E3S Web of Conferences 415, 06008 (2023)
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https://doi.org/10.1051/e3sconf/202341506008

Field investigation and design
environmentally sensitive areaofdebrisnetsinanWilliam Kane1*, Joseph McElhany1, and Brian Forsthoff1
1KANE GeoTech, Inc. 7400 Shoreline Drive, Suite 6, Stockton, California 95219 USA

Abstract. Flexible debris nets are an excellent choice for emergency debris protection. They arelightweight, have a small footprint, and are relatively easy to construct. Available design methodology basedon testing and modelling makes engineering simplified with straightforward calculations. After a huge
wildfire, Montecito, California sustained devasting debris flows. The need to quickly design and constructprotection was made possible proven methodologies under difficult environmental restrictions.

1 Introduction
The community of Montecito, California is one of themost well-to-do in the state. Its residents include many
wealthy and well-known individuals such as talk show
host and entrepreneur Oprah Winfrey and former Prince
Harry, Duke of Sussex, and his wife actress MeghanMarkle, among others.
Montecito is situated east of Santa Barbara,
California between the beautiful Santa Ynez Mountains
and the Pacific Ocean in Southern California, Fig 1.

Fig. 1. View of Montecito, California, from the crest of theSanta Ynez Mountains.

1.1 Thomas fireIn the western United States, wildfires scorch vegetationthat normally anchor soil and changes the soil
chemistry. As a result, rapid runoff and erosion causes
debris flows especially following large fire events inmountainous areas.
In December and January 2018, the Thomas Fire
became largest fire in California history. In early

January 2018, torrential rains stalled in the Santa Ynez
Mountains triggering devastating debris. These flows
overtopped debris basins and creeks resulting in major
property damage, injuries, and loss of life in the areas
below the mountains. This event resulted in the deathsof 23 people, destroyed 10% of the housing, and blocked
U.S. Highway 101, a major California transportation
corridor.
1.2 Geology
Geological evidence indicates the area has undergone
many such events in the past. It is in an approximately 8km wide region between the Pacific coast and the Santa
Ynez Mountains. The area is composed of thick,
Quaternary alluvium including flood plain deposits andprominent alluvial fan deposits most likely resulting
from debris flow events.
The Santa Ynez Mountains are a part of theTransverse Ranges of Southern California. The bedrockis almost entirely composed of interbedded sandstoneand shale beds ranging from the Jurassic Franciscan
formation to Eocene sandstone and shale. Bedding
varies from thick to very thin. These beds exhibit
differential weathering causing large, blocky sandstone
overhangs seen throughout the area. These blocks
eventually weather and collapse, creating boulders ofvarious sizes collect in the drainages. These boulders
then weather into rounded spheroids.
The bedding dip changes throughout the site and isgoverned by extensive folding and faulting throughout
the area. The Mission Ridge Fault is in the western areaof Montecito, while the extensive Santa Ynez Fault runs
along the entire width of the Santa Ynez Mountainabove Montecito. Vertical and overturned beds arefound in the south-eastern area of the Santa Ynez
Mountains of Montecito, [1].

* Corresponding author: william.kane@kanegeotech.comg© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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1.3 Environmental Concerns
California has some of the strictest and most daunting
environmental laws in the world. In addition to thestandards of the United States Environmental Protection
Agency, California also has the California
Environmental Quality Act (CEQA), and projects arealso subject to environmental review by the California
Department of Fish and Wildlife, and the California
Coastal Commission. The County of Santa Barbara,
where Montecito is located, has additional requirements.
Santa Barbara claims it is the originator of the modern
environmental movement. Inspired by g a massive oilspill in the Santa Barbara Channel in 1969, Earth Day
was inaugurated on April 22, 1970.

Fig. 2. Flight path used for aerial reconnaissanceinvestigation.

The mountains are heavily used by residents andvacationers for hiking and mountain biking and clubs
have been formed to protect and maintain the manytrails.

A total of 46 potentially suitable sites were identified
from the aerial phase. Fig. 3 is an example of location
determination from the air photographs.

Prior to obtaining permits for construction
environmental hearings were required and permits
obtained. The typical timeline for a typical project can
range from one to several years. It is not uncommon for
projects to never reach construction because the owners
finally give up or run out of money.

2 Site investigation
The project had a tight timeline. Beginning in May 2018,debris mitigation had to be completed prior to the next
year’s rainy season. After research of debris protection
mitigation strategies, it was decided that temporary
flexible ring net barriers were the best solution. The
barriers would have the smallest environmental
footprint and could be quickly constructed. The
philosophy was to retain portions of the massive numberof available debris close to their source rather thanconstruct large debris basins at the mouths of thecanyons.
Fig. 3. Potential sites determined from the air.

2.1 Aerial Reconnaissance

2.2 Ground ReconnaissanceAerial reconnaissance was made in early May 2018 witha helicopter. The purpose was to identify areas in eachof five major canyons that might serve as optimal
locations for debris catchment and retention, Figure 2.

Ground reconnaissance was performed from June
through September. Each canyon was hiked by teams
using drones, cameras, handheld GPS units, measuring
tapes, and range finders. The locations noted from theaerial reconnaissance were given special attention as
well as new potential sites not observed in the air photos.
This phase located additional barrier locations for a totalof 71 possible suitable sites.

Following the aerial reconnaissance, potential
barrier locations were determined. The criteria usedwere the following:




Channel dimensions
Storage potential
Constructability and access
Maintenance and access
Environmental impacts
Property easements
Anchoring material

3 Debris flow net design
Flexible ring net barriers were selected for the project.
The nets could be constructed quickly without much
environmental impact. However, certain environmental
constraints were imposed on the project. No
construction was allowed in the channel itself.

2

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Therefore, no barriers using post and foundation wereallowed. Construction could only occur in relatively
small areas with no access roads permitted. Therefore,
helicopters had to be used for the entire construction.
Finally, no materials such as anchor grout were allowedin the streams. All construction was strictly monitoredby an on-site biologist to ensure that the conditions ofpermitting were strictly followed.

DEBFLOW program was used in the engineering of theMontecito nets at each individual location.

3.1 Principles of debris flow net design

There are two basic versions of the Geobrugg debris netsystems. The VX net is intended for relatively narrow
channels up to 12 metres wide. The UX net is installedin wider channels (up to 27 metres wide and has posts tokeep the top net support rope from sagging. For the wide
Montecito channels where foundations were not alloweda “Super VX” net was developed. It is a modified VX
net with additional and stronger top net support ropes.
This eliminated the need for foundations in the channelbeds.

Design loads were for the standard Geobrugg nets weredetermined by testing and finite element modelling of
the Illgraben net.
3.2.2 Net Engineering

The principle behind debris nets is to catch debris flows
close to the source, usually high in mountain canyons,stop the massive flow, and then, if desired, allow thematerial to be placed back in the channel to allow naturalprocess to return it safely to the rock/hydrologic cycles.
The basic debris flow protection system consists of acustom ring net engineered to resist the velocities anddynamic and static pressures unique to debris flows.
Support ropes are installed into channel banks andtransfer debris impact and pressure loads from ring nets
to the ground. Excessive energy is absorbed by net
braking elements in the support ropes. In addition, thering net in the system allows the passage of water andfine sediment, eliminating the need to consider any
bulking factor when determining net height.

To produce construction drawings, it was also
necessary to determine the strength of the anchoringrock. Rock and soil properties were estimated during thefield investigation for each installation site.
Anchor design for the nets consisted of determining
the depth required to support the loads on the wire ropes.
The Post-tensioning Institute [4] methodology was used.
The Institute provides design charts with arecommended shear, or bond, strength for a particular
rock/grout combination.

Flexible debris nets can be constructed rapidly withminimal environmental impact and can be combinedwith the existing debris basins to maximize material
storage in the canyons. They have a small construction
footprint and do not change channel flow unless a debris
flow event occurs.

The weathered and fractured sandstone in the Santa
Ynez Mountains was estimated to have a bond strengthof about 700 kilopascals. The maximum test load for thenet anchors was 356 kN. Anchor depths weredetermined to be 3.35 m. Field anchor testing was done
prior to construction to verify capacities.

3.2 Montecito debris net design
3.2.1 Net Design Methodology

3.3 Net construction

As a result of extensive research at its Illgraben test
site, [2, 3] Geobrugg A.G. of Romanshorn, Switzerland
developed a methodology suitable for the design of its
debris flow net systems. A peak discharge is calculated,
and the flow velocity estimated. Once the mass andvelocity are known, design pressures can be determined.
Finally, the design height is calculated. It should be
noted that debris flows tend to be linear features so thatafter an initial dynamic impact, additional surges add
only a quasi-static load to the net, instead of a fully
dynamic load. In addition, the debris material already
impacted and de-watered on the net serves to absorb
some of the energy of subsequent surges. The result isthat much of the debris flow material is not against thenet, resulting in decreased energy absorption and heightrequirements.

A total of six nets were constructed according to plans.
Helicopters were used to ferry materials to the sites.
Anchor drilling was done using wagon drills poweredby compressors. Construction was completed in midwinter 2018-2019. Fig 4. shows a typical completed net.

Geobrugg developed a software program,
DEBFLOW, which determines the appropriate
Geobrugg debris flow system as a function of theparameters given above and site characteristics of aparticular debris flow basin and channel.
TheFig.4. Completed SVX barrier above stream channel.

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In the winter of 2022 – 2023, California experienced
a series of large and powerful storms. Debris flows inwildfire burn areas were common. However, the highest
net in the system was filled and prevented debris fromflowing further downstream into the development
sections of Montecito.

Fig.5. Filled SVX barrier after debris flow event (Photo:Pat McElroy).
The authors would like to thank the following
individuals for their input and assistance in bringing thisproject to construction within a very short time andunder difficult constraints: Les Firestein, The Project for
Resilient Communities; Brian McNeal, Kevin
Wiesman, and Sam Geiges, Access Limited
Construction, Inc.; and Saleh Feidi, Geobrugg NorthAmerica.

References
1.

T. W. Dibblee, Geology of the Central Santa
Ynez Mountains, Santa Barbara County,
California, California Division of Mines andGeology. Bulletin 186. (1966)

2.

Geobrugg, Design Concept, VX/UX Protection
System Against Debris Flow, Fatzer AG,Geobrugg Protections Systems, Switzerland(2003)

3.

Geobrugg AG (2018). DEBFLOW Debris
FlowAnalysis
Program.
CH-8590
Romanshorn, Switzerland (2018)

4.

Post-tensioning
Institute,(2014).
Recommendations for Prestressed Rock andSoil Anchors. PTI DC35.1-14 (2014)4