Mean Higher High Water Research Articles

Large woody debris influences vegetation zonation in an oligohaline tidal marsh

The amount of large woody debris (LWD) in Pacific Northwest estuaries has declined dramatically since Euro-American settlement in the mid 19th century. Little is known about the ecological significance of estuarine LWD. This ignorance impairs protection and restoration of habitat critical to threatened Chinook salmon (Oncorhynchus tshawytscha), as well as other fish and wildlife. This study investigates whether LWD affects the distribution of estuarine shrubs, particularly nitrogen-fixingMyrica gale L. (sweetgale), which dominates the tidal shrub community of the Skagit River estuary, Washington, U.S.A. LWD,M. gale, and other shrubs were surveyed along line transects in an oligohaline tidal marsh and in abandoned agricultural land whose dikes failed more than 50 years ago and which has reverted to marsh. The results demonstrate a strong association between LWD andM. gale. M. gale was very rare on LWD<30 cm in diameter, increasingly more common for LWD between 30 and 75 cm, and always present on LWD≥75 cm. The marsh surface was generally 45 cm below mean higher high water (MHHW), suggesting LWD benefitsM. gale by providing a growth platform at an elevation near MHHW and reducing flooding stress. The largest and most abundant tree in the marsh,Picea sitchensis, averaged only 35.8 cm in diameter, which suggests LWD recruitment from upstream sources is necessary to sustainM. gale populations in the geomorphologically dynamic Skagit marsh. By affecting the distribution and abundance ofM. gale in the estuary, LWD may indirectly affect nitrogen dynamics in the marsh and secondary production of detritivores and herbivores.

Effects of Tides on Maximum Tsunami Wave Heights: Probability Distributions*

Abstract A theoretical study was carried out to understand how the probability distribution for maximum wave heights (ηm) during tsunamis depends on the initial tsunami amplitude (A) and the tides. It was assumed that the total wave height is the linear sum of the tides and tsunami time series in which the latter is decaying exponentially in amplitude with an e-folding time of 2.0 days, based on the behavior of observed Pacific-wide tsunamis. Direct computations were made to determine the statistics of maximum height for a suite of different arrival times and initial tsunami amplitudes. Using predicted tides for 1992 when the lunar nodal f factors were near unity during the present National Tidal Datum Epoch 1983–2001, the results show that when A is small compared with the tidal range the probability density function (PDF) of the difference ηm − A is closely confined in height near mean higher high water (MHHW). The ηm − A PDF spreads in height and its mean height ηo − A decreases, approaching the PDF of the tides and MSL, respectively, when A becomes large compared with the tidal range. A Gaussian form is found to be a close approximation to the ηm − A PDF over much of the amplitude range; associated parameters for 30 coastal stations along the U.S. West Coast, Alaska, and Hawaii are given in the paper. The formula should prove useful in probabilistic mapping of coastal tsunami flooding.

Modeling soil salinity distribution along topographic gradients in tidal salt marshes in Atlantic and Gulf coastal regions

Soil salinity plays a very important role in determining the distribution of vegetation, plant productivity, and biogeochemical processes in coastal marsh ecosystems. Salinity gradients and salinity–vegetation associations in salt marshes have often been observed but rarely explained. A quantitative and systematic study on the soil salinity distribution in salt marshes is not only important to the understanding of coastal marsh ecosystems but also to the development of a potentially useful ecological and environmental indicator. In this research, we developed a salt marsh soil salinity model based on an existing salt and water balance model with modifications to several key features to examine the impacts of tidal forcing, climate, soil, vegetation, and topography on soil salinity distributions of the Atlantic and Gulf coastal marshes. This model was calibrated and validated using field observations from the St. Marks National Wildlife Refuge (NWR) of northwestern Florida, USA. The results showed that the model had good agreement ( r 2 = 0.84, n = 15, P < 0.001) with field observations. We found that the mean higher high water (MHHW) level determines the location of the salinity maximum in a coastal salt marsh. Simulations indicate that tidal irregularity primarily controls the width of the salinity maximum band. Evapotranspiration, temperature, hydraulic conductivity, and incoming tidal salinity significantly affect the salinity maximum band, which may lead to the formation of salt barrens/flats when reaching a threshold level.

Discrimination of Estuarine Marsh Subenvironments (San Francisco Bay, California, U.S.A.) Using a Multivariate Statistical Calibration of Abiotic Sediment Properties

Abstract Multivariate analysis of a suite of sediment characteristics provides a novel, inexpensive, and time efficient approach to characterizing estuarine depositional subenvironments, especially in the absence of diagnostic microfossils and macrofossils. The development, calibration, and implementation of a statistical model are discussed using data collected from the San Francisco Bay estuary, California. Forty-seven surface sediment samples were collected from three marshes within the San Francisco Bay estuary and from six different subenvironments ranging from tidal salt marsh below mean higher high water (MHHW) to tidal freshwater marsh sediments located above MHHW. Sediment samples were analyzed for percent organic content, percent water content, percent clay content, sediment density, and iron concentration. Standard statistical analysis of the data indicates that changes in sediment characteristics exist within and between the individual marshes; however, individually these characteristics are insufficient to distinguish among marshes and subenvironments. Multivariate analysis therefore is employed to consider the array of sediment characteristics simultaneously. The individual marsh research sites are treated as idealized reference types. The results indicate a strong degree of, and statistically significant, predictive capability. These surface data and analyses are used in the interpretation of stratigraphic cores from three sites in San Francisco Bay. The results using this method were broadly consistent with interpretations using traditional methods; however, this method determined more subtle variability than was previously recognized.

The Harmonic Constant Datum Method: Options for Overcoming Datum Discontinuities at Mixed–Diurnal Tidal Transitions*

Abstract The harmonic constant datum (HCD) method is a computationally efficient way of estimating tidal datums relative to mean sea level, without the need to compute long time series. However, datum discontinuities can occur between mixed and diurnal tidal regimes using this method. Solutions to this problem are investigated, with a hypothetical strait that contains a semidiurnal node, using three different procedures: algorithms specifically designed for diurnal tides (DTA), mixed tidal algorithms (MTA) throughout, and cubic polynomial interpolation (CPI) across the diurnal region. DTA creates small discontinuities (≤11% for the strait) in mean higher high water and mean lower low water but does not provide estimates of mean high water or mean low water. MTA gives continuous datums but creates artificial structures in the middle of the diurnal region. CPI provides smooth, continuous datums but does not use the tidal information within the diurnal regions. Which procedure works best depends on the size ...

Will Restored Tidal Marshes Be Sustainable?

We assess whether or not restored marshes in the San Francisco Estuary are expected to be sustainable in light of future landscape scale geomorphic processes given typical restored marsh conditions. Our assessment is based on a review of the literature, appraisal of monitoring data for restored marshes, and application of vertical accretion modeling of organic and inorganic sedimentation. Vertical accretion modeling suggests that salt marshes in San Pablo Bay will be sustainable for moderate relative sea level rise (3 to 5 mm yr-1) and average sediment supply (c. 100 mg L-1). Accelerated relative sea level rise (above 6 mm yr-1) and/or reduced sediment supply (50 mg L-1) will cause lowering of the marsh surface relative to the tide range and may cause shifts from high to low marsh vegetation by the year 2100. Widespread conversion of marsh to mudflat-"ecological drowning"-is not expected within this time frame. Marshes restored at lower elevations necessary to aid the natural development of channel systems (c. 0.5 m below mean higher high water) are predicted to accrete to high marsh elevations by the year 2100 for moderate relative sea level rise and sediment supply conditions. Existing rates of sediment accretion in restored fresh water tidal marshes of the Delta of greater than 9 mm yr-1 and slightly lower drowning elevations suggest that these marshes will be resilient against relatively high rates of sea level rise. Because of higher rates of organic production, fresh water marshes are expected to be less sensitive to reduced sediment availability than salt marshes. The ultimate long-term threat to the sustainability of tidal marshes is the interruption of coastal rollover-the process by which landward marsh expansion in response to sea level rise compensates for shoreline erosion. Bay front development now prevents most landward marsh expansion, while shoreline erosion is expected to accelerate as sea level rises.

Trends in United States Tidal Datum Statistics and Tide Range

Yearly tidal datum statistics and tide ranges for the National Oceanic and Atmospheric Administration/National Ocean Service long-term stations in the United States tide gauge network were compiled and used to calculate their trends and statistical significance. At many stations, significant changes in the tide range were found, either in the diurnal tide range [mean higher high water (MHHW)−mean lower low water (MLLW)], or mean tide range [mean high water (MHW)−mean low water (MLW)]. For example, at San Francisco, the diurnal tide range increased by 64 mm from 1900 to 1998, while at Wilmington, N.C., the mean tide range increased at a rate of 542 mm per century from 1935 to 1999. This analysis suggests that any studies concerned with present or future water levels should take into account more tidal datum statistics than just mean sea level (MSL). For example, coastal flooding and storm damage studies should consider trends in high water levels, since it is the peak values that cause flooding and determine the design of coastal structures. For habitat restoration planning, mean low water and tide range changes should be considered.

Physical Evolution of Restored Breached Levee Salt Marshes in the San Francisco Bay Estuary

AbstractSince 1972 over 940 ha (2,300 ac) of leveed former salt marsh sites around San Francisco Bay have been restored to tidal action, purposely or through natural processes. The evolution of these sites can inform predictions of rates of marshplain evolution and establishment of tidal channel systems. A review of the history of 15 re‐flooded sites ranging in size from 18 to 220 ha (45 to 550 ac) and in age from 2 to 29 years indicates that marshplain vegetation with more than 50% cover was established at nine of the sites within 4 to 20 years. The remaining six sites aged 2 to approximately 20 years continue to be less than 50% vegetated. The evolution of these sites is consistent with the following simple conceptual model of the physical evolution of restored tidal marshes in subsided breached sites. Initially, deposition of estuarine sediment builds up mudflats that allow vegetation establishment once elevations are high enough for vegetation to survive. Sites that are initially lower in the tidal frame take longer to vegetate than those that are initially higher. Three factors appear to retard the time frame for vegetation establishment: limited estuarine suspended sediment supply, erosion of deposited estuarine muds by internally generated wind waves, and restricted tidal exchange. These factors affect evolution more significantly in larger sites. The comparatively short time frame for vegetation colonization and marshplain evolution experienced in earlier, smaller, and/or less subsided breached levee restorations may not necessarily be replicable by simple levee breaching on larger subsided restoration sites now being planned. Our review of the 15 sites also indicates that the formation of tidal channels within the marshes is greatly dependent on whether and how high the site was filled before breaching. Filled sites at high intertidal elevations (above approximately 0.3 m below mean higher high water) can vegetate quickly but after several decades may show little development of tidal channels.

Statistical analysis of modern seed assemblages from the San Francisco Bay: applications for the reconstruction of paleo-salinity and paleo-tidal inundation

The feasibility of developing a seed calibration data set to infer environmental constraints was examined by studying modern seed and vegetation assemblages from 53 stations at three distinct tidal marshes within the San Francisco Bay (salt, brackish and freshwater). Pearson's correlation indicates autochthonous derivation of seeds at the marsh sites. Multivariate statistical analysis of the vegetation and seed data indicates discrete assemblages, which can be used as a proxy for modern salinity and tidal inundation within the estuary. Canonical discriminant analysis indicates that the vegetation and seed assemblages of the three marsh types are statistically distinct. Cluster analysis suggests that marsh vegetation is zoned at the mean higher high water mark at all the sites. The calibration set was then applied to stratigraphic seed data from the estuary to infer changes in paleo-salinity and -tidal inundation during the Holocene. The results of the calibration are compared to earlier reconstructions within the estuary. While the calibrated reconstruction broadly compares to previous reconstructions, a period of inferred lower salinity within the estuary between 3800–2000 cal yr B.P. is calibrated as brackish and not fresh by the calibration model.

Variations in Tidal Level in the Gulf of Mexico and Implications for Tidal Wetlands

Tidal wetland environments have an ecological zonation that corresponds with tide levels, in particular with mean high water. However, mean sea level (MSL), which has shown a persistent rise in the Gulf of Mexico during this century, is the most common reference for water level change. We examine here the relationship between mean sea level and mean high water in describing water level changes in the Gulf of Mexico. The records of monthly mean water level for four stations, Galveston, Pensacola, Cedar Key and Key West, are partitioned into the annual cycle, the long-term trend, and a low-frequency (>10 year period) fluctuation. The trend is the same for MSL and mean higher high water (MHHW) for all stations investigated except Cedar Key, Florida, where MHHW has increased more rapidly than MSL. The low-frequency fluctuations are similar between the stations and the tidal datums. MSL can predict MHHW with discrepancies of up to 5cm owing to the lunar nodal cycle and an annual tidal signal. Low-frequency climatic fluctuations produce greater variations than the nodal cycle, but the difference in frequency can lead to interference between the two in MHHW. The combination of the two can produce sea-level rises in excess of 1cm year−1over several year periods, even in areas having long-term trends of 0·2cm year−1or less.

Biosedimentological zonation of Boundary Bay tidal flats, Fraser River Delta, British Columbia

ABSTRACTBoundary Bay tidal flats on the inactive southern flank of the Fraser Delta have surface sediments consisting almost entirely of very fine to fine, well to very well sorted sands which show a gradual fining‐shorewards trend. Five floral/sedimentological zones form distinct biofacies. These are, from the shoreline seaward, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. The lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from ∼ 12 to 40 days. Similarly, at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to ∼ 2 days, and at the upper limit of the eelgrass zone from ∼ 0·5 to ∼ 0·8 days. These correlations between exposure and zonation are suggested to be causal.In the algal mat and eelgrass zones microtopography of biogenic origin, only a few centimetres high, creates lateral heterogeneity within the zonal biofacies. In the upper sand wave zone, very low amplitude (a∼ 0·1 m) symmetrical sand waves (λ∼ 30 m) of probable storm‐wave origin have a similar effect. In the lower sand wave zone, sand waves (a∼ 0·5 m, λ∼ 60 m) are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones.The densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects: Callianassa californiensis and Upogebia pugettensis, both thalassinidean shrimps; three polychaete worms, Abarenicola sp., Spio sp. and Clymenella sp.; the bivalve Mya arenaria and the gastropods Batillaria attramentaria and Nassarius mendicus. Callianassa excavate unlined temporary feeding burrows, whereas Upogebia build mud‐lined permanent dwelling burrows. Upogebia are restricted to below mean sea‐level where continuous exposure is &lt; 0·5 days, whereas Callianassa extend up to a level, just below mean higher high water, where maximum continuous exposure rises abruptly from 4 to 9 days. This difference in range is probably due to the latter's greater anoxia tolerance—a necessary adaptation for life in an unlined feeding burrow.