Stormwater from runoff enters the subsurface sewer conveyance system through catch basin inlets in roadway gutters, parking lots, depressions, ditches, and other locations, and often not all runoff water from the catch basin enters the inlet and additional water flows in gutters further downstream. There are a few hydraulic aspects to be considered in order to properly model the catch basin-inlet-gutter subsystem:

• Inlets are designed to have certain drainage capacities, and these capacities play an important role in the interaction between sewer subsystems and gutter subsystems. There are well-established design procedures to design inlets based on the design storm data. Once an inlet is set with specific dimensions, its capacity or hydraulic performance is known. In a Bentley SewerGEMS model, the user can optionally input this performance with an inlet capacity rating curve. You can define the tabular relationship between total catch basin drainage flow and the inlet captured flow is presented, or a maximum inlet capacity flow amount. The model dynamically determines the inlet flow.
• When the inlet capacity is set, the excess water above its capacity will flow in the gutter to a downstream point. The gutter can also represent an open channel. Bentley SewerGEMS lets the user specify the gutter cross section just like an open channel; it can be a trapezoidal or generic irregular section, and the user would also specify its Manning’s friction coefficient.
• The following Gutter Shape Types are defined by HEC-22 methodology: Conventional, Parabolic, V-Shaped, and Trapezoidal (Median). With the exception of the GVF-Convex solver, these gutter types are supported by each of our numerical solvers, but differently. HEC-22 Methodology is strictly adhered to for GVF-Rational and Explicit (SWMM) solvers. However, the Implicit (DW) solver models a gutter link element as channel. Generally, tcomputed gutter depth and gutter spread at the interface of the gutter and the inlet opening are computed under assumed uniform flow conditions with the Manning's equation. More specifically, the computed HEC-22 gutter bypass is used by the Manning's equation to estimate the gutter depth and spread at the inlet.

The gutter link is modeled as a channel in the implicit solver. The channel is internally modeled as a rating table of <Depth, Width> pairs for every 20% of Maximum Gutter Depth. That is 6 points (including <0,0> and <max gutter depth value, computed max gutter spread> points). This gutter-as-channel method is used to compute the upstream and downstream pairs of results: 1) Depth (In) and Spread/Top Width (Start) and 2) Depth (Out) and Spread/Top Width (Stop). The terms "Spread" and "Top Width" are used interchangeably even though "spread" refers the width of flow in a gutter, and "top width" is the same measurement but implicitly refers to a channel transect.

This holds true of GVF-Rational, SWMM, and implicit solvers: the software doesn't always compute a Depth and Top Width/Spread. It will always compute Top Width/Spread and Depth together, that is, if the software can compute the pair together at the upstream end the it will compute both of them. Otherwise, the software presents N/A for both of them. At the upstream/start end of gutter link the software computes depth and width of bypass flow in the gutter, right after inlet interception at the upstream node. If the upstream node of the gutter is a Catch Basin, and a physically defined HEC-22 type of catalog inlet, then the software computes these 2 results. Otherwise the software does not attempt to compute them (not having enough physical data to compute them at this time. At the downstream/stop end of the gutter link element the software will always refer to the gutter depth and gutter spread values at the stop node. Therefore, the stop node has to be a Catch Basin with a HEC-22 Inlet for these values to be computed and not appear as N/A.