Orestimba Creek Feasibility Study - Stanislaus County

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.
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