by
Rosi Dagit
Conservation Biologist, Resource Conservation District
of the Santa Monica Mountains
January 1996.
Streambank stability is directly related to soil type, slope of the bank and especially the riparian vegetation. Trees in particular are essential. Their role in dissipating rainfall, encouraging ground water recharge, slowing run-off and retaining soil is well documented. In the Topanga Canyon Significant Watershed, the stream, the utility lines and the roads share common and often competing space within the narrow, steep sided gorges of the mountains. Under these circumstances, there is little latitude for the young stream to pursue its natural tendency to meander without causing substantial interruption of road access. This situation occurs most frequently during the winter rainy season, when the volume and velocity of the water moving through Topanga Creek and its tributaries can be substantial. In particular, the past seasons following the 1993 Old Topanga Fire have provided an extreme situation with tremendous erosion and sediment loads associated with higher than normal rainfall (60 inches in 1994/95).
During 1994-95, numerous projects involving road maintenance or improvement were undertaken along Old Topanga Canyon Blvd. Some of these measures were installed under emergency conditions in order to preserve road access. Others are intended to upgrade existing culverts and bridges.
Due to the importance of the Topanga Canyon Watershed as an Environmentally Sensitive Habitat Area and site containing important riparian and Significant Oak Woodland resources, there is great concern that the seemingly haphazard approach to these "improvement" projects fail to protect the stream integrity as well as road integrity. Given the space limitations inherent in the canyon, striking a balance between these two important components of the system is critical.
On a regular basis, trees along the road are pruned for line clearance by CALTRANS, Southern California Electric, Public Works and on occasion by other utility providers. Unfortunately, much of the pruning has been poorly done, leaving weakened trees which then require further maintenance. Discussions with Southern California Edison have indicated that they may include Topanga in a pilot project using directional pruning, rather than topping or unbalanced crown cleaning. Coordinated efforts between all agencies pruning trees in the canyon is essential if the trees are to receive proper care.
Therefore, the two main issues of concern regarding management of riparian vegetation and trees are: 1) an overall watershed management plan addressing stream volume, velocity and stream bank integrity; and 2) specific construction practices that can protect the existing riparian vegetation.
Besides the lack of a comprehensive plan for managing water flow for the entire length of the watercourse, construction practices commonly observed have been extremely damaging to the riparian vegetation, especially the trees. Work has not been done in compliance with the standards required by the Los Angeles County Oak Tree Protection Ordinance, or those specified in the Malibu/Santa Monica Land Use Plan, (emphasis added).
P68 Environmentally sensitive habitat areas (ESHAs) shall be protected against significant disruption of habitat values, and only uses dependent on such resources shall be allowed within such areas. Residential use shall not be considered a resource dependent use.
P78 Stream road crossings shall be undertaken by the least environmentally damaging feasible method. Road crossings of streams should be accomplished by bridging, unless other methods are determined by the ERB to be less damaging. Bridge columns shall be located outside stream courses, if feasible. Road crossings of streams within ESHAs designated by the LCP may be allowed as a conditional use for the purpose of providing access to recreation areas open to the public or homesites located outside the ESHA where there is no feasible alternative for providing access. Wherever possible, shared bridges or other crossings shall be used for providing access to groups of lots covered by this policy.
P84 In disturbed areas, landscape plans shall balance long-term stability and minimization of fuel load. For instance, a combination of taller, deep-rooted plants and low-growing ground covers to reduce heat output may be used. Within ESHAs and Significant Watersheds, native plant species shall be used, consistent with fire safety requirements.
P85 Earthmoving operations within ESHAs, Significant Watersheds and other areas of high potential erosion hazard (including areas with a slope exceeding 2:1) shall be prohibited between November 1 and March 31 unless a delay in grading until after the rainy season is determined by the Planning Director to be more environmentally damaging. Where grading begins before the rainy season, but extends into the rainy season for reasons beyond the applicant's control, measures to control erosion must be implemented at the end of each day's work.
P179 Avoid major flood control improvements which would limit water flow to or cover groundwater recharge areas.
1.0 Watershed Management Plan
In order to establish the best possible synergy between the stream and the roads along its banks, careful planning and integration of a variety of channel stabilizations techniques is warranted. Topanga Watershed is one of the few remaining streams in Los Angeles County which is not entirely armored. As such, the stream is able to support an unusual diversity of flora and fauna. Despite the numerous residential activities along the stream, many sections remain with intact vegetation, important amphibian habitat pockets, critical nesting sites for numerous raptors and migratory songbirds, and provide significant linkages allowing dispersal of many animals, from foxes to mountain lions.
Each time a new culvert is installed, the flow dynamics downstream are altered. Using Old Topanga Canyon as an example, there are numerous instances where channel modifications have caused downstream problems, necessitating more "improvements". Despite its mostly natural appearance, a large majority of the stream has been impacted by some type of hardscape attempt at bank maintenance. Many of the residential attempts at bank stabilization have relied upon variations of gabion construction, encouraging plant re-establishment. It was interesting to observe that these channel sections have recovering vegetation and limited erosion of the banks downstream. The area below the bridge at Red Rock is a good example of this.
Extensive worldwide use of natural methods for stabilizing banks (planting vegetation, willow wattles, post and wire revetments, deflectors, gabions) has been documented by Kondolf 1993 and Lee 1995. The importance of maintaining the ecological integrity of the Topanga Watershed warrants investigation and demonstration of these techniques in this watershed.
By establishing an overall management plan for the entire watershed, measures which are able to minimize further downstream impacts and prevent further loss of riparian vegetation could be implemented. A full discussion of bio-engineering techniques and their application as an alternative to concrete should be a major consideration of such a plan. In addition, planning ahead would allow for emergency responses that are truly appropriate in the long term maintenance of the road and stream channel.
2.0 Specific Construction/Maintenance Related Damages
In order to better understand the actual impacts of road and channel improvement projects on individual trees and shrubs, an inventory was taken during the months of December 1995 and January 1996. Beginning at Deer Creek Ranch at the top of the Old Topanga Canyon watershed, stops were made at all culverts or sites of road improvements over 3.5 miles. All trees (135) within 10-15 feet of the road were observed. Measurements included: DBH (diameter at breast height), height, condition of the canopy, trunk, and roots. Notes were made regarding slope, height of bank, location of stream channel and substrate. It was beyond the scope of this study to determine which trees were on public or private property.
The value of individual trees was calculated using the International Society of Arboriculture Standard Formula Method. This method uses a basic value modified by the size and condition. The species and location ratings were considered 100% for this study, since these are native trees in a natural setting.
Culvert 3.00 - near newly graded road
A drainpipe emerges just below the oaks (valued at $173,366.89) on the west side of the road. A newly graded bulldozer road falls steeply downslope. There is some erosion on the east side directing flow towards downslope trees.
Culvert 3.24 - 2061 Old Topanga Canyon Road (Chaffee Brothers property)
Most of the trees near this culvert are recovering from being burned in the 1993 fire. On the west side, there is heavy sedimentation and debris from the construction site in the stream channel. The large sycamore alongside the road is buried by piles of sediments. On the downstream side, the roots of the trees are buried on the high side, and a large swath of erosion exists below the outfall. The value of the oaks around this culvert total $26,336.40, the sycamore $72,278.25 and the eucalyptus $3,462.80.
Culvert 3.35 and 3.41 - near the Sprout Farm (1956 OTC Rd.)
Paving up to and on the trunk of several Coast Live Oaks is at least partially responsible for their present state of decline. Just west and approximately 10 feet downslope of these trees, the stream channel is meandering through. Evidence of severe erosion and undermining the roots of the trees stabilizing the bank is clear. It is quite possible that the failure of these trees will cause the bank to fail, and require some remediation. Installation of gabions in the near future could prevent such failure. The value of these trees in their present condition is $46,015.20.
Big Rock Ranch/Calmont School Culvert Installation
Four large Coast Live Oaks have had soil stockpiled over the roots, right up to and burying the trunks for over 6 months. Excavation of the stream channel unearthed numerous roots, which were left exposed. Heavy equipment was working right up to the trunks, severely compacting the soil. Equipment and debris were stored next to the trunks as well.
The value of these trees is $251,455.88 in their present condition. Given the nature of tree response to such impacts, decline should begin to become apparent in 3-5 years. Recovery will depend on how well the trees are treated after completion of construction. Remedial efforts to promote root growth and reduce soil compaction are recommended.
Bridge 750 - Red Rock Road
Proposed renovation of the bridge could have severe impacts on the surrounding trees. Preservation methods and careful planning could minimize those impacts. The value of the oak trees in their present condition is $284,692.38, plus an additional $23,667.60 for the large sycamore.
Culvert 4.96 - 1179 OTC Rd.
During the last big storms, the stream overflowed the road at this location. On the upstream side, the property owner has partially obstructed free flow of the creek, redirecting flow out of the channel and down the driveway. On the downstream side, the bank on the north is armored with a concrete retaining wall and the south side is held in place by several large oaks. A proposal to install a box culvert is in progress.
The value of the trees on both sides of the road is $265,842.71. Loss of these trees during the culvert installation will remove a functional bank stabilizer, and the increased velocity of the water moving past this section of channel could cause severe erosion extending several properties downstream. Preserving the trees and enhancing their stability with gabions may be a more reasonable solution.
Bridge at 971 OTC Rd. - Coastal Commission Permit No.4-95-123
Recent installation of cemented rip-rap at this site has probably caused reduction in channel capacity. The Coastal Commission Permit was applied for after the fact, and includes important mitigation for loss of riparian vegetation. Every effort should be made to retain the remaining trees and shrubs, several of which have been seriously undermined and are in need of stabilization. The value of the sycamores is $48,909.35, the willows over $3,054.00 and the oak $10,354.25 in their present condition.
The largest sycamore has slipped down the bank several feet, and the road shoulder above it has eroded away. Loose sediments remain in the channel and pose a problem when the rains wash them downstream.
Culvert 5.24 - south of 905 OTC Rd
Despite recent efforts at establishing an asphalt berm, the bank is eroding and the berm is beginning to slump in sections. This is an excellent site for revegetation, as well as encouraging the re-emergence of the existing willow community. A young Cottonwood tree (P. fremonti) has become established here as well. This is the only one documented in this drainage.
Water Main replacement impact area from 853 to 740 OTC Rd.
Numerous trees along the road were directly injured by equipment during the installation of the new water line. Equipment and stockpiles of asphalt and soil were routinely deposited underneath the driplines and onto the trunks of trees lining the road. Since there are several areas along the shoulder where the trees would not have been impacted by stockpiling, it is not clear why it was necessary to bury the trees. In fact, several debris piles remain, even though the project was completed quite some time ago.
The value of the trees injured by this project is approximately $104,946.82 for the oaks, and $183,053.72 for the sycamores. This valuation does not include trees located in the channel which have been impacted by sedimentation.
Culvert 5.67 - south of 649 OTC Rd.
Mechanical damage to the oaks near this culvert and an eroded gully downslope from the pipe outfall are factors in the decline of the oaks and sycamores around the site. The oaks are valued at $124,211.15 and the sycamores at $8,375.24.
Portions of the west bank are eroding severely and loss of tree cover in the vicinity will further that problem.
Between Culvert 5.67 and 5.77 - Road shoulder across from slipping slope
Cleared sediments from the east side slope which is continually slipping seem to get stockpiled along the shoulder of the road on the west side. This has buried some oak trunks and created areas of stream bank failure. While the bank is currently heavily covered by oak canopy, and held in place by the root systems, continued decline and loss of those trees will definitely necessitate mechanical support of that stream bank. Not all the oak trees were measured, but the value of those closest to the shoulder is $58,937.46.
Summary
The trees and riparian vegetation serve a critical role in maintaining stream bank integrity, allow for ground water recharge, help dissipate and reduce flow velocity and are an invaluable resource for preserving the environment in the Topanga Watershed. Current management practices disregard common and well understood industry standards for preserving trees during line clearance and construction activities.
There appear to be few procedures in place that attempt to incorporate engineering, hydrological and ecological concerns into the maintenance of the roads. Hence, Coastal Commission permits are submitted after the fact, the Environmental Review Board is rarely consulted and little effort is made to retain and preserve existing vegetation and minimize the channelization of the stream. Many sites identified as problems are well known. An overall management plan that uses a comprehensive hydrological survey, incorporates environmental constraints and attempts to solve problems drainage wide, rather than piece by piece is warranted. This could be presented as a program Environmental Impact Report and receive approval from both the ERB and Coastal Commission, as well as the community at large. By having a plan in place, emergency procedures could be defined and a long term management plan be implemented. The economic benefits of this would be substantial.
The economic value of the trees is only partially represented by the ISA valuation system. In fact, they are worth considerably more. They provide comprehensive slope stability, ground water recharge, velocity reduction and rainfall dissipation at no cost. The amount it costs to retain any portion of the stream channel after the vegetation is removed should also be considered the value of the trees and shrubs. When taken as a whole the value of the trees along the stretch of Old Topanga Canyon surveyed totals over 2.4 million dollars. Of this amount, the protected oaks contribute $862,398.00. It would cost many times this amount to achieve the same level of stream bank protection as they currently provide.
In order to preserve and enhance the function of the existing riparian vegetation, the following best management practices are recommended.
RECOMMENDED BEST MANAGEMENT PRACTICES
1. In compliance with the Los Angeles County Oak Tree Protection Ordinance, any work done within the protected zone of an oak or any other trees within 50 feet of a stream bank should be done by hand.
2. No stockpiling of dirt or equipment should be permitted within the protected zone of the tree (s).
3. Tunneling under roots rather than cutting them should be the standard. Any roots exposed during construction should be protected by wet burlap and reburied as soon as possible. Any cuts should be clean and smooth.
4. Creation of soil or asphalt berms to direct road run-off should avoid direct contact with tree trunks.
5. Line clearance pruning should be done according to ISA standards. Directional pruning and crown reduction are preferred methods. Coordinated efforts by all utility agencies should allow for a schedule minimizing repeated canopy reduction in any given growing year.
TOPPING, the indiscriminate cutting across the tree crown at a single height, is not an acceptable practice. It weakens the tree and sets the stage for further problems due to weak branch attachments.
6. Removal of understory vegetation, or burying such vegetation under permanent rip-rap or culverts should be prohibited except under exceptional conditions.
7. Documentation of existing riparian vegetation should be performed prior to any grading activities.
8. Use of methods encouraging re-establishment of stream vegetation should be preferred over concrete or rip-rap retaining walls.
9. The services of a consulting biologist/arborist should be sought prior to and during both the design and implementation phases of all projects in the Topanga Watershed. Specified monitoring following completion of construction is also recommended.
References:
Kondolf, G.M. 1993. A cross section of steam channel restoration. (in press) Journal of Soil and Water Conservation.
Lee, D. 1995. Highway landscaping: erosion control and other challenges. Erosion Control 2(2):62-67.
Tree Values
Old Topanga Canyon Road
November 1995-January 1996
Appraised valuations based on the ISA Standard Formula: (Diameter (in2) - 19.6 ( %27.5 (basic price) + $1,805.00) ( condition %
Note: Location and species values considered to be 100% for native trees.
Trees within 15 feet of the road along the stream side or immediately adjacent to a culvert are listed.
Project/LocationQ. agrifoliaP. racemosaSalix sp.OtherCulvert 3.0072,340.12
35,427.17
65,599.60Culvert 3.246,600.80
7,920.80
11,814.8072,278.25a. 3,462.80Culvert 3.3523,573.00
418.00Culvert 3.412,833.60
15,323.00
3,867.60Big Rock/Calmont36,906.38
90,675.75
25,500.75
98,373.00Culvert 4.70
Red+Rock Bridge 750194,199.50
5,730.73
13,714.25
18,858.40
52,189.5023,667.60Culvert 4.969,242.48
158,345.58
2,054.80
63,184.80
30,608.10
2,406.95Cheese Rock section (N+to S, stream side)
994 OTC-992 OTC Rd.11,897.20
14,495.95
14,616.75
10,850.70
2,714.9518,345.95
23,562.85
26,296.20
18,345.95
20,966.80
7,384.25
5,774.47971 OTC Rd. area11,814.80
10,354.25
2,750.5
14,616.75
2,750.50
16,976.80
1,527.00
1,527.00across from 926 OTC Rd.24,475.50
10,907.106,198.40
15,414.25
4,950.50
14,562.00
3,680.10b. 6,347.40Culvert 5.114,505.105,858.00Culvert 5.2512,425.109,521.10
11,688.00c. 3,094.65S of 905 OTC Rd.1,203.47
2,750.50
1,083.0059,114.80
2,750.50
1,527.00
5,213.47c. 1,543.60S of 874 OTC Rd.10,354.25
6,848.00
2,627.97
2,146.73
2,461.75
12,694.80
c. 1,353.75S of 860 OTC Rd.
14,542.80b. 6,757.65
d. 1,832.40
d. 1,584.60across from 859 OTC Rd.8,283.40b. 11,076.37S of 853 OTC Rd.15,531.37across from 810 OTC Rd.10,354.25S of 790 OTC Rd.3,003.40
80,293.87across from 774 OTC Rd.55,420.13
23,121.37across from 764 OTC Rd.2,627.97across from 754 OTC Rd.4,100.60
19,6676.75528.204,125.75Culvert 5.43107,168.25
1,155.50
907.80
17,050.50
44,336.10
11,901.37S of 711 OTC Rd.B. 4,125.75by dumpster 655 OTC Rd.9,089.25S of water meters (647 OTC)11,484.20
3,396.75
20,405.40
1,749.408,498.00Culvert 5.67114,543.65
7,934.25
1,733.253,280.12
5,095.12S of hydrant15,531.3726,854.50
1,733.25behind berms on W side east+slope failing1,056.40
4,950.50
1,320.50
1,320.50
18,583.87
6,435.65
15,532.80
5,095.12
1,362.00
3,280.124,600.12
3,712.95Culvert 5.775,095.12
20,790.75
3,736.42
4,129.67
5,429.10
Other species:
Eucalyptus globulus
Umbellularia californica
Populus fermonti
Juglans californica
It was not within the scope of this survey to determine which trees are publicly or privately owned. This table simply provides a list of trees that are being impacted by roadside maintenance or improvements located from the top of the watershed near Deer Creek Ranch, as far south a culvert 5.77. Due to time constraints, it was not possible to complete the survey to the junction with Topanga Canyon Boulevard.
NOTE: Condition rating of the trees represents their status as of December 1995. Several of the trees had experienced considerable decline as a result of construction impacts related to the water main repair and would have had much higher value had they been evaluated two years ago. The trees impacted by the Big Rock culvert project have suffered from soil and root compaction, as well as smothering while dirt was stockpiled for many months. The stress and subsequent decline related to this will not become visible for several years.
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