Update of the Sacramento County Intensity–Duration–Frequency Curves

The Sacramento City/County Drainage Manual provides methods for estimating surface water runoff peak flows and volumes for the analysis and design of drainage facilities in the City and County of Sacramento. The Manual was published in 1996 and included rainfall intensity-duration-frequency (IDF) curves that were developed in a study concluded in 1990. Very few gages with sub-daily temporal resolution were included in the study. In addition, the more recent NOAA Atlas 14 included just two sub-daily gages located in Sacramento County in its IDF analysis.

Since 1990, the County has collected valuable high resolution data though the ALERT system. The region has experienced several large rainfall events in recent years, including the localized rainfall events that occurred in January of 1995 and May 2015 as well as the large atmospheric river event in January 1997. The IDF curves presented in the current manual are being re-evaluated, taking into consideration the additional 28 years of high resolution rainfall data (1990 to 2018) and recent advances in regional frequency analysis methods that reduce uncertainties in IDF estimates especially for more rare events. Potential climate change effects on rainfall extremes are also being considered.

The project involves the following tasks: (1) Rainfall data collection and quality control; (2) Trend analysis in observed and projected data; (3) Climate projection analyses; (4) Generation of at-site depth-duration-frequency curves; (5) Analysis of the spatial variability in extreme precipitation; (6) Development of the final IDF curves; (7) Development of depth-areal-reduction factor based on radar data, and (8) Presentation of a workshop for city and county staff to present the methods used and main results. Preliminary results indicated that for small duration (< 1 day) low frequency storms (<50 years) are currently underestimated, and high frequency storms are overestimated based on the 1996 Manual. For larger duration (1 day or larger), in average rainfall values match well the values proposed in the Manual, even though there are biases (over or underestimation) that are a function of the spatial location within the County.