How to Estimate Sediment Depth Before Hiring a Dredge
By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.
Dredging Services | Boat Lift Installation October 12, 2018, I was standing on the fuel dock at a marina in Fernandina Beach, Florida. The manager pointed to the turning basin and said, 'Ed, we've got two feet of muck in there, the shrimp boats are throwing a wake wider than the channel.' He was convinced it was two feet. I had a probe bar in the truck. I walked to the end of the dock, dropped it in, and hit a hard bottom at 18 inches. The bar went down 18 inches through soft silt before it stopped. That was the first time I realized how often people mistake water depth for sediment depth. The turning basin had four feet of water at low tide, and he wanted to see six. So the actual dredge cut was not two feet—it was two feet of sediment plus one foot of new bottom. The difference is a change order. I've made that mistake myself, more than once. Now I never call a dredge company with a guess. I use five field methods, in order of cost, to get the sediment depth right. You don't need a survey vessel for a small slip, but you do need to follow this process. Here's what I do.
1. Probe the Bottom in a Grid Pattern
Use a 12-foot length of 3/4-inch steel conduit with a pipe cap on top. Mark it every foot with a paint pen. Work from a flat-bottom skiff, and shove it straight down until it stops. Pull it up, count the wet sand line on the side, and record the depth. Take a reading every 25 feet—if you have a tight slip, go tighter. The reason this matters: you are measuring the thickness of the sediment that has to come out, not the water depth. A hydraulic dredge is paid by the cubic yard. Without a grid, you are bidding blind. But be careful: a log or an oyster shell will stop the bar short and make the sediment look thinner than it is.
2. Pull a Core and Look at What You've Got
The probe tells you how deep, but it doesn't tell you what the sediment is made of. That determines the dredge type and the price. I use a 2-inch PVC pipe, cut at an angle on the bottom. Push it down as far as it will go, hold your thumb over the top, and pull it out like a straw. Cut the pipe open with a utility knife and look at the layering. If the first foot is dark, loose muck and then it gets stiffer, a hydraulic cutterhead can handle it. If you pull up stiff blue clay or dense sand with broken shells, you are looking at a mechanical dredge or a clamshell, and the cost per yard roughly doubles. The reason: you are not just paying for volume, you are paying for difficulty.
3. Compare an Old Survey with a New Depth Sounder
Find the last hydrographic survey or chart for your area—many municipal marinas have one on record. Then rent a small boat with a depth sounder and GPS, or use your own. Run a few lines across the slip and record the depths at the same stage of tide. Overlay the new depths on the old survey and subtract. That difference is the net sediment that has built up over the years. Divide by the number of years and you get an average accumulation rate. The reason: this lets you estimate not just the current thickness but how much more you can expect in five years. That matters if you are designing the project for a long haul. But be aware: old charts have limited precision, and compaction means the top of the sediment is not where it used to be.
4. Ask the Dredge Contractor to Do a Pre-Bid Survey
When you send out bid requests, make a pre-bid survey mandatory. Don't ask for a 'reconnaissance' or an estimate over the phone. Tell the contractor you want a written volume estimate based on field measurements, including probe readings and a sounding survey. Most reputable dredgers will do it for free as part of the bid, because they don't want to eat a mistake either. The reason: this forces the contractor to be honest with his price, and it gives you a baseline for a change order if the actual volume ends up wildly different. I learned this in Savannah when a guy bid a marina job by looking at the water from his truck. He was off by 600 cubic yards.
5. Spend the Money on a Sub-Bottom Profiler for Big Jobs
For anything over a few hundred cubic yards, or any job where the sediment is deeper than you can safely probe, hire a hydrographic surveyor with a sub-bottom profiler. This instrument sends a low-frequency pulse that penetrates the sediment and maps the interface between the soft material and the hard bottom. It gives you a full cross-section, not just point samples. I used one in 2019 for a marina in Fernandina Beach and it paid for itself the first week. The reason: with a precise model, you can write a unit-price contract that pays for actual cubic yards removed, and you avoid the single biggest argument in the dredging business—'that's not what I measured.'
Frequently Asked Questions
Can a regular depth sounder tell me sediment thickness? No. A depth sounder reads the water-bottom interface. It cannot see through the sediment. You need a probe, a core, or a sub-bottom profiler to get thickness. How many probe points do I need? Start with a 25-foot grid. If the bottom is variable, close it to 15 feet. Probe more if you hit a piling ghost or an old abandoned structure. In a confined slip, you can cover it with 30 points. What should a pre-bid survey cost? Most dredge contractors include a pre-bid survey as part of their bidding process. If you ask for a separate contract, expect to pay for their time and equipment, but it is usually a small fraction of the dredging cost. The key is to make it a required line item.