Dredge

Dredge
News - 2/27/2020
It has been a pretty mild winter so far with few intense storms that cause sedimentation. There was that atmospheric river that threatened us last December, but fortunately it did not produce the rainfall expected. This is relevant because heavy rain storms bring a lot of sediment and phosphorus into Oswego Lake, requiring periodic dredging to remove.

Our current dredge permit has expired and we are proceeding with an update that will give us flexibility to remove sediment everywhere in the lake, beyond the areas permitted last time. We are considering a different dredge process that will not require machines in the lake and give us flexibility to operate in more locations. It will also not require hand digging, something that occurred frequently as homeowners cleared their boat lifts during the LOIS drawdown as shown in the image above.

Our first project is to remove sediment in the upper canal between the headgate and Childs Road bridge. This may not seem like an obvious area of focus because no boats can access it, but this is a water quality improvement project and will help us remove phosphorus and sediment from water we bring in from the river during summer.

Oswego Canal between the headgate and Woodside Circle has been collecting leaves and tree debris for a long time. This is not necessarily a bad thing because years ago when we ran water for power generation year around all this debris would be pushed downstream into the lake. However, since we have limited our river use to a few months in summer, the leaves fall into the canal sink to the bottom and decay.

Areas in the upper canal have over two feet of silt and organic material that has collected over the years. And every year more material is dropped on the existing pile, creating a deep layer of slowly decaying organic matter. This decay process leaches nutrients into the water, which are carried downstream to the lake during summer. Removing this material will help improve our summer water quality by reducing how much phosphorus is transported to the lake. Of particular benefit will be the making the canal deeper between the headgate and Childs bridge.

When we bring in water from the river it contains suspended sediment and phosphorus adhered to that sediment. A lot of it settles out before getting to the lake, but most of the canal is so shallow there are limited opportunities for this to happen. Our goal is to create a pool between the headgate and Childs bridge where material from the river can settle before reaching the lake. We will inject alum in this pool to enhance phosphorus removal and periodically dredge to remove the accumulated sediment.

This is not an area with easy access so we are exploring ways to hydraulically dredge, essentially using a large vacuum cleaner to collect sediment and pump it into a bag. The material stays in the bag and water leaches out to return to the canal. Our first dredge this coming fall will be with an excavator since there are too many sticks, stumps and rocks to use a hydraulic dredge, but in future years we will schedule this in our hydraulic dredge sequence.

Most of the dredge activity in Oswego Lake is pretty small scale and would not justify creating roads to the lake bed for material excavation. We removed a lot of material during the LOIS drawdown because the city already created access roads and had excavators and dump trucks on the lake bed during their project. We worked with some of these same contractors to remove material from areas like Lost Dog Creek delta that will not be accessible again for many years.

However, aside from the two creek deltas most of the material is deposited near storm drain outfalls. This means there are many small piles of debris that need to be removed instead of one large pile. As a result, we are looking into using hydraulic dredging in all areas that need it in the future. We would be doing small scale dredge projects every winter to clean up debris from these storm drains. This would be less disruptive to the lake then requiring a drawdown and having heavy machinery traveling on the lake bed.

Our permit will not lock us into one method so we can use whatever would be most efficient for a specific area. Our goal is to rent equipment to test the hydraulic dredge method next winter to see how well it works. Sediment pumping is not our largest obstacle, it is getting rid of sediment once it has been collected in bags. This will be part of the testing process, but I am confident we can work something out so LOC can effectively remove debris on an ongoing basis.

It has been a pretty mild winter so far with few intense storms that cause sedimentation. There was that atmospheric river that threatened us last December, but fortunately it did not produce the rainfall expected. This is relevant because heavy rain storms bring a lot of sediment and phosphorus into Oswego Lake, requiring periodic dredging to remove.

Our current dredge permit has expired and we are proceeding with an update that will give us flexibility to remove sediment everywhere in the lake, beyond the areas permitted last time. We are considering a different dredge process that will not require machines in the lake and give us flexibility to operate in more locations.

Our first project is to remove sediment in the upper canal between the headgate and Childs Road bridge. This may not seem like an obvious area of focus because no boats can access it, but this is a water quality improvement project and will help us remove phosphorus and sediment from water we bring in from the river during summer.

Oswego Canal between the headgate and Woodside Circle has been collecting leaves and tree debris for a long time. This is not necessarily a bad thing because years ago when we ran water for power generation year around all this debris would be pushed downstream into the lake. However, since we have limited our river use to a few months in summer, the leaves fall into the canal sink to the bottom and decay.

Areas in the upper canal have over two feet of silt and organic material that has collected over the years. And every year more material is dropped on the existing pile, creating a deep layer of slowly decaying organic matter. This decay process leaches nutrients into the water, which are carried downstream to the lake during summer. Removing this material will help improve our summer water quality by reducing how much phosphorus is transported to the lake. Of particular benefit will be the making the canal deeper between the headgate and Childs bridge.
Debris in Oswego Canal
Oswego Canal between the headgate and Childs Bridge. Stumps and debris are clearly visible in the canal along with a years worth of leaves.

When we bring in water from the river it contains suspended sediment and phosphorus adhered to that sediment. A lot of it settles out before getting to the lake, but most of the canal is so shallow there are limited opportunities for this to happen. Our goal is to create a pool between the headgate and Childs bridge where material from the river can settle before reaching the lake. We will inject alum in this pool to enhance phosphorus removal and periodically dredge to remove the accumulated sediment.

This is not an area with easy access so we are exploring ways to hydraulically dredge, essentially using a large vacuum cleaner to collect sediment and pump it into a bag. The material stays in the bag and water leaches out to return to the canal. Our first dredge this coming fall will be with an excavator since there are too many sticks, stumps and rocks to use a hydraulic dredge, but in future years we will schedule this in our hydraulic dredge sequence.
Sediment bags for hydraulic dredging
Heavy felt bags similar to these will be used to contain sediment pumped from the canal bottom.

Most of the dredge activity in Oswego Lake is pretty small scale and would not justify creating roads to the lake bed for material excavation. We removed a lot of material during the LOIS drawdown because the city already created access roads and had excavators and dump trucks on the lake bed during their project. We worked with some of these same contractors to remove material from areas like Lost Dog Creek delta that will not be accessible again for many years.

However, aside from the two creek deltas most of the material is deposited near storm drain outfalls. This means there are many small piles of debris that need to be removed instead of one large pile. As a result, we are looking into using hydraulic dredging in all areas that need it in the future. We would be doing small scale dredge projects every winter to clean up debris from these storm drains. This would be less disruptive to the lake then requiring a drawdown and having heavy machinery traveling on the lake bed.

Our permit will not lock us into one method so we can use whatever would be most efficient for a specific area. Our goal is to rent equipment to test the hydraulic dredge method next winter to see how well it works. Sediment pumping is not our largest obstacle, it is getting rid of sediment once it has been collected in bags. This will be part of the testing process, but I am confident we can work something out so LOC can effectively remove debris on an ongoing basis.