Dye release Sat May 19th: Balancing Dye Flux

The dye release on Sat 19 May was interesting.    30 gallons of dye were released for 5 hours starting at 0430 (AM!) in approximately 10 m water depth a few meters (3-4 m) above the bed between buoy #6 and #8 in the channel   Dye went up the inlet towards the ICW on the flood.    Dye that had reached the surface could also be observed to enter the marsh at the start of the S-turn.   By the start of the ebb,  the marsh was really pink with Rhodamine concentrations of approximately 10 ppb.    As sampling occured on the ebb tide, concentrations of the order of 1-2 ppb were observed coming out of the inlet.   This had lead some specularion that the marsh stored a signficant fraction of the dye.   But that is not the case.

First the dye was released in the lower water column in 10 m depth.  Only the top 0.5 m at most fo the water column could make it into the marsh.    Most of the dye had to make it’s way up the inlet.   Second, dye was input at 5 mL/sec at a concentration of 2e+8 ppb.   1 mL = 1 cm^3  =  1e-6 m^3.     So the pumped flux of dye into the inlet is about Q=1000 ppb m^3/s.

To balance this coming out of the inlet, one needs a water velocity V,  a depth h, and channel width L, and a concentration D – Assuming a rectangular channel and a uniform velocity and dye concentration.   Then the flux is  Q = L*h*V*D.

Taking representative values for V=1 m/s,   h= 5 m,  L=200 m   this implies that to balance the input flux one needs….. drum roll……   D = 1 ppb.    Which is the order of magnitude that we were measuring coming down the inlet during the duration of the flood.

It will be interesting to see what a more detailed dye mass/volume balance will show.

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