Post-fire Flood-risk Assessment of Sycamore Flat in the Arroyo Seco Watershed in Monterey County, CA

Risky land use decisions have led to large amounts of damages from flooding. Wildfires can potentially alter the risk of flooding by increasing the likeliness of high flow events due to multiple physical processes that increase the volume and intensity of runoff and erosion. In June and July of 2008, the Basin Complex fire burned 240,170 acres of the Arroyo Seco Watershed located in Monterey County, CA. Sycamore Flat is a developed flood plane of the Arroyo Seco River that is lacking a post-fire flood risk assessment that could improve the timeliness of the National Weather Service’s Flood warnings. A HEC-RAS model was developed by collecting and inputing survey data of the Arroyo Seco River and the adjacent flood plane, Sycamore Flat. The point of incipient flooding is 7100 cfs, with a recurrence interval (RI) of 1.9 yrs. Cross sectional geometries were adjusted to reflect 0.5 m increments of aggradation, and the relationship between aggradation and discharge (Q) was identified (y = 478.71x3 1760.7x2 1385.1x + 7087.3, R2 = 0.9997). A Log Pearson Type III analysis on historic peak flow data resulted in an aggradation to RI relationship (RI = 0.0222x3 + 0.0048x2 0.5175x + 1.9143, R2 = 0.9953). The results of this study, in combination with the monitoring of sediment accumulation in the watershed upstream, can be used by the NWS to more accurately indicate the point at which a serious threat exists. This could lead to an earlier evacuation, possibly saving lives or at least an earlier warning that will allow residents time to take actions that will minimize the loss of property.