Gone Very Bad

This article is a continuation of my previous posting about the problem of the culvert replacement project on the West Shore that has gone very bad.

Some background is necessary before understanding the seriousness of the problem. First, the dams in the Trent Severn Waterway's reservoir and flow through lake system were never meant for flood control measures. That means that they were not sized to handle the waters flowing from the watersheds. 

The Loon Lake dam has a capacity of about 17.1 cubic meters per second.

On the West Shore, and for brevity, I will refer to the Gregory Lane causeway with it’s large culvert as the “water crossing”. 

The water crossing’s culvert is impinged upon by waters from Loon Lake, Miskwabi Lake, Long Lake, and Wenona Lake.

According to the MNRF's publicly accessible flood assessment tool, OFAT III, the 100 year return period water flow impinging on the culvert entrance can be 29.56 cubic meters per second. And the 10 year return period water flow is a lesser 17.38 cubic meters per second. I mention the latter flow because 10 year return periods are more common for less travelled, non highway forest access roads.

For the “forest access road” scenario, that means that there’s a ten percent chance of 17.38 cubic meters per second of water impinging on the culvert’s entrance in any given year.

And in reality, it can happen multiple times!

Where things have “gone very bad” relates to the fact that the culvert is sized to handle a flow of close to 9.1 cubic meters per second. According to OFAT III, 9.1 cubic meters per second relates to a 1.25 year return period. And a 1.25 year return period means that property owners face an 80% chance of flooding in any given year. 

That means there’s a big chance of a flood almost every year! 

Unacceptable.

The irony here is that the culvert was supposed to be designed to pass 22.67 cubic meters per second, but it wasn’t. And that amount would have represented just below a 50 year return period for this watershed or a 2% chance of a flood in any given year.

So instead of a 2% chance of a yearly flood, property owners now face an 80% chance of a flood.

And that’s where the replacement project has gone very bad!

And when the 9.1 cubic meters per second capacity is exceeded, water will flow over the lowland properties and the Gregory Road as it finds it’s way toward the Loon Lake dam.

And that’s almost what happened in during the 2016 freshet: waters flowed over some lowlands surrounding Loon Lake while other lowland properties on the lake saw record high water levels. The waters approached the crest of the main road but didn’t quite breach because of the quick actions of the Trent Severn Waterway staff in dropping water levels. The Highway 118 culvert also became submerged as the waters backed up.

Also notable at the time was a 20 inch difference, called a backwater, in water levels from one side of the causeway to the other. That existed because waters couldn’t get through the culvert!

And as I have noted in earlier posts,the Gregory Road, being a dirt and gravel road has an erodible surface and can wash out.

Creating access problems and potentially life threatening conditions.

The culvert sizing problem also compliments the river’s silting problem; creating a totally nasty situation when waters can’t easily be moved for flood control purposes.

To make matters worse, we now have Climate Change issues to factor into an already undersized water crossing and a constrained river. Generally, designers increase culvert and bridge sizes to compensate for those unknown weather conditions. Designing for 100 year return periods is not uncommon.

The problem continues to be made worse because there are currently major management and unresolved design issues between the road owner and the consulting firm that are hindering resolution. 

And because of those unresolved issues, and the cost to correct them, I do not expect the water crossing problem to be fixed in the near future.


Disclaimer 

This article is published by me as a service to my readers and other interested parties. It is based on my own findings, observations, and interpretations. No scientific validation has been performed. While every effort was made to ensure that information was correct at the time of completion, the accuracy and the currentcy of data from other sources cannot be guaranteed. I do not assume and hereby disclaim any liability of any nature whatsoever to any party for any loss, damage, disruption or suffering caused by use of the information contained in this article or as a result of errors or omissions; whether or not such errors or omissions result from negligence, accident, or any other cause. No one should act upon comments made without considering the facts of specific situations and/or consulting appropriate professional advisors. 
All characters mentioned are composites or purely imaginary, and any resemblance to any individual person is coincidental.
 
© 2016 Mike Grinnell