Post-Wildfire Hydrology and Debris Flow Analysis Using Hydrologic Modeling System (hec-Hms)

Post-wildfire hydrology and debris flow prediction is one of the most fundamental indices used to determine operation and maintenance of debris basins. The accuracy of post-wildfire hydrology and debris flow prediction impacts downstream communities in terms of emergency response, flood control, debris flow control, and water quality and supply. Specifically, the upcoming wet season after fires consisting of high intensity-short duration rainfall events threatens to cause debris flows and increased flooding in areas affected by wildfires. The post-wildfire hydrology and debris flow modeling features in the Hydrologic Modeling System (HEC-HMS) are great tools to better understand expected increase in peak flows and debris yields from burned watersheds. The objective of this paper is to highlight the post-wildfire hydrology, debris flow, and debris basin management application tools within HEC-HMS that provide necessary outputs for development of post-wildfire debris flow risk and emergency management strategies. HEC-HMS provides flexible options to simulate post-wildfire hydrology including a simple SCS curve number loss method and a sophisticated dynamic infiltration loss method based on the Pak & Lee fire factor equation (2008). For the subbasin element, HEC-HMS has five different methods for debris yield estimation from burned watersheds including a simple approach using rainfall intensity and a detailed approach using peak flow. In addition to clean out records, HEC-HMS can provide critical information in managing the remaining capacity of debris basins based predicted debris inflow volume using the dynamic reservoir volume reduction feature in the reservoir element. The dynamic reservoir volume reduction feature uses trap-efficiency methods based on sediment/debris inflow from upstream watersheds. Also, for reach elements, HEC-HMS provide several sediment/debris routing methods including the existing seven sediment potential methods and a new Muskingum & Sediment Delivery Ratio (SDR) method. This paper demonstrates the usefulness of applying the post-wildfire hydrology, debris yield prediction, and dynamic reservoir volume reduction modeling features in the HEC-HMS for post-wildfire debris flow risk management and emergency management.