Drainage inlets enable surface water to enter the underground system. The types of inlets have been standardised to provide economy and simplicity of construction and maintenance. Most road agencies have standard drawings for the acceptable range of devices that may be used and designers should familiarise themselves with acceptable local practice.
Inlet pits can generally be:
- side entry inlets – these inlets rely on the ability of an opening in the kerb to capture flow. They are usually depressed at the invert of the channel to improve capture capacity and may or may not incorporate subsurface drainage pits (i.e. rural kerb openings discharging to the verge)
- grated inlets – these inlets are placed directly in the path of the flow e.g. grated gully inlets and inlets for open drains/channels and may need to incorporate subsurface drainage pits
- combination grate and side inlet – these inlets utilise the arrangement of the side inlet with the added capacity of a grate in the channel.
Other types of inlets include those that provide continuous capture such as grated trench drains, kerb drains and slotted inlets. These devices are generally suited to particular situations where conventional inlets, such as those listed above, are unable to be used – for example where there are underground service clashes or very flat road crossfall. In most cases continuous capture inlets are proprietary products.
Side entry inlets
Side entry inlets are vertical openings in the kerb covered by a top slab (Figure 5.3). They may have their throat opening depressed below the gutter invert as well as a locally steepened apron to assist in directing additional flow into the pit. Due to their relatively large openings they are less prone to blockage and are therefore suited to areas of anticipated debris build-up such as within sag vertical curves. They have traditionally been considered more ‘cycle-friendly’ than top entry structures such as grated gullies however today this has less to do with the actual design but rather installation and maintenance practice (i.e. raising covers to match resealing works).
They are of most benefit when used on flatter slopes where low gutter velocity allows greater inflow. In addition, their clear opening is not prone to blockage from regular debris and therefore they are more efficient in sags than grates.
Kerb openings are a form of side entry inlet that have no inlet pit, or interconnecting pipework associated with them. They are useful in isolated, typically rural, locations where kerbing has been installed for channelisation purposes but for which the installation of a full stormwater drainage network may be cost prohibitive, but can be limited by not having sufficient throat drop (depressed throat) leading to the tendency to block quickly due to slow velocities at outlet. The capture performance is similar to a standard side entry inlet having the same opening width. In general, kerb openings in urban environments should be used sparingly in lieu of a pit and underground pipe drainage network because of possible maintenance requirements. Kerb openings should not be considered without consultation with maintenance personnel.
Grated gully inlets
Grated, or top entry inlets consist of an opening in the gutter covered by a grate (Figure 5.4). These inlets require transverse bars for use on public roads that may be accessed by bicycle traffic. However, this bar orientation is susceptible to blockage from debris, particularly on flatter grades, where there are known debris generators like overhanging trees nearby and so need to be of sufficient length to reduce the likelihood of blockage. On freeways, or other roads where cyclists are prohibited, parallel bars, which are less prone to blockage and perform with greater hydraulic efficiency, may be used.
Combination side entry inlets
Combination side entry inlets consist of a side entry pit and grated gully inlet, cast into the same structure (Figure 5.5). Smaller units have the gully inlet parallel with the side entry inlet and of the same length. The side entry inlet, in this case, offers minimal capture increase over the grate alone and provides backup capture in the event that the grate becomes blocked. Larger combination side entry inlets have an extended length of side entry inlet that commences upstream of the grated gully section.
Source: The Local Government and Municipal Knowledge Base http://www.lgam.info/kerb-inlet.
Continuous capture inlets
There are a range of proprietary products that have been designed for situations where flat crossfalls make capture of flows with conventional structures difficult. In these areas of minimal crossfall, gutter flow has a wide spread but has reduced depth. In these locations, it is likely, based on normal design calculations, there would be a need for a significant number of conventional structures in order to contain the spread. A more practical solution is to utilise continuous capture devices such as slotted kerb drain (Figure 5.6) or grated trench drain (Figure 5.7).
Source: ACO Polycrete (reproduced with permission).
Source: ACO Polycrete (reproduced with permission).
Continuous capture inlets have the added benefit of a reduced subsurface cross-section. This enables them to be installed where other constraints such as underground services would otherwise preclude the use of conventional structures. Manufacturers of continuous capture inlets often provide in-house design services for appropriately sizing the devices to suit the site hydrology.
In using these types of treatments, consideration should be made of the potential for increased routine maintenance to ensure their effectiveness.
Other drainage inlets
There are other specialised kerbed drainage inlets, such as slotted inlets, that serve particular situations however these are deemed to be beyond the scope of this Guide. Designers should see manufacturer’s literature or publications such as Brown et al. (2009) for more information.