Flooding on the San Lorenzo River

Flooding of the San Lorenzo River at Big Trees, CA

The San Lorenzo River drains a small basin in the Santa Cruz Mountains in the Central Coast region of California (see map below). Under normal conditions, river flows are low and the river does not present a threat to the surrounding area. Floods, which result in significant property damage and loss, can however develop rapidly due to intense precipitation and the small size of the drainage basin. Recent years with severe flooding include 1982, 1995, and 1998.

The size of the San Lorenzo River basin (106 square miles) complicates efforts at predicting the potential for flooding. Small drainage basins typically have rapid responses to precipitation anomalies, thus reducing warning time and degrading estimates of peak flood stage. Regional and local variability of precipitation within the Santa Cruz Mountains also complicates efforts aimed at flood prediction. Precipitation amounts can vary significantly over short distances due to variability of the terrain. Predicting the total quantity of rainfall for a small drainage basin can therefore be extremely challenging. Forecasts of floods in regions such as the San Lorenzo River basin require good observations of current conditions, as well as an understanding of the evolution of local weather systems.

Short-term forecasting - Forecasters use a wide variety of information for predicting the potential for local flooding. The data include streamflow records from gauging stations, satellite observations of regional cloud structure, returns from local doppler radar, as well as output from numerical weather prediction models.

Effects of longer-term climate variability - While floods on the Big Trees section of the San Lorenzo River are most often associated with episodes of intense precipitation, climatological conditions also influence the likelihood of local flooding. In particular, the southward displacement of the jet stream commonly observed during El Nino events can increase the frequency and intensity of storms arriving in the central coast region of California. This can result in situations, such as that observed during winter of 1998, in which the basin cannot discharge sufficiently between storms. In these cases, subsequent storms, with only moderate precipitation, can increase streamflows to dangerous levels.

For more information about the relationship between flooding in California and El Nino, see the recent report by Monteverdi and Null. Note that relationships between flooding and climatological conditions are inferred from statistical analysis of data from broad geographic regions. The likelihood of flooding in small drainage basins cannot be fully assessed from only long-term climate variations.
This map was provided by the US Census Tiger Mapping Server.

In any region prone to flood emergencies, it is important to have good information regarding local weather conditions, and a well thought-out plan which can be implemented in the event of imminent flooding. The links below point to examples of the types of data analyzed by forecasters in their effort to predict local flooding. (Note that this is only a small subset of the data used in forecasting). Links to agencies which provide information regarding local conditions and information regarding advance preparation for flood emergencies are also listed below.

Examples of data used to assess the potential for flooding:

* Near-real time USGS streamflow data (San Lorenzo River Big Trees station, Bean Creek near Scotts Valley)
* Local doppler radar (SFO, Monterey)
* Western US satellite images (Infrared, Water Vapor; both from NRL)
* Observed precipitation maps (past 1 hour, past 6 hours, past 24 hours)
NOTE: The internet is not a substitute for accurate, timely, local weather forecasts. Much of the data displayed at these links is provisional in nature. Links to example data are provided for educational purposes only.