At the Annual Meeting it was thought by some that the J-hooks at Kelley Park installed in 2009 were the cause of the ice jam which forced the Village to close Ralph Street. Equally, this was also thought to be the cause, in part or whole, in past winters as well. Although, I would have to admit that they may play a role, they are certainly not the sole cause. In fact, if anything, they have enabled the Kayaderosseras to pass ice more easily through that section of the stream by virtue of the fact that the 2 hooks at Kelley have greatly deepened the channel, as they were meant to do.
J-Hooks are rock-vane grade control structures used in streams and rivers to redirect flow away from banks where erosion is an issue and fish habitat quality is moderate to low. They accomplish this by altering the existing flow-path of the stream to create a more pronounced and deeper center-channel by directing and funneling the water in a stream away from the bank.
Additionally, J-hooks will also create a longer and deeper scour-hole on the back side of the “J” or hook-portion of the vane. This is accomplished by installing the rock/boulders at gapped intervals so that the water passes between them, rather than spilling over them. This creates a vortex behind each gap, creating greater shear-stress in the center-channel. The vortices, in turn, shift the bed of the stream left, right, and downstream to create a longer, deeper pool than is possible using a W-weir or Cross-vane structure. However, at bank-full flow (last stop before flood-stage), the entire structure is under water to minimize any influence the structure may have as a barrier to flow which tends to cause the water to move laterally (left, right; typically at 90o to the obstruction) and exacerbate any flooding which may occur. J-hooks and other vane-structures also act to improve other aspects of stream-, water-, and habitat-quality. The vortex of flow-paths between the “crown stones” (gaps) creates both upwelling and down-welling, vertically, in the water column, places sought after by trout and other riverine salmonids. Additionally, they also tend to tumble the water as it moves between the gaps and over the anchor stones, which increases the dissolved oxygen content of the water, lowers the temperature, and even kills off a little bacteria. Through time, J-hooks create a deep pool of fast-flowing water with lots of cover perfect for ambush predators such as trout. To explain this, first you have to know a few important facts about rivers and ice and ice jams. Probably the most important is that it takes roughly a 3:1 ratio of water-depth to ice-thickness in order for the river to break up and carry ice downstream. Meaning the depth of water in the Kayaderosseras must be about 3 times greater than the thickness of the ice. So, if the ice is 12-inches thick, there has to be at least 36 inches of water in the channel to carry the ice.
Second, is being mindful of the conditions which create that ratio; typically, 3 or more days above freezing OR a good dose of rain while snow is still on the ground. These conditions have become fairly common for our winters here (climate change anyone?). We get multiple rain-on-snow weather events; and, during the “January Thaw” we often have 3-5 days where the average daily highs are well above freezing.
So, that creates the conditions ripe for the break up and movement of ice “flows” downstream; this, then, brings the first factor into play…when the water-depth is insufficient to carry the ice, the ice then “anchors” on the river bed or other obstructions (logs, structures/bridges, etc.) and the pile-up begins. On the Kayaderosseras at Kelley Park, this is typically the bend at the far-end of the Park. As many of you know there is a sharp bend and constriction in the river there and the water is, invariably, shallow. Past that bend, the gradient of the river also changes dramatically and the water becomes “flat”; meaning the gradient is less-steep and the river becomes wider, shallower, smoother and slower. All these factors (the gradient, the bend, the depth, the width of the channel, etc.) amount to the ice falling out of the water column to form the jam. This past winter, this is exactly where the head of the jam formed.
As I said above, the J-hooks have been working to deepen the channel, protect Kelley Park from flooding, protect the parking lot and bank on that side for being carried away in a flood and improve that reach of the stream since they were installed nearly 10 years ago. Although they may act as an obstruction, depending on how deep the flow in the river is, at bank-full run, there is approximately 2-3 feet of elevation difference between the top-of-bank (parking lot) and the tops of the crown-stones of the J-hooks. That is no accident. They were designed and constructed that way so that material being carried by the river at bank-full or flood-stage would not get hung up on them.
While the J-hooks can act as an obstruction at lower flow conditions (base flow), such flows in the KC typically do not carry ice because that 3:1 ratio isn’t there to break up and move the ice. More-typically the water will flow under the ice at base-flow and over the ice when runoff is elevated, but, not at bank-full stage. In either instance, the J-hooks are not a factor because the ice isn’t a factor.
Now, I may well eat my words for breakfast someday. But, in the meantime, now that we know this is a semi-regular occurrence, we will have to monitor conditions to make more well informed judgements about when the jams will form, what the cause/s are, and, if there’s anything that we can do to alleviate the conditions. But, for now, I’d say the pluses of the J-hooks have it over the minuses.