1.0 watershed master planning
The broad master plan elements of the Topanga Creek Watershed Management Plan are identified and described in this appendix. These elements are required in Topanga to protect existing life, property, infrastructure, and the stream system from significant negative impacts due to flood hazards.
1.1 hydrology
Planning elements related to hydrology focus on quantifying the magnitude of the runoff and runoff reduction. Management of the flood hazard requires an accurate assessment of the flood threat. Assessment is accomplished through the application of hydrologic models to predict stream levels as a function of rainfall intensity and a variety of other factors, such as the ability of the soil to absorb water. Accurate prediction of stream levels will allow us to make rational decisions regarding the reduction of the intensity and magnitude of the flood hazard that are compatible with the resource protection goals, the needs of the community, and the policies (see Appendix B) of the Topanga Creek Watershed Management Plan (TCWMP). Reduction of flood hazard is best accomplished by decreasing the rate of runoff through either increased surface absorption or detention of runoff (Sections 1.1.2 and 1.1.3).
1.1.1 validation of the hydrologic and hydraulic models
The hydrologic and hydraulic models used to define the stream flow characteristics of the major watercourses in Topanga need to be validated. Fortunately, there is ample historic data for the Topanga watershed to modify the models as necessary. The validation procedure to be followed consists of using the data (i.e., rainfall intensity and stream heights) recorded in Topanga Canyon over the last 50 years, for a variety of storms, as the basis for checking the existing DPW model. The output from the model for a range of storms, in terms of maximum stream heights, is compared with the actual data for maximum stream heights at various places in the Canyon, particularly with the stream gage that was located near the mouth of the Canyon (i.e., just below the îS-curvesî). Where the model compares poorly, it is to be adjusted in a rational manner so that it agrees with the measured data. A range of rainfall intensities, amounts of watershed burn, and soil saturations is to be used in the validation so that the results of the model may be believed across the range of conditions for which the model is likely to be used. This range should include the storm scenarios (i.e., a spectrum of storm and site conditions) selected for evaluating various planning and runoff mitigation options. Suggested storm scenarios covering a spectrum of storm and site conditions for this evaluation are given in Table C-4.
Findings:
Accurate data are required to evaluate flood hazards with hydrologic models.
Validated hydrologic models are essential to the implementation of the TCWMP.
Recommendations:
C1.1.1-1 The stream gage in Topanga Creek should be re-established by DPW.
C1.1.1-2 All estimates of stream flow characteristics and elevations for watercourses in Topanga Canyon should be performed with hydrologic models that have been validated using data from Topanga Canyon.
1.1.2 reduction of peak flow runoff
Runoff detention provides an excellent means with which to reduce the peak flows of the Capital Storm and maintain peak flows as they were prior to development. Depending on the extent of the detention system constructed, the peak flows associated with the 50-year design storm (Appendix A, Section 1.3) could be reduced to those associated with a lesser storm with a 25, or even a 10 year return interval. Water is impounded only in those storms which exceed the return interval of the lesser storm. To determine the actual reduction in flow and the equivalent return interval of the associated reduced storm requires some calculations and a specific knowledge of the detention system used. Once the peak intensity passes, the detained runoff immediately releases and the system is again ready to perform its function.
Detention basins have proved effective elsewhere [7] providing not only flood protection but scenic and resource value as well (Figure C-1). Runoff detention offers an effective means to actually reduce the flood hazard in Topanga Canyon because:
with our small watershed the needed detention time will be fairly short and thus the size of the basin can be fairly small.
the threat is from short, localized and intense rainfall which again implies that short detention times are acceptable.
there are many side canyons which provide a number of sites where small unobtrusive detention basins could be constructed.
functional (serendipitous) detention basins already exist near roadways where roadfill of a side canyon has created a basin and where drained by an undersized culvert (as many are) they can reduce the peak flow.
Two major forms of runoff detention are envisioned in the Topanga Creek Watershed Management Plan: basins and ponds. The primary difference between these two concepts is size. Detention ponds would be maintained and built by individual home/property owners while the detention basins would probably be maintained/built by the State, County, owners of a major development, or the community.
Detention basins. To illustrate how detention basins would work, two areas in Topanga were examined. The locations of these areas are identified on the map in Figure C-2 and pictures of these two areas are shown in Figures C-3 and C-4. Detailed topography for these two sites is given in Figure C-5. An estimate of the capacity needed at these two sites to reduce the runoff of a 50-year storm to that of 10 and 25-year storms is given in Table C-1. Using Figure C-5, a rough estimate of the footprint for a detention basin was made as listed in Table C-2. Table C-2 also lists an estimate of the maximum average depth of water in the basin. To generate Table C-2, the 50-year storm flows computed by DPW were used to estimate the needed detention to reduce the effect of the 50-year storm to that of roughly 10 and 25-year storms. Reducing the storm from a 50 to a 10-year event, for example, means that the peak flow at the mouth of the basin will be no more than that of a 10-year storm for the rainfall of a 50-year storm. The greater the volume of water detained, the greater the reduction in flood hazard. These are first cut estimates and need to be refined with a more detailed analysis, but they do indicate that the basin volume needed is consistent with the physical layout of the sites.
Figure C-1. Detention basin in Corona del Mar; the outlet for the basin is in
the center of the picture while the top of the dam is in the foreground.
The site 1 detention basin will be of particular interest to Red Rock Canyon residents because of their repeated concerns with flooding. Constructing the detention basin at Site 1 could give the homeowners protection in the Capital storm. The site 2 detention basin (at the outlet of Zuniga Canyon) illustrates that there already some de facto detention basin sites (i.e., upstream from road fills) that for little or no cost may provide flood protection.
If the detention basins at sites 1 and 2 can detain even more runoff than shown, they could provide significant reductions in the flood hazard for a considerable distance down stream from these locations. For example, allowing a greater depth a Sites 1 and 2 could reduce the flood hazard for much of the creek along Old Topanga Canyon Road as well as the Township Site and Red Rock Canyon. Several locations in the Topanga watershed were identified as potential sites where detention basins could be constructed. Also a number of sites exist (e.g., Figures A-3 and C-6) for the serendipitous basins (i.e., those formed by road fill).
Siting criteria for detention basins will be needed to ensure that their impact on the community and the riparian habitat is minimal. These criteria would include: cost-benefit analysis depending on the amount and location of downstream development; priority to sites that mitigate impacts of existing development (e.g., Summit Pointe); the use of appropriate types of fill; and appropriate native vegetative cover; the manner in which the outlet structures are built; acceptable limits on the extent of grading, the size of the basins, and the kinds of locations considered (i.e., avoiding parklands and riparian habitats); and protection for trees and other significant flora. Detention basins that are to mitigate the runoff from a new development must be on-site.
Detention Ponds. Yards, fields and other relatively flat surfaces may be converted to provide ponding to reduce the peak flow during the 50 year design storm. The intent is to provide a simple solution that the residents can employ to reduce runoff or compensate for the increased runoff created by an improvement, such as a room addition. Ponds are simply constructed by placing a berm of 1-2 ft height around any reasonably flat area. The outlet for runoff from this area is configured such that no water is impounded unless rainfall intensities exceed those of the Reduced Design Storm (e.g., a 10 or 25-year storm). Lawns, patios, driveways, or any reasonable flat surface may be used; also a depressed area upstream from a road fill could be used.
A good example of the application of ponding is illustrated by a site where extensive grading was performed several years ago. This site, where two pads were created, was mentioned in Appendix A (Section 2.1) and is depicted in Figure A-2. The lower pad is bermed so that it could readily provide ponding and thus lessen peak flow runoff. This could be accomplished by reconfiguring the existing drain pipe, shown in Figure C-7, so that impounding of runoff occurred for the 50-year storm.
Findings:
Detention basins can be an important tool for reducing the existing flood hazard in Topanga Canyon. Detention basins offer an excellent means to reduce peak flow runoff in the major watercourses and are especially appropriate for a small watershed like Topanga Canyon. Criteria for their siting will be needed to minimize the impact of the basins on the riparian habitat. Some of these criteria were mentioned above.
Ponding provides a means for individual property owners to reduce peak flow runoff or mitigate peak flow runoff increases caused by changes (e.g., new paving) on their property. Ponding also reduces sedimentation and erosion by slowing runoff velocities and volume.
With only a modest change at some sites, peak flow runoff could be significantly reduced.
Figure C-2. Topographic map of portion of upper watershed in Topanga
(near Old Topanga Canyon Road).
Figure C-3. Potential location for detention basin, site 1.
Figure C-4. Potential location for detention basin, site 2.
(a) Site 1.
(b) Site 2.
Figure C-5. Detailed topography near detention basin sites pictured in Figures C-3
and C4. Figure C-2 shows a larger section of the topography.
Figure C-6. Other potential sites for detention basins (serendipitous sites).
Table C-1. Estimate of detention basin capacity needed to reduce the peak flows from
a 50-year storm to those of a 10 or 25-year storm.
Peak Flow Rate QEstimated Detention Capacity
(30 min duration)SiteStation50-yr storm (calculated by DPW)10-yr storm (estimated)
25-yr storm (estimated)
10-yr storm
25-yr storm1
(Red Rock)
2
(Zuniga)274
236540 cfs
360 cfs386 cfs
257 cfs448 cfs
299 cfs190,000 ft3
125,000 ft3
115,000 ft3
75,000 ft3
Table C-2. Estimate of detention basin depth needed to reduce a 50-year storm to
a 10 or 25-year storm.
SiteArea Available for Basin
a 10-yr storm
volume avg. depth
a 25-yr storm
volume avg depth1
(Red Rock)
2
(Zuniga)20,000 ft2
20,000 ft2 190,000 ft3
(30 min duration)
125,000 ft310 ft
7 ft115,000 ft3
(30 min duration)
75,0006 ft
4 ft
Figure C-7. Depicts increased erosion where drain pipe installed below the pad level;
if installed differently, decrease of peak flow runoff could result.
Recommendations:
C1.1.2-1 Employ detention basins to reduce peak flow runoff.
C1.1.2-2 Develop criteria for the siting and construction of detention basins. The primary purpose of these criteria is to insure that the impact of the basins on the riparian habitat is minimal.
C1.1.2-3 Employ ponds to mitigate the increase in peak flow runoff and sedimentation engendered by the development of small parcels (e.g., grading a building site, building a house) and additions to existing residents and businesses (e.g., paving).
C1.1.2-4 Develop a funding mechanism for building detention basins.
C1.1.2-5 Assess existing serendipitous detention basins in Topanga (i.e., those formed by road fill of a side canyon) as to their present and long term potential to act as detention basins. Prioritize the importance of each basin to flood hazard mitigation. Insure that these serendipitous detention basins are not destroyed by culvert upgrades and infilling.
1.1.3 runoff suppression
A number of measures may be used to increase infiltration of water and thus suppress the generation of runoff. These methods to mitigate the flood hazard have the distinct advantage that the runoff is never generated in the first place, and has the added advantage of reducing the amount of sedimentation.
The ponds and detention basins mentioned above will increase infiltration somewhat, but more significant infiltration can be achieved by requiring modifications to the grading, brushing, paving, and construction practices that exacerbate peak flow runoff as well as foster excessive erosion and sedimentation. There are three main approaches for increasing infiltration.
Increase surface porosity: this may be achieved by using alternative paving materials or different types of backfill.
Decrease runoff velocity: this may be attained by lengthening drainage courses or by putting energy dissipaters in them.
Increased diffusion: this may be accomplished by channeling runoff into open fields or onto lawns rather than concentrating it in swales or drainage pipes.
There is a variety of techniques to carry out each approach. In this section a number of practices are described which exacerbate peak flow runoff. Abatement procedures relating to these practices would only be triggered when they significantly increase the flood hazard. Suggested thresholds defining these limits are listed in Table C-3. Final determination of the threshold values should be based on hydrologic models, which are to be used to establish the efficacy of the thresholds and abatement methods.
Grading and brushing. Several instances of large scale grading for single or small lot subdivisions have occurred in Topanga (Figures A-2 and A-8) causing concern as to their accumulated effect on the flood hazard. The use of swales and other conveyances to guide the runoff off such properties should be restricted. Properties where significant grading has occurred, as defined in Table C-3, should be required to mitigate their peak flow runoff to the extent that it does not exceed that which would have occurred prior to the grading.
Similar restriction should be imposed for those wishing to use clear cut or disking methods to remove brush from around a property (Figure A-7). Hand clearance of brush should be strongly encouraged. Properties where clear cut or disking type brushing exceeds the threshold listed in Table C-3, should be required to mitigate their peak flow runoff to the extent that it does not exceed that which would have occurred if hand clearance methods had been used.
Paving. The widespread use of impervious surfaces for paving should be discouraged with an effective set of standards that require runoff mitigation measures (e.g., porous pavement) for paving above the threshold criteria of Table C-3. Also attention should be paid to the manner in which pavements are drained so that runoff is not concentrated or channeled creating excessive erosion along the paving shoulders and lessening the absorption of the runoff. Generic designs should be made available to the general public. One such design for a concrete driveway is sketched in Figure C-8. This design is not only superior to the standard concrete driveway in regards to reducing runoff, but may also be stronger and more durable because of the gravel base course.
Construction practices. Construction practices, which lead to increased peak flow runoff and sedimentation, are wide spread in Topanga Canyon including impervious surfaces; runoff conveyances such as concrete swales; piles of soil or other erodible material left uncovered in the open; and the dumping and spreading of soil over the side of slopes, on creek banks, or as road shoulder fill. Moreover, the often observed practices during construction projects, such as piling soil around the base of trees, cutting their roots, etc. markedly lessens the ability of trees and vegetation to absorb runoff, retard its flow, and hold the soil.
Regulations for erosion control during construction exist, but have not been adequately observed to be effective in preventing damage from both erosion and sedimentation. The efficacy of both the existing regulations and their implementation need to be comprehensively re-evaluated and revised to eliminate any damage caused by construction related erosion and sedimentation.
Drainage practices. Present drainage practices invariably follow the custom of collecting and conveying runoff as efficiently and quickly as possible to a propertyís boundary, often as a concentrated flow. The cumulative impact of such runoff on downstream neighbors, the environment, and its contribution to the overall flood hazard is typically not accounted for and needs to be assessed. Reduction in this runoff is most effectively dealt with at its place of origin (e.g., at the mouth of the gutter spout, the edge of the pavement, etc.). Once the runoff has been concentrated, it is much harder to get into the ground or slow its travel, and thus ultimately reduce the amount of peak flow runoff.
Roads. Roads are a major contributor to runoff, erosion and sedimentation in the watershed. Problems from roads are created by inadequate surface drainage control and by construction of unstabilized, erodible, cut and fill slopes.
Findings:
Controls on the practices described above can provide an effective means to reduce the flood hazard. Controls should limit the amount of additional peak flow runoff that may exit the property boundary.
More protection is needed for natural systems (e.g., trees) that provide runoff and erosion protection.
Recommendations:
A set of recommended regulations and practices have been developed to provide runoff mitigation. The central feature of these recommendations is that they are triggered only when the practice encountered contributes substantially to the flood hazard. This trigger is defined in terms of threshold values (Table C-3). The mitigation requirement is to be no change in peak flow runoff at the property boundaries before and after any development above the thresholds mentioned in the table (e.g., the added runoff from any paving over 1000 ft2 would have to be mitigated on site) . A suggested protocol to be used to demonstrate that no increase in peak flow has occurred due to the development is given in Table C-4. The protocol is defined in terms of three different scenarios for rainfall intensity and soil saturation. That is, the ìno increase in runoffî standard would have to be demonstrated for a variety of storm conditions..
C1.1.3-1 Ensure that all drainage plans specify that runoff is delivered to a natural drainage channel or public drainage device at non-erosive velocities with the fine sediments retained on-site.
C1.1.3-2 For those properties having impervious paving that exceeds the areas given in Table C-3, any increases in peak flow runoff and sedimentation (i.e., over the unpaved condition) are to be mitigated on site.
C1.1.3-3 Where clear cutting or disking is used to remove brush over an area exceeding the limits specified in Table C-3, any additional runoff and sedimentation, which is generated over that due to hand brushing techniques, is be mitigated on site.
C1.1.3-4 Grading, road building, and any other practice which disturbs an area of soil over the limits specified in Table C-3, should demonstrate that any additional peak flow runoff and sedimentation (i.e., over the undeveloped condition) is mitigated and retained on site.
C1.1.3-5 Protect trees and vegetation from harmful practices to reduce runoff and sedimentation, and increase absorption of rainfall.
C1.1.3-6 Develop regulations that endorse the basic notion that passing whatever runoff a particular property generates onto its downstream neighbors is no longer an acceptable practice.
C1.1.3-7 Assess a proportional fee to fund costs of off-site flood hazard mitigation (e.g., detention basins).
C1.1.3-8 Evaluate current grading standards and inspection procedures and develop standards and practices that will effectively prevent any sediment transportation from construction sites.
C1.1.3-9 Require that all new roads with either cut or fill slopes shall be replanted or retained to prevent erosion.
C1.1.3-10 Develop a road management program for both public and private properties to correct existing adverse conditions at major sites of road induced slope erosion by constructing remedial drainage improvements, slope plantings or retaining structures.
Table C-3. Criteria which trigger runoff mitigation measures.
practicecriteriapaving1000 ft2, 1000 ft2 per acrebrushing
(clear cutting or disking)3000 ft2, 3000 ft2 per acregradingfor volume: 1500 ft3, 1500 ft3 per acre
for surface area: 1000 ft2, 1000 ft2 per acreTable C-4. Protocol for determining potential increases in peak flow runoff due to development (i.e., a series of storm and soil conditions are used to assess changes in runoff) .
storm occurrence intervalsaturation level in soil 1none 50none 50saturated, representing 4th day of a storm
Figure C-8. Generic design for pervious pavement.
1.1.4 need for additional data for hydrologic models
Differences in peak flow runoff under a variety of conditions will need to be calculated to evaluate the consequences, in terms of changes in peak flow runoff, of the various practices mentioned in Section 1.1.3. To perform these ìbefore-and-afterî studies effectively will require more precise information about the inputs to the hydrologic modelsófor example, more precise measurements for runoff coefficients, especially in those situations for which little information is available. Runoff coefficients (Appendix M) are used in the hydrologic model to determine the amount of runoff absorbed prior to its reaching a watercourse.
1.2 land use
The land use element of the Topanga Creek Watershed Management Plan (TCWMP) addresses the uses, development standards and intensities needed to protect the community from the adverse impact of increased runoff and flood hazard. Land use regulation is an appropriate tool to protect sensitive resource areas, regulate flood hazard areas, and ensure that development is sited and implemented in such a way that adverse impacts on runoff characteristics from vegetation removal and alteration of the terrain are minimized.
Development practices that reduce the amount and rate of runoff mitigate the downstream flood hazard at the source of the problem. In an area where the undeveloped stream channels are an important natural resource, this is the preferred alternative for management of the flood hazard. Where appropriate land use is not implemented and stream capacity is exceeded by runoff volume, destruction of the natural channel and/or artificial channelization occurs.
There are three adopted local land use plans that establish development policy in the Topanga Creek watershed.
The Malibu/Santa Monica Mountains Interim Area Plan; the IAP applies in the upper watershed of Topanga and was adopted in 1981.
The Malibu Local Coastal Program Land Use Plan; the LCP applies in the coastal zone of Topanga and was adopted in 1986.
The Santa Monica Mountains Comprehensive Plan of the Santa Monica Mountains National Recreation Area; the SMMCP applies throughout the SMMNRA and was adopted in 1979.
These plans have been in effect for at least 10 years, yet remain largely unimplemented by Los Angeles County.
The land use recommendations of the TCWMP are described as a series of specific development standards to mitigate hazardous conditions and to protect important resource areas (see Section C2.2). The land use policies and recommendations of the TCWMP are consistent with, and supported by, the policies of all three adopted local area plans (see Appendix J). The elements of these plans related to flood hazard mitigation, water quality, and resource protection are the basis of the TCWMP.
Findings:
Establishing appropriate land use practices to mitigate flood hazard and protect sensitive resource areas is the first priority of an integrated watershed management plan.
The land use recommendations of the TCWMP emphasize mitigation of flood hazards by reducing the rate and volume of runoff and by removing development from high hazard areas.
The policies of the adopted local area plans (Appendix J) support the land use recommendations of the watershed management plan.
Recommendations:
C1.2-1 Adopt the recommendations of the TCWMP to implement the policies of the local area plans.
1.3 channel management
Maintaining the function and stability of the stream channel is an essential management goal. Uncoordinated efforts by residents and agencies have created unintentional downstream problems. The management plan includes a comprehensive, watershed-wide view of streamflow. Under the recommendations of the TCWMP, site-specific projects will be required to take into account their effects on the whole stream system.
1.3.1 Stabilization of creek banks
New designs are needed where it is necessary to stabilize creek banks to protect roads, structures or riparian habitat from stream erosion. Existing problems in the channel are predominantly associated with the roads, where the extensive use of concrete rip-rap has been used to stabilize creek banks to protect the adjacent roadways. This has proven structurally as well as environmentally unsound due to gradual degradation and undermining of the rip rap; increased erosion of unprotected slopes; and permanent loss of riparian habitat, particularly large overstory trees.
Cost-effective designs for embankment stabilization structures (e.g., various forms of gabions) are required which are structurally, hydraulically, and environmentally sound. General types of engineering designs should be prepared so that they are available and can be employed for emergency repairs and by property owners. Concrete should not be used for erosion protection in any watercourse in Topanga. Design information for gabion, rock fill, or other stabilization techniques for use in the smaller drainages should be made available to private property owners.
1.3.2 Channeling of main watercourses
The channeling of watercourses through piece meal infilling and stream bank armoring is to be prohibited. Placing fill in or near the floodplain and lining the channel is to be governed by permit and adhere to the regulations and BMPs described in Section 2.3. Such practices tend to concentrate the flow and increase its depth and velocity, which increases the flood hazard for the downstream neighbors.
1.3.3 Filling of low points
Low points created by the topography of Topanga Canyon provide an excellent means of retarding runoff by creating mini detention basins (see Section 1.1.2). Infilling these areas should be discouraged.
1.3.4 Hydraulic structures
Changes in streamflow dynamics and subsequent erosion problems often result from alterations of hydraulic structures, such as culverts and bridges. Comprehensive watershed modeling can be used to identify how these structures affect stream levels, hydrologic capacity, and possible downstream impacts. The function of numerous structures in Topanga should be evaluated as part of the overall hydrologic watershed model.
An example of where such a study may have been beneficial is the recently completed culvert project in Red Rock Canyon (shown in Figure C-9), which because of various constraints (i.e., existing road and adjacent home elevations) was constructed by the County to a standard less stringent than the DPW 50-year design storm. This project is a good example of where a trade study to clearly identify the consequences of the various design options would have been highly beneficial.
Some County bridges are undersized and in danger of flooding during the Capital storm, which raises the issue of public safety and the need to upgrade bridges and develop a plan (e.g., placing warning signs on the bridges) to reduce the risk to the public. One of the worst of these bridges is shown in Figure C-10.
Findings:
A coordinated watershed model is needed to direct channel management
Coordinated channel management will lessen the need for emergency repairs.
Preserving and restoring channel capacity by replacing inadequate structures can provide a cost-effective means to reduce negative downstream impacts.
In some instances, the 50-year design storm standard may be inappropriate (e.g., the County culvert project in Red Rock Canyon) and a lesser standard may be used. Also flood hazard protection (e.g., berms) may be acceptable. However, trade studies are needed in these situations to clearly identify all the consequences.
Recommendations:
C1.3-1 Create a hydrologic watershed model that identifies undersized or poorly located/designed structures and provides guidelines for replacement.
C1.3-2 Prepare a survey of the existing major hydraulic structures so that significant deficiencies may be identified and a plan made for their remediation.
C1.3-3 Develop a set of general designs for embankment stabilization, flood walls, and other devices that may be needed to lessen the flood hazard at a particular location. These designs are to be specifically related to the situations encountered in Topanga, require little maintenance, and be consistent with the protection of the environment.
C1.3-4 Discourage the use of concrete in the floodplain or along creek banks. Where grouted rip rap exists, replacement with more appropriate materials should be made over time.
C1.3-5 Erosion control should be performed only with porous materials that allow the infiltration of runoff. Energy dissipaters should be used to ensure that water velocities remain low.
C1.3-6 DPW should incorporate into its capital improvements program the replacement of undersized bridges.
C1.3-7 Create an emergency warning and evacuation system for those bridges and major roadways where a substantial public safety hazard exists. This is especially important for the Topanga Elementary School.
C1.3-8 Assign the WMC the responsibility of ensuring that flood hazard protection at one location does not increase the flood hazard at another.
C1.3-9 Allow a lesser standard than the 50-year design storm where site conditions warrant. The need for the lesser standard must be demonstrated by a professional engineer and approved by the County.
1.4 Coordination with other programs
Coordination with other programs is an important means of developing and implementing the Topanga Watershed Management Plan. Watershed management activities in other areas that have provided a valuable resource in developing the Topanga Watershed Management Plan, include the California Department of Water Resources Model Floodplain Management Ordinance[18], The Malibu Creek Natural Resources Plan [4,6], and the Santa Monica Bay Restoration Plan [20].
Findings:
Coordination of watershed management activities with other watershed management groups through shared information and regional implementation strategies is one strategy to maximize limited public and private resources.
Recommendations:
C1.4-1 Coordinate the Topanga Creek Watershed Management Plan with the Santa Monica Bay Restoration Plan and the Malibu Creek Watershed Council, particularly as they relate to stream protection and land use practices in the Santa Monica Mountains.
Figure C-9. Newly installed culvert in Red Rock Canyon.
Figure C-10. County highway bridge, at risk of washing out in the 50-year storm.
2.0 development standards
2.1 implementation
To meet the goals of the Topanga Creek Watershed Management Plan (TCWMP), specific development standards (practices) need to be adopted. The Malibu/Santa Monica Mountains IAP [26] specifically refers to the establishment of a Santa Monica Mountains Community Standards District for the Santa Monica Mountains. The TCWMP supports a Santa Monica Mountains Community Standards District as a zoning district to implement the land use policies recommended in the management plan. The development standards recommended by the TCWMP are supported by the policies of the adopted local land use plans (Appendix J).
Findings:
The development standards recommended by the TCWMP are supported by the policies of the adopted local land use plans (Appendix J) and should be implemented through a Santa Monica Mountains Community Standards District.
Recommendations:
C2.1-1 Adopt a Santa Monica Mountains Community Standards District to implement the development standards of the TCWMP.
2.2 hazard mitigation
Land use planning anywhere in the Santa Monica Mountains must recognize and address the regular occurrence of natural disasters including flood, fire, erosion, and landslides. While the TCWMP is directed towards mitigation of flood hazard, the plan recognizes that comprehensive and integrated development standards are required for mitigation of all hazards.
Equal weight must be given to fire, flood and slide protection in establishing development standards. Vegetation removal for fire protection increases mass movement on slopes and increases stream loadings and downstream flooding. It is unacceptable for mitigation of one hazardous condition to exacerbate another. The goal of the TCWMP is to develop strategies that make the Topanga community resistant to natural hazards of all types. The watershed management plan therefore incorporates policies that integrate management of all forms of natural disaster that regularly occur within the community.
Findings:
The development standards recommended provide for the mitigation of all recurrent natural hazards including flood, fire, erosion, and landslides, as well as for protection of important natural resources.
Recommendations:
C2.2-1 Establish sufficient slope setbacks for new structures for fire protection; prohibit ridgetop development; allow only limited vegetation clearance on slopes greater than 3:1 (30' or less). All fuel modification and fuel management plans required under Section 11.702(a) of the Fire Code shall comply with these standards.
C2.2-2 Require development setbacks from oak and riparian habitats.
C2.2-3 Prohibit alteration of stream channels or floodplains; prohibit development within floodplains; require development setbacks from stream courses.
C2.2-4 Establish and implement a cooperative program for clearing stream obstructions.
C2.2-5 Establish BMPs for any work in stream courses and adjoining habitat areas.
C2.2-6 Prohibit any increase in the rate of peak runoff from new development.
C2.2-7 Control erosion and sedimentation. Retain sediment and runoff on-site. All drainage must be conveyed and released in a non-erosive manner at nonerosive velocities into natural channels or to an approved public drainage device.
C2.2-8 Establish maximum limits on the amount of grading allowed.
C2.2-9 Establish maximum limits on the amount of impervious surfaces allowed.
C2.2-10 Provide plans to control runoff and erosion from roads/driveways. All cut and fill road slopes must be replanted or retained to prevent slope erosion.
C2.2-11 Protect water quality by monitoring and regulating the location of septic systems.
C2.2-12 Prohibit new development where inadequate road access exists for emergency ingress and egress to the main roadways of Pacific Coast Highway, Mulholland Highway, Topanga Canyon Boulevard, and Old Topanga Canyon Road.
C2.2-13 Land use density in undeveloped areas of core habitat should be low, 5-40 acres/unit. Infilling should be allowed in existing developed neighborhoods where infrastructure is adequate. Land use density shall be determined by a development constraints matrix and be consistent with all of the land use policies.
3.0 Environmental Site Planning Techniques
Prior to designing a new development or renovating existing structures, analysis of site-specific environmental constraints related to hazard mitigation should be required. Constraint factors to be considered should include, but not be limited to, access and water supply; topography [slope and aspect]; fuel load; proximity to drainage channels; biological resources; visual resources; and archaeological resources. While this is currently done on a limited basis, more in-depth review is recommended.
Findings:
A careful review of site constraints prior to designing new development or renovations is a cost-effective means to prevent potentially hazardous or environmentally damaging development and to identify effective mitigation measures to protect life, property and natural resources.
Recommendations:
C.3.0-1 Require a pre-design constraints analysis to identify site-specific hazard mitigation problems prior to design.
4.0 npdes stormwater runoff best MANAGEMENT
practices
Topanga Creek watershed is becoming increasingly developed without a coherent management approach to water quality. These issues have not been seriously addressed due to lack of data. Basic information regarding volumetric storm water effects, existing surface and ground water quality and pollution sources need to be gathered.
NPDES requirements are not being met and Best Management Practices have not been uniformly applied throughout the watershed. This is a major concern because of the problems associated with pollutants reaching the shallow alluvial aquifer and deeper bedrock water system used by many residents for drinking water. Water quality at Topanga State Beach is also a direct reflection of problems in the upper watershed. Regular sampling at the mouth of the creek indicates that unhealthy levels of pollutants are found, especially after heavy rains.
Findings:
The levels of pollution in the runoff and creeks of Topanga and the pollution sources have not been established.
Recommendations:
C4.0-1 Perform a baseline water quality study and monitor changes yearly.
C4.0-2 Identify the major sources of pollution and develop realistic programs to address them.
C4.0-3 Develop an anti-dumping campaign, focusing on rewards, heightened public education, signing, and a hot-line to reduce this form of pollution. Pursue grant money to fund these efforts.
5.0 maintenance AND EDUCATION program
The Topanga Creek Watershed Management Plan will require a long term maintenance program of Best Management Practices to be successful. Maintenance is the responsibility of both residents and public agencies. Working together with a common purpose should reduce costs and provide greater protection for sensitive stream resources. Clearly defined methods for conducting maintenance need to be identified and agreed upon by both the residents and public agencies. Maintenance practices which can reduce the flood hazard include those related to: road shoulder, bridge, and stream crossing maintenance; stream bank and channel maintenance; line clearance; protection of riparian vegetation; eradication of channel clogging exotic flora; prevention of illegal dumping; and prescribed burns (to reduce sedimentation and erosion, and to increase infiltration). Finally, the plan will need to establish clear communication pathways related to clearly established responsibilities.
The support of the community is essential to the successful implementation of the Watershed Management Plan and its recommendations for Best Management Practices. Sustained involvement of residents through local organizations such as TASC, Topanga Canyon Town Council, Topanga Canyon Creekside Homeowners Association, Topanga Chamber of Commerce, and the RCDSMM is envisioned. Community meetings regarding the plan and its key components will allow all participants to have a voice during its planning and implementation. Since the plan relies heavily upon implementation of new Best Management Practices, education will be particularly important to help establish those practices necessary to protect health, safety and the environment.
Several of the following recommendations have already been implemented in the community. The Creekside Homeowners Association has been meeting to discuss improved methods of flood hazard protection and several local experts have toured the watershed and addressed public meetings.
Findings:
To be successful, the watershed maintenance program will require the cooperative efforts of residents and community organizations working with Federal, State, and County agencies and will require community education and public outreach programs.
Recommendations:
The watershed maintenance program should include, but not be limited to, the following Best Management Practices associated with each aspect of long term maintenance:
5.1 Communication and community awareness
C5.1-1 Disseminate information to all affected property owners as recommended by the CRS Repetitive Loss Plan (see Appendix A, Section 3.1.2) including: the design manual for retrofitting flood prone residential structures; information on elevating residential structures; the handbook on flood emergency and residential repair; and information on flood-proofing techniques and systems.
C5.1-2 Distribute brochures with ìuser friendlyî information on water quality, vegetation management, flood hazard reduction, maintenance of slope stability, etc.
C5.1-3 Publish a series of articles in the Messenger newspaper on the watershed plan.
C5.1-4 Coordinate volunteers to provide hands-on implementation of BMP's.
C5.1-5 Establish demonstration sites for detention basins and holding ponds
C5.1-6 Have experts address community meetings.
C5.1-7 Identify who is to be called when a problem is identified, establish a response time, and create a list of agreed-upon possible responses.
C5.1-8 Clearly define who is responsible for what actions.
C5.1-9 Establish a twice annual monitoring schedule to identify problems before and after the rainy season.
C5.1-10 Hold an annual public meeting to discuss watershed management concerns.
C5.1-11 Establish an appeal process to address conflicts that arise due to proposed remedial actions.
5.2. Environmental Review Process
C5.2-1 Compile annually a list of flood hazards and sites of potential slope failure.
C5.2-2 Identify appropriate solutions to the hazards and have the solutions approved by relevant agencies (i.e., Coastal Commission, US Fish and Wildlife Service, Calif. Department of Fish and Game, US Army Corps of Engineers, etc.).
5.3 Road Shoulders (DPW and Caltrans)
C5.3-1 Use only non-erodable approved materials to construct fills, backfills, embankment stabilizations, and road shoulders.
C5.3-2 Prohibit importation of any fill debris material from outside of the watershed.
C5.3-3 Prohibit placement of any materials within the protected zone of a tree, or a minimum of 10 feet from the trunk.
5.4 Streambank Stabilization
C5.4-1 Design hardscape to preserve and enhance vegetation whenever possible.
C5.4-2 Limit use of grouted concrete rip rap only to those areas where gabions, bioengineering efforts, etc. are not possible.
C5.4-3 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. No stockpiling of dirt or equipment should be permitted within the protected zone of the tree(s).
C5.4-4 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.
C5.4-5 Creation of soil or asphalt berms to direct road run-off should avoid direct contact with tree trunks.
C5.4-6 Removal of understory vegetation, or burying such vegetation under permanent rip-rap or culverts should be prohibited except under exceptional conditions.
C5.4-7 Documentation of existing riparian vegetation should be performed prior to any grading activities.
C5.4-8 Use of methods encouraging re-establishment of stream vegetation should be preferred over concrete or rip-rap retaining walls.
C5.4-9 The services of a consulting biologist/arborist should be sought prior to and during both the design and implementation phases of all projects. Specified monitoring following completion of construction is also recommended.
C.5.4-10 Require analysis with the hydrologic model prior to installation of any streambank hardscape to identify any impacts that could alter channel capacity or stream flow dynamics and to identify potential downstream impacts.
5.5 Stream Channel Maintenance
C5.5-1 Remove any large debris that could create a flood hazard by obstructing the creek channel.
C5.5-2 Plan strategic placement of boulders on a stream-wide basis to reduce stream velocity during peak flow.
C5.5-3 Require that plants and animals be protected during any construction within or adjacent to the stream channel.
5.6 Line Clearance/Utility Maintenance
C5.6-1 Reduce impacts by coordinating line clearance pruning among all concerned agencies.
C5.6-2 Prohibit topping (cutting trees straight across without regard to branch structure).
C5.6-3 Apply directional pruning and crown reduction as pruning standards. Use ISA standards for pruning.
C5.6-4 Prohibit pruning between January and April in order to protect nesting raptors.
5.7 Repairs of Bridges and Stream Crossings
C5.7-1 Replace caissons, concrete retaining walls, and other support structures in accordance with BMPs to protect stream resources and prevent downstream impacts by altering flow dynamics. Use hydrologic model to estimate impacts.
5.8 Riparian Vegetation Protection
C5.8-1 Establish and maintain a revegetation program in order to encourage quick re-establishment of riparian vegetation.
C5.8-2 Create a list of appropriate species for planting under utility wires in order to reduce problems related to tree/utility line interactions.
5.9 Exotic Flora Eradication
C5.9-1 Develop a program to eradicate giant cane (Arundo donax), castor bean (Ricinus communis), periwinkle (Vinca minor), tree tobacco (Nicotania glauca), German ivy (Senecio sp.). and Algerian ivy (Hedera sp.).
5.10 Illegal Dumping
C5.10-1 Prevent illegal dumping by limiting vehicle access, placing appropriate signage, imposing and enforcing penalties (fines, misdemeanor offense).
5.11 Prescribed Burns
C5.11-1 Reduce flood hazard and catastrophic erosion and sedimentation by carrying out controlled burns.
6.0 Watershed research and Restoration Programs
To accomplish the goal of thoughtful maintenance and environmental protection, a number of research and restoration programs should be carried out. Together, these programs should provide the data needed to make informed decisions about appropriate methods for maintaining road safety and preserving stream function and ecology. Funding for these studies will be sought from a variety of sources.
6.1 Water Quality Analysis
Goal: To identify sources of point and non-point pollutants entering the stream; to determine any impacts of these pollutants; and to recommend specific measures to eliminate or mitigate pollution problems.
Time Line: 2-3 years
6.2 Sediment and Erosion Control Analysis
Goal: To identify specific sites contributing to high levels of sediment and erosion flow into the stream. Evaluate and refine appropriate Best Management Practices.
Time Line: 2-3 years
6.3 Creek Bank Stabilization Study
Goal: To locate areas where slopes are unstable. Evaluate bio-engineering, gabion, and other environmentally sound solutions and recommend appropriate standards for specific sites. Perform a similar study for sites where grouted rip rap has been placed.
Time Line: 2-3 years
6.4 Tree Inventory
Goal: To identify all trees that are within the projected flood zone that maintain streambank stability. Determine possible remedial efforts that could improve existing tree health. Identify locations needing revegetation in order to enhance streambank stability.
Time Line: 1 year
6.5 Biological Inventory
Goal: To monitor the biodiversity of the watershed and identify potential indicator species which could alert us to major shifts or losses. To involve the community in maintaining diverse habitats to support the large number of plant and animal species within the riparian zone.
Time Line; 5 years
6.6 Exotic Flora and Fauna Eradication
Goal: To identify areas impacted by invasive exotic plants and animals. Establish an eradication program.
Time Line: 5 years
6.7 Restoration of Topanga Creek Estuary
Goal: To reestablish a functional estuary at the mouth of Topanga Creek.
Time Line: 5-10 years
Findings:
A series of studies are needed to provide the data necessary for long term management decisions.
Recommendations:
C6.0-1 Seek a variety of grants to fund restoration programs and basic research necessary to implement the watershed management plan. Major issues include: water quality analysis, sediment and erosion control analysis, embankment stabilization methods, tree inventory, biological inventory, exotic plant eradication, and restoration of the Topanga Creek estuary.
7.0 EARLY WARNING SYSTEMS AND EMERGENCY RESPONSE
Warning systems and accompanying emergency response have long been recognized as effective ways to save lives and reduce flood damages. The excellent response and coordination of disaster services by the County that is currently in place is acknowledged to be one of the best programs in the state, if not in the country.
Emergency Operations Center. The Topanga Coalition for Emergency Preparedness has recently created the Topanga Emergency Operations Center (EOC). This is the first effort to create an EOC in a community in the unincorporated area, which was made possible with the generous support of Supervisor Zev Yaroslavsky. In an emergency, the EOC will interface and cooperate with County and State authorities and act as the link between the community and local fire and sheriff departments, Red Cross, and California Office of Emergency Services (OES). The EOC will monitor official and non-official reporting to assure communication of correct information. The EOC will also assist emergency crews and residents. EOC volunteers will staff a bank of four phone lines serving a public information hot line.
Early warning system (EWS). To provide local flash flood information it is recommended that the EOC communications system be able to tie into the stream gage at the lower end of the canyon and to the Fire Station rain gage. In Topanga, where most employment lies outside the canyon and families remain at home, a source of reliable information regarding access and related hazardous conditions is a critical factor during emergencies.
Comprehensive Disaster Management. Dr. Daniel Botkin, Director of the Center for the Study of the Environment at UC Santa Barbara has proposed a system of comprehensive disaster management that utilizes the highest technology available to minimize loss of life and property. Their program proposes improved preparation, prevention, planning, warning, and response for virtually every kind of disaster, but has initially focused on a pilot program for wildfire disaster management along the Santa Barbara coast. The prototype plan from the pilot program is meant to be applied statewide, nationally and internationally to manage wildfire and other natural disasters
The wildfire management program is of immediate interest for our watershed management plan. It will provide a cost benefit analysis to compare current fire policy with proposed fire management practices; prepare an Environmental Impact Report that assesses beneficial and adverse impacts of various fire management actions; develop sample laws, ordinances, and strategies to reduce risk of fire damage; develop a comprehensive plan to implement fuel modification procedures through county, state and federal fire agencies; and provide a statewide, national, and international prototype to cope with wildfires and other natural disasters such as floods, mudslides, hurricanes, and earthquakes. The benefits are clear when remembering the massive costs and human suffering of recent fires and other natural disasters in California.
Findings:
Early warning systems can increase public safety by providing information regarding access and location of potentially hazardous conditions.
Comprehensive disaster management including preparation, prevention, planning, warning and response has clear social and economic benefits and should be incorporated into the watershed management plan.
Recommendations:
C7.0-1 Develop an early warning system through the EOC that can provide an accurate assessment of potential flood hazards in Topanga Canyon. Tie this system into the weather service and the real time data collected by the various government agencies.
C7.0-2 Develop a system whereby the major commuter routes can be closed to non residents when the potential flood hazard exceeds a designated threshold.
C7.0-3 Incorporate into the TCWMP the findings and recommendations of the Comprehensive Disaster Management Plan based on the wildfire prototype project currently being developed at the Center for the Study of the Environment at UC Santa Barbara.
8.0 economic feasibility and considerations
Several possible funding mechanisms exist to finance elements of the Topanga Creek Watershed Management Plan. Among the most common are grants from government agencies, special assessments, bonds, and service charges that can be employed in combination or singly, for various features of the Plan. The methods used are generally selected depending on the scope of the program, the authority available through state or local statutes to impose a funding method, existing local policies and practices, the local political atmosphere, the severity of the flood hazard, and the cost and difficulty of the mitigation. Benefit Assessment Districts are another source of funds specific to Topanga Canyon watershed. Economic and social benefits would include protection for life and property, reduced hazard from peak flow runoff, reduced erosion and sedimentation, improved water quality, improved scenic characteristics, improved recreational resources, and enhancement of water related habitats.
8.1 Grants
The TAC has applied for grants through the Resource Conservation District of the Santa Monica Mountains over the past three years. We are reapplying for funds under Public Law 566, specifically for small rural watersheds, administered by the Department of Agriculture, Natural Resources Conservation Service and for EPA grants under the Clean Water Act 205(j) and Clean Water Act 319 (h). The California Resources Agency Environmental Enhancement and Mitigation Program for Related Transport Projects is another potential source of funding. We will also apply for an EPA Educational Proposal for Print, Film, Media to fund a video documentary of the Topanga watershed planning process. With the assistance of our sponsor, the RCDSMM, we are studying other potential grant sources (see list in Appendix F) and believe with the completion and adoption of this report that the TAC will be in a good position to win funding support to implement elements of the TCWMP.
8.2 county support
The elements of the TCWMP deserve the support of DPW. This would include capital improvements such as construction of detention basins and bridge replacements, implementation of the maintenance BMP's identified in Section C5.0, and design support for the some of the general engineering techniques for streambank stability.
8.3 Fees
Some of the costs of flood hazard management should be tied directly to those that contribute to its increase. The feasibility for including an assessment for flood hazard mitigation facilities with permit fees for new developments should be investigated.
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