Introduction

Modelers commonly use a deterministic approach to develop design storms and floods. In this process, they often select the median precipitation-frequency values for rainfall depth and assume that the frequency of the rainfall event matches the frequency of the resulting flood. Modelers apply a hypothetical temporal storm pattern or scale precipitation depths from a single historical event. Rainfall depths are further adjusted using a single regional areal reduction factor. Additionally, modelers must assign fixed values to rainfall-runoff parameters such as initial soil moisture and baseflow conditions. While deterministic modeling is standard practice and sometimes necessary for design storm analysis, it underrepresents and overlooks the variability in both storm characteristics and basin parameters (Ahmadisharaf et al., 2018; Yu et al., 2019).

The deterministic design storm approach could result in inaccurate predictions and potentially misleading conclusions. For example, although NOAA Atlas 14 provides upper and lower confidence intervals for precipitation depths across durations and frequencies, these uncertainty ranges are often disregarded during model development and analysis.  Temporal patterns are often critical to estimating the peak flow value and placing the heaviest rainfall near the beginning, middle, or end of a storm can significantly alter peak flow predictions. 

Variability within storm parameters and basin parameters can be incorporated in HEC-HMS.  In this application guide, part 1 will demonstrate how to vary NOAA Atlas 14 frequency storm depths, temporal patterns, and depth areal reduction factors using the Hypothetical Storm Meteorological Method.  Part 2 will demonstrate how to incorporate basin variability, specifically initial moisture and initial baseflow.  The model will be using the Uncertainty Analysis compute type.

Study Area

The San Lorenzo watershed is situated in Northern California, within the Santa Cruz Mountains. The San Lorenzo River drains an area of 106 square miles and flows through the city of Santa Cruz before emptying into Monterey Bay and the Pacific Ocean. The watershed falls within "Region 7" of the A14 temporal distributions. The San Lorenzo watershed is particularly susceptible to flooding caused by atmospheric rivers storms and lead to rapid runoff in the Santa Cruz Mountains.  San Lorenzo watershed also has a flow gage (San Lorenzo River at Big Trees, 11160500) that has daily and peak flow data going back to 1935.  A flow frequency curve was created in this tutorial: Modeling Flow-Frequency Relationships using HEC-HMS and used for comparison against the model results.  

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