Robert Barkau's UNET 1-D unsteady flow model includes the Preissman (note spelling) slot in Chapter 7 of the users manual. UNET is a Corps of Engineers model. In addition to a brief discussion of theory and application, there are references:

Streeter & Wylie, 1967
Parmakian, 1963
Liggett, 1975
Cunge et al, 1980.

Neil Jordan
NJordan@EXPONENT.COM