Measuring porosity in flume experiments with a sand bed

1. After completing a flume experiment, carefully drain the flume of just enough water so that water surface is at the sand bed.
2. Collect (say with a short PVC pipe) a section of sand.
3. Measure the mass of this section (=m_wet)
4. Put sample into oven to cook off all water.
5. Measure the mass of the dry sample (=m_dry)
6. Given knowledge of sand density (=rho_sand) and water density (=rho_water), then porosity (lambda) can be computed as:

lambda = 1/((m_dry*rho_water)/((m_wet-m_dry)*rho_sand)+1)