Appendix-A

Roadside gutters

Tutorial 02 introduced junction nodes and conduit links as the basic elements of a drainage network. An example of the parallel pipe and gutter conveyance arrangement that will be used in this tutorial is shown in Figure 3.1. It consists of a below-grade circular pipe connected to manhole junctions on either end, plus an above grade street and gutter channel also connected to the same two manhole junctions. The details of the gutter inlet and drop structures that make the actual connection with the manholes are not important for our purpose. The parameters needed to characterize this type of node-link arrangement are as follows:

Manhole Invert Elevation

The invert elevation of the manhole junction is the elevation of the bottom of the manhole relative to the model’s datum (such as mean sea level). The invert elevation establishes the junction’s vertical placement in the SWMM model.

Manhole Maximum Depth

The maximum depth of the manhole junction is the distance from its invert to the ground surface elevation where street flooding would begin to occur. If this maximum depth is left set at zero, SWMM automatically uses the distance from the junction’s invert to the top of the highest connecting link, which would be the top of the gutter/street channel in this particular tutorial.

Surcharge Depth

The surcharge depth of a junction is the additional depth of water beyond the maximum depth that is allowed before the junction floods. SWMM uses this depth to simulate pressurized conditions at bolted manhole covers or force main connections but it will not be used in this tutorial. Different types of surcharge will be discussed later in this tutorial.

Conduit Offsets

The inlet offset for a conduit is the distance that its inlet end lies above the invert of the junction that it connects to. A similar definition applies to the offset for the outlet end of a conduit. In the parallel pipe and gutter system, the elevations of the gutters are set above the pipes using their inlet and outlet offsets (Figure 3.1).

System Design Criteria

The general drainage system criteria that will be used in this tutorial are listed below. These criteria are based on those defined for the city of Fort Collins (City of Fort Collins, 1984 and 1997). Figure 3.2 shows the different elements of the street considered in these criteria. Two storms will be used to design the drainage system: a minor or initial storm (2-yr) and a major storm (100-yr). The initial storm is one that occurs at fairly regular intervals while the major storm is an infrequent event.  In this tutorial, the streets are classified as “collectors”. Note that the general drainage criteria presented here apply only to this tutorial and will change depending on the location of the system being designed. The criteria are:

The minimum gutter grade (S_L in Figure 3.2) shall be 0.4%, and the maximum shall be such that the average flow velocity does not exceed 10 feet/second

The cross-slope (S_x in Figure 3.2) of all streets will be between 2% and 4%

The encroachment of gutter flows onto the streets for the initial storm runoff will not exceed the specification presented in the second column from the left in Table 3.1. The encroachment of gutter flows on the streets for the major storm runoff will not exceed the specification presented in the third column from the left in Table 3.1

The pipe system should carry the 2-yr storm and work as an open channel system.

Table 3.1 : General drainage system criteria for Fort Collins (City of Fort Collins, 1984 and 1997)