UGA
Skidaway Institute of Oceanography
search
Research About US Education Faculty Publications R/V Savannah
 Tidal Asymmetry 

Factors that distort tidal currents in estuaries and tidal creeks

The distortion of the M2 tide in shallow estuaries plays an important role in sediment and salt transport. Factors such as friction and channel morphology generate shallow water overtides such as M4 and M6. When these are added to the M2 tidal current, maximum ebb and flood are shifted closer to high or low water resulting in a tidal current that is distorted from the M2 component. Whether the shift goes toward low or high water depends on the hyposometric curve of the surrounding salt marshes.

 
Spatial Variability in Tidal Current Asymmetry in a Shallow Tidal Creek
Cheryl Burden, Jack Blanton, Trent Moore, Julie Amft and Susan Elston
Introduction
The Asymmetric shape of the curve describing water level and velocity over an M2 tidal cycle is the result of overtide generation such as M4. Overtide generation is caused by non-linearities due to bottom friction and continuity constraints. Overtides become important when a/h becomes large, where a is tidal amplitude and h is bottom depth.
Scope of Study
The generation of the M4 and higher harmonic overtides occur commonly in shallow tidal creeks. We are conducting studies in Okatee Creek, a shallow tidal creek in South Carolina (Fig. 1).
The M2 amplitude in the creek is about 1 m and a/h approaches O(1) near the end of the creek (Fig. 2).
We describe the asymmetric properties of water level and velocity in the upper reaches of Okatee Creek using fixed monitors (Table 1). Instruments designated "SBE" recorded sub-surface pressure, temperature and salinity at 0.1 hr intervals using either Sea-Bird Electronics MicroCats or SeaCats. Those designated "RDI" were Acoustic Doppler Current Profilers (ADCP) set to record vertical profiles of horizontal currents in 0.5 m bins at 0.2 hr intervals. We used 600 KHz RDI Workhorses.
Instruments moored at stations shown in Fig. 1 provided simultaneous measurements of temperature, salinity, sub-surface pressure and current profiles. Station distances (km) are measured from the mouth of the creek and depths (m) are for low water. Recording dates for Stations A, B and C were 28 February to 27 April, 2001. For Station D, the dates were 27 March to 27 April, 2001.

Comparison of tidal currents at Stations B and C
A plot of the currents at Stations B and C over three tidal cycles at spring tide (Fig. 3) shows significant changes in their characteristics within a 7 km distance.

Note the following in Figure 3:
• Strength of currents changes dramatically
• Ebb stronger than flood at Station B
• Flood stronger than ebb at Station C
• Acceleration changes abruptly at certain parts of flood and ebb phases
• Maximum ebb occurs soon after high water
• Maximum flood occurs just before high water

Harmonic constituents of vertically averaged currents at Stations B and C are shown in Table 2.

M2 and M4 tidal harmonics at Stations B and C. Amplitude (amp) is in m/s and phase (phs) is in degrees.

Table 2 shows the following:
• M2 currents at C lag those at B by 14 minutes
• M2 amplitude diminished by 0.4
• M4/M2 uniform over station interval

Comparison of tidal water level along the creek

We have plotted an example of spring and neap tide water levels (Fig. 4). Note that low water at Station D at spring tide does not have a sharp end but decreases slowly until rising rapidly as the tide comes in (Fig. 4). This suggests that water level at spring tide is prevented from falling to the same level farther downstream because extensive shoals that emerge at low water between Stations C and D prevent complete drainage of the water at Station D. This does not occur at neap tide when these shoals remain submerged.
Harmonic analyses of subsurface pressure were used to compare how M2 and M4 tidal harmonics change along the creek (Table 3).

Note the following in Table 3:
• M2 tide at A lag those at C by 35 minutes, with negligible phase change between C and D
• M2 tidal amplitude uniform despite abrupt decrease in energy of M2 current at C
• M4/M2 increases slightly from mouth to head of creek

Discussion and Summary
The M2 water level constituent changed little over the channel length (Table 3). But there was a remarkable decrease in the M2 current amplitude over the 7-km distance separating Stations B and C (Table 2). This indicates that friction dissipates much of the tidal energy along the portion of the creek where a/h increases rapidly.

In order to provide a qualitative estimate of friction at Stations B and C, we calculated terms in the vertically averaged momentum balance and chose values of drag coefficient (Cd) that closely balanced the observed water slope. (See Equation 1).

Equation 1 leaves out the non-linear advection term which is surely to be important in channels with large variations in cross-sectional width and depth.

A plot of the three terms in Equation 1 (Fig. 5) reveals that acceleration is negligible, and that the most important terms are water slope and friction. More importantly, the drag coefficient required at Station C must be larger by a factor of five in order to make the curves similar in magnitude.

Note the following for Station C as compared to Station B:
The water slope is quite irregular, perhaps due to the heterogeneous distribution of marshes near the head of the tidal creek.
There is a much longer slack period at low water

The tidal currents at Stations B and C show asymmetric characteristics typical of those in creeks having extensive salt marshes along the bank. The fact that strongest ebb and flood currents occur close to the time of high water are consistent with high salt marshes that become flooded relatively late in the tidal cycle (Dronkers, 1986; Blanton et al. , 2001).

The large expanses of peripheral salt marshes cause large volumes of water to be stored during part of the tidal cycle. Large intertidal storage induces larger ebb currents and weaker flood currents (Friedrichs and Aubrey, 1988). This was the case at Station B. However, as the head of the creek is approached, the increasing value of a/h causes the crest of the tidal wave to catch up with the trough thereby inducing stronger flood currents. This was observed at Station C and we speculate that flood currents become more dominant as the head of the creek is approached.

Acknowledgments
Dr. Chunyan Li provided many helpful comments concerning standing and progressive tidal waves. We also thank Raymond Thomas who operated the R/V Gannet , Skidaway Institute of Oceanography's estuarine research boat and Jay Rosenzweig of Savannah State University who operated the R/V Sea Otter , the boat used to deploy and retrieve the instruments. We gratefully acknowledge support from NOAA's Coastal Ocean Program through a grant to South Carolina SeaGrant Consortium entitled "Tidal Circulation and Salt Transport in a Tidal Creek-Salt Marsh Complex".

References
Blanton, J., G. Lin and S. Elston 2001 . Tidal Current Asymmetry in Shallow Estuaries and Tidal Creeks. Continental Shelf Research : in press.

Dronkers, J. 1986. Tidal asymmetry and estuarine morphology. Netherlands Journal of Sea Research 20 : 117-131.

Friedrichs, C.T. and D.G. Aubrey 1988. Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27 : 521-545.

Related Links:

Sponsors:

Other references:

Blanton, J.O., G. Lin and S.A. Elston.  2002.  Tidal current asymmetry in shallow estuaries and tidal creeks.  Continental Shelf Research 22: 1731-1743.

Blanton, J.O. and F.A. Andrade.  2001.  Distortion of tidal currents and the lateral transfer of salt in a shallow coastal plain estuary (O Estuário do Mira, Portugal).  Estuaries 24: 467-480.

 


Text Only Version

• Skidaway Institute of Oceanography • 10 Ocean Science Circle • Savannah, GA 31411 • USA • (912) 598-2400 •
Copyright © 2017 Skidaway Institute of Oceanography. All rights reserved.