Evaluation of Subterranean Subsidence at Seal Beach National Wildlife Refuge
the purpose of this study was to determine the amount of subsurface subsidence at naval weapons station seal beach nwssb areas which include seal beach national wildlife refuge sbnwr since the last extensive survey in 1994 since no benchmarks were established on the tidal marsh data from the surrounding nwssb was used for the sbnwr rates of subsidence could vary between terrestrial and tidal environments since benchmarks were only terrestrial but if subsidence was occurring in terrestrial areas surrounding the sbnwr we assumed the same surface subsidence in the tidal marsh we sought to update established benchmarks and examine sub surface elevation changes from continuously operating reference stations cors established by the national geodetic survey ngs this information will be critical for modeling sea level rise at sbnwr the sustainability of the marsh may have important implications for conservation strategies of habitat and endangered species results from our ngs benchmark surveys showed that subsidence is still occurring at nwssb at a rate of 4 13 mm yr se 1 21 mm yr since the last survey in 1994 the nearby cors elevation trends confirm local subterranean subsidence and were within the range of measured local subsidence on the refuge however the differing elevation trends in the cors demonstrates the need for local measurements and site specific data the mean rate of subsidence from the measured ngs benchmarks is in agreement with results from a broader study of elevation change done for the los angeles basin fig 10 shen et al 2011 subterranean fluid extraction along with tectonic action is believed to be causing the regional patterns in subsidence and uplift bawden et al 2001 and the cyclical patterns in the continuous cors data are likely showing patterns of withdrawal and recharge from precipitation figs 5 9 the long term sustainability of sbnwr tidal salt marsh ecosystem faces threats from regional and local subsidence sediment availability and sea level rise the current rate of sea level rise in southern california is 2 1 mm yr nrc 2012 and when combined with subsidence rates sbnwr is experiencing a relative sea level rise rate three times more 6 23 mm yr than that of similar southern california marshes not experiencing subsidence compared to the nrc 2012 sea level rise projection curve we found that the current rate of relative sea level rise at sbnwr with subsidence is now similar to the rate expected in 2036 for areas of southern california without subsidence