MONTECITO EMERGENCY DEBRIS FLOW MITIGATION PROJECT
Year
May 2019
Location
San Ysidro, Buena Vista, and Cold Springs Canyons
Six Geobrugg flexible debris flow nets have been installed in San Ysidro, Buena Vista, and Cold Springs Canyons. The construction was closely monitored by a supervising biologist, geologist, and a geo-tech engineer to ensure that the environmental conditions of our permits and authorizations from the multiple governmental agencies were carefully maintained.
Project Description
The Net Project is a debris flow prevention and mitigation system that will be located in five canyons north of the community of Montecito in Santa Barbara County, California: Cold Spring Canyon, Hot Springs Canyon, San Ysidro Canyon, Buena Vista Canyon and Romero Canyon. The Project involves the installation of 6 Geobrugg flexible debris control nets.
The basic debris flow protection system consists of a steel ring net engineered to resist the velocities and dynamic and static pressures unique to debris flows. Support ropes are installed into channel banks and transfer debris impact and pressure loads from ring nets to the ground. Excessive energy is absorbed by net braking elements in the wire support ropes.
The net design calls for a minimum elevation of three feet above the water surface of the low-flow channel to allow for natural stream processes and wildlife use. This space between the water surface and the bottom of the net will be maintained except during high-flow or debris flow events.
The nets are pre-fabricated to specification for each location. Net installation will be done by Access Limited Construction (ALC) with oversight from DRS Engineering. A biologist will be onsite to conduct wildlife surveys, monitor for permit compliance, and provide oversight of construction and maintenance work.
ALC has prepared a Work Plan (ALC 2018) that details the method of installation at each of the net locations. The Work Plan describes access, staging, equipment, and materials to be used at each net location. The method entails general procedures that are adapted to the specific characteristics unique to each site.
In general, equipment and materials will be deployed by helicopter. An Emergency Action Plan (EAP) describes how and under what circumstances work will be curtailed in the event of inclement weather. The EAP explains how weather data will be monitored and analyzed and what thresholds will be used to trigger evacuation protocol.
Maintenance of the debris nets may be necessary if damaged and in need of repair. Annual and post-event inspections will be conducted by the geotechnical engineers and ALC. Minor maintenance can typically be done using tools and materials transported by hand and foot. The need for removal of accumulated debris will depend on frequency, intensity, and the amount of precipitation experienced in the surrounding watershed.
Intense and localized rainfall events as occurred on January 9, 2018 have potential to mobilize soil and debris. The debris retention system will be monitored as described by ALC. Should the nets accumulate sufficient material to block the channel, equipment will be mobilized to the location via aerial transport once streamflow has subsided sufficiently to allow safe access.
When the watersheds are revegetated to pre-Thomas Fire levels, estimated in approximately five years depending on natural drought and vegetation re-growth cycles, the net systems will be removed entirely, generally by helicopter, under the supervision of biologists.
Performance Security
Given the loss of life and over $2 billion in assessed property values – and the high risk of debris flows this winter as outlined in the BGC Engineering Inc. analysis –the Santa Barbara and Montecito communities are supporting and will support all aspects of the Project financially.
TPRC’s installation agreement with ALC sets forth -- in addition to mobilization and installation costs -- the expenses of (a) annual maintenance, (b) complete removal in about five years, and (c) interim debris management in the event of a debris flow.
As a condition of TPRC pulling its permit, it will post the County’s standard “Corporate Surety Faithful Performance Bond” – or any other performance or surety instrument required by and satisfactory to the County – to fully guarantee the payment in advance of such annual maintenance, removal and debris management costs.
Budget for First 6 Nets in Montecito Canyons
Six nets have been permitted at precise locations determined by geotechnical and environmental experts and located by latitude and longitude: two nets in Cold Spring Canyon, two in San Ysidro Canyon, and two in Buena Vista Canyon (which lacks any debris basin).
Project Chronology
Concept design and permit processing.
Intent to File Subsequent Application
As the mitigation report by KANE GeoTech, Inc. dated October 5, 2018 describes, geotechnical engineers identified 71 net sites in the above five canyons that have the potential to catch significant quantities of debris above Montecito during the next five years. The map below shows the locations of the first 6 permitted nets. TPRC intends to seek permits for additional nets in 2019.
Location of Nets
The yellow flags below show the locations of permitted nets to be installed in Cold Springs, San Ysidro and Buena Vista Canyons. The green flags are proposed for Hot Springs and Romero Canyon on US Forest Service land.
