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Areal-reduction factors for the Colorado Front Range and analysis of the September 2013 Colorado Storm




Hulstrand, D. M., author
Kappel, W. D., author
Colorado State University. Department of Engineering, publisher

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Information about extreme precipitation is of interest in hydrologic engineering applications such as dam design, river management, and rainfall-runoff-relations. These require knowledge on the spatial and temporal variability of precipitation over an area. In order to obtain areal average values for hydrologic modeling purposes, point rainfall amounts are often transformed to average rainfall amounts over a specified area. This is addressed using depth-area curves which require the use of areal reduction factors (ARFs). The Colorado Department of Transportation (CDOT) Flood Hydrology Committee tasked Applied Weather Associates (AWA) to derive 24-hour ARFs for the Front Range of Colorado for area sizes of 1- to 1000-sqmi. In addition, basin specific ARFs for the September 2013 rainfall event were calculated for four basins (Boulder Creek, St. Vrain Creek, Big Thompson River, and Thompson River basin). This study was initiated due to areal limitations and potential issues associated with NOAA Atlas 2 ARF curves. AWA analyzed storm events along the Front Range of the Rocky Mountains extending from northern New Mexico through southern Canada, including the September 2013 event. Each storm event utilized in the analysis represented meteorological and topographical characteristics that were similar to each other and to the September 2013 event. These storms were selected to derive storm specific ARFs which represented to the meteorological and topographical characteristics of the four basins. The individual storm ARFs were utilized to derive a site-specific set of 24-hour ARF values to be used in the hydrologic analysis of four basins along the northern Front Range of Colorado.


Presented at the Protections 2016: 2nd international seminar on dam protection against overtopping: concrete dams, embankment dams, levees, tailings dams held on 7th-9th September, 2016, at Colorado State University in Fort Collins, Colorado, USA. The increasing demand for dam and levee safety and flood protection has motivated new research and advancements and a greater need for cost-effective measures in overtopping protection as a solution for overtopping concerns at levees and dams. This seminar will bring together leading experts from practice, research, development, and implementation for two days of knowledge exchange followed by a technical tour of the Colorado State University Hydraulic Laboratory with overtopping flume and wave simulator. This seminar will focus on: Critical issues related to levees and dams; New developments and advanced tools; Overtopping protection systems; System design and performance; Applications and innovative solutions; Case histories of overtopping events; Physical modeling techniques and recent studies; and Numerical modeling methods.
Includes bibliographical references.

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areal reduction factors
extreme precipitation events
probable maximum precipitation
September 2013 rainfall
Colorado flooding


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