Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 Total value of damageable property is comprised of the structural and content values described for the parcels within the 0.2% annual chance floodplain. Table 2-3 shows the total structure and content values by category and economic impact area. In total, the study area has just over $300 million in estimated damageable property. Table 2-3: Value of Damageable Property Within the 0.2% (1/500) Annual Chance Floodplain October 2011 Prices ($1,000’s) Land Use Structural Value Content Value Rural Urban Rural Urban TOTAL Residential 17,706 123,204 8,853 61,602 $211,365 Commercial 0 23,732 0 25,030 $48,763 Industrial 0 13,593 0 20,014 $33,607 Public 0 4,541 0 2,123 $6,664 TOTAL $17,706 $165,070 $8,853 $108,769 $300,398 12
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 3.0 Depth-Damage Relationships and Flooding Characteristics 3.1 Determining Flood Depths For structure and content damages, depth of flooding relative to the structure’s first floor is the primary factor in determining the magnitude of damage. The GIS database, provided by Stanislaus County contained spatially referenced polygons for each parcel in the study area. In this study, ground elevation was assigned to the centroid of each parcel using GIS. Foundation heights, which were determined via survey data from a previous Orestimba study, were verified by a new survey sample in 2007, where surveyors used the bottom of the wainscot of the residence. The results were very similar to heights estimated visually during field visitation and ranges from past Corps studies (1-1.5ft). These heights were then added to the assigned ground elevation to establish first floor elevations. Water surface elevations (WSEL) from the Flo2D model were provided in the form of shapefiles for the 20%, 10%, 5%, 2%, 1%, 0.5% and 0.2% ACE probability events. Details of hydraulic data and the Flo2D model can be found in the Hydraulics Appendix. The WSEL were referenced to the Flo-2D grid cells containing parcel centroids and by subtracting first floor elevations, depths were determined for each structure within the flood plain. Flooding depths in general were rather shallow with very few structures facing depths greater than 2 feet. Individual depths were assigned for each and every structure in the study area for the 20%, 10%, 5%, 2%, 1%, 0.5% and 0.2% ACE probability events and these water surface profiles were then imported into the FDA models. This data was used for establishing expected damages for each event. 3.2 Depth-Damage Relationships Damages to structures and contents were determined based on depth of flooding relative to the structure’s first floor elevation. To compute these damages, depth-damage (DD) curves were used. These curves assign loss as a percentage of value for each parcel. The deeper the relative depth, the greater the percentage of value damaged. The sources of the relationships were different depending on land use. For residential structures, the IWR DD curves were used in accordance with EGM-04-01. The non-residential structure DD curves used for this study are the standard 1998 FEMA Federal Insurance Administration non-residential curves based on insurance claims and empirical damages nationwide. These curves represent average Nonresidential structures throughout the nation and are appropriate for the structures within the Orestimba Study area. DD curves for non-residential contents were taken from the American River Watershed Economic Re-evaluation Report (ERR) expert elicitation for short duration flooding. Depth Damage relationships are shown in the tables below. 13
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Orestimba Creek Feasibility Study - Stanislaus County

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