6.4 Tidal prism revisited

To put the various tidal transport results in perspective and because of the continuing physical changes in the channel, mud flats and marsh, including the addition of the restoration area, we have updated Smith's model based on more recent data acquired during 2002 and 2003. ADP current measurements were also acquired as part of this data collection effort. We have used the following approach to obtain updated estimates of the tidal prism for Parsons Slough, for the location near the H1B, and finally, for the entire Slough. Additional details concerning the methods and results can be found in Broenkow and Breaker [9].

Smith's parameterized cross-section model was employed together with a recent bathymetric map from Malzone [4] in conjunction with the volume continuity equation given earlier (Eq. (2)) to estimate the mid-channel tidal currents and the tidal prism. Using the model with a U-shaped channel, sloping mud flats, and level Salicornia marsh, we first calculated the updated cross-sectional area. By integrating the cross-sectional area along the length of the Slough, we then obtained a new water volume. Finally, by integrating the channel, mud flat and marsh widths, we obtained updated surface areas. These values were then used in Eq. (2) to estimate the volume transports during a half tidal cycle through the seaward-most sections of Elkhorn Slough and Parsons Slough. The values entering the model were subsequently adjusted within reasonable limits given the uncertainties involved to produce volume transports that were generally consistent with the recently-acquired ADP current measurements.

The computed mid-channel current speeds are shown in Figure 17a and indicate values consistently in excess of 150 cm/s over the first 2 km from the entrance of ES. The tidal prism for Parsons Slough was estimated to be 2.4 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> and that for the H1B to be 4.9 106 <sup>m</sup><sup>3</sup> . The results for the entire Slough produce a somewhat larger tidal prism than those predicted by the earlier results (Figure 17b). The maximum tidal prism, i.e., the difference between the cumulative high and low tidal volumes, is approximately 7.6 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> and can be inferred directly from Figure 17b. The contribution to the cumulative volume from Parsons Slough corresponds to the step increase observed in Figure 17b between 2 and 4 km from the entrance to ES. The model estimate for Parsons Slough itself represents over 30% of the tidal prism for the entire Slough.

#### 6.5 Classification

It is difficult to compare ES with some of the more well-known estuaries along the West Coast, such as Puget Sound, the Columbia River, and San Francisco Bay, because its characteristics differ significantly, particularly its spatial scales and recent evolutionary development. One estuary that is similar in some respects, however, is Morro Bay, located approximately 150 km south of ES just north of Pt. Conception. Morro Bay is a bar-built estuary or barrier-lagoon [35]. Like ES, it has a well-defined entrance channel that feeds into the bay itself with a bay interior that is essentially marine-dominated. In summer, salinities increase by several parts per thousand (ppt) inside the bay due to excess evaporation. Although the tidal prism for ES and Morro Bay are similar, the surface area of Morro Bay is almost twice that of ES [21]. Perhaps the largest difference between these estuaries is that while ES is growing rapidly, Morro Bay appears to be filling gradually [36].

A 30-Year History of the Tides and Currents in Elkhorn Slough, California DOI: http://dx.doi.org/10.5772/intechopen.88671

Figure 17.

The upper panel (a) shows the computed mid-channel current speed using the continuity model. The lower panel (b) shows the cumulative tidal volume at given stages of the tide. The tidal prism is the difference between the high and low tide cumulative volumes. The dotted line represents the results from Smith's [10] model.

Formally, a slough is a swamp-like region, inlet, or backwater. According to most accounts, e.g., [1], ES certainly qualified for the name prior to 1946 before the entrance to Moss Landing Harbor was created. However, now the name is inappropriate since its character has changed dramatically through direct tidal exchange between ES and Monterey Bay. However, some of the tributaries that feed into ES are still sloughs in the formal sense. ES appears to be unique in this region because it continues to expand, whereas many other inlets/estuaries appear to be filling in over time. Various terms have been used to describe Elkhorn Slough such as a "seasonal estuary" or as a "tidal embayment." Both terms are appropriate. Because of vigorous tidal forcing, and because density stratification from fresh water discharge occurs only in winter, vertical mixing is intense especially at constrictions such as the H1B and the Parsons Slough railroad trestle. ES is nearly vertically homogeneous in the main channels where most observations have been made. Although ES is vertically well-mixed, it is not necessarily laterally homogeneous. Although the data are few, the observations of temperature and salinity in the Slough, particularly in the wider portions, indicate cross-slough gradients that are consistent with increased warming and higher salinities in these shallower regions. During the summer, ES is a negative estuary because excess evaporation produces higher salinities in the upper slough leading to decreasing salinities toward the mouth. Values greater than 37 ppt have been observed leading to what some authors call "hypersaline" conditions where salinities in this case were higher than salinities in offshore waters. During winter when increased precipitation often occurs, fresh water fluxes increase with additional input from the adjoining sloughs, causing ES to resemble a true estuary with salinities increasing toward the Bay.
