4.4 Tidal Zone Information and Database
The across-shore elevations of estuaries can be identified and mapped. The three broadest divisions are the upper supratidal, the intertidal , and the lower elevations of the nearshore subtidal. Tidal variations and variations in morphology of estuaries and their adjacent coastlines will change the absolute measure of the elevation associated with each of these boundaries. Therefore, the definition of tidal zones will usually be somewhat subjective (Howes et al., 1994).
In BC, the marine limit is equal to the storm log line and marks the boundary between the terrestrial upland and the supratidal splash zone. The higher high water line, usually described as the previous large tide's higher high water swash mark, follows the approximate elevation of the boundary between the supratidal and the intertidal. The lower boundary of the intertidal is the chart datum, defined in BC as the level of the "lowest normal tide" (Thomson, 1981) and commonly referred to as "zero tide". Note that the higher high water and lower low water have slightly different definitions for Canada and the United States. Canadian marine charts show chart datum at the "lowest normal tide" where the US defines chart datum at "mean lower low water". As a result, the US chart datum is slightly higher elevation than the Canadian chart datum (Thomson, 1981).
The Tidal Zone database summarizes features relative to different elevation zones of an estuary. The three major zones are schematically illustrated in Figure 4 and are:
Supratidal (may also referred to as the Backshore): The zone that extends inland from the higher high water line (HHW) of the mean tides. The upper boundary of this zone is equivalent to the landward line of marine processes (i.e. the storm surge limit or storm log line that is the lower limit of strictly terrestrial vegetation such as mosses). These areas may be inundated as a result of exceptionally high tides, tidal damming and/or freshet conditions. The supratidal zone corresponds to the `A' zone of the Physical Shore-zone Mapping system (Howes et al., 1994).
Intertidal: The area of tidal influence occurring between chart datum (defined by Canadian Hydrographic Service as the plane below which the tide will seldom fall (Thomson, 1981) and higher high water line (HHW) of the mean tides. Higher high water can often be recognized as the recent high tide swash line. The intertidal zone is often subjectively divided into `upper', `middle' and `lower', and corresponds to the `B' zone of the across-shore description in Howes et al., (1994).
Subtidal: The area below zero chart datum and usually covered by brackish or marine water in an estuary. The 20m isobath dividing shallow, nearshore waters from deeper subtidal environments may be considered the practical limit of the estuarine study area. The subtidal zone corresponds to the `C' and `D' zones of Howes et al., (1994).
Elevation is known to provide an important control on biota in estuaries from both submergence times and from the relative effect of salinity that is related to submergence. In general, biota, especially vegetation, follow well-defined zonation patterns (see Gray and Scott 1987 for an excellent example) based on elevation and related submergence time and there has been considerable discussion in the literature as to whether there are any "critical elevation" controls to zonation; Raffaelli and Hawkins (1996) conclude that there are no definitive breaks for elevation control of species zonation - slope, substrate, elevation, tidal regime, wave exposure and freshwater influence are all factors that contribute significantly to zonation.
In a comparison of different estuaries, elevation becomes an important consideration as the relative elevations of various biological communities in relation to known elevations (zero tide, log line etc.), can be used to infer estuarine processes (wave energy, desiccation, etc.).
Provided in this Section is a description of how elevation can be mapped and incorporated into the elevation zone database supplied in Table 7. This methodology is separated into three components discussed below.
Figure 4 - Schematic representation of across-shore elevations and definitions for elevation zones.
Table 7 - Description of the elevation zone database.
Field Name |
Type |
Width of Field |
Description |
EstuaryID |
Alphanumeric |
14 |
Unique identifier for Estuary (eg. SEYMWSD0001890x) |
PolygonID |
Alpha |
13 |
Unique identifier for elevation polygon |
Zone |
Alpha |
15 |
Name of the elevation zone (e.g. intertidal etc.) |
AreaP |
Numeric |
50 |
Area of subunit in square meters |
Slope |
Numeric |
2 |
Slope of subunit in degrees |
Upper_elev |
Numeric |
4 |
Elevation in metres above/below chart datum |
Lower_elev |
Numeric |
4 |
Elevation in metres above/below chart datum |
Elevations within the estuary may be difficult to measure. A methodology for computing elevation from tidal curves is outlined below and worked examples are provided in Table 8.
Step 1: Airphoto pretyping
When planning an estuary elevation survey, locate one or more across-shore sampling transects from the airphoto. Transects should be selected to include a range of vegetation and substrate types, as well as including channels.
Step 2: Compute Elevations
In certain instances, transect surveys will not be feasible in the larger estuaries, therefore elevation data will have to be interpolated. This step provides a methodology to compute elevations using the Canadian Hydrographic Service Tidal Tables (CHS 1998).
Estimations of tidal elevations between the marine limit and the zero datum can be obtained for individual estuaries by consulting tide tables of the nearest reference port. Table 8 provides a method and examples of determining intermediate tidal elevations using the Canadian tide tables. The position of those elevations within the estuary can then be determined by observation of the position of the water line at a known tide level (e.g. using tide prediction tables, find the time at which the water level will be at the prescribed elevation and shoot a picture(s) of the estuary at that time). Alternatively, the waterline can be walked with a recording GPS to fix different elevations within the estuary.
Definition of general elevations of across-shore zonation in an estuary are included as qualitative descriptions of general locations within the estuary. When available, an elevation survey, with height in decimeters above zero datum should be included in the mapping. Surveys may be confounded by the differences in the datum used for land-based survey and that of the chart datum. Care must be taken to ensure that estuary elevations are tied to marine chart datum.
Table 8 - Methodology for computing for tidal zone elevations.
The Supratidal Zone is the zone from the "marine limit" or break in terrestrial vegetation, which will correspond to the elevation of measured extreme tides, to the elevation of Higher High Water, Mean Tides (HHWMT). This elevation corresponds approximately to submergence of ~5% of the time.
The remaining zones, the upper, middle and lower intertidal zones, are less likely to consistently correspond to vegetation or faunal breaks due to exposure, substrate, aspect and runoff effects. These intertidal zones are easily calculated and useful for generally characterizing different portions of the estuary.
The calculated elevations should be plotted on the airphotos and later digitized into polygons. If the estuary is suitable for transect surveys, then elevations from the transects should be used to verify the computed elevations prior to digitizing.
Where possible, at least one across shore transect running from the log line to the zero tide should be surveyed. Elevation measurements should be made in metres, and the elevation of each change in substrate or biological community type noted on the elevation zone field sheets. Using a combination of the computed elevations in conjunction with field surveys, the elevation zone database should be updated.
