ATTACHMENT to Contract

Scope of Work and Project Schedule

Tasks – A1

Task A: Literature Review of Similar Restoration Projects

Given the complexity of both the ongoing physical processes in the slough and the scale of restoration that will be required to reverse the active marsh degrading processes, a review of comparable sites can help inform and guide the Elkhorn Slough Tidal Wetland Project. More importantly, the direct experience of the consultant team in analyzing and implementing management plans on a wide array of large scale estuarine and wetland projects will be essential in crafting the optimal management approach using the project tools. Therefore, many of the projects we will site in the literature review will be prior PWA and HTH projects. In addition our coastal geomorphologists and team biologists have extensive experience in the Gulf Coast (Louisiana) and the UK where comparable types and scales of erosion are occurring. These will provide a valuable addition to reference systems along coastal California and the Pacific Northwest.

The literature review will include the following categories:

  1. The response of major slough channels to altered tidal exchange: PWA has conducted a variety of projects on major slough channels where the tidal prism has either been increased (leading to erosion, as at Elkhorn Slough) or decreased (resulting in channel aggradation). In the literature review, we will depict how a combination of hydrodynamic modeling and geomorphic prediction tools were used to predict the equilibrium channel condition in the largest San Francisco Bay wetland restoration projects (Sonoma Baylands, Warm Springs Marsh, and Cooley Landing Marsh).
  2. Tidal Wetland Erosion: We will document projects and locations on the west coast where wetland erosion has occurred. A comparable form of wetland erosion is currently occurring in the Marin wetlands near Larkspur. We have been monitoring both edge erosion (where the marsh edge has retreated 450 ft over the past 150 years) and interior marsh erosion (more recent) which is strikingly similar to that occurring in the north side Elkhorn wetlands. In addition, our team staff (especially John Bourgeois, HTH), will provide both direct project experience, research and literature review from the Louisiana coastal wetlands where sediment starvation and inundation have resulted in the loss of millions of acres of tidal wetlands. Literature on erosion mechanisms, modeling studies, and solution approaches developed there will be included. We have ongoing research relationships with two key Louisiana researchers, Dr. Denise Reed and Dr Eric Webb (formerly of HTH) who are leading efforts to halt erosion and restore the Gulf Wetlands. These experts provide valuable insights for Elkhorn Slough Tidal Wetland Project.

Three of PWA’s senior coastal geomorphologists (Dr. Steven Crooks, Dr. David Brew, and Jeremy Lowe,) have research and work experience with comparable problems in the extensive tidal salt marshes bordering the English coastline. Over a 15 year periods the estuaries along the southeast coast of England have lost between 10% and 55% of their salt marsh habitat. The mechanisms of loss include bank retreat in wave exposed areas, but the majority of these marshes are found in back estuarine settings protected from wave energy. Internal erosion, as pannes and sloughs enlarge and vegetation thins to be displaced by mudflat is widespread and pervasive. The loss of these highly valued marshes has been the subject of a considerable amount of research by Dr. Crooks of PWA. The factors identified by Dr Crooks include many of the same factors at Elkhorn Slough: increasing rate of relative sea-level rise, low sediment availability, human impacts on sediment transport pathways (ebb currents dominate the system causing a net export of sediment), and increasingly poor drainage of marsh soils. Dr Crooks will contribute the results of his own research, restoration methods being tested in Europe, and input from other renowned British marsh geomorphologists (Dr Allen Pye, Dr Jonathon French, Dr. Jon Pethick, Dr David Stoddard.)

  1. Tidal Inlet prediction and management: There is extensive literature, with excellent west coast applications, on the behavior of tidal inlets. However, most of this literature was previously generated for application to harbor inlets. Bob Battalion, PWA’s senior coastal engineer, has compiled an extensive library of these studies for application to prior PWA projects. However, while this data will be valuable in understanding the impacts of potential projects on the Moss Landing Harbor Entrance, it is more complex to apply to Alternative B (potential to recreate a new slough entrance directly to Monterey Bay, north of the harbor entrance. In the mid-1980’s PWA extended the prior work on harbor entrances (developed by Professors O’Brien and Johnson at UC Berkeley) to tidal estuaries with direct ocean connections. This work was developed in conjunction with a major wetland restoration project at the Tijuana Estuary, where inlet closure had become a significant problem. This work included a compilation of both wave data and tidal prism data from all of the major tidal inlets along the west coast. The categorization of data into those systems that are open, closed or intermittent represents the largest compilation of such data currently available.
  1. Restoration of subsided tidal wetland systems: Prior experience and literature on restoring subsided wetlands (comparable to Parson’s Slough wetlands) will guide their future restoration. Our restoration of the Sonoma Baylands site in north San Francisco Bay involved the placement of several million cubic yards of dredge material to raise the marsh surface (about 2 meters) to an elevation where natural sedimentation and revegetation processes could be restored. This remains the largest and most successful subsided wetland restoration project on the west coast and a valuable guide to possible fill placement at the Parson’s slough wetlands, and possibly help raise the north side wetlands.
    In addition to our experience with restored sites, we are currently the prime consultant (with HTH Associates) developing the restoration plan for the South Bay Salt Ponds, involving the restoration of over 15,000 acres of subsided former tidal marshes in south San Francisco Bay. This project includes extensive literature review on the relationship between sediment supply and the restoration of subsided wetlands.
  1. Management and restoration of eroding slough channels: The goal of the current project is not simply problem understanding; it is the development of the most implementable, cost effective and sustainable system with the least adverse impact. This will require first controlling the erosion in the primary slough channel, then implementing a program to reclaim the wetlands. Information on similar projects to control the main slough erosion problem is essential to the successful management of the interior and adjacent wetlands. In Oakland, we are currently designing the plan to reconnect Lake Merritt (formerly, a major tidal wetland) with San Francisco Bay. The plan must control the size and depth of the connecting channel to protect adjacent infrastructure and the BART (subway) tunnel, which passes under the slough channel. By controlling the range of tidal exchange, and preventing erosion with concrete sills, we will maintain adequate tidal exchange to support improved wetlands will controlling erosion in the main slough channel.

The literature review will be based on a combination of our own extensive experience, scientific literature review, reports from comparable analysis and restoration projects, and discussion with other experts throughout the world. A memorandum summarizing findings from the literature review will provide valuable background information that will help refine the work program for subsequent tasks, and provide guidance on the optimal solutions.

Tasks – A3

Task B: Develop Alternative Conceptual Designs

In this task, PWA design staff will refine the identified large-scale alternatives to a level that can be adequately analyzed and modeled in Tasks C, D, E and F. The preferred approach (one or two) will then be further refined and developed in Task G to preliminary design level.

The key element of Task B is developing the alternatives to a level that demonstrates feasibility and is suitable for tidal inlet assessment, hydrodynamic model testing, morphological assessment, and future habitat characterization. The alternatives include:

Option 1: No Action

Option 2: Restore historic conditions

Option 3: Decrease the size of the opening under Highway 1

Option 4: Decrease the tidal prism by muting Parsons Slough/other wetlands

Option 5: New alternative developed by PWA

It is important to recognize that PWA has previously analyzed and modeled all four of these options at a preliminary level in our 1992 study. In that study, we obtained bathymetric maps of the pre-harbor opening to the north to characterize historic conditions. We used available data to model existing conditions at the time (no action; Option 1) and used our understanding of the slough problems to identify options 3 and 4, and conduct preliminary testing of them. In the current study, we will incorporate the more detailed bathymetry now available to refine these options to the required level:

  1. Option 1. No Action: The “no action” alternative is important for several reasons. It is important to understand how the project site will evolve and what the equilibrium conditions will be when it does stabilize. These will provide an important comparison with the other options for final decision-making of the preferred approach. In addition, the no action alternative is required by CEQA to demonstrate purpose and need and to compare potential impacts of the other options.
  2. Option 2. Historic Mouth Conditions: Opening a new mouth to the north will provide a comparable wave and littoral sediment climate to conditions that existed prior to the new harbor mouth opening in 1947. This will require closure of the existing (at the Highway 1 Bridge), construction of a new Highway 1 bridge to the north, and excavation of a new slough mouth through the beach. PWA has the detailed bathymetry available to determine the mouth conditions for the pre-1947 configuration. However, it should be recognized that the current slough conditions produce a tidal prism significantly larger than the historic slough bathymetry. As a result, the initial equilibrium conditions of the new mouth/tidal inlet would be larger than historic conditions. We would use the tidal inlet analyses and our own database of tidal marsh outlet geometry to predict this initial condition. Over time, as sediment deposition (or active placement of material to fill eroded parts of the slough and constriction of the tidal prism from Parsons Slough) occurs the new mouth will be expected to gradually decrease to a smaller size, likely close to the pre-1947 conditions. In addition to the new mouth and closure of the existing channel at Highway 1, the preliminary design will calculate the appropriate channel excavation required to connect the existing channel to the new mouth.
  3. Option 3. Decreased mouth opening under Highway 1: In 1992, PWA Project Manager Jeffrey Haltiner teamed with Robert Battalio (with M&N at that time) to develop a preliminary design for a barrier at Highway 1 that would mimic the tidal damping effects of the historic shoaling mouth at the Bay. Different barrier heights were tested to determine the height of the barrier needed to reduce the erosive shear stress at various locations up the slough. Mr. Battalio and his team will refine this concept for use in the current model study. Determining the barrier dimensions sufficient to reduce slough erosion will require a trial and error method using the hydrodynamic model developed in Task D. Therefore, an initial barrier height will be estimated, and subsequently refined.
  4. Option 4. Reduce the tidal prism in Parsons Slough: Our prior work demonstrated that the 1984 opening of Parsons Slough added up to 40-percent additional tidal prism to the total tidal exchange at the mouth. At that time we recommended reducing the size of Parson’s slough at the Railroad Bridge, as this provides a stable location to construct the required structure. We will evaluate the size of opening required to dampen the tidal exchange to that characteristic of a restored (mature) marsh in Parsons Slough. We have extensive data on the natural morphology of tidal marshes that provides a relationship between an equilibrium marsh and the tidal prism. Creating an opening of that size will immediately reduce the erosive effects in Elkhorn Slough, while providing adequate tidal exchange to support a restored marsh. Use of fill material would greatly accelerate the restoration of Parsons Slough, considering the limited sediment supply currently available.

The task deliverable will include sketches, dimensions of openings, sizes of structures and a description of assumptions made to characterize each of the options. In addition, the PWA team will work with the client team to determine if there are any other feasible options that should be considered, or refinement of the current options that would improve their function or implementation.

Tasks – B2

Task C: Predict Inlet Dynamics

There are two tidal inlets of concern in this project: The existing one at the current harbor mouth, and the potential future one at a more northerly (historic) location. The potential functioning and stability of each of these is essential in assessing a potential alternative. It is important to recognize that while the area of interest for Elkhorn Slough tidal processes under current conditions is the Highway 1 Bridge, the actual tidal inlet is at the harbor mouth and this controls conditions in the Elkhorn Slough, as well as internal harbor processes (dredging, flow etc).

The ability of an inlet to remain open is primarily a function of the amount of sediment deposited near its mouth due to wave-induced sand transport, and the scouring effect of tidal currents. Since the proposed restoration alternatives will affect the tidal prism (reduce scour potential) and/or relocate the inlet mouth (increase wave exposure), these actions have the potential to alter the inlet behavior. The goal of a new opening would be to recreate the shallow historical opening that produced the muted tidal regime pre-1947. However, it would be important for the new Elkhorn Slough mouth to maintain a continuously open connection to Monterey Bay. The changes in tidal prism could also affect the littoral sediment budget and result in greater need for navigation dredging at Moss Landing Harbor.