WORK PLANS - Laserfiche WebLink

Home Browse Search

4. SUBSIDENCE Step 2:Conceptual model Highly organic soils(histosols)predominate southern and Central Staten.Subsidence on Staten Island,as in much of the Delta,is primarily caused by the oxidation of organic soils such as those found on Staten(Deverel and Rojstaczer,1996).As Delta islands are dewatered for agriculture,the organic soils receive an influx of oxygen that drives decomposition.Delta soil organic matter is positively correlated with subsidence and GHG emissions as COz is emitted from the oxidation of organic matter. Explanation (Deverel and Leighton 2010,Deverel et al.2016b).Much of southern Staten is characterized by a Proposed Wells substantial layer of peat soil with between 18 and 47%organic matter content.The present-day „.w subsidence rate in southern Staten Island has been estimated at 0.8 inches per year(Deverel and �I Leighton,2010)and the area is up to 26 feet below sea level.Sea levels are predicted to rise from 0.033 to 1.031 inches per year to 2100(htto://www.opc.ca.gov/webmaster/fto/odf/agenda items/20180314/ltem3 Exhibit-A OPC SLR Guidance-rd1odf)The combination of sea-level rise and land subsidence increases the hydraulic gradient between the adjacent channels and the island floor. This leads to more seepage beneath levees which can reduce levee stability(Deverel et al.,2015). Drainage ditches must be deepened periodically to keep up with the sinking island floor to maintain an aerated root zone. v The implementation of rice and wetlands on the organic soils of southern Staten island has the potential to stop and reverse subsidence.Deverel et al.(2016)indicated that rice cultivation essentially stops or substantially slows oxidative subsidence.The flooded conditions in the rice fields establish anaerobic conditions at the water-soil interface which substantially slows decomposition.Managed and permanently flooded wetlands will stop subsidence due to the same mechanism(Deverel et al.,1998, 2014).The key difference between the two systems is that rice fields will only be flooded during the wz growing season while wetlands will be permanently flooded and rice grain is harvested whereas all the wetland biomass stays in the wetland.Since the wetlands are flooded year-round,organic matter production outpaces anaerobic decomposition and accretion occurs. Since the early 1990s permanently flooded wetlands on Twitchell Island were used to study how to halt and reverse subsidence(Deverel et al.1998;Miller et al.2008,Miller et al.2008;Deverel et al. Miles 2014).Since 1997,soil has accrued at a rate of 1.3-2.2 inches each year and accretion is still continuing 0 0.125 0.25 0.5 after 20 years.In comparison,farmed areas lose up to 1 inch of soil per year.The available data indicate that flooding rice fields during the warmest months when soil oxidation rates(Deverel et al.2016b). HYDR FOCUSI Staten Monitoring Wells-Plan Yew Figure Subsidence in the Delta has been the subject of several scientific studies which will be integrated in this PROJECT 5629 IDATE-. =412020 monitoring sector.The list includes Deverel and Leighton(2010),Deverel and Rojstaczer(1996),Deverel et al.(2012),Deverel et al.(2016),Hatala et al.(2012),Kirk et al.(2015),Deverel and Rojstaczer(1998), Deverel et al.(2014),Deverel et al.(2007a),and Ewing et al.(2006) Step 3:Hypotheses Restoring wetlands will reverse subsidence H S 1 a. on the project areas