text: In 1923, Strickler published a report examining 34 formulas for the computation of flow in pipes and open channels and related experimental data.  
  The report validated the Gauckler formula and by inference, the Manning formula. Strickler proposed that the Ganguillet-Kutter n-value, used to characterize hydraulic roughness in the Manning formula, could be defined as a function of surface roughness, .   
  Strickler’s equation introduces a new emperical coefficient which must be determined experimentally to define n-value.  However, unlike n-value, which has units of T/L,  has units of length, and at least in theory, is a measurable quantity. A measurable quantity is potentially useful for channel design and stream restoration engineering where the design value of hydraulic roughness may be unknown. 
  Stricker proposed that for a fixed boundary, surface roughness could be defined by the median grain size of a river’s bed material. He also noted that the onset of sediment transport, the mobile boundary condition, increased the observed hydraulic roughness.
  For fixed boundary, gravel bed rivers, Strickler’s equation can be quantified as:
  where  is the median grain size in meters.
  Later researchers produced variations on Strickler’s equation proposing different measures of surface roughness and corresponding variations in the empherical coefficient.  For example, Strickler’s equation has been used to estimate n-values for riprap lined channels from stone gradation.
  The equation also describes the scaling of hydraulic roughness in Froude scaled, physical hydraulic models. 
  In 1933, Johann Nikuradse published a study of hydraulic roughness in pipes that validated Strickler’s observations of the influence of surface roughness in turbulent flows.