Project Alternatives: The Parsons Sill Project History

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1 Project Alternatives: The Parsons Sill Project History Project Impetus Over the past 150 years, human actions have altered the tidal, freshwater, and sediment processes that are essential to support and sustain Elkhorn Slough s estuarine habitats. Historic sources of sediment supply to Elkhorn Slough have been cut off, reducing the slough s resilience to increased tidal flow. In addition, local stakeholders and scientists began observing tidal erosion (deepening and widening) along Elkhorn Slough and the loss of marshplain habitat following the construction of Moss Landing Harbor in 1947, which included dredging a new ocean inlet to replace the previous shoaling tidal inlet (Oliver et al. 1988, PWA 1992, and VanDyke and Wasson 2005 in PWA et al. 2008). Tidal erosion along the slough channel and the loss of vegetation in the marsh interior have been mainly attributed to construction and maintenance dredging of the Moss Landing Harbor entrance, which increased tidal flow to and from the estuary. Other factors contributing to the erosion problems include intentional and unintentional levee breaching, subsidence of marsh areas including Parsons Slough, accelerating sea level rise, and changes to biological processes (PWA et al. 2008). As a result of these changes, approximately 50 percent, or 1000 acres, of the tidal marsh in Elkhorn Slough has been lost since Almost 73,250 cubic yards (CY) of sediment are exported each year from Elkhorn Slough into Monterey Bay from habitat erosion. Bank erosion rates along the main channel of Elkhorn Slough range from 1 to 2 feet per year. These rapid changes contribute to the estuary s ongoing marsh loss and estuarine habitat erosion, degraded water quality conditions, increased levels of pollution, eutrophication, and increased numbers of invasive species. They also impact neighboring private lands, public access sites, and railroad and road infrastructure (TWP 2007). A comprehensive range of alternatives were considered during development of the proposed project, with the intent of conserving and restoring the Elkhorn Slough. Figure 1: 1946 Moss Landing Harbor was created when the Army Corps of Engineers dredged a channel through the sand spit at the mouth of the slough. The historic slough mouth (at the bottom of the photo) later closed with sand buildup.

2 Tidal Wetland Project Large Scale Restoration Actions The Tidal Wetland Project (TWP) is an initiative of ESNERR that is focused on developing and implementing strategies to conserve and restore estuarine habitats in the Elkhorn Slough watershed. TWP retained a team led by Philip Williams and Associates, Ltd. (PWA) to assess three large-scale restoration alternatives aimed at addressing tidal erosion and marsh loss in Elkhorn Slough. As summarized below, the alternatives considered in the final report prepared by PWA et al. (2008) included construction of a new ocean inlet north and east of Moss Landing Harbor, construction of a tidal barrier at Highway 1, and restoration of subsided tidal marsh in Parsons Slough. New Ocean Inlet: The PWA team evaluated creation of a new ocean inlet and a new channel to reconfigure Elkhorn Slough s connection with Monterey Bay. Under this alternative, a new ocean inlet would be located at the approximate historic location of the old Salinas River mouth, where it would connect to the main portion of Elkhorn Slough by a channel excavated to the north and east of Moss Landing Harbor. This alternative also included construction of a barrier under the Highway 1 bridge to block tidal exchange between Elkhorn Slough and Moss Landing, as well as a new bridge where Highway 1 would cross the new channel connecting to Elkhorn Slough. Although it was anticipated that construction of a new inlet north of Moss Landing would substantially reduce the rate of sediment loss from the estuary, the potential effects of this type of construction on water quality, sensitive natural resources, and land use would likely be substantial. In addition, the cost to construct this alternative was high (estimated at $100 million) and once implemented, the alternative would not be reversible. Figure 2 : The new Ocean Inlet option included excavating the channel and reconnecting it to the ocean through the historic inlet location. An alternative to this option included new inlet north of the north jetty, and an extension of the north jetty eastward to the vicinity of the south abutment of the existing Highway 1 bridge.

3 Tidal Barrier at Highway 1: The PWA team also evaluated construction of a partial tidal barrier at the Highway 1 bridge. Construction of this barrier would reduce tidal exchange between Elkhorn Slough and Moss Landing Harbor, but would not completely eliminate the hydraulic connection. Two different barrier crest elevations were considered in the PWA report. It was anticipated that construction of a tidal barrier at the Highway 1 bridge would likely result in a moderate reduction in the rate of sediment loss in Elkhorn Slough. Potential impacts, such as impacts to water quality, wildlife passage, and navigation, would likely be significant. The alternative was considered somewhat reversible since at least a portion of the tidal barrier could be removed in the event that unanticipated adverse effects were realized as a result of its construction. Project costs were estimated at $30 million. Figure 3: The Tidal Barrier at Highway 1 option.

4 Restoration of Parsons Slough: Unlike construction of a new ocean inlet or a tidal barrier at the Highway 1 bridge, the PWA report found that a restoration alternative in Parsons Slough would not directly modify the tidal inlet of Elkhorn Slough. Instead, it would reduce the erosion potential in the main channel of Elkhorn Slough downstream of the Parsons Slough mouth by reducing the tidal prism in Parsons Slough, including South Marsh. This was attributed to the fact that Parsons Slough, due to its size (430 acres) and relatively large-scale subsidence (elevations which have subsided by approximately 3 feet below mean higher high water [MHHW]), represents approximately 37 percent of the total tidal prism of Elkhorn Slough (PWA et al. 2008). Figure 4: The Parsons Slough Restoration option included the construction of a water control structure (shown) and/or raising the slough bed with sediment fill.

5 The PWA report considered two ways to reduce the tidal prism in Parsons Slough: (1) use sediment fill to raise the bed of Parsons Slough to elevations appropriate for a natural marsh (approximately MHHW); and/or (2) construct a new water control structure downstream of the UPRR bridge in Parsons Slough Channel to manage tidal inundation characteristics in Parsons Slough. The PWA report anticipated that adverse effects resulting from implementation of restoration alternatives in Parsons Slough would be less than those associated with either a new ocean inlet or a tidal barrier at the Highway 1 bridge. Subsequent estimates of project costs by Moffat & Nichol (2008) were estimated at $4 million, a fraction of the cost of the other alternatives. The reversibility of the alternative was considered possible. In general, the PWA team concluded that there were no small-scale restoration approaches, such as bank protection, that would halt the trajectory of habitat conversion in Elkhorn Slough. The humaninduced impacts to Elkhorn Slough during the 20th century, coupled with future sea level rise, create an unfavorable future environment for vegetated marshplain habitat. In addition, none of the alternatives identified in the PWA report would replace the loss of sediment supply to the system, created by the re-routing of the local rivers directly to Monterey Bay. It was determined that sediment supply would be needed to rebuild intertidal areas, resupply Elkhorn Slough channels, and maintain intertidal areas against rising sea levels. A restored fluvial source of sediment, or an artificial sediment supply, with periodic resupply to offset the inundation impacts of rising sea levels, would be required to restore an expansive marshplain, and replace the sediment volumes lost over the past 60 years. PWA recommended that the alternatives presented in their report be considered as a phased program and possibly implemented sequentially, based on a monitoring program and various pilot studies that demonstrate effectiveness and need for additional action. The Parsons Slough restoration alternative presented in the PWA report was further vetted in a draft restoration plan for Parsons Slough, as described below. Parsons Slough Restoration Plan Based on an evaluation of the effectiveness, cost, potential impacts, risk, and long-term reversibility of each of the alternatives evaluated in the PWA study, ESNERR determined that the project most able to advance the goals of the Tidal Wetland Project Strategic Plan in the near-term would focus on management of tidal exchange in Parsons Slough. A Draft Restoration Plan for Parsons Slough (Restoration Plan) was completed by Moffat & Nichol in 2008; the Final Restoration Plan was completed in January 2010 (ESNERR et al. 2010). The purpose of the Restoration Plan was to develop a range of project alternatives at Parsons Slough to accomplish ESNERR s restoration objectives, as recommended by the PWA study. Three basic wetland restoration alternatives, with several subalternatives, were evaluated in that document. High Water Control Structure: This alternative would involve construction of a water control structure across the Parsons Slough channel at the UPRR bridge to mute the high tide but not the low tide. Tidal muting would lower high water levels in Parsons Slough to provide for hydrologic conditions (inundation frequencies) suitable for the establishment of vegetated salt marsh. The proposed structure would mute tides to a maximum target elevation of +2.5 feet on the average tide, while allowing water levels to drop to existing

6 extreme low tides of approximately -1.5 feet. This alternative also considered the addition of sediment at one or more locations to raise the existing elevation and further restore salt marsh habitat. The Restoration Plan found that this alternative would reduce both tidal scour at the entrance of the channel and consequent sediment loss from the marsh. It would also immediately reduce the tidal prism, would result in salt marsh habitat creation over the short-term, and would require minimal to no near-term sediment additions for restoration. However, the water control structure would be complex and would likely require intensive maintenance and management to function properly. Habitat created would be extensive in area, but diversity would be less than in full tidal systems. The structure would also be a partial barrier to passage of some fishes and mammals. This alternative considered various combinations of culverts and tide gates in the water control structure to accommodate anticipated sea level rise, although maintaining target tidal elevation over the very long-term (100 years) with sea level rise might not be possible. In addition, under this alternative, tidal flushing would be detrimental to water quality, particularly in the summer season. Construction of a high water control structure intended to substantially lower the tidal frame of Parsons Slough was eliminated from further consideration because impacts to water quality from this alternative would likely be significant, and because maintenance of the structure would be complex and expensive over the long-term. Figure 5: The High Water Control Structure Option for the Parsons Slough Restoration would have muted high tide but not low tide.

7 Subtidal Sill: This alternative included construction of a subtidal sill (with the crest at -2 feet) in the Parsons Slough Channel downstream of the UPRR bridge to arrest channel scour and contain sediment within Parsons Slough. This alternative also considered addition of sediment at various locations within Parsons Slough to raise the existing grade relative to the tides and restore fully tidal salt marsh habitat. The Restoration Plan estimated that approximately 2.3 million CY of sediment would need to be added to the Parsons Complex under this alternative to raise the elevation to a level suitable for large-scale colonization of vegetated salt marsh. The Restoration Plan found that construction of a subtidal sill would only address tidal scour in the portions of the main channel that are located south of the entrance to Parsons Slough, and the resultant sediment loss from the marsh in Parsons Slough. It would not immediately reduce the tidal prism in the main channel, and would not result in salt marsh habitat creation in Parsons Slough over the short-term, unless sediment was added as fill. The sill structure would be simple to repair and maintain, and would allow passage of most fish and marine mammals. With the addition of fill, habitat created over the long-term would be extensive in area, and habitat diversity would be that of a fully tidal system. From a long-term management perspective, the Restoration Plan found that the sill could be raised to anticipate sea level rise, and that tidal flushing would be similar to existing conditions, which would minimize potential impacts on water quality. Figure 6: The Subtital Sill alterntive would reduce tidal scour south of the entrance to Parsons Slough without reducing the tidal prism of the Elkhorn Slough channel. Some alternatives also included sediment additions (not shown).

8 Internal Dikes: This alternative considered construction of dikes across various portions of the Parsons Complex, including the Five Fingers area, South Marsh restoration cells, and Rookery Lagoon. The dikes would either be equipped with culverts and tide gates to connect the areas to the rest of the Parsons Complex, or would include open channels. Similar to the other alternatives, this alternative also considered sediment additions to reestablish tidal marsh vegetation behind the internal dikes. This alternative would not address tidal scour at the Parsons Slough Channel, or the associated loss of sediment from the marsh. However, it would immediately reduce the tidal prism, would result in salt marsh habitat creation over the short-term, and could require minimal to no near-term sediment additions for restoration. Tide control structures associated with the internal dikes would require regular maintenance to function property. Habitat created would be extensive in area, but diversity would be less compared to full tidal systems. The tide control structures would be a partial barrier to fish and marine mammal passage. The tide gates could be modified to anticipate sea level rise, although, over the long term (100 years), maintenance of targeted tidal elevations may not be feasible. Tidal muting would lead to reduced flushing and impaired water quality in the summer season. In addition, the Restoration Plan found that this alternative could lead to the loss of marsh in the main body of the Parsons Complex over time, making the long-term benefits limited. Figure 7: Various configurations were considered for the Internal Dikes option, which would have reduced the tidal prism but would not have reduced tidal scour.

9 The subtidal sill alternative, with no filling of lower areas and no additional diking, was selected by ESNERR as its preferred alternative in the Restoration Plan. Other alternatives considered in the Restoration Plan included construction of a subtidal sill in combination with the addition of sediment in Parsons Slough, as well as construction of internal dikes in Parsons Slough. Both of these alternatives met several of the project objectives and have the potential to lessen one or more of the potential impacts associated with the proposed project. It is anticipated that the addition of sediment to Parsons Slough behind a subtidal sill would lessen impacts on water quality compared to the proposed project by reducing the residence time of water. However sediment placement for salt marsh restoration would also displace habitat presently used by large fish. Similarly, construction of internal dikes within Parsons Slough could prove less restrictive of marine mammal movement into and out of the Parsons Slough Channel compared to the proposed project, but would result in less of a reduction in the tidal prism of Elkhorn Slough. Low Sill Alternative A draft design for the construction of a subtidal sill structure across Parsons Slough Channel was completed in January 2010 (Ducks Unlimited, Inc. et al. 2010). The design included construction of a fixed base structure, spanning the Parsons Slough Channel, and an adjustable weir attached to the center of the fixed base structure. The adjustable weir was envisioned to consist of a series of flashboards that would be installed with a hand operated hoist maintained on-site. A notch approximately 25 feet wide, located in the center of the adjustable weir, would be left open at all times to allow water to flow between Parsons Slough and Elkhorn Slough. The invert of this notch would be at an elevation of -5 feet, and would provide for the passage of water at all tide levels. The width of this notch could be adjusted in response to water quality and habitat conditions in Parsons Slough by removal of one or more of the adjacent flashboards. Later in January 2010, ESNERR met with the UPRR to discuss the potential effects of the sill and adjustable weir on the adjacent railroad embankment. Currently, water flowing over lower elevation sections of the UPRR tracks during extremely high tides washes away ballast (gravel) around the tracks, resulting in the need for frequent and expensive maintenance. UPRR expressed concerns that operation of the sill could exacerbate these conditions by lowering the height of the high tide in Parsons Slough and creating a substantial difference in the tide levels on either side of the bridge. During that meeting, UPRR indicated that the sill would need to be operated such that during seasonal high tides, the head differential be equalized on either side of the railroad embankment to minimize the potential of washing ballast out from under the tracks. This would require that the sill structure be operated in either the most open configuration (all flashboards removed), or some combination of open adjustable bays, when tides in excess of about 6 feet were anticipated. The maintenance requirements and potential liability implications associated with this level of management prompted ESNERR to consider additional options for configuration of the subtidal sill. Several alternative configurations to protect the railroad from the tide level differential were considered, including incorporating erosion protection on the reach of the railroad being inundated; constructing a vinyl sheetpile wing wall on either the east or west sides of the railroad embankment to displace water levels from the railroad embankment to the sheetpile wall; and moving the proposed sill to the east side of the Parsons Slough channel (Gentzler pers. comm. 2010b). Preliminary review of these potential alternatives by stakeholders and resource agencies identified a variety of concerns, ranging from an increased visual impact associated with a wing wall, to more severe construction impacts associated with working on the eastern side of the

10 railroad bridge. In light of these considerations, ESNERR determined that construction of a low sill (i.e., construction of the base structure without the adjustable weir component) would minimize risk to the UPRR tracks and would meet the primary project objective of reducing tidal scour in Parsons Slough Channel, although to a lesser degree than a structure including the adjustable weir. This low sill alternative was selected by ESNERR for construction. Figure 8: The Low Sill option, which was selected by ESNERR, is anticipated to reduce tidal scour in Parsons Slough Channel to a lesser degree than the adjustable weir depicted in Figure 6. Conclusion Construction of a sill in the Parsons Slough Channel, rather than at an alternate location (i.e., Highway 1 bridge, or within Parsons Slough), most effectively addresses tidal scour at the confluence of Elkhorn and Parsons Slough, a primary objective of the proposed project. The proposed sill structure would be simple to repair and maintain, would allow passage of most fish and marine mammals, and is anticipated to result in only minimal impacts on water quality. The footprint of the sill has been designed to meet a variety of water quality and biological resource objectives identified by regulatory and resource agencies, and represents the minimal area necessary to meet those criteria (about 0.30 acre of permanent impacts to waters of the State). As such, the proposed project represents the lowest impact feasible alternative that achieves the stated project objectives.

11 Sources Cited Ducks Unlimited, Inc., URS, and Dixon Marine Services, Inc Task Percent Design Report. Parsons Slough Sill Project. Prepared for the Elkhorn Slough Foundation. January 15. Elkhorn Slough Tidal Wetland Project Team Elkhorn Slough Tidal Wetland Strategic Plan. A report describing Elkhorn Slough s estuarine habitats, main impacts, and broad conservation and restoration recommendations. 100 pp. Gentzler, S., Hydrology and Hydraulics Group Manager. 2010b. Memorandum from Gentzler, URS Corporation, to Bryan Largay, Elkhorn Slough Foundation, regarding an upstream alignment of the sill structure. January 20. Moffatt & Nichol Draft Report of Existing Conditions for the Parsons Slough Complex Wetland Restoration Plan. Prepared for the California State Coastal Conservancy and Elkhorn Slough National Estuarine Research Reserve. November pp. Oliver, J.S., J. King, M. Hornberger, and D. Schwartz Erosion of wetland habitats by increasing tidal currents from harbor construction and breaching dikes in Elkhorn Slough, California. Final Draft Report, National Estuarine Research Reserve, National Oceanographic and Atmospheric Administration. Philip Williams & Associates, Ltd. (PWA) Elkhorn Slough Tidal Hydraulics Erosion Study, Technical report prepared for the U.S. Army Corps of Engineers, San Francisco District. PWA, H.T. Harvey & Associates, 2nd Nature, E. Thornton, and S. Monismith Hydrodynamic Modeling and Morphologic Projections of Large-Scale Restoration Actions: Final Report. June 6.