Runoff in MIKE-SWMM Melissa Figurski

Subroutine Schematic OVRLND subroutine does runoff.

Surface Flow Routing Elements Subcatchment (overland flow) Channel (trapezoidal/parabolic channel flow) Pipe (circular channel flow) Control Structures (weirs and orifices)

Discretization “Mathematical abstraction of the physical drainage system” Turn basin into rectangles Fine vs. Coarse Discretization How the computer sees your basin. More on pg. 80

Discretization

Subcatchment Overview Discretize basin into subcatchments Calculate runoff from each subcatchment Combine outflow from each subcatchment for total runoff Total runoff is computed through a

Subcatchment Subareas Three subareas in each subcatchment represent impervious w/ storage, impervious w/o storage, and previous w/ storage. Actually, subcatchment can consist entirely of one or more types of subareas. Width of impervious area is ratio of impervious area to total impervious area.

Non-linear Reservoir d2 – d1 = i* + WCON [d1 + 0.5(d2 – d1) – dp]/5/3 Manning’s equation for flow. W is width of subcatchment. Non-linear eqn. from combination of Maning’s and continuity equation. Eqn. for time step – solve for d2 with Newton-Raphson method. Plug d2 into Manning’s equation for outflow in that period. i* = rainfall excess d2 – d1 = i* + WCON [d1 + 0.5(d2 – d1) – dp]/5/3 Dt 1.49 W S1/2 WCON = – A n

Reference Storm Water Management Model: Version 4, User’s Manual. Section 4: Runoff Block, Appendix VI