Phase 1:Topanga Creek Watershed Restoration

October 2000-December 2001


Erosion and sediment delivery are an important component of non point source pollution in Santa Monica Bay coastal watersheds. Excessive sediment loading in these watersheds has been blamed for the loss of valuable habitat for numerous aquatic animals. While there is much talk about the amount of sediment contributed from the upper watershed regions, there is also a perception that beaches at or near the watershed outlets are not receiving sufficient replenishment. The Los Angeles County Department of Beaches and Harbors has recently received funding to re-institute a survey of beach conditions to evaluate how things have changed since the last surveys were done in the 1980s.

Beach replenishment and protecting watershed terrain are both important. The seeming disconnect between too much sediment upstream and not enough at the beach needs to be understood in order to integrate appropriate management practices that will foster whole system function.

Potential sources of excessive sediment loading in the watershed include grading and other construction activities, road construction and maintenance, urban runoff, altered fire frequencies, etc. A recent study (Ambrose and Orme, 2000) of the Malibu Creek Watershed found that sediment loads into that system have been dramatically influenced by some of these factors. However, the Malibu study did not quantify the sediment loads and its influences due to a lack of field data. Furthermore, the Malibu study did not evaluate the relationship between current road maintenance practices and sediment loading. The standard road maintenance practice, which is to stockpile eroded soils in loose berms along road shoulders, may contribute significantly to the total sediment loading in a watershed.

Topanga Creek watershed is an ideal area to conduct further study on erosion and sediment delivery in the northern Santa Monica Bay. It is relatively small but confronts the same issues related to sediment delivery as in other watersheds. The information learned by conducting an erosion and sediment delivery study in this watershed could easily apply to other Bay watersheds.

Finally, and most importantly, information on erosion and sediment delivery, as well as a clear understanding of the existing hydrologic conditions specific to Topanga Creek are crucial for developing a plan to restore the historical Topanga Lagoon. Historical photographs of the mouth of Topanga Creek, the 1876 U.S. Coast survey map and a 1924 USGS geological map show the extent of the former lagoon. This lagoon provided both estuarine and riparian habitat that is critical to numerous species, including endangered Tidewater Gobies and Steelhead Trout. The beach side of Pacific Coast Highway is under the care of the Los Angeles County Dept. of Beaches and Harbors, and the upstream portion is privately owned by the LA Athletic Club. The private property is under option for purchase as parkland. This provides a golden opportunity to explore the possibility of restoring some of the area to a lagoon and enhancing the riparian habitat along the creek.



1. Identify as far as possible the sources (natural and anthropogenic) of erosion and sediment delivery. Quantify the amount of sediment from each of these sources. Identify the patterns of and key processes for erosion and sediment delivery throughout the Topanga Creek Watershed. Determine the extent of excessive erosion and sedimentation occurring in the watershed.

2. Identify possible solutions for reducing excess sedimentation in the watershed while maximizing potential for beach replenishment.

3. Identify physical parameters that need to be addressed in watershed streambank restoration and lagoon restoration planning efforts.


The contractor shall be responsible for the performance of the work as set forth herein below and for the preparation of products and reports as specified in this Exhibit. The Project Director shall promptly notify the Contract Manager of events or proposed changes which could affect the scope, budget, or schedule of work performed under this agreement.

All sampling locations will be placed on either public lands or along public road right of ways. In some cases, private property may be used with written permission from the owner.

A standard Procedure for data collection and analysis should be submitted to the Contract Manager for review and approval before the performance of the scopes.

Quarterly progress reports will be submitted to the Contract Manager.

TASK 1: Identify as far as possible the sources (natural and anthropogenic) of erosion and sediment delivery. Quantify the amount of sediment from each of these sources. Identify the patterns of and key processes for erosion and sediment delivery throughout the Topanga Creek Watershed. Determine the extent of excessive erosion and sedimentation occurring in the watershed.

1A. Produce a map of the Topanga Creek Watershed that assesses Quaternary geomorphology, identifies and documents sources of sediments within the Topanga Creek Watershed.

Using present and historic aerial photos, and existing data regarding vegetation, fire history, development, and topography, generate a preliminary map of the surficial sedimentary geology of the basin that incorporates present and historic data.

Ground truth the preliminary map by conducting a field evaluation of morphosedimentary units within the basin to examine their role as sources of sediment to stream channels. This map will be revised as necessary during the project and a final map submitted as part of the project report.

1B. Evaluate the role of the hillside system as a sediment source.

In each of the major subdrainages of the Topanga Creek Watershed, a minimum of 2 sites will be selected that illustrate the nature, magnitude and frequency of sediment movement by overland flow, debris flow, landslide occurrence, and roadside soil stockpiling. In Old Topanga, and within Topanga State Park, more than 2 sites will be needed in order to accurately reflect the sediment contributions from those subdrainages.

A series of 0.5 square meter erosion plots and erosion pins will be installed on public hillslopes within the sites chosen and will reflect variations in substrate, slope declivity, slope orientation, vegetation cover, development disturbance and fire history, including recent fire events. Data from these plots will provide erosion rates in kg/m2 of hillside. Data from erosion pins will be used to monitor deep seated landslides and sediment movement on slopes.

The erosion plots and pins will be monitored monthly during the dry season, weekly or more as needed during the wet season, subject to accessibility. End measurements will include sediment yield in weight/ unit area and/or in weight/ depth per unit time.

Field data will be calibrated against precipitation amount and intensity, and against slope and other pertinent variables. Sites will be mapped on a GIS overlay. Data will be analyzed using a range of statistical techniques starting with bi-variate and multi-variate correlations, and analysis of variance. Where possible, a mean value of potential erosion per subbasin will be extrapolated, but owing to downslope storage, this does not imply total sediment yield for a basin. However, long term basin yields can be extrapolated from these types of data.

1C. Determine the role of the channel system as a sediment source and sink. Establish the nature, magnitude and frequency of channel change in the main and tributary channels attributable to bank collapse, fluvial erosion, and sedimentation.

A network of channel cross-sections and long sections will be established at locations defined from field investigation and preliminary mapping, and will be representative of the basin and sub-basin conditions. At each location chosen, markers will be installed for each cross-section by using stakes or existing infrastructure for reference. Cross sections will be defined as reaching to the extreme maximum flow of the channel. Long sections will vary in length depending on site conditions, with a minimum of 10 meters on either side of the cross section.

All cross-sections will be surveyed monthly during the dry season and after each storm even during the wet season. Data collected will include: channel width, bank height, slope, channel depth, hydraulic ratios and stages, sediment characteristics and suspended sediment load.

A record of scour and fill of pools, runs and riffles for specific sections will be established. Measurements will include width, length, depth, and note substrate characteristics.

Depth integrated samples (taken within the entire water column from surface to substrate) will be collected from specific sites as needed, in order to characterize suspended sediment load. This information will augment the data collected on Total suspended solids at the water quality monitoring locations.

Total suspended solids data (measured in mg/l) collected monthly by the Topanga Stream Team will be integrated into the project data.


1D. Evaluate the contribution of roadside berms as a sediment source.

Sites along the roads, both bermed and unbermed, will be selected and monitored with either erosion traps, erosion pins or PEEPS (Photo-electronic Erosion Pins), funding for which is being provided by a Coastal Conservancy grant. Data collected will provide measurements of slope wash and dry ravel, which will be converted into dry weight per unit area per unit time (g/sq.m/d).

This data will allow a comparison between the rate and amount of sediment yielded by the berms to the normal background rates.


1E. Evaluate the role of the Estuary - Ocean interface as a sediment constraint:

Establish 5 cross-sections across the width of the lagoon between the ocean and the bridge, and monitor the position, width and depth of the thalweg on the South side of the bridge at Pacific Coast Highway at the mouth of Topanga Creek.

Calibrate dynamics of wave energy (wave height and period) at the mouth of Topanga Creek/estuary by reference to deep water wave parameters taken from Catalina Ridge Buoy, and observed tidal data referenced to the Santa Monica Pier, and to discharges recorded at the Topanga Stream gage at mile marker 2.2 Topanga Canyon Bvld. If possible, augment these measurements during storm events using trained volunteers to collect data according to the Littoral Environmental Observations data sheet (produced by the Coastal Engineering Research Center, US Army Corps of Engineers). This data includes wave height, period, tide, etc.

Field data will be gathered into a database and GIS overlay that illustrates the seasonal and temporal characteristics of the estuary-ocean interface. This information will be reviewed in light of entire watershed processes and provide critical parameters needed to evaluate restoration designs.


TASK 2. Develop possible solutions for reducing excess sedimentation in the watershed, while maximizing potential for beach replenishment.

Identify key physical attributes and dynamic processes influencing the rate of erosion and sediment delivery in the watershed. Identify and estimate the relative contribution from natural and human influenced processes.

Develop recommendations regarding hillslope and channel management to control excessive sediment loading, with special focus on Best Management Practices that will address road shoulder maintenance concerns, Prioritize actions that could meet both the needs for beach replenishment, water quality improvement and unstream habitat restoration.


TASK 3. Evaluate the potential for restoring portions of the historic lagoon as well as streambanks in the watershed. Identify physical constraints that affect restorations.

Evaluate the potential for restoring portions of the historic lagoon as well as streambanks in the watershed. Develop parameters that need to be addressed in a lagoon restoration plan. The evaluation should Integrate information from previous beach condition surveys and information from LA County Dept. of Beaches and Harbors on current conditions.

In addition, as part of a Fish and Game grant, presence/absence of steelhead trout and tidewater gobies will be noted during the surveys conducted under Task 1. The data will be used to evaluate the role of the present and restored lagoon in providing habitat for these endangered species.


TASK 4. Develop Final Report

Prepare a final report describing the results of the study. Include in the final report a data summary report, a report on the results of data analysis, a discussion of the study results and report recommendations. Include also all GIS maps produced under this project.

Deliver 10 copies of a draft final report to the Contract Manager for review. Modify the draft report in response to the reviewer’s comments. Deliver the modified final report as follows (including GIS maps):


Exhibit B
Task 1.   Source Identification and Quantification  
  1.A. Create Quaternary Geomorphology map 10/2000 - 10/2001

Evaluate the role of hillside system as sediment source 10/2000 - 10/2001


Evaluate the role of the channel system, road Berms as sediment source and sink 10/2000 - 10/2001

  1D. Evaluate the contribution of roadside berms 10/2000 - 10/2001
  1E. Evaluate the role of the estuary-ocean interface 10/2000 - 10/2001
  1F. Conduct data analysis and develop GIS 10/2000 - 10/2001
Task 2.  

Develop possible solutions for reducing excessive Sediment 08/2001 - 12/2001

Task 3. Evaluate Restoration potentials 08/2001 - 12/2001
Task 4. Develop final report and recommendations 08/2001 - 12/2001

Erosion and Sediment Delivery Study


Personnel Cost Match Total
Project Coordinator ($30/hour x 120 hours) 2400 1200 3600
Geomorphologist (Dr. Antony Orme) $50/hour 25000 25000 50000
Field Assistants ($14/hour x 1070 hours) 15000 15000 15000
GIS Technician ($18/hour x 125 hours) 2250 2250 4500
Personnel subtotal 44650 28450 73100
Materials Cost Match Total
erosion pins 1000 0 1000
erosion troughs 2000 0 2000
sieves 2000 2000 4000
GIS computer equipment 0 4000 4000
ArcView 3.2, spatial analyst, etc. 0 4000 4000
historic aerial photos 0 800 800
printing and duplication 500 500 1000
transportation ($0.32/mile x 100 miles) 320 100 420
Materials subtotal 5,820 11,400 17,220
PROJECT SUBTOTAL 50,470 39,850 90,320
15% administrative overhead 7570 0 7570
TOTAL PROJECT COSTS 58,040 39,850 97,890

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