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

Abstract

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.