MITIGATION MANAGEMENT AND MONITORING PLAN BAYFRONT PARK WETLAND RESTORATION MILL VALLEY, CA

Transcription

1 MITIGATION MANAGEMENT AND MONITORING PLAN BAYFRONT PARK WETLAND RESTORATION MILL VALLEY, CA PREPARED FOR: THE CITY OF MILL VALLEY PARKS AND RECREATION DEPARTMENT 180 CAMINO ALTO AVENUE MILL VALLEY, CA PREPARED BY: PHILIP WILLIAMS & ASSOCIATES 550 KEARNY STREET, SUITE 900 SAN FRANCISCO, CA PHYLLIS FABER WETLANDS BIOLOGIST 212 DEL CASA MILL VALLEY, CA DIANE RENSHAW CONSULTING ECOLOGIST 607 PACO DRIVE LOS ALTOS, CA MAY 11, 2007 BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 1

2 MITIGATION MANAGEMENT AND MONITORING PLAN BAYFRONT PARK WETLAND RESTORATION MILL VALLEY, CA A. INTRODUCTION In 1993 the City of Mill Valley was the lead agency for the then proposed development of the Mill Valley Community Center, located at the northern end of Richardson Bay. To offset loss of 0.04 acres of seasonal wetland filled by the development, the City of Mill Valley required that 3,500 square feet of seasonal wetlands be established within the Bayfront Park area. In 1999 the City had its consultant, Philip Williams & Associates, prepare a conceptual design for a tidal marsh in Bayfront Park to fulfill the wetland mitigation requirement. The Mill Valley Community Center has now been constructed. The proposed restoration will reconfigure and enhance an existing filled wetland area, creating a net gain of approximately 0.12 acres (5,280 square feet) of low, middle, and high marsh habitat, exceeding the minimum of 3,500 square feet required by the City of Mill Valley as mitigation. B. RESPONSIBLE PARTIES The City of Mill Valley is the project applicant, and the project work will be overseen by the Mill Valley Parks and Recreation Department. The project mitigation plan has been prepared by Philip Williams & Associates, Phyllis Faber, Wetlands Biologist, and Diane Renshaw, Consulting Ecologist. The City of Mill Valley will be the sole party responsible for the successful implementation and completion of this mitigation measure. C. PROJECT REQUIRING MITIGATION 1. Location: The project site is at the northern end of Bayfront Park in Mill Valley, Marin County, CA (USGS 7.5 San Rafael topographic quadrangle (37 53' 50"N, ' 27"W) (Figure 1). 2. Brief Summary of Overall Project: The proposed project will improve habitat values on the site by lowering elevations over as much of the site as is practical, implementing a conceptual wetland mitigation design prepared for the City of Mill Valley by PWA (1999) (Figure 2, 3). This will result in a greater exposure to the twice-daily tides with a resulting increase in the extent of cordgrass, pickleweed, and salt grass (Figure 5). The benefits of this downward shift of elevations include improved habitat potential for clapper rail, saltmarsh harvest mouse, and the numerous invertebrate species that live in the mud and provide food for shore BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 2

3 birds. In addition the increased tidal inundation will increase the productive contribution of the marsh to the Bay ecosystem. Existing exotic upland vegetation will be cleared and removed from the site prior to excavation activities. An area of existing marsh vegetation will be preserved on the outboard side of the project site to provide a vegetation buffer between the lowered site and the tidal channel. The majority of the earthwork and grading will be the removal of fill and excavation of material to lower the overall site to elevations at which tidal salt marsh vegetation will establish. Some of the grading and excavation will occur in existing wetland areas; these areas will be reconfigured and are expected to reestablish as lower tidal marsh habitat. Soil will be excavated and hauled off-site for disposal at a suitable upland location to be determined by the contractor. The material to be excavated consists of fill of an unknown origin. Composite soil sampling will be conducted prior to any grading activities to characterize possible pollutants at the project site. This information will be provided to the contractor to ensure proper disposal of the excavated material. Excavation activities will be conducted to minimize the potential release of sediments by timing construction in the sub-tidal zone to coincide with low tides as well as by maintaining the vegetation buffer described above. It is anticipated that construction equipment will be working from the existing upland area, but soil conditions may require the use of mats in some areas of the project site. Construction activities will be scheduled to begin outside the Clapper Rail breeding season (February 1 to August 31) and will end before October 15 to comply with City of Mill Valley grading requirements. It is anticipated that the restoration activities will be conducted between September 1 and October 15, 2007 to comply with these restrictions. 3. Site Characteristics: a. Physical characteristics The 0.3 acre (13,000 square feet) project site was historically part of a large area of salt marsh fringing upper Richardson Bay (Figure 6). Over the years the site and much of the surrounding area was filled and the hydrology modified by various urban uses. The proposed project site has been partially filled to above tidal elevations and is currently a combination of upland fill and wetland areas. The current elevation of the site ranges from approximately 1.5 feet NGVD to 7.5 feet NGVD, with the majority of the site situated within an elevation range of 3.0 feet NGVD to 7.5 feet NGVD. The length of shoreline adjacent to the project site is approximately 210 feet. b. Biological characteristics BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 3

4 Vegetation at the site is a mix of upland species and species typical of tidal estuarine salt marshes in San Francisco Bay. It includes cordgrass (Spartina foliosa), found around MSL to MHW (1.5-2 feet NGVD); pickleweed (Sarcocornia pacifica, formerly Salicornia virginica), found from MHW to MHHW (2-3.5 feet NGVD); saltgrass (Distichlis spicata), found above MHHW to EHW ( feet NGVD); and a variety of upland plants and grasses at higher elevations (>4.5 feet NGVD). Biological resources at the site were analyzed in the Mitigated Negative Declaration adopted for the Mill Valley Community Center project in Biological reports prepared by Entrix, Inc. in 1991 and 1993 looked at the tidal marshland habitat near the project site for sensitive plants and wildlife and reviewed contemporary California Department of Fish and Game Natural Diversity Data Base (CNDDB) records for sensitive species in the project site vicinity. Three species of wildlife (salt marsh harvest mouse, California clapper rail, great blue heron) and two plants (Marin knotweed, Pt. Reyes bird s beak) were reported by CNDDB from the project vicinity, but no sensitive plants or animals were found in the tidal marshland at this proposed mitigation site. D. MITIGATION DESIGN 1. Basis for Design: The mitigation site was chosen for its proximity to the original project site, because it is owned by the City of Mill Valley, and because it was historically part of a large marshland fringing upper Richardson Bay. The site has been partially filled to above tidal elevations. Because it is still adjacent to free-flowing tidal waters it presents an opportunity to restore subtidal and low tide functions through removal of artificial overburden and selective grading to lower overall elevations at the site. 2. Proposed Restoration Project: a. Proposed enhancement actions: PWA conducted vegetation surveys at and near the project site in 1999 and This information was compiled and compared to develop target vegetation criteria for mudflat, cordgrass, pickleweed, and salt grass. The elevation ranges are summarized in Table 1. Table 1: Vegetation Elevation Ranges Vegetation Range Vegetation Type (feet NGVD) unvegetated mudflat < 0.5 cordgrass (Spartina foliosa) pickleweed (Sarcocornia pacifica) salt grass (Distichlis spicata) upland vegetation >4.5 BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 4

5 Table 2 shows the area of habitat that will be created versus existing habitat that will be impacted by the restoration project. Table 2: Summary of Habitat Areas Existing Habitat Area Impacted by Project (ft 2 ) Estimated New Habitat Area Created by Project (ft 2 ) Net Area (ft 2 ) Vegetation Type unvegetated mudflat cordgrass (Spartina foliosa) 0 2,880 2,880 pickleweed (Sarcocornia pacifica) 710 3,770 3,060 salt grass (Distichlis spicata) 2,330 1, upland vegetation 10,750 5,470-5,280 Figure 2 shows the grading plan for the restoration project. b. Target vegetational communities: upland plantings, wetland communities The reconfigured wetland areas will be allowed to revegetate naturally. Based on experience at other North Bay marshes, PWA (1999) expects that pickleweed will establish in the middle marsh area within the first year, with dense coverage within about five years. Cordgrass is expected to colonize and establish in the low marsh within three to five years, with dense coverage within ten years. Over the long term (15 to 50 years) the site is expected to evolve into a dynamically stable system, with a mix of vegetation types characteristic of natural wetlands (PWA, 1999). Upland areas will be planted with a variety of native plants. The concept plan specifies the use of native vegetation that will allow the restored wetland area to be visible from the adjacent pedestrian trail. Native plants used in the mitigation plantings will be derived from local stock. Upland plantings will be irrigated as needed and maintained by the City of Mill Valley. Native shrubs and perennials that will be planted on the upland area will be selected from appropriate species, including the following: Arroyo willow (Salix lasiolepis), toyon (Heteromeles arbutifolia), California rose (Rosa californica), coyote brush (Baccharis pilularis), gumplant (Grindelia stricta), sea lavender (Limonium californicum). All restoration plantings will be raised from stock taken from local sources, or will be cuttings or seeds collected from local plant material. Other native species suitable for the upland area may be substituted on the recommendation of a qualified native plant restoration specialist. BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 5

6 Upland areas not planted with native shrubs will be seeded with a mix of saltgrass (Distichlis spicata), meadow barley (Hordeum brachyantherum), yarrow (Achillea millefolium), California brome (Bromus carinatus), annual lupine (Lupinus sp.), California poppy (Eschscholtzia californica) and/or other native species recommended by a revegetation specialist approved by the City. Time of application, application rates, and irrigation requirements will be determined by the native plant restoration specialist. d. Maintenance Upland plantings will be irrigated from an underground City-owned recycled water line already in place at the edge of the restoration area. Irrigation to the upland plantings will be installed and maintained in good working order by the Parks and Recreation Department, who will also be responsible for maintaining the plantings in a healthy condition. Irrigation will be used for the first two or three years to allow the plantings to become established, and may be discontinued when the plantings are capable of surviving without supplemental summer water. Invasive weedy species noted in the annual survey will be removed from the project site. E. Success Criteria and Monitoring 1. Success Criteria: Upland plantings will be replaced as needed to provide 50% survivorship or a satisfactory visual screen at the end of the five year monitoring program. Upland plantings are not part of the wetland mitigation requirement. The wetland mitigation will be deemed successful if, at the end of five years, there is a minimum of 3,500 square feet of low, middle, and upper marsh habitat. The presence of indicator species (Spartina foliosa, Sarcocornia pacifica, Distichlis spicata) and/or evidence of tidal inundation will define the extent of the tidal marsh habitat. 2. Monitoring: a. Methods 1. Photo documentation. Photos will be taken of the existing site before restoration work begins. Following the final grading and installation of upland plantings, a minimum of three photo stations will be established at the site. Stations will be positioned to record the low marsh area, the middle and upper marsh area, and the upland area. Stations will be marked by pvc posts sunk in the ground to a point not over six inches above ground. Photos will be taken at a fixed height from these stations, duplicating the view of prior years. Photos will be taken at the same season at least once each year for the 5-year monitoring period. Photographs will be included in an annual report to be provided to interested agencies. BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 6

7 2. Vegetation Analysis An annual survey of vegetation at the site will be undertaken in July or August (to capture maximum growth and bloom of marsh vegetation). Two transects each with 50 foot intervals will be established in the first year after grading and will record percent plant cover and species distribution. These data are expected to be adequate to determine if the project objectives are being met given that a range of elevations appropriate for species establishment is known. Pickleweed should begin to establish by the second year and cordgrass by the third or fourth year. Any weedy species will be noted for corrective action or other maintenance, as described above in section D. 2. d., Maintenance. b. Schedule Following completion of the restoration project, the restoration area will be monitored by the City of Mill Valley or by its consultant on an annual basis for a minimum of five years. c. Corrective Management If annual surveys show that the tidal wetland areas are not performing as expected, the City will consult with PWA or another qualified wetland consultant for recommended corrective measures. If, at the end of five years, the target wetland area has not been obtained, the City will consult with interested agencies and with PWA or another qualified wetland consultant, and will institute corrective measures, which may include selective regrading of the site. If performance standards are not met at the end of five years, and if corrective management is required at that time, another five year monitoring program adhering to the specifications of this document will be initiated. d. Reporting Annual monitoring will be conducted in July or August, and an annual report will be prepared by the end of October. Copies of the annual report will be kept on file at the City of Mill Valley Parks and Recreation offices. Copies of the annual report will also be provided to all interested agencies. BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 7

8 REFERENCES Entrix, Inc Impact assessment of the proposed community center for the City of Mill Valley. Prepared for the City of Mill Valley Impact assessment of the preferred site for the new Mill Valley Community Center. Prepared for the City of Mill Valley Parks and Recreation Department. Philip Williams & Associates (PWA) Mill Valley Community Center tidal wetland mitigation conceptual design. Memorandum report prepared for Don Hunter, City of Mill Valley. BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 8

9 FIGURES 1-6 BAYFRONT PARK TIDAL WETLAND RESTORATION 1. VICINITY MAP 2. GRADING PLAN 3. GRADING CROSS-SECTION 4. BCDC LIMITS 5. VEGETATION HABITAT AREAS 6. HISTORIC TIDAL MARSH BAYFRONT PARK WETLAND MITIGATION PLAN CITY OF MILL VALLEY PAGE 9

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14 Vegetation Type Existing Habitat Area Impacted (ft 2 ) Total New Habitat Area Created (ft 2 ) Net Change in Habitat Area unvegetated mudflat cordgrass (Sparina foliosa) 0 2,880 2,880 pickleweed (Sarcocornia pacifica) 710 3,770 3,060 salt grass (Distichlis spicata) 2,330 1, upland vegetation 10,750 5,470-5,280 (ft 2 )

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