NAVIGATIONAL SAFETY ISSUES

The proposed layout has cross current fairways ranging from 185’ to 365’. Using an average distance of 266’ as an example, at the required “idle speed” of 1 mph (app 1.47 fps); the time necessary for straight-line navigation from the shoreside slips to the exit channel near Towhead Island is approximately 3 min. If one adds a very conservative 45 sec for backing and turning, the total time to transit the full length of the fairway will be approximately 3 min 45 sec.

The width of the fairways ranges from 96’ to 200’ at their widest point. Using the average fairway width of 138’ and considering the commonly present crossing current of 1 mph (1.47 fps), the current will drive any vessel attempting to transit the full length of the fairway at the required no wake speed into downstream vessels and structures in approximately 1 min 33 sec.

Subtract the width of a 50’ vessel and the reasonable expectation that a it should be more or less centered as it transits the fairway, and this time to collision is reduced to less than 1 min.

The following conclusions must be drawn of vessel operation within this design.  This design offers absolutely no margin for operator error or mechanical failure. 

1. Backing and turning of a large vessel while maintaining a centered position within the fairways will be extremely difficult.
2. Controlled entry and exit with the prevalent beam-to current will be extremely difficult.
3. Loitering at the fairway exits to wait for passing traffic will be impossible.
4. Maintaining a lawful idle speed within the fairways will be impossible. Considerable applications of power and a lateral crablike vessel attitude will be required to counter the current as vessels transit the fairways.
5. A single screw vessel with its inherent limited maneuverability will be impossible to safely operate.
6. Mechanical failure of any vessel operating in the fairway will result in an uncontrolled collision.
7. Frequent collisions will occur between vessels attempting to enter and exit the fairways and the docks and moored vessels downstream.
8. Operation of vessels in swift water conditions and evacuation of vessels in advance of flooding will be impossible.
9. The average weekend recreational boater will not be able to safely operate a vessel in this marina and even a very experienced pilot will be challenged.
10. The presence of any wind whatsoever will significantly increase the already extremely high navigational difficulties posed by this design.
11. As site-specific actuarial data is compiled, insurance for vessels resident to this marina will be either very expensive or unobtainable.

 DREDGING ISSUES

Major annual dredging of the current marina consists of a preliminary deepening of the approaches and inner fairways in late spring by very large commercial vessels using scraping and wheel washing techniques.

 

The dredge first has to work its way into the Harbor using its scraper and clearing the ensuing silt by wheel washing. This photo shows this in process in 1998. The next step is to repeat the process in the Towhead Channel, the upper end of the Inner Harbor and then the lower end. As can be seen, it is a rather violent affair and can only be safely accomplished in the absence of other vessels.

Smaller resident vessels accomplish the ensuing routine scraper dredging of individual slips into the voids created by the commercial vessel. Since this can be done individually, it is only necessary to move one vessel at a time and is often accomplished when vessels are out on the river for the day. Even in normal years, the river attempts to fill the Harbor with silt throughout the boating season and this activity is continuous. The traditional downstream orientation of The Harbor is particularly suited to this activity. For routine clearing of slips, it is necessary for the current to assist in the dispersion of the resulting silt downstream

The proposed designs will nearly double the area requiring routine dredging. The absolutely necessary early season, large scale clearing of the area with large commercial vessels will be impossible: These large vessels cannot be safely positioned and operated in long cross current fairways and the violence of this activity is a significant hazard to recreational vessels whose presence is to be expected year round.  

Even if all recreational vessels were removed and a large dredge somehow properly positioned to clear the fairways into the Towhead Channel, this would still not clear the individual slips. Nearly continuous scraper dredging of the slips by smaller vessels will still be necessary. Unfortunately, the orientation of the slips will not allow the river to clear the material downstream. On the upstream side of the docks, the current will simply push it back where it came and/or into other slips downstream. The only feasible way to clear this design of accumulated silt is to use a cutter dredge that utilizes a floating pipeline to pump the resulting material out into the main river. This technique is burdensome and prohibitively expensive to the point it is not a practical consideration.  

The proposed design has also not taken into account the effects flooding will have on the adjoining shoreline. The areas shown in orange on this illustration can be expected to retain large volumes of mud and debris in the wake of even a moderate flood. These will collect on otherwise insignificant obstructions such as steps, rails, curbs, posts, etc. This material will have to be routinely removed by hand, skip loader, and dump truck to inland disposal areas. Disposal of this material into the marina is environmentally damaging and if allowed, would only add to the dredging problem  

 

DEBRIS ISSUES

This photo shows the typical day-to-day debris that routinely transits the area. The developers have assumed this material follows a straight-line downstream and will be blocked by the point at Beargrass Creek. The fact is se debris are just as susceptible to movement by wind, waves, and eddies created by land masses, docks, and vessels so it can enter from any direction except from directly downstream. The discharge eddy at the mouth of the creek actually pulls debris into the area and any heavy upstream rain in the creek result in a steady parade of debris making a rather amazing u-turn as it exits the creek with the majority ending up in The Harbor.

The traditional design of the current harbor with its downriver slanted docks minimizes this problem. Even as small to medium sized debris constantly enter The Harbor; they generally shed off the slanted boats and docks and eventually float downstream and out of the area with only minimal encouragement by boat owners. The protective Harbor Island also plays an important role. As has already been stated, wind and waves have nearly as significant a role in the movement of debris as the current. Although wind, wave, and current driven debris still enter the area through the relatively small entrances at either end, the Harbor Island blocks much of these.

The Islands most important contribution is to block large destructive items such as trees. Although wind and flood sometimes deposit these in the inner Harbor, under normal conditions, they have too much mass and resultant inertia to make the turn at the entrances. They generally remain out in the Towhead Channel and exit the area harmlessly.

The proposed removal of the Harbor Island will greatly increase the likelihood these will routinely enter the proposed marina. Once the large and small debris described enter the proposed design, they will form large tangled rafts on the upstream facing vessels and structures. These will be impossible to remove against the current and over the very long distances to the Towhead Channel. Although all vessels in the marina will be affected, those in the upriver facing slips will be held captive as even an arm sized branch is sufficient to cause expensive propeller damage. These dense collections of debris will make the operation of all vessels in the area extremely perilous year round

This illustration shows the areas that will likely be subject to catastrophic debris impacts and accumulation during times of major flood. During a prolonged event, this build up will encompass the entire moorage. As the river tops the Beargrass Point and drives into vessels and docks at considerable velocities, huge volumes of larger items will be added to the mix.

Besides the obvious vessel damage this will cause, during an extended event, the continued accumulation will result in a massive hydrostatic load that will overpower the load bearing capacity of even the most well designed floating structures. 

The photo below shows what happens when a large flood and debris flow encounters a marina complex eerily similar to the one proposed for the RiverPark Place development. Notice the debris accumulation in the upriver slips at the bottom of the frame. It is clear the forces involved are awesome. This incident occurred in 1969 in Ventura CA. Several citizens and emergency response personnel perished as a result. This is what will eventually happen if this project is approved.