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Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach CA

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Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach CA SENT VIA EMAIL Technical Memorandum DATE: July 17, 2009 To: Neal Conatser, Assistant Engineer, Marin County Department of Public Works From: Michael Love P.E., Principal Engineer, Michael Love & Associates [email protected] / ph: 707-476-8938 / fax: 707-476-8936 Subject: Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach CA. Project Background Easkoot Creek in Stinson Beach, California is a tributary to Bolinas Lagoon in western Marin County. The creek drains a watershed of approximately 1.59 square miles (FEMA, 1997) of steeply sloping hills that border the Pacific Ocean before flowing through the town of Stinson Beach and into Bolinas Lagoon. Due to a lack of channel capacity, the stream frequently overflows its banks between the Arenal Avenue and Calle de Arroyo crossings (Figure 1). Nearly half of this 2,000-foot channel reach is located within the National Park Service (NPS) Golden Gate National Recreation Area (GGNRA) and is adjacent to several parking lots. The current-day stream reach of Easkoot Creek downstream of the GGNRA parking lots crosses under a series of streets, collectively referred to as the “Calles”. The Calles from Calle del Pinos to Calle del Arroyo frequently experience flooding from Easkoot Creek, compounded by occasional flooding from wave overtopping to the west. Regular out-of-bank flooding also occurs upstream of the Calles within the GGNRA parking lots and in the area of the Parkside Restaurant. Currently, flooding within lower Easkoot Creek is exacerbated by sedimentation that reduces channel capacity. Sedimentation and the resulting decrease in channel capacity likely contributed to the out-of-bank flooding that occurred at the Park Entrance during a relatively small flow event on January 25, 2008. The out-of-bank flow traveled northwest through the parking lot and onto the wave slope through a gap in the dunes (Neal Conatser, Marin County Flood Control and Water Conservation District, personal communication). Historically, sediment from lower Easkoot Creek was routinely and extensively dredged to maintain hydraulic capacity. In recent years, only limited dredging at specific locations adjacent to the road crossings has been conducted. Routine dredging of the channel is viewed as unsustainable due to cost and the regulatory restrictions associated with the presence of steelhead trout (Oncorhynchus mykiss), a Federally listed threatened species that resides in the stream. Over the past 35 years, there have been numerous studies of flooding and flood control options for Easkoot Creek. Reports prepared by M. M. Sadjadi (1971) and by William Spangle and Associates July 17, 2009 Page 2 (1984) developed several alternatives to alleviate flooding. These studies recommended diverting or bypassing high flows directly to the ocean through a bypass channel or some type of overflow feature while maintaining “normal” flows in the stream channel downstream of the bypass point. Several potential bypass points were identified, with the preferred point located at the existing channel bend near the Parkside Restaurant. This overflow would cross through the existing GGNRA parking lot and/or picnic area before discharging onto the wave slope. Both studies indicated that sediment management would be a critical component of the bypass channel. The recommendations of the two engineering studies, as well as the results from more recent studies of the Bolinas Lagoon watershed, were incorporated into recommendations for the Bolinas Lagoon Ecosystem Restoration Project developed by an interagency working group (Working Group, 2008). Recommendations that directly relate to Easkoot Creek included: 1. Investigate utilizing a portion of the Golden Gate National Recreation Area Stinson Beach Parking Lot as seasonal floodplain for Easkoot Creek 2. Conduct a qualitative sediment source analysis (human induced) of the Bolinas Lagoon watershed and seek remedies for problem areas 3. Improve floodplain access and remove sediment deltas along the eastern shore of Bolinas Lagoon consistent with flood protection Though dated, the bypass flow recommendations of the two engineering studies merit further exploration. At the time the reports were developed, protection of native fisheries was not a noted concern. Additionally, more limited dredging and sedimentation in the channel associated with large flow events in Easkoot Creek has highlighted the need to develop a sediment management plan in conjunction with any proposed flow bypass alternatives. In July 2006, the Marin County Flood Control and Water Conservation District (District), initiated discussions with NPS regarding various high-flow bypass alternatives that would be sited in the GGNRA to address flooding issues in lower Easkoot Creek. In addition to concerns regarding infrastructure and maintenance, NPS has expressed concerns regarding the creation of a bypass system that discharges floodwaters directly into the ocean. Of particular concern is the protection of the steelhead trout, which would be adversely affected if diverted into the ocean. Purpose of Memorandum At the request of the District, Michael Love & Associates (MLA) has researched relevant background material and investigated the feasibility of providing adequate fish protection for juvenile steelhead for the previously proposed high-flow bypass system, as well as two additional alternatives proposed by the District. This memorandum also presents the results of readily available studies of sedimentation within Easkoot Creek and identifies additional considerations that might be incorporated into further feasibility studies and engineering design. This memorandum does not assess the flood flow attenuation benefits or actual costs or feasibility of constructing the alternatives The three high-flow bypass alternatives for which the District requested MLA to assess protection of native fisheries are: Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 3 1. Direct high-flow bypass channel to the ocean, as described in the William Spangle and Associates (1984) report. 2. Lateral weir/spillway to divert high flows into a detention basin with return to the existing stream channel and high-flow bypass channel to the ocean. 3. Floodplain/seasonal wetland restoration that overflows to the ocean, restoring historic processes that were associated with a wetland located at the site of the current-day Calles and GGNRA parking lots. Review of Background Materials Historical Geomorphic Conditions Historic documents indicate that Easkoot Creek flowed into a seasonal wetland referred to as Willow Camp. The stream was also hydrologically connected through subsurface and potentially surface flow to a coastal lagoon referred to as Poison Lake or Willow Camp Lake (NPS, 2004 and Darren Fong, 2009 Personal Communication). During larger flow events, Easkoot Creek at Willow Camp occasionally overflowed to the ocean rather than flowing into Bolinas Lagoon. Poison Lake and adjacent wetlands were reportedly filled in the 1950’s to expand the parking lot for beach use (Spangle, 1984, NPS 2004). The Spangle and Associates (1984) report indicated that flooding issues along Easkoot Creek seemed to have increased after the filling of the area and suggested that re- establishing this flood bypass would alleviate sedimentation and flooding issues in lower Easkoot Creek. The 1854 U.S. Coastal Survey chart and 1926 U.S. Coast & Geodetic Survey chart (with topography taken from earlier surveys) indicate that portions of Calles and GGNRA Stinson Beach parking areas were historically a wetland that received the waters of Easkoot Creek (Figure 2, Figure 3, and Figure 4). This wetland area appears to have been formed by coastal sand dunes on as many as three sides and uplands to the northeast. The 1854 chart shows the area (approximately 20.5 acres) hatched distinctly different from the salt marsh and a mudflat to the northwest, indicating that it was a wetland or marsh (Tetra Tech, 2001a). The 1926 chart shows a similar area hatched as a wetland. Both the 1906 plat map (Figure 5) and the 1926 chart referred to this wetland area as “Willow Camp,” suggesting the marsh area was a seasonal freshwater wetland. A differently hatched area represents the extents of Poison Lake, historically in the present location of the GGNRA South Parking Lot (NPS, 2004, Darren Fong, Personal Communication). On the 1906 plat map, much of the GGNRA parking lot areas are shown as an area of dense willows, with a ditched channel flowing northwest closely following the current-day channel alignment of Easkoot Creek. The ditch is shown ending at a willow stand near the current-day Park Entrance, with multiple flow arrows that suggest it drained into a willow wetland with no distinct channel. The map extents stop at approximately Calle del Pinos, adjacent to the Park boundary. The 1926 map shows a channel within the wetland that drains to Bolinas Bay. Historical accounts and the maps indicate that under normal flows, the creek followed an alignment from the wetland to Bolinas Lagoon similar to the present-day alignment (Tetra Tech, 2001b). Review of Background Information and Flood Control Alternatives for Easkoot Creek, Stinson Beach, California. Michael Love & Associates July 17, 2009 Page 4 Another channel is shown on the 1906 plat map, diverging from Easkoot Creek near the current-day Parkside Restaurant. This channel flowed
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