Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Introduction
criteria for all of Maricopa County, the Policies and Standards manual for each jurisdictional entity
should be referenced for specific guidance for incorporated areas.
The rainfall-runoff modeling procedures that are contained in the manual are physically based.
That is, the procedures are based, to the extent practical, on the physical processes that occur
during the generation of storm runoff from rainfall. While the basic procedures are physically
based, this does not assure that the rigorous application of the procedures will, in fact, reproduce
the actual rainfall-runoff phenomenon of any storm that has occurred or may occur in the future.
However, the procedure, when applied with good hydrologic and engineering judgement, should
yield consistent results for design purposes.
Throughout the development of the Hydrology Manual, three benchmarks were continually
applied in judging the applicability of individual procedures and the overall methodologies: accuracy,
practicality, and reproducibility. Accuracy is a measure of how well the results of the
procedure reproduce the physical process being simulated. Although accuracy is highly desired,
it is theoretically impossible to achieve in an earth science such as hydrology, and in a practical
sense, accuracy is not feasible to assess except for a few situations where adequate verification
data are available. Relative accuracy was assessed throughout the development of the procedures
in the manual through testing and verification against recorded data.
Practicality is a user’s decision regarding the best and most appropriate level of technology to
apply considering the information that is available, the anticipated uses, the consequences of
error, and the desired or required output. Whereas both simpler procedures and more sophisticated
procedures are available, the adopted methodologies provide a compromise between
these two extremes, and the best practical level of technology is judged to be recommended in
the manual considering the state of current hydrologic knowledge of arid and semi-arid lands.
Reproducibility is a characteristic that provides reasonable confidence that consistent results will
be achieved by all qualified users. Reproducibility is highly desirable for a design standard in
order to eliminate, to the extent possible, unnecessary conflicts over the interpretation and application
of the design method. Reproducibility is achieved through clear and concise manual procedures
and user guidance. Every effort has been made toward this end.
A brief discussion of the content of each chapter of the Hydrology Manual follows:
Chapter 1 Introduction - The introduction states the purpose, scope and limitations, and general
use of the manual.
Chapter 2 Rainfall - The characteristics of severe storms in Maricopa County are documented
as a setting for defining the design rainfall criteria. Procedures and information are provided for
the determination of depth-duration-frequency statistics of storms in Maricopa County. These are
1-2 August 15, 2013