Environmental Science & Engineering - www.esemag.com - June 2004
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Curb inlets can improve service and reduce cost

By A.R. Perks and R.G. Hewitt

Some Canadian municipalities have been installing curb inlets for roadway storm drainage because they are easier on bicyclists and traffic, and less costly to install and maintain than traditional grated inlets. The curb inlets make it easier for snow clearing as well – no more damaged snowplow blades and inlets in winter.

Curb inlets (also known as side opening inlets) are set into the face of the curb rather than in the road gutter itself (see Figures 1 & 2). This means that there are no iron grates in the traffic lane for cars and bicyclists to avoid. Not only is this more convenient, but because the underlying support structure does not have to withstand the constant pounding of traffic, there is less settlement and damage to the inlet itself – not to mention the reduced wear and tear on cars and bikes.

Figure 1

However, until recently, no reliable data was available on the capacity of the curb inlets used in Ottawa to capture the roadway runoff, and so the design information needed to be updated. To overcome this problem, R. V. Anderson Associates Limited (RVA) and the University of Ottawa were engaged by the City of Ottawa to assist in developing the necessary information, such as flow capture based on varying road and cross fall slopes, for designing the new inlets as part of the City’s roadway design procedures.

RVA had previously worked on the City’s storm sewers and design criteria, and the University of Ottawa had previously done research into roadway drainage for the Ontario Ministry of the Environment, and still possessed the specially designed laboratory flume (scaled down model) for testing the standard grate inlets used by most cities.

For the curb inlet project, the flume (model) was set up to represent the City’s curb inlet design at a one-half scale, and a series of experiments was carried out under different flow and roadway conditions, such as road/gutter slope, cross fall (ie. side slope) and gutter shape to establish hydraulic capacity.

Figure 2
The laboratory results clearly showed that under most practical roadway conditions, the curb opening inlets have substantially less hydraulic capacity than the standard drop inlet gratings. The curb inlet “efficiency “ is between 10% and 20% for most roadway conditions. The standard inlet captures approximately 60%-70% of the street flows under similar conditions.

In addition, the tests showed that the addition of a depression in the curb at each inlet does not dramatically improve the efficiency of the flow capture rate.

The main factor is simply that the City of Ottawa curb inlets are less than 1 metre in width, due to the potential for snow and ice accumulation. Similar curb inlets in the US, where curb opening inlets are more widely used, are in the range of 2-5 metres in width along the curb. This width is required to create sufficient hydraulic “draw” to turn the flow in the gutter into the curb opening inlet.

For the City’s narrow design, under all but the flattest street slopes (i.e. less than 0.5% - 1% slope), the gutter flow just by-passes the inlets.

However, when installed at low points or sumps, the curb opening inlets act in a different way, as basic orifices or broad crested weirs. Then, their capacity increases under the hydraulic head available, and they behave comparably to the standard grate inlets.

The resulting problem is that using typical inlet spacing derived for standard grated inlets, the street gutters will be forced to carry significantly more flow, and potential for flooding will exist at the next low spot in the road, for example. While the problem may not manifest itself for many years along a given street, eventually a storm will overtax the system and create either traffic or flooding problems.

The City of Ottawa updated its design standards and overland flow routes to accommodate the new data. Other municipalities should also update their drainage design standards to allow for the design of curb inlets based on road slope, cross fall, gutter shape, spacing and lateral spread.

In addition, municipalities might consider wider inlets, using upstream flaring and/or gutter depressions to maximize capture, and be especially cautious on steeply sloping streets where unintended ponding might occur at low points. Additionally, overland flow routes may need to be provided at these locations.
A.R. Perks, P.Eng., FEIC, is Director, R.V. Anderson Associates Limited in Ottawa.
Contact e-mail: aperks@rvanderson.com.
R.G. Hewitt, P.Eng., is Director, Infrastructure Services, City of Ottawa.

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