Habitat Creation in Tidal Zones: Hydrologic Issues in West Coast Applications

The creation of effective tidal marsh habitat can be streamlined using hydraulic design tools as well as through the proper identification of applicable reference sites. The frequency of inundation corresponding to vegetation zones observed at reference sites can be used to design the elevation zones of proposed tidal marshes. Seasonal fluctuations in salinity levels can affect the suitability of vegetation types in tidal zones. Project success also depends to a large extent on protection from wave action. This paper discusses issues encountered in designing habitat enhancement projects in tidal zones, including analysis of historical tidal records, application of various hydraulic modeling tools, identification of applicable reference sites, establishment of desired elevation zones for plant communities, and accounting for water quality fluctuations arising from freshet conditions. Tidal marsh habitat may be desired in areas previously outside of elevation zones suitable for vegetation establishment. In these cases, the configuration of the inlet channels and project site topography must account for the proper balance of sediment transport through tidal energy and freshwater inflow from landward sources in order to ensure a lasting project life with minimal maintenance. Introduction This paper includes three case studies; one each in Alaska, Washington, and Oregon. • A weir constructed at the mouth of Fish Creek in Anchorage, Alaska, impounds freshwater during low tide while allowing tidal inundation of the marsh area at higher tides. The existing marsh serves as a reference site for a Copyright ASCE 2006 World Environmental and Water Resources Congress 2006 2 constructed marsh proposed by the Port of Anchorage, Alaska two miles to the north. The proposed marsh, protected by a dike and controlled by an adjustable weir, would provide 10 ha (25 acres) of tidal marsh habitat with vegetation zones designed for periodic inundation. • An undersized culvert under Kingston Road in Kingston, Washington separates a tidal estuary from Puget Sound. A larger replacement structure would reduce scour problems and prevent excess velocities that currently result in a fish passage barrier. • A culvert under a private railroad connects the Willamette River in Portland, Oregon to a historic floodplain. Typical tidal cycles do not inundate a significant area of the floodplain under existing conditions; a replacement culvert and site grading would establish desirable vegetation zones. Tide Levels The target habitat sought in tidal zones depends on the magnitude of the tide cycles adjacent to the project site. Tide levels vary significantly along the West Coast of the United States. Table 1 compares tidal datums for the three sites in Alaska, Washington, and Oregon where habitat enhancement projects are proposed. The sites exhibit extreme, moderate, and minimal tide fluctuations, respectively. Table 1. Comparison of Tidal Datums (Source: NOAA) Tide Anchorage, Alaska Kingston, Washington Portland, Oregon Mean Higher High Water 8.888 m 3.350 m 1.086 m Mean High Water 8.669 m 3.091 m 0.927 m Mean Tide Level 4.676 m 1.975 m 0.501 m Mean Low Water 0.686 m 0.860 m 0.076 m Mean Lower Low Water 0.000 m 0.000 m 0.000 m Each of the sites exhibits semi-diurnal tidal fluctuations. Figure 1 shows a typical 25hour cycle at the Port of Anchorage for spring and neap tide cycles, with two high and two low daily slackwater conditions. Because the frequency of the astronomical tides mimics the fundamental period of Cook Inlet and Knik Arm, the semi-diurnal tidal fluctuations are among the highest in the world, at times exceeding 10 meters (33 feet). Observed water surfaces are sometimes significantly higher than the tide level due to storm surges and waves. Figure 2 shows a typical 72-hour tidal fluctuation at Kingston, Washington, in the Puget Sound, where the mean tide range is 2.2 meters (7 feet). Figure 3 shows a typical monthly tidal fluctuation in the Willamette River at Portland Oregon. Although Portland is located over 100 miles inland along the Columbia and Willamette Rivers, the tidal energy is transferred from the Pacific Ocean through backwater effects in the rivers. Mean tidal ranges at the Columbia River mouth are very similar to those in Puget Sound; the mean tide range at Portland, however, is Copyright ASCE 2006 World Environmental and Water Resources Congress 2006 only 0.5 m (less than 2 feet). High runoff events can mask the tidal phase; the longterm fluctuations in Figure 3 are due to changing river levels. 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 1 E le va ti o n (f t M L L W ) Lower High 8/ T id al E le va ti o n (F ee t M L L W ) Copyright ASCE 2006 10 m Spring Tide Neap Tide Higher High Water (HHW) Water (LHW)