type Discretizations = enumeration(centr_diff "Central Difference Scheme - Basic", centr_diff_AllowFlowReversal "Central Difference Scheme - Allows Reverse Flow (but not yet zero flow)", upwind "Upwind Scheme - Basic", upwind_AllowFlowReversal "Upwind Scheme - Allows Flow Reversal (and zero flow too)", upwind_smooth "Upwind Scheme with Smoothing");