If an inlet type is not the "Full Capture" type and there is a bypass gutter from the inlet connecting to downstream (i.e. the inlet is On Grade), the surface inflow to the inlet will be divided into captured flow (the flow that makes into the inlet node and adds to the subsurface system) and bypass flow (the flow that goes to the surface gutter and flows downstream). The model will determine the captured flow, bypass flow, inlet gutter depth and spread based on the inlet types. If the inlet type is “Catalog Inlet” the HEC-22 method is used to calculate all of the flows, depth and spread If the inlet type is “Inflow-Capture Curve”, “Percent Capture”, or “Maximum Capacity”, the captured and bypass flows are determined by the capture curve, percent or maximum specifications, the inlet gutter depth and spread, then are determined by the bypass gutter hydraulic calculation (Manning equation) based on the bypass flow.

The above described calculation algorithm, however, is applicable only when the inlet is not surcharged from the underlying conduit, since the HEC-22 method does not apply to such an overflow condition. When the inlet is surcharged (i.e., the inlet node hydraulic grade line is above the ground rim elevation), the Saint Venant equations based dynamic calculation is applied instead of the HEC-22 based calculation. In order to achieve a smooth transition in switching such equations, the model needs a small transition depth within which the equations are gradually switched. The smaller the transition depth is, the more accurate results are but the model will be less stable. The new calculation option gives user a control over the transition depth. Typical values are on the order of 0.1-0.5 feet and a default value of 0.5 has been proven to be satisfactory for most conditions.