Riverine Flooding from Tropical Cyclone Hilary was Not Exceptional in the Palm Springs Area

Southern California experiences some of largest floods within the contiguous United States. Riverine flooding induced by Tropical Cyclone Hilary are placed within this context for the Coachella Valley and adjacent mountain tributaries, in the Palm Springs area. Preliminary streamgage data were combined with previous streamgage-based analyses performed within the flood potential framework , and merged with large flood evaluation, trend detection, and frequency analyses in the Flood Potential Portal , a decision support system for understanding, quantifying, and communicating about flood hazards.

The Coachella Valley is drained by the Whitewater River, with headwater streams in the San Bernardino and San Jacinto Mountains. These mountains experience some of the highest flood potential in the United States, and the highest west of Texas. The Coachella valley itself experiences relatively lesser flood potential, with floods being only 1/10th the magnitude, on average, than the mountainous headwaters. During the floods induced by Tropical Storm Hilary, on the evening of August 20, 2023, substantial floods emerged from the mountains in the headwaters of the Coachella valley, though at locations with streamgaging data these floods were only (at most) half the size of floods that can be expected to be yielded from these watersheds, and half the flood magnitude that infrastructure should be designed to safely pass. In the Whitewater River, streamgage data indicate that peak discharge can be expected to be 2.2 to 4.7 times larger than the preliminary value for the flood magnitude experienced on August 20th (21,100 cubic feet per second, cfs); infrastructure should be able to pass more than twice this discharge in the vicinity of Rancho Mirage. None of these floods of August 20th were extreme, as quantified using the flood potential method. Large floods are relatively common during the summer in these desert areas of the valley and areas to the south, from monsoon and tropical cyclone activity, though are less common (but still occur) in the San Bernardino and San Jacinto Mountains during the summer season.

Hence, it can reasonably be concluded that Tropical Cyclone Hilary induced flooding that was not extreme or exceptional in the Coachella Valley, but instead produced floods that can be expected to occur in any year when such meteorologic conditions emerge. Substantially larger floods can be expected in the future, as informed by floods of the past within this area. Communities and infrastructure should be resilient to floods substantially larger than what occurred on August 20th. And continued monitoring of streamflow at streamgaging stations is needed to detect and adjust for trends, due to such mechanisms as climate change; the U.S. Geological Survey (USGS) needs to be properly funded for these monitoring activities.

Supporting information:

• The San Bernardino and San Jacinto Mountains are in flood potential zones 20 and 20S, which have flood potential index values of 47.4 and 29.4. Zone 20 is in the 97th percentile, while zone 20S is in the 87th percentile. Zone 20 has some the highest flood potential in the contiguous United States. (Source: Mapping tool in Flood Potential Portal .)
• The Coachella Valley is in zone 18W, which has a flood potential index of 5.3 (34th percentile). Floods in this valley and other portions of zone 18W are 5.3/47.4 = 11% of the magnitude, on average, as floods in zone 20.
• Floods typically occur during the winter in zones that include the San Bernardino and San Jacinto Mountains (November to March), but also occur in August and September (in 1954, 1976, 1983, and 1984). Floods in zone 18W occur with similar frequency during both the winter and summer months (seasonal peaks in large floods in February and August), with summer mechanisms of monsoon and tropical cyclones, and the winter mechanism of atmospheric rivers.
• Trends in large flood magnitudes, frequency, and flashiness are not increasing in Southern California. However, increasing trends are present for smaller-scale floods in zones 20 and 20S (annual peak discharges with an annual probability > 0.25). With the flooding of this past year, these analyses need to be recomputed, once the USGS finalizes the data. (Source: Mapping tool in Flood Potential Portal .)
• Snow Creek streamgage (USGS 10256500): 2380 cfs, 8/20@1700 (preliminary); record peak discharge: 9900 cfs, 1/25/1969; expected flood potential Qefp = 4300 cfs; Q100,index = 3810 cfs; Q100,regional regression = 4500 cfs; flood extreme index = Ef = 4300/2380 = 0.55; floods are extreme if Q > maximum likely flood potential discharge = Qmlf = 6880 cfs (Sources: USGS and Watershed Analysis tool in Flood Potential Portal .)
• Tahquitz Creek streamgage (USGS 10258000): 1900 cfs, 8/20@2245 (preliminary); record peak discharge: 2900 cfs, 11/22/1965; Qefp = 3950 cfs; Q100,index = 4530 cfs; Q100,regional regression = 5640 cfs; Ef = 1900/3950 = 0.48; floods are extreme if Q > Qmlf = 6320 cfs (Sources: USGS and Watershed Analysis tool in Flood Potential Portal .)
• Whitewater River streamgage (USGS 10259100): 21,100 cfs, 8/20@1745 (preliminary); record peak discharge: 27,500 cfs, 2/14/2019; Qefp = 98,200 cfs; Q100,index = 66,900 cfs; Q100,streamgage FFA = 51,000 cfs (including Hilary peak; bimodal/multimodal magnitudes present); Ef = 21,100/98,200 = 0.21; floods are extreme if Q > Qmlf = 162,000 cfs (Sources: USGS and Watershed Analysis tool in Flood Potential Portal .)