1. Field of the Invention
The present invention relates generally to a method and apparatus for maintaining sufficient water depth in harbors and waterways to accommodate deep draft vessels. More particularly, the invention concerns a method and apparatus for accomplishing daily resuspension of newly deposited sediments using a submerged, turbulent water discharge, thereby eliminating the need for periodic dredging and its associated adverse environmental impacts.
2. Discussion of the Invention
Introduction - Most present day harbors are located in bays and estuaries which formed at the mouths of major rivers. These harbors have natural water depths of about 20 feet. At this depth, ebbing water currents are sufficiently intense to carry sediments, introduced by the rivers and streams, through the harbor and out to the ocean.
Although 20 foot water depths were adequate for early sailing vessels, the introduction of deeper draft vessels around the turn of the century created the need for greater water depths. Bucket and suction dredges were introduced to deepen the navigation channels and berthing areas. With these increased depths came the need for continual maintenance dredging. This process of progressive deepening and subsequent maintenance dredging continues to this day. For example, today's major ports and harbors have water depths of 40 feet or more, reflecting the massive size of modern deep draft tankers and bulk carriers.
Increasing the depth of channels and berthing areas reduces the scouring action of natural water currents. As a result, these quiet water areas have become natural collection points for sediments which in the past would be transported through the area and out to sea. Furthermore, the deeper one dredges these areas, the faster they fill in. As a result, maintenance dredging has become an increasingly burdensome problem.
After 150 years of dredging, dredged material disposal areas are becoming scarce. The problem has been compounded by recent environmental regulations which severely limit available options for dredged material disposal. These regulations came about through the recognition that harbor silts and clays have a proclivity for absorbing heavy metals and other toxins. As a result, the cost of dredging has risen sharply in recent years.
Conventional dredging techniques focus on the efficient removal of consolidated bottom sediments. Conventional dredges achieve their economies of scale by removing large volumes of sediment relatively infrequently. As a result, harbor channels and berthing areas are typically dredged every year or two. In between dredgings, sedimentation continues to occur, so that towards the end of a dredging cycle, ship operators are forced to carry only a partial load to avoid grounding.
The shortcomings associated with conventional dredging methods have led to the search for alternative methods of maintaining adequate water depths. One approach has been to interrupt the sedimentation process, either by preventing the influx of sediment to, or the deposition of sediment within a harbor or waterway area. This approach, termed sedimentation control, has proven most effective in harbor berthing areas. An important benefit of the sedimentation control approach is that the costly dredge material disposal problem is avoided.
Another prior art sedimentation control method involves the use of the scour jet array. This system typically consists of a series of near-bottom water jets positioned along the face of a wharf or dock. The jet array is powered by a high pressure water pump, which discharges to a common manifold pipe. The jets are connected to the manifold pipe by a series of valves and hoses.
Although scour jet array systems are generally effective in preventing unwanted sedimentation, they are quite expensive to build and operate. Tests have shown that the most efficient method of producing a water jet with a sizable range of scour is to use a high flow rate, low velocity discharge. High flow rates, however, require large pumps and pipes, thereby increasing the cost of the scour jet array system.
An alternative method of generating a high flow rate, low velocity discharge is to use a motor driven propeller. A number of agitation dredging and sedimentation control systems have been built using open mounted, near-bottom propellers to produce the required scour flow. One type of prior art propeller system is described in U.S. Pat. No. 3,449,915, issued to T. E. Cummings.
The prior art propeller type systems typically suffer from several fundamental problems: (1) the open propeller design is subject to fouling or damage by waterborne debris; (2) the unprotected propeller can be buried by sediment avalanching from adjacent undredged banks; (3) the propeller type systems utilize high salinity (and, therefore, less efficient) bottom water in producing a turbulent scour plume; and, (4) the exposed propeller poses a hazard to divers and marine mammals.
The method and apparatus of the present invention overcomes the drawbacks of prior art systems through the use of an array of turbine chimney scour units of highly novel design. Compared to conventional prior art dredging methods, the turbine chimney scour system of the present invention is less costly and considerably less damaging to environmental water quality. By continually preventing sediment deposition, the turbine chimney scour system avoids the costly problem of dredged material disposal. Additionally, reducing the residence time of the resuspended sediments minimizes the very serious environmental problem of adsorption and release of heavy metals and other toxins. The turbine chimney scour system also minimizes the impact to benthic organisms which are periodically destroyed by conventional dredging methods.
The turbine chimney scour system of the present invention is also considerably more effective and less environmentally damaging than the prior art agitation dredging methods. The prior art agitation dredging methods typically function by remobilizing consolidated bottom sediments, allowing ambient currents to carry them from the area. For example, one common prior art method utilizes a specially-designed vessel to suck sediment off the bottom and discharge it horizontally across the water surface. The objective is to raise the material high enough within the water column so that it is carried a significant distance downstream by ebbing currents. Agitation dredging is typically performed only once or twice a year. The lengthy period of time between dredging is critical, for it allows the sediments to consolidate, greatly increasing their shear strength and resistance to resuspension. As a result, agitation dredging methods are not particularly effective, since they fail to fully resuspend the consolidated bottom sediments, causing them to deposit a short distance downcurrent.
In contrast to agitation dredging, the apparatus of the present invention is typically activated on a daily basis, fully resuspending the thin layer of newly deposited sediment. This provides three important benefits: (1) water turbidity is less because less sediment is resuspended during each operation; (2) transport distances are greater because the sediment is fully resuspended; and, (3) fewer toxins are released because the bottom sediments have a negligible residence time before being resuspended.
Compared with the pump based prior art scour jet array systems, the turbine chimney scour system of the present invention is less costly and considerably more efficient. Cost savings accrue primarily through the use of individual turbine fan units to produce the required jet discharge. This eliminates the need for a long and costly large diameter piping system. Moreover, by utilizing a high flow rate/low velocity discharge, the turbine chimney scour system requires about 1/10th the input power to achieve the same scour performance.
The turbine chimney scour system of the present invention is readily distinguishable from and clearly superior to the prior art open propeller scouring system for several reasons. In the first place, the turbine fan is a different and intrinsically more efficient propulsion method than an exposed propeller. The ducting around the turbine fan acts to produce a higher discharge velocity per unit power input. Enclosing the propulsion device, also, protects divers and unsuspecting marine mammals from accidental injury.
Secondly, the ability to readily adjust the intake location of the apparatus allows low salinity, oxygen rich surface water to be used for scouring. Tests have shown that low salinity water reduces the cohesive strength of the bottom sediments and is more effective in resuspending fine bottom sediments. Additionally, the higher oxygen content of the ingested surface water provides improved water quality near the bottom, which tends to be anaerobic with consequently high bacterial levels. The combination chimney/elbow configuration of the apparatus, also, provides excellent protection of the turbine fan unit from sediment avalanching of adjacent mud banks. Field tests of propeller based scour jet array systems have shown this to be a serious problem.
Another advantage of the apparatus of the invention resides in the novel design of the screened intake which helps to prevent damage to the propulsion unit from waterborne debris. The design, also, prevents the ingestion of fish during operation. The latter is an important permitting consideration in areas having significant marine fisheries (e.g. the salmon fishery in the Pacific Northwest). As an added feature, the flow from the turbine fan can be briefly reversed following operation, helping to clean the screen of any accumulated debris. Still another advantage of the apparatus of the invention is the gravity support frame which permits rapid installation and recovery of the turbine chimney scour units for routine maintenance.