New tools help crews rate the hazards of landslides, debris flows, flooding, and more.

By Carol Forrest, Marcus Quigley, Nathan Jacobsen, Michael Harding, Cid Tesoro, and Doug Isbell

In the days and weeks following the devastating Cedar, Paradise, and Otay wildfires of October 2003 that blackened in excess of 375,000 acres, San Diego County and the City of San Diego separately undertook the tasks of conducting assessments of post-fire hazards and mitigating potential impacts. The process of rapid assessment of post-fire hazards and the emergency mitigation of primary and secondary impacts required efficient collection, processing, and analysis of field data and conditions. Both the county and the city contracted with GeoSyntec Consultants to assist with these monumental efforts.

FIGURE 1

Satellite imagery was used to create this pre-burn PHIRE analysis.

FIGURE 2

The post-burn analysis aided hazard-assessment and-mitigation efforts.

GeoSyntec used a number of recently developed tools and techniques during the post-fire hazard assessment, mitigation, and implementation process to improve the efficiency of the collection of field data during the assessment and improve the ability to make time-critical engineering decisions before the imminent onset of winter rains. These tools included rugged personal digital assistants (PDAs) equipped with integral global positioning systems (GPS) and multispectral satellite imagery, and automated feature analysis of post-fire imagery to delineate burn areas based on satellite imagery and to refine estimates of burn severity and watershed response. The increased potential for post-fire hazards and impacts was qualitatively evaluated using 1-meter pan-chromatic and 2.4-meter multispectral satellite imagery and the Spatial Analyst extension of ArcMap 8.3. Factors considered in the model were slope steepness, soil erodibility, and burn severity, which were combined to form a relative erodibility index. With the enormous size of the burned watersheds, this analysis allowed a rapid assessment of the hazards and impacts to values at risk (VARs).

A ruggedized PDA with integral GPS for data collection.

Project Manager Carol Forrest in Harbison Canyon.

The post-fire hazard index of relative erodibility (PHIRE) analysis of the post-fire satellite imagery allowed GeoSyntec to focus the fixed-wing aircraft and helicopter overflights on the most critical areas. The fixed-wing overflights (at 4,000 to 5,000 feet) and helicopter overflights (at 500 feet) further narrowed the areas to be covered on the ground with field-assessment teams. This rapid, multilevel approach to post-fire hazard assessment saved the city and county considerable time and money and facilitated the rapid deployment of site-specific mitigation measures to the most critical areas.

The ground survey teams employed PDAs equipped with integral GPS running Jetstream, a rapid-development relational database engine used for consistent data gathering. Information was gathered on drainage features, surviving vegetation, hydrophobic soils, burn severity, receiving waters, infrastructure, and surviving homes. The PDAs were downloaded every night into a whole-project field data management system and correlated with the aerial and site photographs.

Concurrent with development of a comprehensive hazard-mitigation plan, the city and county initiated early-action measures including public assistance (erosion control materials and guidance to homeowners), cleaning out storm drains, cleaning out sediment-retention structures, and protecting storm drain inlets.

The hazard evaluation and priority establishment was performed in a manner consistent with the approach taken by GeoSyntec staff in previous fires, which was first to rate the hazards (e.g., landslides, mudflows/debris flows/high sediment loads, flooding, rockfalls, retaining structure damage), and then to rate the impacts of those hazards (e.g., public health and safety, public and private property damage, damage to infrastructure, transportation route damage, damage to receiving waters). Based on these assessments, each site was given an overall hazard rating, and the sites with the highest hazard rating became the high-priority sites for development of hazard-mitigation plans.

A crew from the California Conservation Corps (CCC) installs temporary gravel bag check dams.

CCC crews installing fiber rolls on a hillside.

Selection criteria for mitigation measures included effectiveness, implementation cost, maintenance cost, environmental compatibility, regulatory acceptability, availability, suitability, and longevity. Specifications for candidate mitigation measures were developed, which in some cases included development of customized specifications for post-fire application. The mitigation measures included sediment control measures, erosion control measures, trash racks and debris flow devices, evacuations and warnings. Soil bacteria (Mycorrhizal inoculum) were used in limited areas where native seeding (with nine native seed species) was applied to burnt slopes.

Mitigation measures were selected for the high-priority sites, and hazard-mitigation plans and specifications were developed, using the satellite imagery as the base layer. GeoSyntec, on behalf of the city and county as applicants, coordinated with the Natural Resources Conservation Service under its Emergency Watershed Protection Program and with the Federal Emergency Management Agency for reimbursement of the eligible projects.

As the mitigation plans were finalized, materials were ordered and labor forces were contracted. Labor forces included hand labor crews (e.g., California Conservation Corps and Urban Corps) who were trained to construct temporary grade control measures, barriers and diversions, and slope interrupter devices. Experienced hydraulic erosion control contractors were retained to apply hydraulic mulch (wood fiber, tackifier, native seed, and mycorrhizae) and bonded fiber matrix. Construction contractors were retained to install trash racks and debris flow devices, such as k-rail.

Hydroseeding units apply mulch to steep slopes in Harbison Canyon.

Despite the mitigation measures, some areas were still at risk of flooding (because of steep slopes and short times of concentration) and warranted development of an evacuation and warning system. This system included identification of the homes at risk, installation of additional rain gauges, development of a three-stage warning system, and issuance of pagers linked to the county’s Emergency Alert System to homeowners.

Another result of October 2003 wildfires was the complete burning of the watersheds of three San Diego reservoirs: San Vicente, El Capitan, and Otay. A rapid assessment was conducted to quickly identify VARs and mitigation measures to help protect public heath, water quality, and infrastructure associated with the reservoirs. The reservoirs are used for non-contact recreation (boating and fishing), as well as their primary function to provide drinking water to San Diego.

Green hydroseeding areas delineate watershed mitigation in Harbison Canyon.

Erosion control methods on up-gradient slopes were not considered practical because of the vast size of the watersheds at each reservoir, so in-reservoir treatment systems were evaluated, selected, and designed. Mitigation measures included spillway debris booms, creation of sediment basins in tributaries using geotubes (geosynthetic tubes filled with dredged material), turbidity curtains deployed within the reservoir near the mouth of tributaries to partition sediment-laden runoff, and alum dosing to enhance settling of sediment particles.

The first test of the mitigation measures occurred December 25, 2003, with a storm that brought approximately 0.7 inch of rainfall to the county. This event triggered mudflows and debris flows in locations that were predicted by the PHIRE analysis. These debris flows caused the closure of some roads and affected some property, but did not damage any homes. Additionally, as predicted, there were high-sediment and -debris flows into the reservoirs.

Field engineering during implementation and changed conditions throughout the winter resulted in the need to update the plans. As-built plans and as-costs were also required for funding reimbursement. Because it will take years for the watersheds to recover, the city and county will face other issues, including problems in subsequent winters with the next level of priority sites, site disturbance from debris removal and the reconstruction process, and possibly permanent drainage design modifications necessitated by changed post-burn site conditions.

Overall, GeoSyntec found that use of the recently developed tools and techniques during the hazard assessment, mitigation, and implementation process significantly improved the efficiency of the collection of field data during the assessment, and improved the ability to make time-critical engineering decisions, which were vital given the magnitude and complexity of the task at hand. Although the repercussions of the fires will persist for quite some time, these tools, combined with a trained labor force, appropriate mitigation measure technologies, and a defensible plan, facilitated a timely and appropriate response.

Carol Forrest, P.E., Marcus Quigley, P.E., and Nathan Jacobsen, P.E., are with GeoSyntec Consultants in San Diego, CA. Michael Harding is with Great Circle International in San Diego. Cid Tesoro and Doug Isbell work for the County of San Diego.

EC - January February 2005