How to dig up the bottom of a river – the endless mechanical circus.

I love digging underwater, but loathe of the endless mechanical circus. Don’t get me wrong – being able to vacuum up sediment with a giant hose is far better than having to lug bucket  after bucket of backdirt up and out of your unit. And as far as archaeology is concerned, nothing can beat floating weightless over the excavation. You can move and work in ways that are impossible on land.

Trowelling sediment into the dredge. - © 2011 Douglas Inglis

The cost of these luxuries is the ridiculous amount of equipment we use. On the Aucilla River Geo-archaeological Project, our excavation rate was determined by two things: soil density (either hard, very hard or complete fluff) and whether-or-not the equipment decided to work that day. Our dredge was a finicky, cumbersome and generally bloody-minded device with the default attitude “I do not wish to work today.”

We worked with what we had – and motivated it with expletives.

We used a power jet dredge made several decades ago for sluicing gold from rivers. The power jet was a beast and worked amazingly well after years of abuse and disuse. Our problems lay elsewhere. The list includes trouble with the intake, the water pump, the pump deck, the pressure hose and the suction hose. Coincidentally, this is also a list of every other component in the dredging configuration that could possibly fail. (Actually this is not quite true – the floating screen deck could fail, but never sank. The design was Ed Green’s, and had been improved over several decades of excavation in the river. The screen deck would, however, occasionally lose a float and go all topsy-turvy when too many people got on one end or the other.)

Our setup on the river looked like this:

Our dredge configuration on the Aucilla River. - © 2012 Douglas Inglis

This is how a power jet dredge system should work (but didn’t):

You turn the pump on (this was near to impossible). It sucks water through the intake (basically never). The pump propels the water through the pressure hose into the power jet (if we could prime it). Inside the power jet, a high pressure nozzle creates a venturi effect, creating suction in one direction and expelling water in another. Water, sediment and everything else is sucked up through the long ten meter suction hose (a 50/50 probability if all other components worked). The diver would then use the dredge to vacuum up dirt from the unit as they excavated. It would get sucked to the surface where it emptied onto the floating screen deck.

The reality of underwater archaeology is that whatever can fail, will. There are almost always problems, and you have to joyfully live with them. For one, our Honda pump could be a bitch to pull start. As the season got on, it got harder and harder. This was exacerbated by the fact that the pump was floating in the middle of the river on a very small and precarious barge. I fell off several times. It was a mild hazard and actually flipped completely over once. This drowned the pump, but could have been much worse for the individual trying to start it.

Eddie on the screen deck - the precarious pump deck floating in the back. - © 2011 Douglas Inglis

To get the pump to suck, you have to prime it (fill it with water) first. Our pump could not hold a prime to save its life; by the time we started it, the water would drain out. The seal between the intake hose and pump was terrible, as was the seal between the intake hose and the intake head. The head hung in the water, filtered out twigs and was theoretically designed to prevent water from draining out. It never worked even once. We overcame the problem of bad seals by judiciously smothering Vaseline over everything. Getting suction remained a speed test. We had to force feed gallons of water into the pump, put the pressure hose back on and start the pump all before it drained. It could be exhausting.

Throughout the season, the pressure hose kept chaffing on the side of the pump barge and screen barge. We kept patching the hose with duct tape, buffering the sharper edges of the barges, and cut and repaired the hose when necessary. Thankfully, we got all our hoses and fitting from a local store run by Aggies. As this was an Texas A&M project, they were exceedingly helpful and did not laugh too much at our troubles.

When everything cooperated at last, water would spout out the power jet, and the dive team would cheer. It was really that much of an ordeal – we would actually cheer when we finally go the equipment working.

We usually had one diver excavating at a time. As you scrape away dirt with a trowel you Hoover it up with the long 4″ corrugated suction hose. It was difficult to wrangle the large semi-flexible hose using brute force. It went where it wanted. Not only was it heavy and unwieldy, but it pulled with the suction and current. To mitigate the downstream forces, we tied it off to the dive barge about ten feet back from the dredge head. This gave us a much shorter length to work with.

Excavating underwater, with the suspended dredge descending into the unit. - © 2011 Douglas Inglis

Instead of running the dredge hose along the river bottom, we hung it on the excavation frame so it descended vertically into the pit. This was one of the most useful aspects of Jim Dunbar’s ingeniously designed frame. Because the frame took some of the weight, its dredge hose was easier to control. This also prevented it from rubbing against the sides of the unit and collapsing our carefully excavated walls.

Inevitably nodules of clay, rocks and roots could all jam up the corrugated uptake hose. When this happened, the suction would drop off. It was always a bad sign.

Unclogging the dredge was an awkward and embarrassing task.

If you were lucky, someone on the screen barge would notice that exhaust water was clear, indicating that no sediment was being sucked up. If the screener was feeling industrious they would try to clear the blockage – which meant getting soaked. The most effective method was to plug the exhaust (with your hand, arms, etc.) and try to reverse the flow of water through the hose, effectively blowing the clog forward and dislodging it.

If this did not work it was time to shut off the pump and beat on the hose.

The diver would have to take the heavy 4″ semi-flexible hose, hold it over their head, and jump up and down underwater.  The idea was that this would shake the clog out. No dignity was involved. If this did not work, you took your flashlight, shone it through the somewhat transparent hose and moved along its length until you found where the clog was. You then beat it into submission with a heavy hammer. As a last resort, you grabbed another diver, disassembled everything and tried to bring it to the surface to figure out what was wrong. Usually the clog would tumble out in the process, and we would put it all back together again.

It was amazing how much we could excavate even with the constant problems – that is just underwater archaeology for you.

Doug taking a cat nap on the screen deck. - © 2011 Douglas Inglis

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Categories: Archaeology, Aucilla River, SCUBA

Author:Doug Inglis: divingarchaeology.com

I study the archaeology of seaborne exploration and contact. I am passionate about public history and outreach, and write about nautical archaeology at http://divingarchaeology.com

6 Comments on “How to dig up the bottom of a river – the endless mechanical circus.”

  1. JB Pelletier
    January 28, 2012 at 10:28 pm #

    Nice, I love dredging, nothing beats it, nothing beats hammering the dredge when it dies the 5th time that day…lol. I have taken 2 continuing ed classes on small engine repair just to make life easier these days.

    • January 29, 2012 at 1:19 am #

      Beating on the dredge gets kind of cathartic after awhile – or is that just me? I should really take some small engine repair classes too – they would probably be the only courses I take where I was guarantied to use to material!

  2. Terry Ethridge
    April 22, 2013 at 10:30 am #

    Try welding up a mesh sphere of 1/8 in wire 8 to 10 in dia. and installing it over your dredge intake.

    • May 6, 2013 at 5:05 pm #

      Good suggestion. Most of our jams occurred when we were removing the overburden of leaves, sticks, rocks and debitage that had slumped down over the years. In those situations, the objective was to remove the large stuff. The screen would work well for finer work however. I have tried pvc drain ends, but stuff always jams in the grid.

      • Terry Ethridge
        May 6, 2013 at 5:23 pm #

        What is your process for obtaining a permit to excavate a river bottom. Is it a County or State application? Also there is a type of pitch fork with 6 to 8 tines about 2 inches apart, good ones are stainless, curved and have a formed basin near the wooden handle. Makes easy work of the large material in either a shovelling or raking motion. Terry

  3. May 6, 2013 at 5:30 pm #

    Every scenario is different. You usually have to get permission from number of agencies; the process varies from state to state, and country to country. I was not involved in the permitting process in Florida, but I know we were accountable to the Bureau Archaeologist Research and the Florida Division of Historical Resources. I am starting a project in Tobago in three weeks. We had to go to the national assembly for permission there. It is always a lot of paperwork.

    I wish we had one of the fork-scoops you describe. They would have been useful in that scenario. Thanks for the comment!

    Doug

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