It might be interesting to discuss this specific case or your own where you've succeeded (or failed) to take adequate safety precautions for unusual dives. I obviously have a great deal of time on my hands.... 
Nevertheless, at the risk of boring people silly, I wanted to share my recent mud gate dives that involved differential pressure. They really made me take a good look at my tendency to downplay risk. I'm sometimes guilty of taking a "rosy scenario" view and then go "whistling past the graveyard".... Maybe some others can relate to that.
In this case, my safety precautions certainly weren't fail-safe, but I think they were more than adequate. I invite you to be the judge. Some may think the risk was minimal and the precautions excessive. Others may think the risk was significant and not addressed sufficiently. Feel free to weigh-in, criticize or to share your own similar experiences evaluating risks and precautions for unusual dives.
Dive report: Opened a dam mud gate, Sept 12, 2010.
First, I don't have commercial dive training, but have enjoyed the challenge of a few simple underwater jobs in the past, including some that have involved minimal differential pressure. This mud gate job would be a little different because, at a depth of 7 to 9', it was a little deeper. Even the small differential pressure of 4psi could present problems depending on the amount of surface area involved. In this situation, there was the possibility, however remote, of creating an opening of 20x20 inches in the worst-case scenario, that is a total failure of the gate. I was briefly tempted to take the rosy view, but it quickly passed.... :shocked2:
A little background. This dam is part of an old historic granite mill complex. I've felt fortunate to have been involved with this historic mill and happy to provide underwater services to them at very low cost. They always provide the assistance of their facilities man, who is a very capable fellow, although a non-diver. He is completely capable to do line tending, etc. I wanted to avoid using a safety diver to keep the costs down, if at all possible.
The impoundment in front of this dam was small, probably 50' in diameter and relatively shallow, dropping down steeply near the dam to a maximum depth of 9 feet at the bottom of the mud gate. I visually estimated the incoming natural flow in the feeder stream to be no more than 3 gallons per second. Leaking of the dam had stabilized the impoundment level at 9', about 7' below the maximum level of 16'.
For over 20 years, this mud gate had been inoperable, stuck in the closed position. Efforts to open the gate had snapped the rusted control shaft years before. They now needed the mud gate opened and the impoundment drained so they could repair or replace the gate. No schematics were available, so the design of the mud gate was unknown. The mudgate sealed off a 20x20" rock-lined straight 10' chute.
After discussing the goal and safety issues with the facilities man, the plan was set:
I geared up and then climbed down a ladder to get to the water's edge. Up close, the water still looked dark and murky and the scattered layer of foam on the surface gave it a pretty crappy look. Once I submerged, it turned out to have surprisingly good vis of about 10'! I was in my drysuit and the 64-degree water felt great!
Taking pictures along the way, a few fin kicks brought me down the sloped, mostly sandy bottom to the mud gate. There was a faint rumble of moving water. Large sheets of thick plastic were sucked into a slightly open corner of the gate. The plastic was also wrapped around the vertical shaft of the gate.
A real good sign was that the gate was not a worm-geared drop-gate as surmised by the facilities man. Instead, it was a butterfly-type that would probably be much easier to open. In addition, as for the risk of entrapment, its vertical pivot shaft would split the 20x20" opening and, when opened, the butterfly plate would stick out, thereby virtually eliminating the chance of a diver's body making a full seal on the opening if sucked in. Plus, the butterfly valve shaft was held securely at the top by a pillow block bolted into the granite. It was unlikely to suddenly fall away and leave a huge opening. Beautiful! No worries now!
I poked around taking some pictures to at least have documentation in the event that the valve couldn't be opened. I pulled away most of the plastic and surfaced after a total of 20 minutes. Once back to the shore, my tender tossed me the tools I requested and the end of the rope tether.
I tied on the rope tether and then entered the water with a claw hammer, a 24" pry bar, and a military folding shovel. After descending to a position by the near side of the mud gate, I signalled the tender so he would take up the slack and secure the rope at that length. Then I went up to the surface to confirm the securing of the rope on shore and review future signals. We expected there might be some confusion if I pulled against the tether inadvertently, but we agreed on slow, steady extraction when in doubt. This eventually happened once during the dive.
I resubmerged and began to work on the mud gate, which was solidly frozen in place. The butterfly plate wouldn't budge even with the pry bar, so I took to pounding the shaft and pillow block to loosen the corrosion and accretions. (My tender was told to expect the hammering.) Then I shoveled away sand from the bottom third of the gate. Fortunately, the vis quickly cleared as water was sucked into the 1-inch wedge-shaped opening at the top of the butterfly plate. After about 15 minutes of pounding and prying, the butterfly plate moved an eighth of an inch, then a little more, until it felt ready to be forced open.
Just for the sake of experiment, I placed the shovel blade over the now 3-inch wide opening to feel what 4psi differential was like on that small surface area. It was substantial! I realized how one might get into a suction situation and because of poor leverage or grip become trapped, especially if a larger surface or greater pressure differential were involved.
I laid into the pry bar and finally got the mud gate fully open and the rushing water created a loud rumble. My tender later said the water shooting out the other side of the 20x20" chute was very loud. I didn't test the suction....
Within a few seconds I finned to the water's edge, which had already receded about a foot. Ten minutes later, the impoundment was nearly completely empty.
Around that time, unfortunately, the thick bottom sediments slid down the steep natural granite gorge into the face of the mud gate, half plugging it with sticks and muck, keeping it from draining enough to uncover the mudgate.
That's when my third "dive" began, as I went back down and waded through the sticky 3-foot deep muck, staying on scuba for added safety. Then, for a half-hour, I shovelled and cleared away muck in front of the mud gate until the bottom stopped advancing, all the while on scuba....
They were pretty safe dives until that third one.... the final clearing of the mud gate almost killed me! Not to mention the cleaning of my gear afterward!
Pics follow.
Dave C
Historic mill dam with non-functional mud gate valve.
Dark and murky-looking water.
The mud gate valve after some clearing.
Typical amount of flow through the open mud gate (image from weeks later at same water level)
Impoundment mostly drained
Impoundment mostly drained
Nevertheless, at the risk of boring people silly, I wanted to share my recent mud gate dives that involved differential pressure. They really made me take a good look at my tendency to downplay risk. I'm sometimes guilty of taking a "rosy scenario" view and then go "whistling past the graveyard".... Maybe some others can relate to that.
In this case, my safety precautions certainly weren't fail-safe, but I think they were more than adequate. I invite you to be the judge. Some may think the risk was minimal and the precautions excessive. Others may think the risk was significant and not addressed sufficiently. Feel free to weigh-in, criticize or to share your own similar experiences evaluating risks and precautions for unusual dives.
Dive report: Opened a dam mud gate, Sept 12, 2010.
First, I don't have commercial dive training, but have enjoyed the challenge of a few simple underwater jobs in the past, including some that have involved minimal differential pressure. This mud gate job would be a little different because, at a depth of 7 to 9', it was a little deeper. Even the small differential pressure of 4psi could present problems depending on the amount of surface area involved. In this situation, there was the possibility, however remote, of creating an opening of 20x20 inches in the worst-case scenario, that is a total failure of the gate. I was briefly tempted to take the rosy view, but it quickly passed.... :shocked2:
A little background. This dam is part of an old historic granite mill complex. I've felt fortunate to have been involved with this historic mill and happy to provide underwater services to them at very low cost. They always provide the assistance of their facilities man, who is a very capable fellow, although a non-diver. He is completely capable to do line tending, etc. I wanted to avoid using a safety diver to keep the costs down, if at all possible.
The impoundment in front of this dam was small, probably 50' in diameter and relatively shallow, dropping down steeply near the dam to a maximum depth of 9 feet at the bottom of the mud gate. I visually estimated the incoming natural flow in the feeder stream to be no more than 3 gallons per second. Leaking of the dam had stabilized the impoundment level at 9', about 7' below the maximum level of 16'.
For over 20 years, this mud gate had been inoperable, stuck in the closed position. Efforts to open the gate had snapped the rusted control shaft years before. They now needed the mud gate opened and the impoundment drained so they could repair or replace the gate. No schematics were available, so the design of the mud gate was unknown. The mudgate sealed off a 20x20" rock-lined straight 10' chute.
After discussing the goal and safety issues with the facilities man, the plan was set:
- The goal was to clear obstructions and pry open or even remove the mud gate
- The first dive would be non-tethered and would be to evaluate and photograph the mud gate and determine what tools to bring on the subsequent tethered dive
- On the second dive, I would wear a polypropylene rope tether and have a line tender
- We would use basic line-pull signals to communicate "okay" status or the need to be extracted
- The tether would be secured on shore at a length that would limit my proximity to the mud gate opening- it would allow my hands to reach the area of differential pressure, but not allow my body to be sucked onto the opening
- After opening the mud gate, the diver would exit the water while the impoundment drained
I geared up and then climbed down a ladder to get to the water's edge. Up close, the water still looked dark and murky and the scattered layer of foam on the surface gave it a pretty crappy look. Once I submerged, it turned out to have surprisingly good vis of about 10'! I was in my drysuit and the 64-degree water felt great!
Taking pictures along the way, a few fin kicks brought me down the sloped, mostly sandy bottom to the mud gate. There was a faint rumble of moving water. Large sheets of thick plastic were sucked into a slightly open corner of the gate. The plastic was also wrapped around the vertical shaft of the gate.
A real good sign was that the gate was not a worm-geared drop-gate as surmised by the facilities man. Instead, it was a butterfly-type that would probably be much easier to open. In addition, as for the risk of entrapment, its vertical pivot shaft would split the 20x20" opening and, when opened, the butterfly plate would stick out, thereby virtually eliminating the chance of a diver's body making a full seal on the opening if sucked in. Plus, the butterfly valve shaft was held securely at the top by a pillow block bolted into the granite. It was unlikely to suddenly fall away and leave a huge opening. Beautiful! No worries now!
I poked around taking some pictures to at least have documentation in the event that the valve couldn't be opened. I pulled away most of the plastic and surfaced after a total of 20 minutes. Once back to the shore, my tender tossed me the tools I requested and the end of the rope tether.
I tied on the rope tether and then entered the water with a claw hammer, a 24" pry bar, and a military folding shovel. After descending to a position by the near side of the mud gate, I signalled the tender so he would take up the slack and secure the rope at that length. Then I went up to the surface to confirm the securing of the rope on shore and review future signals. We expected there might be some confusion if I pulled against the tether inadvertently, but we agreed on slow, steady extraction when in doubt. This eventually happened once during the dive.
I resubmerged and began to work on the mud gate, which was solidly frozen in place. The butterfly plate wouldn't budge even with the pry bar, so I took to pounding the shaft and pillow block to loosen the corrosion and accretions. (My tender was told to expect the hammering.) Then I shoveled away sand from the bottom third of the gate. Fortunately, the vis quickly cleared as water was sucked into the 1-inch wedge-shaped opening at the top of the butterfly plate. After about 15 minutes of pounding and prying, the butterfly plate moved an eighth of an inch, then a little more, until it felt ready to be forced open.
Just for the sake of experiment, I placed the shovel blade over the now 3-inch wide opening to feel what 4psi differential was like on that small surface area. It was substantial! I realized how one might get into a suction situation and because of poor leverage or grip become trapped, especially if a larger surface or greater pressure differential were involved.
I laid into the pry bar and finally got the mud gate fully open and the rushing water created a loud rumble. My tender later said the water shooting out the other side of the 20x20" chute was very loud. I didn't test the suction....
Within a few seconds I finned to the water's edge, which had already receded about a foot. Ten minutes later, the impoundment was nearly completely empty.
Around that time, unfortunately, the thick bottom sediments slid down the steep natural granite gorge into the face of the mud gate, half plugging it with sticks and muck, keeping it from draining enough to uncover the mudgate.
That's when my third "dive" began, as I went back down and waded through the sticky 3-foot deep muck, staying on scuba for added safety. Then, for a half-hour, I shovelled and cleared away muck in front of the mud gate until the bottom stopped advancing, all the while on scuba....
They were pretty safe dives until that third one.... the final clearing of the mud gate almost killed me! Not to mention the cleaning of my gear afterward!
Pics follow.
Dave C
Historic mill dam with non-functional mud gate valve.
Dark and murky-looking water.
The mud gate valve after some clearing.
Typical amount of flow through the open mud gate (image from weeks later at same water level)
Impoundment mostly drained
Impoundment mostly drained
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