Co-Chairmen 

 G. C. Richardson 

 R. Baca 



Physics of I 



Dissolved Gases 



and Engineering Solutions 



PHYSICS OF NITROGEN 

 SUPERSATURATION 



The condition of supersaturation occurs when water 

 falling over a spillway entrains large volumes of 

 air as it plunges into the stilling basin. Atmospheric 

 gases are driven into solution by the high pressure 

 of the impacting water. Because the condition of 

 supersaturation is a chemically unstable condi- 

 tion, a natural degasification process occurs which 

 releases the excess gas in solution. The rate of gas 

 release across the air-water interface is generally 

 controlled by atmospheric pressure and water tem- 

 perature. The degasification rate can be estimated 

 from the relation: 



MODELING OF GAS REGIMES 



There are two distinct gas regimes which require 

 modeling, namely the near-field and far-field re- 

 gimes. The near-field program involves describing 

 the processes of entrainment and supersaturation 

 which occur in the stilling basin. This problem 

 represents a considerable challenge to any modeling 

 effort because it involves a description of a highly 

 turbulent phenomenon. The far-field problem refers 

 to the problem of describing motion and distribu- 

 tion of nitrogen-supersaturated water given the 

 level of supersaturation at the stilling basin. This 

 problem has been modeled with considerable 

 success using one-dimensional transport models. 



Sr = k(C-C s ) 



where C is the concentration of dissolved nitrogen 

 (N 2 ), C s the equilibrium saturation level and k is a 

 rate coefficient. The saturation concentration is 

 principally a function of temperature. At 1 atm 

 pressure, the nitrogen solubility data is adequately 

 described by the relation: 



C s = 23 



0.55808 T + 0.00763 T 



where the temperature, T, is given in degrees 

 centigrade. The rate coefficient, k, is a function of 

 temperature, the degree of turbulence and the 

 interfacial area over which the gas transfer occurs. 

 One of numerous correlations for k proposed in 

 the literature is the relation: 



k = 



/D Z U 



1.028 



(T-20) 



h* 



MITIGATION ACTIVITIES 



0) Consideration of methods to reduce the levels 



of dissolved gas supersaturation in the Columbia- 

 Snake River systems caused by operation of hydro- 

 electric facilities led the Corps of Engineers to two 

 approaches: A reduction in spill volume and modifi- 

 cation of spillways to reduce the degree of air 

 entrainment to stilling basin depths. Several methods 

 of achieving these goals were discussed, including: 



• Additional upstream storage reservoirs to 

 reduce the flow during the run-off season and re- 

 lease the water later in the year. However, there is 

 much opposition to construction of additional dams 

 due to their anticipated environmental impact. 



• Increase water flow through power houses, 

 thereby reducing the volume passing over the spill- 

 way. There are two methods to do this; either by 

 installing additional generators in skeleton bays of 

 the Snake River dams or by passing water through 

 the skeleton bays which have no turbines. Both 

 approaches have been used, but present plans are 

 to install a full complement of turbines by 1979. 

 The use of slotted bulkheads to break the force of 



(3) the water was tested and successfully aided in re- 



(2) 



where D z is the film diffusion coefficient, U is the 

 average flow velocity and h the mean water depth. 



Richardson: Corps of Engineers, Walla Walla Washington; 

 and Baca: Battelle-Northwest, Richland, Washington. 



778 Round Table Discussion 



