Types of Stormwater Inlets from HEC12 and HEC22

Note:  Types of Stormwater Inlets from HEC12 and HEC22

Stormwater Inlets consist of four main types (http://onlinemanuals.txdot.gov/txdotmanuals/hyd/storm_drain_inlets.htm) with most common shown in Figure 1.

1.   Curb opening inlets either at a sag or continuous on the street,
2.   Grate Inlets either at a sag or in combination with a Curb opening
3.   Slotted Drains in parking lots which can be simulated as curb opening inlets and
4.   Combination inlets either at a sag or continuous on the street which combine a curb opening inlet and a grate inlet for the stormwater runoff

A sag inlet is the end of the line for the runoff because the flow and its debris load have no other place to go as described in the HEC-22 and HEC-12 manuals and a continuous grade inlet is designed to capture the entire runoff flow but if the flow is too large or the inlet is clogged the bypassed flow can travel past the inlet and flow on down the street to a new inlet.   The interception of a sag inlet is ultimately 100 percent but the amount of interception by a continous inlet is variable and is governed by the width of the opening, the grade of the street, the splash over velocity and the amount of side and flontal flow in a grated or combination inlet which is governed by the width and the length of the grate.  Any flow in a continous opening inlet that is not captured ends up as bypass flow and travels down the downstream link or conduit (Figure's 2, 3, 4, 5 and 6).

Figure 1.  Common Types of Stormwater Inlets on Streets

Figure 2.  Continuous Grate Inlet(1) and Sag Curb Opening Inlet(4)

Figure 3.  Curb Opening Inlets(2)

Figure 4.  Continuous Curb Opening Inlet(2)

Figure 5: Grate Inlets and Combination Inlets (1, 3 and 5)


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