The term bank height as used in dredging practices represents the face of material to be dredged. It is often left out of consideration when estimating production rates, which could be detrimental to the expected outcome of any dredging project. While most people generally think of the pumping capacity of a dredge, the excavation mechanism feeding the dredge and abundance of available material is equally important. This document will identify the importance of bank height through simulations using a cutter suction dredge.
While other factors play into the big picture with the excavation mechanisms (i.e. compaction, suction velocities, material size, etc), our focus will be a balanced cutterhead system so these factors are not limitations. The purpose of this document is to focus strictly on the relationship between bank height and the effects on production.
The Dredge Model
The model used for demonstrative purposes in this document is an 8"x8" Moray Class underwater pump swinging ladder dredge, 18 foot dredging depth capability, 200 hp available to the underwater pump drive, 25 hp cutter drive, swing width of 13' and 70% operating efficiency.
The Dredge Site Model
The dredge will be operating at maximum depth (18 feet), pumping in 8" SDR17 HDPE pipe, the discharge distance is 1,500 feet, the terminal elevation is 20 feet and the compaction rate is soft (equal to 6 blows per foot using a Standard Penetration Test). The material to be dredged is defined as well-graded fine sand with a d50 size of 0.0049"and d85 size of 0.0245".
The Layman's Theory
In the simplest of explanations, if material is not available in quantities equal or greater to the potential throughput of a dredge, you will experience a decrease in design or theoretical production rates. The dredge is capable of producing a higher throughput because it can actively feed the material to be dredged faster than the material is available to the dredge. This should not be confused with a "cutter-limited" state, but strictly due to the non-availability of material.
Things to Consider
The dredge model for this illustration represents a small dredge contractor's type. The swinging ladder configuration, by nature, has a limited channel width capability because the dredge stays stationary and the ladder swings approximately 30% either side of center. This type of dredge was chosen to show the dramatic effects on production with limited bank heights. There are also other correlations that work in conjunction with bank heights that add to the efficiency or inefficiency of the dredge including; channel width, repositioning time, cutter diameter in relation to the suction pipe diameter and overall operating efficiency. The key is to keep the suction in material at as high of a percentage of the time as possible.
The first graph set uses the dredge and dredge site models and looks at a bank height of 1,000 feet so we can review this example dredge in a perfectly unlimited situation. All graphs provides data in cubic yards per hour and have a correlating graph showing the concentration of the dredged material as a percentage by weight (Cw).
The second set of graphs looks at a bank height of 18 feet, equal to the maximum dredging depth for the dredge. You will start to notice a significant decrease in the throughput as the dredge becomes limited by the amount of bank height available. It is important to note at this point that the design throughput of this dredge model remains at 222 yd3 in every example. The expected throughput takes into account the limitation of bank height and the operating efficiency constant of 70%.
Now let's review what happens as we lower the bank height to see what we typically find as a "real world" situation. The bank height will be lowered to 5 feet, which would be typical for a small contract dredging job (ex. lake restoration job). Notice the drop on both throughput and concentration as we lower the bank height.
More commonly for dredge contractors with focuses on environmental, lake restoration or industrial type dredging projects, a very shallow bank height is generally the case through a large portion of the dredging project. As you will find in the next series of graphs, a bank height of 2 feet starts to affect the throughput dramatically. One would think that a 2 foot layer of material for an 8" suction pipe would provide the dredge with the potential to achieve decent production rates. But based on how fast the dredge can swing through the material, reposition and start dredging again you find that bank height is a very important consideration to any dredging project. The graphs below further demonstrate this example.
Another interesting point to make is that the average dredge operator pays little attention to vary the engine speed once a line length is established and the dredge conditions are relatively static. So the throttle is set to X and they perform the normal dredge functions; swinging the ladder, advancing to the next cut, controlling the cutter speed, etc. This creates a very inefficient system because of the waste of horsepower. In the examples above, the expected horsepower requirement for the dredge pump was 155 hp, 140 hp, 136 hp, 133 hp respectively. This translates to an expected horsepower per yard3 per hour rate of 1 hp/yd3, 2.2 hp/yd3, 3.2 hp/yd3, 5.3 hp/yd3 respectively. In the fourth example you are paying a lot just to pump water.
While there are mathematical formulas that have been derived to perform the calculations required to calculate a certain dredging job and certain bank height, it is highly recommended that you contact the manufacturer of the dredge to work with you to generate estimates to be used for bidding purposes.
For additional information of to have a case study review performed for your dredging project, please contact: Bob Wetta, President - Dredging Supply Company, Inc. Phone: (985) 479-1355.