Delta Island Subsidence Reversal A Criteria-Based Approach to Modeling and Evaluation Matthew E. Bates, Jay R. Lund Abstract Elementary modeling is used to estimate the probability distribution of elevations and water depths at failure for thirty-six subsided islands in the California Sacramento-San Joaquin River Delta, with a potential subsidence reversal rate of 4 cm/yr. Given estimated annual probabilities of failure, potential elevation gains are expected to be limited to one to two meters, before flooding. While in many scenarios this may seem insufficient to justify project investment, we propose that criteria exist for which one to two meter gains are significant. We introduce a criteria-based approach for evaluating which islands might be promising candidates for subsidence reversal. Based on a fixed subsidence reversal rate, this approach compares the annual probabilities of failure with estimated elevations in each year, then predicts the likelihood of specific islands flooding within the specified depth range and recommends relevant subsidence reversal strategies. As an example, we evaluate the implications depth-at-flooding has for the subsequent aquatic habitat. Egeria densa, a noxious waterweed currently combated by the State of California, is used as a sample species for choosing the depth-based success criteria. For some islands, even modest gains in elevation promote ecological benefits, moving islands out of the depth range favored by unwanted invasive species such as Egeria densa. This method and approach might be useful for better integrating subsidence reversal activities into long-term solutions for the Delta. Delta Subsidence Reversal Results Conclusions The California Sacramento-San Joaquin River Delta (the Delta) is an often unstable landscape At a subsidence reversal rate of 4 cm/yr, it will take 134 years before our deepest island reaches mean sea level. Within 40 years, only eight The risk of flooding dominates this system, and increasing the rates of subsidence reversal is whose fate is commonly debated. Far from the dynamic tidal estuary of pre-European times, of our shallowest islands could reach mean sea level. Significant elevation gains are plausible, but occur on extended time scales (Table 1). not expected to greatly change the anticipated depths at levee failure (Table 3). According to today’s Delta is a fixed system of islands and levees built by various groups and individuals Conversely, probabilities of failure increase quickly with time. Many islands reach a 50% chance of flooding within fifteen to twenty years. even elementary modeling, subsidence reversal operates on an extremely long time horizon. adhering to no uniform standard. As Delta islands were drained for agricultural production, At the rates modeled, withstanding floods for 40 or more years is unlikely for almost all islands. Our deepest islands have a 70% - 95% chance Separately, the probabilities of leave failure increase rapidly over time and approach 100 the rich organic soils once shallowly submersed have been exposed to decades of microbial of flooding before the -1.5 meter level is reached, and will likely be problematic no mater what strategies are tried (Table 2). Though annual percent long before most islands have gained significant elevation. This implies that the oxidization. The result is a collection of “islands” that rest meters below mean sea level (msl). risks of failure differ, even the lesser risks add up quickly. Thus, initial depth is the only characteristic that correlates well with flooded depth. goals of achieving mean sea level through subsidence reversal are unrealistic and that we need to modify our goals to include elevations below mean sea level. The social and Subsidence reversal has been the subject of ecological criteria that might determine realistic goal elevations for flooded islands are considerable policy and scientific discussion The colors in Table 1 are tied to our sample criteria-based depth ranges. In a simple yet tractable view, we suppose Egeria densa invades complex. As a tractable example of how this criteria might be used to evaluate candidate and experimentation in the Delta. Three most readily from 1.5 to 4.6 islands, we have presented an example scenario using the supposed invasion range of the recent publications make it possible to meters (5 to 15 ft) below noxious waterweed Egeria densa. Minimizing the chance of islands flooding in this range roughly project the effects of subsidence msl. This criteria is leads to clear island rankings and potential strategies based on expected outcome (Table 4). reversal across the Delta: introduced as an example framework to demonstrate Combining these results with economic considerations (Figure 4), leads to a field of tradeoffs Initial Elevation: Mean island elevations further analysis. It shows from which policy makers can select suitable projects. have been estimated by Mount and Twiss that by broadening our (2005) for thirty-six Delta islands in the year definition of subsidence- 2000. Estimated island elevations range reversal success, we can from -0.16 m to -5.36 m msl with an average allow at least a few projects elevation of -2.78 m msl. operating at reversal rates similar to the USGS average Subsidence Reversal Rate: Scientists from to be reasonably Table 3. Differences in flooded elevation based on differing subsidence reversal rates. the USGS (Miller et al. 2008) have spent considered. over a decade testing subsidence reversal Selection Criteria: marshes in the Delta. They have found local Investing in subsidence Initial Elevation in Island Name Probability of flooding Strategy 2000 (m, msl) subsidence reversal rates that range from reversal before islands flood into Egeria zone -0.5 to +9.2 cm/yr, an average 4.0 cm/yr. incurs construction and Coney Island -0.16 0 Union Island -0.27 0 Retain as marshy habitat. Roberts Island -0.73 0 maintenance costs, and Subsidence reversal may help Hotchkiss Tract -0.94 0 Annual Probability of Failure: A DWR Delta keep up with sea-level rise. No comes at the cost of Canal Ranch Tract -0.97 0 Figure 1. Map of the Sacramento-San Joaquin threat based on the selection Risk Management Strategy report (URS Grand Island -0.98 0 foregone agricultural criteria 2009) calculates the annual risk of flooding Delta showing estimated mean elevations in Brack Tract -1.41 0 the year 2000 (from Mount and Twiss 2005). revenue. While operational Terminous Tract -1.52 0 from seismic and flood-inflow events for Dead Horse Island -2.06 0.16 costs are expected to be Ryer Island -1.88 0.26 Good potential for subsidence each island. Wright-Elmwood Tract -2.10 0.35 similar throughout the reversal to provide benefits based Quimby Island -2.22 0.41 on the selection criteria , but may Delta, significant per-acre King Island -2.38 0.48 Probability of Failure and Initial Elevation be risky. Bradford Island -2.24 0.56 revenue differences exists Bethel Tract -1.99 0.58 amongst the islands. These Orwood Tract -2.52 0.72 Medford Island -2.45 0.73 price differentials, when Palm Tract -3.08 0.75 combined with a depth- Jersey Island -2.34 0.78 Brannan-Andrus Island -2.81 0.86 Very risky, subsidence reversal Woodward Island -3.27 0.89 based criteria for success, not recommended as a means of Victoria Island -3.16 0.91 achieving the sample criteria. are expected to influence Twitchell Island -3.17 0.91 project selection and also Holland Tract -3.73 0.91 Tyler Island -2.61 0.91 lead to a need for tradeoffs Jones Tract -3.26 0.93 (Figure 4). Staten Island -3.29 0.96 Tables 1 & 2. Annual elevation and probability distributions for subsided Delta islands, with a reversal rate of 4 Rindge Tract -4.38 0.97 Venice Island -4.29 1.00 Continue farming these islands to cm/yr. Three color-coded depth ranges are shown, roughly based on the anticipated invasion ranges of middle- Sherman Island -3.73 1.00 encourage further subsidence, McDonald Tract -4.65 0.93 based on selection criteria? Figure 2. Initial elevation (Mount and Twiss 2005) and annual probability of failure (URS dwelling invasive waterweeds like Egeria densa. Avoiding the Egeria range is a sample project success criterion. Bacon Island -4.82 0.72 Bouldin Island -5.09 0.52 2009) for the thirty-six Delta islands included in this study. Subsidence reversal may safely Empire Tract -5.13 0.50 reduce flooded volume without Webb Tract -5.15 0.49 Least expensive Most effective encroaching on Egeriarange.zone. Mandeville Island -5.36 0.32 Table 4. Probability of islands flooding within a supposed Egeria densa invasion range, with corresponding island rankings and depth-based strategies for subsidence reversal. Dead Horse Methods Ryer Wright-Elmwood Quimby Based on an initial elevation and an annual rate of gain, the elevation of each island can be King Bethel Bradford modeled over time using the formula dt = do + t * r, where dt is the subsided depth in any References given year, do is the initial island elevation, t is the number of years since subsidence reversal was initiated, and r is the estimated annual elevation gain, here estimated at 4 cm/yr. Miller, R.L., Fram, M., Fujii, R., and Wheeler, G. (2008). “Subsidence reversal in a re-established wetland in the Sacramento—San Joaquin Delta, California, USA.” San Francisco Estuary and Watershed Science, 6(3), Article 1. Independently, the probability of islands flooding over time is modeled with the recurrence- Mount, J.F. and Twiss, R. (2005). “Subsidence, sea level rise, seismicity in the Sacramento-San Joaquin Delta.” San t Francisco Estuary and Watershed Science, 3(1), Article 5. interval formula Pf = 1 – (1 - Pa ) , where Pa is the annually independent probability of levee failure and Pf is the probability of at least one levee breach within t years.