Imperial County Hydrology Manual (CA)

A new hydrology manual was developed for Imperial County to provide guidance and recommendations on computational techniques and criteria for the estimation of runoff, discharges, and volumes for use in hydrology submittals to the County. The manual is organized into three primary sections: precipitation analysis, small area hydrologic procedure (Rational Method), and large area hydrologic procedure (NRCS Hydrologic Method).

Within each section, there is a general description which provides a brief overview of the topics covered.  This is followed by sub-sections with main topics relevant to the larger section.  Where applicable, procedures to perform detailed calculations are provided.  Online data resources are included with website links and a description of the website owner and data type in case the web page should be moved by the owner.  Related tables and figures are generally located immediately adjacent to the relevant text.  Example problems demonstrating the use of methods are located at the end of the section, and equations used in a section are numbered according to the section number and order of appearance in the section. The precipitation analysis section of the manual addresses topics including rainfall depth and intensity, depth-area reduction factors, temporal storm distribution, and a worked example.

The small area hydrologic procedure section of the manual includes typical Rational Method guidance for selection of the runoff coefficient, time of concentration, and other related parameters.  The rainfall intensity is determined from the NOAA Atlas 14 Point Precipitation Frequency Estimates to equal the time of concentration.  The use of the Modified Rational Method for junction analysis is also presented, along with worked examples. The large area hydrologic procedure section of the manual includes a description of the NRCS precipitation losses and runoff, watershed delineation, curve number determination, rainfall and the antecedent runoff condition (ARC), ARC adjustment values, preparation of incremental rainfall distribution, determination of excess rainfall amounts, S-graph selection, hydrograph calculation, and how to transition from the Rational Method to the NRCS hydrologic method.

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