An investigation of the effects of water velocity on inorganic phosphorus influx to a sediment

Measurements of the net influx of soluble reactive phosphorus (SRP), to a river bed-sediment, illustrate the importance of the water velocity and hydrodynamics in controlling the transfer rates. Experiments are reported using a characterised bed-sediment, with associated fauna, contained in a flowing-water channel. The results show a systematic increase in the net influx of SRP with increasing water velocity. 

A mathematical description of the influx was sought by modelling the experimental results using the Elovich equation, a boundary-layer model and a parabolic rate equation. In fact all three kinetic equations produce a good representation of the experimental data and it is concluded that further research is needed, in well-defined hydrodynamic conditions, to distinguish between the boundary-layer model and the parabolic equation. 

The boundary-layer model leads to an inverse relationship between the boundary-layer thickness (z/μm), and the water velocity (v/cm s^-1, viz z ≈ 2500/v). In comparison, the parabolic equation of the form: influx of SRP (μmol m^-2 = k_p [SRP-EPC₀]², where EPC₀ is the concentration at which the influx is zero prior to the sorption of phosphorus by the sediment and k_p is the rate constant which leads to a velocity dependence, k_p* = 0·714v + 1 where k_p* is the reduced rate constant, k_p* = k_p(v)/k_p(0). 

The semi-empirical Elovich equation in the form: influx of SRP (μmol m^-2) = ( 1 b) ln(1 + abt) where a and b are the Elovich parameters and t the time, gives a convenient description of the net influx of SRP to bed-sediments downstream of a point-source of pollution. The parameters calculated from the results obtained from the experimental channel are used to estimate the SRP flux to the sediment for a distance of up to 5 km downstream of a point-input of SRP.