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

Wong [13] used an ENDECO 714 current meter set at a height of 1.6 m above the bottom to measure the flow near the H1B for a 2-week period in September 1986 (Figure 6—second panel). He found maximum speeds of 80 cm/s during the ebb tide. Because these measurements were made within the bottom boundary layer, he estimated the values to be approximately 20% lower than the corresponding freestream velocities, assuming a logarithmic velocity distribution in the bottom boundary layer [15]. From measurements made at the H1B during a spring tide, Wong estimated speeds as high as 113 cm/s on ebb and 75 cm/s on the flood. More recent observations at the entrance to Parsons Slough (Figure 1) approximately 4 km from the entrance to Moss Landing Harbor indicate maximum flooding and ebbing velocities of 150 and 170 cm/s, respectively. These current speeds are significantly higher than those measured by Wong at the H1B, a result of the narrow entrance to Parsons Slough, and thus are not representative of the flows encountered in the main channel of ES. Wong found that cross-channel velocities were less than 3 cm/s at all locations, consistent with highly channelized flow. Wong's data indicated that velocities near the mouth were approximately 50% greater than Clark's measurements, apparently due to the increase in tidal prism resulting from the addition of the South Marsh to tidal flooding and to the continued effects of erosion over the 16 years between the two sets of measurements [5]. Finally, Wong [13] found that overall, over 90% of the variance in current speed in ES is caused by the tides.

We made current measurements in 1992 with an InterOcean S4 current meter suspended from the H1B, 3 m above the bottom. Again these measurements indicate increasing tidal flows at the H1B where maximum speeds on the ebb tide approaching 120 cm/s were observed (Figure 6—third panel). The higherfrequency ripples superimposed on the tidal current record are primarily due to seiche period oscillations in Monterey Bay [16]. Our most recent record (Figure 6 bottom panel) shows a 1-month sample from a 14-month time series made with an InterOcean S4 current meter located 1.1 m above the bottom (almost certainly within the bottom boundary layer) in the main channel approximately 250 m east of the H1B. Modulation of the primary diurnal and semidiurnal tidal currents by the spring and neap tides produces maximum ebb currents during the spring tide (105 cm/s) that are three times greater than the maximum ebb currents observed during the neap tide (35 cm/s). Adjusting for boundary layer attenuation, the near surface ebb and flood speeds could reach 125 and 42 cm/s, respectively.

During December 1993, Malzone and Kvitek [6] used an S4 current meter to measure currents near the time of maximum ebb at four locations along the main channel of ES (Figure 7). These results show a steady decrease from slightly over 100 cm/s at the mouth, to approximately 50 cm/s at 8.5 km inland. This decrease in current speed is consistent with the reduced tidal prism landward of each measurement location. The large decrease in current speed between 2.2 km and 6.5 km is caused by the addition of waters that drain from Parsons Slough at about 3.5 km from the H1B.

Vertical current profiles in ES have been examined on several occasions. Wong [13] constructed vertical current profiles from observations acquired during May 1987 and April 1988, approximately 200 m from the H1B during peak flow on the ebb tide (Figure 8). He found a significant reduction in speed in the deepest 3–4 m with vertical shears as high as 10 cm/s/m. His data showed no well-defined core of maximum speed. He made these measurements by lowering and raising a single S4 current meter. Wong used his data to estimate a roughness length for the bottom and a thickness for the bottom boundary layer using standard boundary layer parameterizations. From a logarithmic decay law for the boundary layer, he estimated a bottom roughness length of 6.5 cm. Using this figure, he calculated a

#### Figure 6.

Current measurements acquired in Elkhorn Slough at or near the Highway 1 Bridge (H1B). The top panel shows Clark's [14] first measurements in ES using a paper recording TSK current meter. In the second panel, Wong's [13] measurements are shown where an S4 meter suspended from the bridge was employed. In the third panel, our 1992 measurements duplicating Wong's method are shown, and finally, in the fourth or bottom panel, our S4 current measurements from January (2002) are shown using a bottom mounted current meter at 1.1 m above the bottom and 200 m east of the H1B.

friction velocity for the bottom, and used the friction velocity to estimate a thickness for the bottom boundary layer of 3.3 m, following Komar [17].

Vertical profiles constructed from current meter data collected closer to the H1B in February 1994 at different stages of the tide [4], indicate less vertical shear in the upper 5 m, but increased shear in the bottom boundary layer where the thickness of the layer itself appeared to be less than 2 m (not shown). These results taken near

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

#### Figure 7.

The red crosses show main channel current speed measurements made near maximum ebb tide (taken from [6]). The solid black line shows results of the continuity model (redrawn from Figure 17). Greatest changes are found inland of the Parsons Slough entrance.

#### Figure 8.

Vertical current profiles in ES show deceasing current speed in the bottom boundary layer which extends up to 3 m above the bottom [13]. All times are PST + 8 h. (a) 20 May 1987, 22:30; (b) 30 May 1987, 23:00; (c) 9 April 1988, 07:30; and (d) 22 April 1988, 08:30.

the H1B where the primary channel is about 100 m wide, suggest that velocity shear near the bottom may be large enough to mix the entire water column above. Wong's and Malzone's results also demonstrate that tidal current speeds had increased during the 6-year interval between their observations.

Until 2002, no measurements of the tidal currents in and around Parsons Slough and the adjoining South Marsh had been made. This overlooked area contributes at least 30% to the tidal prism of ES, as we discuss in Section 6. As a result, ADP observations of the currents at the entrance of Parsons Slough were acquired on 20 November 2002 over a half-tidal cycle. The results are shown in Figure 9. We gauged the ebbing flow from the ESNERR area with nine transects made up of ten stations each spaced about 10 m apart across the 90 m wide channel. Tidal currents during the ebb cycle were acquired with speeds often in excess of 100 cm/s with a maximum speed of 112 cm/s observed during a 2-h period when the ebbing flow was most intense. Ebbing flow in this channel is deflected clockwise (looking in the direction of flow), and maximum speeds were always observed in the 5 m-deep channel close to the southern (right) bank consistent with centripetal acceleration. Horizontal shear near the south bank approached 10 cm/s/m.

Wong [13] observed a large time lag and reduced amplitude between the tide in the ES main channel and the tide in the ESNERR suggesting that the tide was restricted, indicative of tidal choking as encountered by Kjerfve and Knoppers [18] in a coastal lagoon along the U.S. East Coast. Our observations show that the principle tidal range in the ESNERR (Table A1) is virtually identical to that observed at the SP railroad trestle, and that HHW in the South Marsh lags that at the SP railroad trestle by only 20 min, not necessarily consistent with tidal choking. These results also indicate that relatively large volumes of water are exchanged between Parsons Slough and ES itself. Clearly, additional measurements of the tidal currents through the entrance of the Parsons Slough/South Marsh area are required to better understand this relatively new and overlooked portion of the Slough.

#### Figure 9.

ADP vertical sections of along-channel currents landward of the SP railroad trestle at the entrance to Parsons Slough. The south shore is on the right. These data were obtained through an ebb tidal cycle 20 November 2002. The integrated transport was 2.4 106 m3 .

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

Figure 10 shows a time series obtained from an S4 current meter deployed close to the location where the ADP measurements were acquired and during the same period. Vigorous tidal flows are observed that approach 100 cm/s during the ebb tide and values that approach 60 cm/s during the flood. Higher frequency oscillations superimposed on the tidal records are due to the natural seiche oscillations of Monterey Bay [16].

All of the current measurements made in Elkhorn Slough until 2003 were acquired at a single location in the cross-slough direction, usually in the main channel. Hence, cross-channel shear had not been measured, although its importance in making volume transport calculations was well recognized. Without some knowledge of cross-channel current variability, it is impossible to obtain even crude estimates of the water volume exchange between the Slough and the Bay over a complete tidal day.

To address this shortcoming and, at the same time, obtain a more reliable estimate of the tidal prism, we made a series of acoustic Doppler profiler (ADP) measurements (Pinkel [19]) at 10 locations across the channel, 250 m east of the H1B on 2 January 2003 (Figures 1 and 11). We held a small boat stationary using a mooring line stretched across the 190-m-wide channel of the Slough. The data were binned in 0.25 m increments and averaged for two minutes to reduce Doppler noise to less than 1 cm/s. The lack of vessel motion ensured that high quality data were acquired. We completed six cross-channel profiles during a 6.9-h flood tidal cycle with a tidal range of 0.43 to 1.19 m referenced to MLLW.

At the beginning of the flood cycle, the typically 10–20 cm/s flow was centered over the northern (right) and southern (left) channels (Figure 11). As the flow increased, core speeds of 50 cm/s were observed near the mid-channel shoal. At maximum flood, the 60 cm/s core remained near the center of the channel, and not in the 4.5 m deep channel located near the north bank. As the flow subsided, the current core moved toward the southern channel. The vertical sections in Figure 11 show a core maximum

#### Figure 10.

S4 Current meter time series acquired on 2 January 2003 during the ADP cross sections shown in Figure 9. Shown are the along-channel and cross-channel velocities as well as the total speeds. Note the large variability caused by seiche motions in Monterey Bay. The shaded bars show the periods of the six ADP cross sections.

#### Figure 11.

ADP vertical sections of along-channel currents 250 m east of H1B show vertical and horizontal shear. The north shore is on the right. These data were obtained through a flood tide cycle on 2 January 2003. The integrated transport was 4.9 106 <sup>m</sup><sup>3</sup> .

with near surface speeds of 62 cm/s about midway through the tidal cycle. Greatest horizontal shear occurs between the core and the north shore (right side of Figure 11) but does not exceed 2 cm/s/m. Flow at all depths was in the flooding direction. Because of the time required to acquire the data, these profiles were, of course, not synoptic.
