Study assesses climate change risks to the Delta’s vulnerable communities, ecosystems, water supply, and flood management
Delta Adapts: Creating a Climate Resilient Future, simply called Delta Adapts, is the Delta Stewardship Council’s climate change study consisting of a first-ever climate change vulnerability assessment and adaptation strategy for the Sacramento-San Joaquin Delta and Suisun the Marsh. The study will help the Council assess specific climate risks and vulnerabilities in the Delta and, in coordination with a diverse group of stakeholders, develop adaptation strategies to address those vulnerabilities.
The Delta Adapts study consists of two phases:
Phase 1: A vulnerability assessment to improve understanding of regional vulnerabilities due to climate change in order to protect the vital resources the Delta provides to California focusing on State interests and investments.
Phase 2: Develop an adaptation strategy that details strategies and tools that State, regional, and local governments can use to help communities, infrastructure, and ecosystems thrive in the face of climate change.
The Council staff is in the final stages of completion of Phase 1 and is preparing a draft report that will be available for public comment in early 2021. At the November meeting of the Delta Stewardship Council, they shared some of the key findings of the analysis.
Harriet Ross, Assistant Planning Director with the Planning Division of the Council began by noting that the climate change is already altering the physical environment of the Delta and that the Delta will continue to experience climate change through hotter temperatures, more severe wildfires, and prolonged droughts. The Delta Stewardship Council authorized the climate study in 2018, and in early 2019, the Council endorsed a set of resilience goals that built upon the coequal goals.
She also noted that over the long term, climate change in the Delta is expected to adversely affect human health and safety, lead to economic disruptions, diminish water supply, degrade water quality, shift ecosystem function and habitat qualities, and increase the challenge of providing basic services. Many of these impacts will disproportionately affect disadvantaged communities.
“Although the extent of those impacts in the future is not exactly known, Delta Adapts will help the Council to assess specific climate risks and vulnerabilities in the region, and our next phase of the study in preparing the adaptation strategy will represent a big step in identifying ways to address them,” she said, noting that the study was conducted in coordination with a diverse group of stakeholders over the last two years. “The specific goals of the climate study are to inform future Council work, help the state prioritize future actions and investments, provide a tool kit of information for local governments to use in their regulatory documents, and serve as a framework to be built upon for the Council and others in the future,” Ms. Ross said.
Delta Adapts is a regional planning level study that covers the entire Delta and Suisun Marsh, consistent with the Council’s regulatory authority, and is designed to inform policy. Ms. Ross acknowledged that a lot of other climate assessments of the Delta have been done by state agencies and individual cities and counties, but these typically focus on assets those agencies own or analyze vulnerabilities based on certain climate conditions. The Delta Adapts study covers a much broader range of asset categories and climate change conditions and is designed to be complementary with all of the other existing efforts.
The study looks at a broad range of climate stressors, including changes in air temperature, precipitation, hydrologic patterns, and sea level rise, and the corresponding climate hazards of extreme heat, wildfire, drought, and flooding. During the development of the vulnerability assessment, staff had an ongoing collaboration with agency partners, and built off of existing models and data, working hard to ensure the studies are complementary, especially across state agencies.
They held stakeholder briefings to ground-truth the data and verify the results, and reached out to community-based organizations for assistance in structuring engagement to reach vulnerable communities in the Delta. There was a technical advisory committee consisting of experts on the system and in climate change who really provided invaluable input into the technical approach and analyses and have reviewed all of the work. A stakeholder workgroup consisting of local agencies, cities, counties, environmental groups, water districts, and others provided data and input early into the process.
“Overall, through all the engagement, the feedback we’ve received has been largely positive,” said Ms. Ross. “Most were impressed with the scale and comprehensiveness of our analysis, especially with our flood approach where we were able to explore different aspects of climate change in a way that has really never been done before. We heard feedback that our probabilistic flood maps, which shows the likelihood of flooding, are easier to understand when compared to more traditional flood maps. It was also noted that we made a concerted effort to address stakeholder concerns and ran to grab many of the issues that were brought up.”
Staff is still working on the draft vulnerability assessment right, but today they will review some initial key findings.
PRELIMINARY RESULTS: EQUITY ANALYSIS
Next, Avery Livengood, Senior Environmental Planner, gave the results of the equity assessment. She noted that a key part of the equity approach was to evaluate social vulnerability to climate change, so the first step was to review studies done by other state and regional agencies to see what data and indicators that they have used to identify social vulnerability in general. Next, they conducted a literature review to identify factors that increase vulnerability to the specific climate hazards that the Delta Adapts study is focusing on.
The results are listed on the slide. “What we found is that many of the factors cut across all three of the hazards that we looked at,” said Ms. Livengood. “For example, preexisting health conditions such as asthma tend to make people more sensitive to the effects of flooding, heat, and wildfire. Income level is another example because it affects the household’s capacity to recover from extreme events.”
The list was then used to develop a custom social vulnerability index for the Delta. The index overlay has 14 factors, which made it possible to identify communities that have multiple intersecting characteristics that increase their vulnerability. Those communities with the highest vulnerability in the Delta are shown on the map in dark orange; these communities scored in the 70th percentile for more than half of the 14 indicators. During phase 2 of Delta Adapts, the map and the index will be used to identify and prioritize equitable adaptation strategies.
“Because this work is new, we’re still working to determine how exactly we are going to use it,” said Ms. Livengood. “That’s why we’re really excited that the index already has one real-world application with the 2021 Delta Science Proposal Solicitation. Staff published the social vulnerability index on the web map so it’s now publicly accessible, and it allows users to view the data and to explore individual indicators on the map. Funding applicants are directed to use this map to evaluate how their project will address one or more of the factors that contribute to vulnerability within a specific community. The map tool also makes the information accessible to anyone in the general public.”
Ms. Livengood acknowledged that what she just presented was focused on social vulnerability, but the equity and technical memorandum will be much broader and will lay out how the principles of equity can be addressed throughout the Delta Adapts initiative. Their approach has been to engage early and often with local stakeholders.
“For more than a year now, we’ve been contacting community-based organizations and service providers in the Delta for their feedback and ideas, and we’ve made a lot of additions to their work based on their recommendations,” she said. “A few examples, we added a food security indicator to the index. We also added evacuation routes to our asset database so that we can report on whether any of those routes are at risk of flooding in the future, and we’re currently working to produce an educational video about Delta Adapts so that people that don’t have time or wherewithal to read a long report will still be able to learn about our key findings.”
She concluded by acknowledging and appreciating the contributions of the organizations listed on the slide, and they will continue building on the collaboration going forward.
PRELIMINARY RESULTS: FLOOD HAZARDS
Andrew Schwarz, Supervising Engineer, then discussed flood hazards and the flood hazard maps. He began by explaining the approach and analysis that was taken to analyze flooding. Because flooding in the Delta is a very complex system to understand and to model, the flood model was built on existing tools that were adapted and improved in order to take advantage of the work that has already been done in the Delta.
“We built a model that can consider a very wide range of future climate change conditions, including changes in tide and storm surge, sea level rise, and Delta inflows,” said Mr. Schwarz. “We really wanted a model that would help us improve our understanding of this system and not just test one scenario or another scenario. Finally, we really wanted a model and some tools that were very flexible. We know that climate change information is constantly changing and being updated; in a few years the IPCC will release new scenarios and projections of climate change and we want to be able to ingest those quickly and update our analysis. So the tool that we built is very flexible and can be updated without having to issue another contract to a contractor to do this all over again in just a few years.”
Mr. Schwarz then presented the first map of flood hazards in the Delta under current conditions, noting that there are a few caveats: All of the maps assume no additional building or reconstruction of levees, so levee improvements stop at where they are at today. Similarly, up in the watershed, no additional improvements are assumed to those levees for flood control measures, because how much of that work would be done and where and when it will occur is unknown, so the easiest assumption is just to use today’s conditions and then look at those possible trajectories into the future with further adaptation as adaptation strategies.
The first flood map looks at current conditions; the darker the blue, the higher the flood risk. Under current conditions, much of Suisun Marsh is exposed to flooding, with the levees being overtopped during a ten-year event or an event that would have a 10% chance of occurrence in any year which is a fairly low level of flood protection. Conversely, throughout most of the rest of the Delta, there is a fairly low risk of flooding – it would take a 200-year event or an event that would have a less than half a percent chance of occurring in any one year. The blue colors in the southern Delta are generally restoration areas that we want to see flooded more frequently or areas with known flood deficiencies.
“Under a 100-year event, at current conditions, an event that would have a 1% chance of occurrence in any one year about 2% of the Delta’s 625,000 people would be exposed to flooding,” said Mr. Schwartz.
At 2050 conditions, there is substantial additional flooding throughout the Central and South Delta, including the Stockton area; levees are overtopped with much smaller storm events. The darker colored areas on the map represent flooding under an event that would have something between a 10-50 or 50-100 year recurrence of flooding. During a 100 year event at mid-century conditions, nearly 65,000 people will be exposed to flooding, including 11,000 people living in communities with the highest social vulnerability.
“That level of flood risk might make it challenging to make continued agriculture investments, especially in permanent crops and high-value crops,” he said. “In addition, several urbanized and urbanizing areas are exposed to flooding, increasing the potential for significant economic disruption and loss, and impacts to socially vulnerable populations.”
Moving out to 2085 conditions, most of the south and central Delta are exposed to flooding on less than a ten-year recurrence, so it doesn’t take much of a storm to start seeing massive flooding throughout the Delta.
“Another way to think about this is over a ten-year period toward the end of the century, these islands would have a 65% chance of occurring because that ten-year storm has a 10% chance of occurring in any individual year,” said Mr. Schwarz. “At 100-year storm event, 20% of the Delta’s population, over 120,000 people, would be exposed to flooding and of those, over 20,000 people would be living in areas with the highest social vulnerability. We move up to a 200-year storm event, 44% of the Delta’s population would be exposed to flooding, so nearly a doubling of the population that would be exposed to flooding, going from a 100-year to a 200-year event.”
He noted that most of the additional people that get added that would be more exposed are actually in the north Stockton area and the Pocket area of Sacramento, so this highlights where targeting investments in additional flood protection areas can protect tens of thousands of people from flooding.
“It’s also interesting to note on this map that even under these significant substantial changes in climate change conditions, the North Delta remains relatively secure and not prone to high flood risk,” said Mr. Schwarz. “That highlights the value of past flood management investments that have been made, particularly along the Sacramento and American Rivers, and most importantly, the Yolo Bypass, which allows us to accommodate huge additional inflows.”
It has always been known that there are parts of the Delta that are driven by the river system and other parts that are driven by the ocean processes, and the slide shows where that line of transition happens in the Delta. The red dots show areas driven primarily by flood risk from the river system; the green dots are the areas that are most vulnerable to sea level rise, and the blue dots are the transition zones where they are vulnerable to both of those processes.
“This is important because as we move to adaptation in our next stage, we’re really going to need to focus on the source of vulnerability and look at adaptation strategies that are going to address that vulnerability,” said Mr. Schwarz. “You can think about things like improving the bypass or upstream storage to manage those inflows into the Delta that will help those red dotted areas, but it’s not going to do much for green areas. For those areas, even if you reduce inflow from the rivers, the sea level rise is what’s going to challenge them, so we’re going to need to focus our adaptation on understanding what is driving vulnerability.”
“What does all of this mean?” he continued. “We already knew a lot of the Delta would flood with the expected change in climate change conditions, and we knew that flooded areas would disproportionately affect vulnerable communities. But now we have a very good idea of where the greatest flood impacts will occur in terms of people affected and approximate economic losses, so we can target future levee investments to protect those communities. We also know where the most socially vulnerable communities are so we can ensure that investments are equitable and focused on these communities. We know what drives flooding in different areas of the Delta, we can now structure adaptation strategies to address the cause, and design different adaptation strategies that are needed for flooding caused by increasing river flows or in sea level rise increase. This also allows us to task adaptation strategies to see how effective they would be at addressing these concerns.”
PRELIMINARY ANALYSIS: WATER SUPPLY
The analysis considered how the water supply system is sensitive to changes in temperature, sea level rise, and precipitation and how that can affect water supply reliability.
“Just like the sea level rise, the riverine process, and the flood risk, it’s really important for us to understand which of these climate processes are really driving vulnerability in the water supply system as well because our adaptation strategies will be targeted to address those vulnerabilities,” said Mr. Schwarz. “We have a lot of different levels of certainty about climate projections. We understand that temperatures are going to go up, that’s relatively certain, but how precipitation will change is a bit less certain, and so that should factor into how we do adaptation as well.”
Higher temperatures are the most certain and pose the greatest risk to the water supply system. More variable precipitation, while fairly certain but not as certain as temperature, was especially impactful during dry periods, and that sea level rise, which is fairly certain as well, is of less concern to the water supply system relative to the other factors.
“It doesn’t drive reductions in water supply reliability in the same way that the other two factors do,” he said.
Higher temperatures mean more rain, less snow, and more runoff in the core winter months when it can’t be captured in reservoirs because of the need to provide flood protection; in March, April, and May when reservoirs are no longer managed for flood protection and now managed for water supply, the snow is really mostly gone and there isn’t enough spring snowmelt to refill those reservoirs.
More variable precipitation means there will be more years that will be wet, more years that will be dry, and fewer that will be closer to average. In addition, wet years just don’t provide much benefit to the system actually because the system that we have today really can’t capture that additional runoff. And in those additional dry years, they really intensify and expand the drought conditions.
“Overall we found that climate change will reduce Delta exports in all year types, from wet to dry, but the impacts will be disproportionately greater in dry years,” said Mr. Schwarz. “So the dry year ability to deliver water falls much, much more significantly than our wet year ability to deliver water from the Delta. Climate change will also chronically reduce reservoir storage in all years, meaning that less water can be carried over from one year to the next, increasing vulnerability of droughts, and the impacts of those droughts when they do occur. All this means greater water shortages, especially in dry years, and generally lower reliability of Delta water.”
The analysis also considered how drought conditions may change in the future, and estimated that droughts similar to the drought experienced in 2012 to 2016 will be five to seven times more likely to occur under 2050 conditions.
“That was a really rare event we just lived through – about a 500-year event historically, so very rare,” he said. “And that’s going to be something like a 100-year to a 70-year reoccurrence in the future. It’s not surprising that it will become more common, but the degree to which an extremely severe drought would occur in the future should really be cause for concern and additional planning.”
PRELIMINARY ANALYSIS: ECOSYSTEM
Dylan Chapple, Senior Environmental Scientist then reviewed the preliminary results for the Delta Adapts ecosystem analysis. He noted that the past ecosystem loss in the Delta has big implications for climate change impacts; the Delta ecosystem has already been heavily compromised compared to the historical state with 98% of historical freshwater tidal wetlands have been lost since the 1800s. Climate change will further compromise ecosystem health without substantial investments in the Delta’s ecosystems.
The analysis considered over 170,000 acres of natural ecosystems which were grouped into unleveed ecosystems with tidal, riverine, and floodplain connections, and ecosystems protected by levees. They analyzed the impact of 6 inches through 6 feet of sea level rise on these particular ecosystems across the Delta landscape.
Unleveed tidal wetlands are critical habitat for a number of species and the target of thousands of acres of restoration investment. For the analysis, they partnered with the USGS to model wetland accretion which describes the ability of these wetlands to change elevation and persist in response to sea level rise. Wetland elevation is a key determinant in ecosystem function, so the risk is defined as either the transition of a higher elevation marsh to a lower elevation marsh or as the complete drowning of wetlands when it becomes a mudflat or open water.
Under up to 2 feet of sea level rise, tidal wetlands are able to maintain their elevations and risk is low. However, under 3.5 feet of sea level rise by 2085, 53% of freshwater wetlands in the Delta and 100% of brackish tidal wetlands in Suisun Marsh are at risk. Under 6 feet of sea level rise, 100% of tidal fresh water wetlands in the Delta are at risk.
“Restoring tidal wetlands as soon as possible will allow them to maintain their elevation and reduce their risk of being lost,” said Mr. Chapple.
Tidal wetland connections to upland areas can allow these ecosystems to migrate and persist in response to sea level rise; however, upland transition space is extremely rare in the Delta. The Browns Island and Sherman Lake wetlands, shown on the left, account for the majority of tidal wetland acreage in the Central Delta region, but they are islands and have no connections to surrounding upland areas. In the Cache Slough – Yolo Bypass region shown on the right, wetlands in the Lindsay Slough area which is at the bottom left corner of the image, does have adequate connections to upland space, but the Liberty Island wetlands have only limited upland connections.
“Future investments in tidal wetlands that are better connected to upland transition space will substantially reduce the risk of wetland loss in the Delta,” he said.
For leveed ecosystems, flood maps were used to identify where ecosystems are at risk due to levee overtopping resulting from a combination of different sea level rise scenarios and a 100-year storm event, which has a 1% chance of occurring in any given year. For 1, 2, and 3.5 feet of sea level rise, 48%, 63%, and 78% of leveed ecosystems are at risk of flooding, respectively. Deeply subsided islands in the Central Delta and managed wetlands in Suisun Marsh are at the highest risk.
“This analysis looks at current conditions, and it doesn’t take into account future investments to update levee systems,” Mr. Chapple said. “So for the leveed ecosystems we have in the Delta, levee maintenance and the pursual of ecosystem restoration efforts like subsidence reversal wetlands that reduce flood risk will help protect leveed ecosystems in the Delta and Suisun Marsh.”
SIGNFICANCE AND NEXT STEPS
The Delta Adapts study is the first comprehensive climate change study in the Delta. As a result of the first phase, new comprehensive flood and water supply models have been developed that are flexible to new information. These are all open source models so others can use them and include their own information.
Also, Delta Adapts developed a socially vulnerable index that allows people to see geographically where these communities are located and they are defined; this allows a focus on where adaptation should occur and equitably in the future. They have also done extensive community outreach to establish those relationships with various organizations and service providers who are helping to structure outreach efforts to the vulnerable communities. Lastly, they have worked collaboratively with agency partners in developing the methodology, sharing data, and verifying the results.
For the next steps, the project team will present the remaining results at the December meeting of the Delta Stewardship Council. A public draft of the vulnerability assessment is expected to be released in early January with a 30-day public review period. Following that, they will be revising and finalizing the vulnerability assessment, and beginning the adaptation strategy shortly after that point.
DISCUSSION HIGHLIGHTS
Councilmember Frank Damrell asked about saltwater intrusion. To what extent does that impact this study and to what extent can we draw any conclusions?
“We looked at sea level rise as a component, particularly of the water supply system, so the time of the year we worry about saltwater intrusion is during the drier, warmer period where we’re managing for water supply,” said Andrew Schwarz. “That was where we really looked at whether salinity was going to penetrate further into the Delta. And the way the system operates and what’s programmed into the model is that the highest priority for the system to manage is to meet the salinity requirements and the other water quality regulations in the Delta first, so the first available water in the system goes to meet those requirements, and then water supply deliveries are made after that, basically.”
“What our modeling showed is, for the most part, the system is able to even with 2 feet of sea level rise manage those regulations most of the time, so in most year types, we don’t see salinity penetrating further into the Delta at all because we don’t allow it to, basically, because the system is managed to keep pushing that salinity out,” Mr. Schwarz continued. “But in the future, and even now, we see occasionally in rare years where there’s just not enough water in the system to keep that salinity out to manage all the regulations. That will happen in the future too and it will become more severe and more common in the future where we will have years where we have acute penetration of salinity deeper into the Delta, that will really affect in Delta water users because the water can become too saline for them to use. We did look at that, but we don’t see it happening on a regular basis. It’s not really a chronic problem; it’s more of an acute problem during droughts.”
Councilmember Oscar Villegas commented, “We should be sounding the alarm loud and clear for everybody to hear that the system that we have currently is not designed to withstand the multiple pronged challenges that we’re going to face going forward under many of these scenarios that you’ve laid out. The real takeaway for me is that the way we manage our system right now is going to have to change. We are currently in a space where the entire water system that we have coming through the Delta is driven in large part by a series of seasons where we have snowpack and we manage the water accordingly. And if in fact much of what we’re saying is that that pathway for how we manage our water is no longer going to be what we are receiving, we’re going to need to increase storage capacity because we’re going to need to have the flows to manage saltwater intrusion and we’re going to need the water flows to manage the water temperature for fish, we’re going to need all these things in a very different way, so I’m just astounded about how complex this process is … “
“This is designed to drive policy, and if folks are not recognizing the magnitude of what we’re dealing with, by the time it’s front and center for most of us and throughout the state that doesn’t deal with Delta issues on a daily basis, it will be too late to address systemic challenges,” continued Councilman Villegas. “I would offer that this report and the way it was presented I think is so succinctly, it should be required viewing as part of allowing folks to make public policy going into the future statewide … “
During the public comment period, Osha Meserve with Local Areas of the North Delta pointed out the multiple benefits of levees: “It protects the communities, industry, and agriculture in the Delta, the cities within and next to the Delta, and also habitat projects. As important and not understood as well is how much the levees protect statewide infrastructure, things like highways, pipelines, communications, and of course the state’s water supply through SWP and CVP rely on water being able to get through the Delta safely. Even with the tunnel, the proposal, the way it’s presented now is that the south Delta facilities would continue to be used, so the need for water to be able to flow safely through the Delta is a continuing need … Levees are critical to resiliency and it’s not an option to defund the levees in the future for a variety of reasons. Maybe we need to get more creative about how to put that funding together in a fair way … “
