaborations between dam owners and scientific institutions. These practices, used by other water districts, serve as an example from which MMWD and NMWD can benefit. Response = Agree with the finding Statement: NMWD agrees that the expertise of scientific institutions and the tools and practices they develop have proved useful for a select group of dam owners in California. Many of the largest dams in California are owned and operated by either a federal agency or the California Department of Water Resources, a state agency3. It is important to note that Forecast Informed Reservoir Operations (FIRO) is only fully practiced at one dam in California: Lake Mendocino4. It is still considered a pilot program and being studied at only three other locations in the state: Lake Oroville, New Bullards Bar, and the Prado Reservoir. Lake Mendocino is a dual-purpose reservoir, providing both water supply storage and flood control storage (the former is the responsibility of Sonoma Water and the latter, the US Army Corps of Engineers (USACE); overall dam safety is a USACE responsibility). FIRO practices at this dam provides better st 3 Top 3 largest dams and owners: Shasta Dam – US Bureau of Reclamation; Oroville Dam - CA Department of Water Resources; Trinity Dam - US Bureau of Reclamation. Overall dam safety and the flood control operation of this dam is the responsibility of the US Army Corps of Engineers, not Sonoma Water. Page | 4 orage management within these two distinct volumes within the reservoir. Stafford Lake doesn't have a flood control pool, only an available volume for water supply storage. NMWD has been monitoring the progress of the Advanced Quantitative Precipitation Information (AQPI)5 system development in the region through our partnerships with Sonoma Water as well as Marin County. As part of that project, a series of new X-band radars have been installed in the Bay Area and a new C-band radar is planned for a site in Marin County. As the project progresses NMWD will continue to evaluate its role and level of future. It is anticipated, but not fully understood, that AQPI will have a benefit for NMWD in regards to dam safety. The responsibility to develop engineering criteria or guidance and associated regulations lies with state and federal agencies tasked with dam safety. These agencies have the expertise and funding for those efforts as well as existing partnerships with other state and federal agencies with a scientific mission that allows vetted and verified scientific research to be applied to engineering criteria. The science-based agencies utilize the tools and practices noted in the Report's findings. The Center for Western Weather and Water Extremes (CW3E) is the preeminent organization involved with the science of atmospheric rivers. NMWD staff have been actively participating in webinars and email updates, including AR forecast products from the CW3E as well as the California-Nevada Drought Early Warning System for the last several years. There are currently only 14 water providers in California that are active members of CW3E's Water Affiliates Group (WAG). See explanations for R1 and R6 below for more information on this topic. Report Recommendations