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K Grindstone Joe Association N Flood Determination & Impacts San Joaquin County April 7, 2020 Page 2 of 3 database query at the Project location shows that the only maps available from the BAM Website at the Project Location are the FEMA effective FIRMS. Thus, the CVFPB does not have a Designated Floodway at this location and does not qualify as within the floodway under definition (3), unless the CVFPB has provided the County with maps not available to the public. The Project location does not meet the definition of a Floodway for definition (4). While the project location is located between the levees adjacent to Little Potato Slough and not at or above the special flood hazard area (SFHA) elevation (9 feet NAVD) it is not the watercourse itself. There is no FEMA or State Reclamation Board (now known as the CVFPB) designated floodway at this location. Definition (5) would require a determination from the County's Floodplain Administrator. Definition (1) is typically determined through hydraulic modeling. Understanding how these models calculate water surface elevations (and Floodways), the Project is extremely unlikely to cause a measurable increase in water surface elevations, let alone rise to a one -foot increase and based upon the changes in volume and cross sectional area, it is highly likely to cause a decrease in regulatory elevations (though not at a measurable scale). Floodways are calculated by "filling" the regulatory floodplain until the water surface elevation has increased by one -foot from the regulatory (or 100 -year) water surface. In a one-dimensional modeling scheme, the conveyance areas are reduced from the fringes towards the main channel until a difference of one foot has been achieved between the regulatory and the "encroached" model. A similar approach can be undertaken in some two- dimensional models, though volume tends to be more critical than a cross sectional area reduction in those schemes. The type of model described in the FIS for the Project area is typically either a one-dimensional/two- dimensional dynamically coupled hydraulic models or two-dimensional hydraulic models. Both potential schemes and their impacts are described later in this letter. In evaluating the project specific topographic changes along with existing site information, the below metrics were developed. It should be noted that based upon past experience, the Delta Hydro -dynamic model is not available for public use, however, typical values modeling grid cells were assumed. From a hydraulic analysis point of view, the project consists of two main components: the removal of the existing above -grade wastewater holding tanks and the installation of a single sub -grade wastewater holding tank. One existing wastewater holding tank is a 10.25 foot diameter, ground supported tank; the remaining four tanks were approximately five foot long by four foot wide, by 3 foot high. The existing tanks are completely above grade and will be removed as part of this project. The proposed single wastewater holding tank is entirely subgrade. However, to ensure it is properly anchored and compliant with County code, one foot of fill will be placed above grade (see previously provided Project plan sets). Typical ground elevations were obtained by reviewing available USGS LiDAR2 at the project site. The project site ground elevations range from 6-7 feet above NAVD (or two to three feet below the base flood elevation). A cross section was drawn from the crowns of the levees adjacent to Little Potato Slough and through the project location to determine the approximate cross-sectional area. The approximate extents of demolition and construction for the site were measured from aerial imagery and the minimal amount of grid cells needed to model the project location was determined using an assumed 25 -foot by 25 -foot grid cell (a typical grid cell size used in two-dimensional modeling). The characteristics of the project were then compared against the larger location 2 USGS LiDAR Collected in 2017 under Task Order G16PD01047