Gradient of pressure or concentration

Flow rate across a unit area = (conductivity) x (driving gradient) 

Numerical examples follow. First, for flow of water, let the material of the dike have a conductivity factor or transmissivity of 2 x 10 kg/Pa-m-sec and let the difference in pressure from one side to the other be 0.008 MPa (for example, because of a difference in water level of about 30 inches or 75 cm). The average pressure gradient would be 0.002 MPa/m and the flow rate would be 0.04 kg/m -sec—a cupful in 5 seconds for every square meter of cross-section area material with such a high transmissivity would not be suitable for dike purposes. 

Second, for diffusion of salt, let the material of the dike have a conductivity factor or diflfusivity of 5 x 10 m sec. This is a value for the wet sand en masse regardless of the geometry of the tiny water pathways that permeate it. The salt content of seawater is about 3 g per 100 g or 3 kg per 100 kg a cubic meter of seawater has a mass of about 1100 kg so the salt content is about 33 kg/m if the concentration changes linearly from the salty side to the freshwater side, the change is 8.3 kg/m then the diffusion rate for salt is 4 X 10 kg/m -sec. A year is about 3 x 10 sec, so that this equals 1.2 kg or nearly 3 lb of salt crossing per square meter per year. 

To express the same result using moles, take the molecular weight of NaCl as 60 so that 1 kg-mole = 60 kg. Then the concentration in seawater is 33/60 or 0.55 kg-mol/m and the diffusion rate is 0.02 kg-mol/m -year. 

The purpose of the examples is to bring three points to attention as follows. 

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