Now that our first cohort of overwintering clams is in the Shellfish Resilience Lab’s saltwater tank, we are learning how to maintain the lab’s systems. Some of what we are learning may be unique to the Gouldsboro lab, but some of it is knowledge that other towns might use if they decide to raise and overwinter clams. This post shares some of what we’ve learned from the cold days and low tides this January and February.
According to the tide chart, the afternoon low tide on January 3 in Prospect Harbor would be 2.0 feet below Mean Lower Low Water (MLLW — see the note at the bottom of this post for more information about MLLW) and would occur at 5:10 PM. That’s an unusually low tide. Over in Bunkers Harbor, Dana Rice was finishing up his day’s work in the area around the Shellfish Lab when he heard odd, gurgling noises coming from the pump. Dana runs his business out of the building that houses the lab, so he is often on the wharf and floats unloading lobsters and doing other things. It was a good thing he was there because the tide was so low that it had dropped below the intake on the lab’s pump.
On the right is a picture of the pump. The water comes in through the rectangular holes that you see about one-third the way up from the pump’s bottom and is pushed out the pipe on top.
To understand how the water could fall below the pump’s intake ports and what trouble this can cause, it helps to know a little bit about how the water gets from Bunkers Harbor into the tanks in the lab and, if you are not from this area, a little bit about the tides here.
I will start with the tides. The difference between low tide and high tide in this part of Gouldsboro is usually between 10 and 13 feet. On January 3, 2022, the high tide earlier that day was 13.3 feet above MLLW. So, when it dropped to 2.0 feet below MLLW, that was a 15.3-foot swing … a big one!
The picture at the top of this page is from last March when there was a similarly low tide. I took it from a float that moves up and down on the pilings you see in the foreground. The shellfish lab is in the building you see at the top of the photo. Notice that there is water in the center of the harbor even during such a low tide. So, that is where we put the pump. Last September, we attached it to the piling that you see at the far left edge of the photo at the top of the page.
The pump is suspended in a plastic pipe. As you can see in the photo on the left, the pipe is positioned toward the bottom of the piling. We were not exactly sure how high the pump’s intake was off the bottom but figured it was between three and four feet. What happened late in the afternoon on January 3 was that the water level dropped below the intake. When Dana heard gurgling and bubbling sounds he went into the lab, turned the pump off, and called Mike Pinkham. Mike was nearby and measured the water level off the bottom when he arrived. It was 41 inches. So, we knew the intake was higher than that.
The tide was already headed back in when Mike arrived. He tried turning the pump back on and it immediately started pumping water again. That suggested that we might just be an inch or two away from having the pump low enough to keep working even on extreme low tides.
The tide charts estimated that the low tide the following evening would be only 1.9 feet below MLLW — or 1.2 inches higher than on January 3. So, on January 4 Mike and I were in the lab at 6:03 PM, the scheduled time for the low tide that evening, to see whether an extra inch made a difference. 6:03 turned to 6:04 and then to 6:05 and the pump kept running. Mike was out on the float at 6:03 and measured a water level of 43 inches. So, we figure our pump’s intake is about 42 inches of the bottom and have evidence that is close to being low enough. This summer, when we won’t care so much about getting wet and cold, we will pull the pump back up out of its plastic tube and extend it downward another 6 inches or so.
Siphoning and Anti-Siphoning
The photo at the top of this page shows that the tanks inside the lab are a long way up from the water in the harbor. A one-inch diameter black plastic pipe (you can see it in the picture of the pump attached to the piling) runs from the pump all the way up to the lab, a change in height of about 30 feet. When the pump stops, the water runs right back down all that vertical distance back to the ocean.
Inside the lab, the pipes that bring water to the tank run down into the tank, below the tank’s water level, as you can see at right. We did this because the water runs constantly and we wanted to keep it from splashing. But having the tank’s delivery pipe sticking into the water means that when the pump stops, it not only empties out the black plastic pipe but also siphons water out of the tank until the level is below the delivery pipe. To keep that from happening, we installed an anti-siphon valve — a valve that lets water flow in only one direction — into the delivery pipe and the tank.
When Dana heard the gurgling noise in the lab on January 3, he recognized it because he had heard it before, back in December. There is a plastic filter screen around the pump to keep seaweed and other things that could clog the pump from getting into it. But, one cold day in December, Dana heard gurgling and surging noise from the pump and then it stopped. Mike and Jim McLean, pulled the pump up out of its plastic housing and found that the filter screen was clogged seaweed. When they cleaned out the seaweed and restarted the pump they found that the delivery pipe was full of water that was turning to ice. They were eventually able to get the water flowing but learned yet another lesson about the system: You really do want to let the water siphon back out of the delivery pipe — especially when it is freezing cold outside. So, Mike and Jim removed the anti-siphon valve. So, now the water level in the tank drops if the pump stops, but we can get it started again, even on a cold day.
Keeping Life Interesting
Around 5 PM on February 1 the low tide was projected to be 1.8 feet below MLLW. We’d seen worse and so were not concerned. But what we had not considered is that there had been a particularly intense storm over the weekend on January 29 and 30 and there was still a lot of ocean swell, enough to affect the water level in Bunkers Harbor. By luck, Mike was at the lab cleaning out the snow around our backup generator when Josh Scott, who works for Dana, heard the sounds of the pump acting up again. Within a few minutes, the water was back and everything was working again, though there was more silt in the tank all of a sudden.
It was a good reminder that MLLW is just an average and tide charts are just calculations based on averages. Ocean swell can have its own impact on the water level.
More about MLLW …
In Maine, low tide happens twice a day. Most days, one of the low tides is lower than the other, sometimes by a lot! Mean Lower Low Water is the average of the lower low water heights in a particular location, averaged over a 19 year period. The estimated low tide and high tide for each tide cycle are relative to this average lower low tide level.