5.3.3 Inlet Capacity
The capacity of the various categories of drainage inlets can vary significantly depending on a number of factors such as road geometry, size and configuration of the inlet and the amount of depression allowed in the gutter adjacent to the kerb opening. In addition to the geometric influences, the capacity of inlets can be reduced by the presence of debris and detritus under storm conditions, which can result in partial blockage of the drainage inlet and the design must allow for this, see Section 5.5.3 – Blockage.
A flush inlet is one in which the normal gutter cross-section is continued through the location of the inlet without any alteration to the cross-sectional shape.
A depressed inlet is one in which the crossfall of the gutter is increased locally, so that the grade of the kerb invert is depressed for the length of the inlet. Depressed inlets provide greater efficiency than flush inlets primarily because the modified gutter shape contains a greater proportion of the overall gutter flow in the area where the inlet is located. Depressed inlets are generally shown on the drawings with suitable transitions.
The gradient of the road (which typically is also that of the gutter) impacts on the performance of an inlet. As the gradient increases, so too does the velocity of flow within the gutter. Side entry inlets rely on flows being redirected into the opening, so as the velocity increases, it becomes increasingly difficult to redirect and capture these flows. Aprons with deflectors (Figure 5.8) are sometimes used to assist with capture, but their effectiveness varies.
Grated inlets are also impacted by steeper gradients and flows across the top of the inlet can eventually reach velocities where they bounce across the entire length of the grate. This needs to be considered by the designer in the grate design.
The size and configuration of drainage inlets can vary greatly and so with it their ability to capture flows. Many inlet structures have had their capture rates independently tested over a variety of gradients and crossfalls to produce performance curves. Examples of performance curves used in Victoria are shown in Appendix A. Due to the number of different drainage inlet configurations, it is beyond the scope of this Guide to provide a comprehensive list of performance curves. Designers should check with their local jurisdiction regarding availability and use of this information.
Section 5.5 – Design Theory provides empirical relationships for calculating the capture characteristics of inlets where specific performance charts are not available.