American Society of Civil Engineers


Investigation of Flood Hazards on Alluvial Floodplains


by Philip J. Shaller, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618), Douglas Hamilton, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618), Macan Doroudian, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618), Parmeshwar L. Shrestha, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618 E-mail: pshrestha@exponent.com), Jene Lyle, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618), and Andrea Cattarossi, (Exponent, 320 Goddard, Suite 200, Irvine, CA 92618)
Section: Hydraulics and Structures, pp. 1-12, (doi:  http://dx.doi.org/10.1061/40792(173)421)

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Document type: Conference Proceeding Paper
Part of: Impacts of Global Climate Change
Abstract: Investigations were carried out to delineate the 100-year flood hazard for a project area located in the upper Coachella Valley of Southern California. A multi-disciplinary approach was adopted, integrating elements of geologic mapping, mapping of geomorphic features, detailed topographic data, hydrologic inputs, and numerical simulations of flood propagation. The San Andreas Fault dominates the geology, groundwater conditions, landforms and surface water hydrology in this area. Splay faulting between the major faults may be responsible for the formation of aligned channels and ridges on the floodplain northwest of the project area. Active uplift and a shift to intermittent stream flow during the Holocene have resulted in incision of Morongo Wash into the alluvial plain. As a result, most of the flooding in this area is confined to the existing channels, particularly Mission Creek and Morongo Wash. In contrast, the alluvial fan at the mouth of the neighboring Long Canyon watershed is the result of a typical desert watershed dominated by occasional flash-flood type activity. Historical aerial photo coverage of three floods that struck the area indicate that flooding on the Long Canyon fan follows typical alluvial fan flooding patterns, and that associated with Morongo Wash and Mission Creek follows typical riverine flooding patterns. A two-dimensional flood routing model was applied to assess the potential risk of future 100-year flooding at the subject property from Morongo Wash, including a forced overflow scenario. To adequately characterize flood propagation and flood routes, it is critical that subtle topographic features be accounted for in the numerical model. For preliminary flood routing investigations, the USGS DEM data are generally a good source of topographic information. However, the relatively coarse resolution of the USGS DEM data and the significant age of the source survey measurements suggested the need for more accurate topographic data. Elevation data from Light Detection and Ranging (LiDAR) technology were therefore used. Model simulations were evaluated to predict the maximum discharge entering the project area as a result of temporal flood inflows from Morongo Wash, as well as the maximum water depths and extent of flooding in the interim floodplain.


ASCE Subject Headings:
Floods
Flood plains
California