300
                      Mid-plane interface                 Mid-plane interface
     350-                                    200u)400                                      u),- -400-                   

     450                                   >-600


        500-  ~Side-walledge           -     80

     5501                                   10000            200   400    600   800  1000            200   400   600   800   1000
                     X pixels                               X pixels

  Figure 4-3 A sample edge image and the corresponding interface image as a result of pruning.

    The interface images of a whole sequence of 200 are then averaged, thereby obtaining an average interface for a particular tunnel condition. Also, the standard deviation value of the fluctuations of the interface location is calculated as


                                   1001000                  ,2
                                 0~j(K0K) Z~j&K
                                       1000l000                                  (4.1)

                                       jl                  )


where I is the a column vector of Y pixel values at]*' X pixel, K =[1, 2,.,1016], & denotes element by element multiplication and L ] denotes rounding off to the nearest integer value. The advantage of computing the standard deviation value is two fold. First, it gives a measure of the fluctuations of the air-water interface. Second, it is essential to identify outlier points. An average interface image and the corresponding standard deviations (from mean at an average interface image pixel coordinate) are shown in Figure 4-4.