Whitemud Watershed Drains and Crossings
Design standards for drains and crossings have been a complicated issue for the District. There are demands for larger works to better service the land but there are also concerns that larger works contribute to problems downstream and of course budgets are limited.
The conservation district is responsible for all man made drains within the Whitemud Watershed. These are coloured blue in our maps. It is important that we try to provide a similar level of service appropriate to the land along all the 1400 miles of drain and the 1200 crossings the District is responsible to maintain.
We know that it would be far too expensive to construct drains for spring floods. Therefore we have accepted the Provincial agricultural standard. This standard should get the water from a 3 or 4 inch 24 hour rainstorm off cultivated land in about three days, before there is significant crop damage. Because rainstorms vary considerably in intensity and conditions before the rain event can be so variable it is difficult to make a hard and fast rule but significant damage due to the failure of the drain in the summer should not occur on average more than once every 15 years or so.
Crossings are part of the drainage system and transportation system. Therefore it is important that the crossing not restrict the drain and since failure might result in a road being closed for several weeks crossings are designed to a higher standard. The Board has adopted the 10% flood frequency design standard. In other words the design capacity of the crossing should not be exceeded more than once every ten years on average. This does not mean the crossing will fail every ten years but it is likely the road will be overtopped and some damage will occur roughly once in ten years on average. The important feature of this design standard is that it provides the same level of service throughout the watershed. If it is a Whitemud crossing installed in the last 25 years it should not have serious problems with floods expected to occur once every ten years on average. Often we are able to incorporate overflow spillways with crossings to further reduce the risk of failure.
The district's annual budget is about $1 million over 60% goes into drain maintenance and close to 20% goes into crossings. These percentages may reverse in a few years. The Whitemud is responsible for about 1200 crossings with a replacement value of about $20 million. Few crossings last longer than about 60 years so almost all our crossings will have to be replaced in that time. In other words we need to spend an average of $330,000 ($20,000,000/60) on crossing replacements every year, increasing that amount for inflation. The need for replacements has been low the last few years so it is expected that crossing costs will become a serious strain on the budget. In the last 25 years 350 crossings have been replaced. Therefore it seems likely about 800 will require replacement in the next 35 to 45 years. It is estimated these will cost roughly $12,000,000 in 2011 dollars or about $345,000 per year on average.
Currently the Board puts an emphasis on drain maintenance rather than major improvements. Our work is mostly limited to mowing and cleanouts. This gives the biggest bang for the dollar. A well maintained drain has two or three times the capacity of a poorly maintained drain and over the long haul requires less work.
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