Drainage Design in Southwestern Alberta

Alberta’s southwestern corner is a small area which produces by far the highest amount of precipitation in the province.  To see how extreme the precipitation differences are, just take a look at Alberta Transportation’s “Runoff Depth Map” (to the right; Click to enlarge) which governs one of the methods used to determine design flows for bridges and culverts in the province.  The small southwestern corner is the only area containing the maximum “runoff depth” of 150 mm, while most of the province is 40 mm or less.

The design of bridges and culverts requires some special attention in this area.

Unit Runoff

When comparing the sizes of various streams to their drainage areas, the tremendous amount of runoff in this area becomes clear.  In the prairies, design flows for bridges and culverts are usually around 0.1 – 0.2 cms/km².  In other words, a drainage area of 10 km² will translate into a design flow for bridges and culverts of about 1 – 2 m³/s.

When you’re in the foothills, or have a hilly drainage area, design flows need to be around 0.3 – 0.5 cms/km².

In the southwestern corner of the province, in which I have done a fair bit of work over the last 10 years, I have a guide of around 1.0 cms/km².  That means a drainage area of 10 km² will produce 10 m³/s of design flow.  This is literally 10 times that for a flat prairie site, but the evidence suggests this is correct.

The Belly River at MVLA headworks has a drainage area of about 130 km²

For example, the photo to the right is the Belly River at the Mountain View, Leavitt and Aetna (MVLA) Irrigation District headworks.  It is a few kilometres from Waterton National Park, within the small highest runoff depth region of 150 mm.  It has a drainage area of about 130 km², and it is apparent that the design flow at the site is probably also in the range of 130 m³/s (unit runoff of 1.0 cms/km²).  The stream bottom is about 15 m wide, the stream banks are 25 – 30 m wide, and the flow velocity is at least 2 – 3 m/s.

Alberta Transportation does not have design flow information, but lists a drainage area of 155 km² for the bridge on Highway 5 (4 km downstream of the photo).  The bridge is a 4 span, 84 m long structure.

This site had a drainage area of only 8 km²

Likewise, for a recent site we were working on, a drainage area of only 8 km² corresponded to a channel which was 2 m wide, with 2 m high channel banks and a 2:1 channel sideslope.  In most areas of the prairies a drainage area of this size would result in a depression in a plowed field, but in Alberta’s southwestern corner it was a decent channel that required careful attention to the design flow to ensure the resulting culvert was not undersized.

Irrigation

This is also the reason why irrigation works so well in southern Alberta.  The huge precipitation volume in the southwest corner is stored behind dams (Oldman, Waterton, St. Mary, etc.) and a system of canals sends the water eastward, where some of the province’s driest areas are located within a short distance.