Abstract
The Sacramento – San Joaquin Delta has been facing the major issue of land subsidence within the last century. This has created a high risk of failure of existing levees that have been protecting the area from flooding. Land subsidence below sea level is also creating the risk of salinity intrusion into the freshwater system of the Delta. Wetlands in the Delta help reduce oxidation of the land and compaction of the peats, which assist with reversal of land subsidence. These wetlands need to be re-established and maintained in the Delta. The study performed by Miller et al. (2008) on the West Pond on Twitchell Island in the Delta established in year 1997 shows that maintaining the permanent shallow depth of the water in a wetland with dense marsh vegetation helped to increase land elevation by 4 cm/year in 9 years. Therefore, it is important to study this wetland to understand its hydraulic characteristics. As a wetland is very big and it is difficult to study on-site, we tried to simulate West Pond using a mesocosm located at Sacramento State. A common way of studying the hydraulics of a water body is with the help of tracer study. We have used Rhodamine WT for this tracer study, which is also a common type of a dye used in tracer studies. The results calculated show that the Sacramento State mesocosm has a similar low hydraulic efficiency, with a hydraulic efficiency parameter of 0.11 compared to 0.04 for the West Pond and a peak residence time of 7.72 hours compared to 9.37 hours for the West Pond. The Sacramento State mesocosm shows less short-circuiting, with a short-circuiting parameter of 0.42 compared to 0.63 for the West Pond, a lower Peclet number of 1.09 compared to 5.58 for West Pond, a lower dispersion coefficient of 0.40 m2/hr compared to 698.02 m2/hr for West Pond and can be represented by a lower equivalent number of Continuous Stirred Tank Rector’s (CSTR) in series, 1.39, compared to 3.40 for West Pond. Analysis of West Pond and the Sacramento State mesocosm hydraulics and comparing them with studies performed by Dieberg et al. (2005) and Lin et. al. (2003) helped to show that such differences may be common for wetland mesocosms. To simulate hydraulics of wetland more accurately some options are to match the inflow and outflow locations, connect mesocosms in series, and match the nominal hydraulic residence time of the wetland. This study provides better directions for future studies using mesocosms to understand and improve hydraulics of wetlands, especially for land subsidence reversal.