Black Point Site, Wetland Delineation, Novato, California

Located in Marin County, this area was of particular interest due to the above average rainfall received. Use of one and two-dimensional numerical modeling from climatologic and hydrological data was conducted to delineate jurisdictional wetland boundaries because the above average rainfall conditions made delineating this area difficult, if not, impossible. Wetlands are delineated utilizing the U.S. Army Corps of Engineers, 1987 Wetland Delineation Manual under the normal 2-year, 24-hour return interval storm event. The 1987 Manual focuses on 'hydrology' as inundation and/or saturation within 12 inches of the land surface and emphasizes that the presence of water has an overriding influence on characteristics of vegetation and soil due to anaerobic conditions. Therefore, it is generally difficult to delineate wetland areas under above or below average rainfall conditions.

The focus of this study was to develop a method in which wetland hydrology could be assessed when above average conditions exist. The methods developed predicted a process by which long term daily climate data was utilized to create an average hydrologic period to drive the model. The area of study is known as the Black Point site and is located in Marin County, California, where above average rainfall has been experienced within recent years. A hydrologic model was developed and calibrated to 1992 precipitation data, thereby, simulating groundwater elevations within the study area. Initially, the model was used to predict surface ponded conditions in 1997, which could be independently verified. The study proved successful and further development of a 2-dimensional model was conducted to predict the degree of saturation beneath the ground surface to effect anaerobic conditions under the Corp's regulatory definition. An average precipitation year was constructed from the long term climate record and used as boundary conditions within the model to define wetland hydrology.

 A numerical model was developed using the computer program SWMS_2D.  The program solves a set of partial differential equations to determine the location of the water level and the moisture content in the soil as a function of space and time. The model was 'calibrated' to the 1991-1992 water year, as this year had the most reliable water level data. The model was then used to define wetland hydrologic conditions at the Black Point site. Final acreage from wetland boundary determinations made by Corps personnel in 1997 and the 1997 wetland model results differed by 0.5 acre in a 40 acre site or by approximately 4%.