1. Classification of Estuaries by Hydrodynamics
Hansen and Rattray, 1966, Limnol. Oceanogr., 11,
Looking at Partially Mixed estuaries and ignoring lateral variability:
momentum balance:
continuity:
salt balance:
equation of state:
Non-dimensionalizing:
current velocity as river flow
salinity as a function of x and z

2. Results are cast in terms of two dimensionless parameters for the case of zero wind stress:
1) The tidal mixing parameter M :
; B2 is the width of the estuary
2) The gravitational circulation based on the Estuarine Rayleigh number:
We can use surrogates of the above two parameters to characterize the estuary:

3. A) The circulation parameter, which is the ratio of the net surface current us to the mean freshwater velocity through the section Uf
The larger this ratio, the stronger the gravitational circulation. This ratio is typically  1
B) The stratification parameter, which is the ratio of the top-to-bottom salinity difference  S to the mean salinity over the section S0
At mixed conditions,  S = 0

4. The diffusive fraction of the total upstream salt flux  in an estuary can be determined as a function of these two parameters (circulation and stratification)
gravitational convection ceases; upstream salt flux entirely by diffusion
diffusion is unimportant; upstream salt flux almost entirely by gravitational convection
both advective and diffusive fluxes are important in the horizontal balance

5. 2 1
10-1
10-2
S / S0
us / Uf
 = 1
 = 0.99
 = 0.9
 = 0.5
 = 0.1
 = 0.01
Note that the advective component of salt flux is not necessarily proportional to salinity stratification.
gravitational convection ceases; upstream salt flux entirely by diffusion
diffusion is unimportant; upstream salt flux almost entirely by gravitational convection
both advective and diffusive fluxes are important in the horizontal balance

6. 1) No vertical structure in u
No Mixing
10 1
10 0
10 -1
10 -2
10 -3
S / S0
us / Uf 1b 1a 2a 2b 3b 3a 4 NM JF Sh Sl
J17
J11 Ch Cl Mh
M l
Types of Estuaries
1) No vertical structure in u
seaward flow at all depths
diffusive flux of salt dominates
1a) well mixed
1b) stratified
 = 1
 = 0.99
 = 0.9
 = 0.5
 = 0.1
 = 0.01
2) Flow reverses with depth
advective and diffusive fluxes of salt contribute
2a) well mixed
2b) stratified
3) Strong gravitational circulation - advective flux of salt is dominant
4) Salt wedge

7. Estimate  from flushing rate estimates
Officer and Kester, 1991, ECSS, 32,
Flushing Rate R F
Fint
The greater F, the lower  ,
indicating less diffusive and more advective effects

8. Geyer’s new classification
short
rapid flushing
Mississippi
Fraser
Merrimack
highly stratified
Snohomish
Columbia
Hudson high
UR/(βgsoh)1/2
partially-mixed
Hudson low
James
Chesapeake
long
slow flushing
Delaware
A framework for prognostic estuarine classification. The axes are the forcing variables: tidal velocity and
“freshwater velocity”, non-dimensionalized by the a densimetric velocity scale. Stratification variations are mainly
represented by the vertical position on the diagram. The length and flushing times depend on both parameters.
Additional research may lead to a more quantitative approach using this framework.
well-mixed
UT/(βgsoh)1/2