Research on scouring around the Sweepstakes

In my last post I finished off by stating that the fact that there is an environmental concern in the west end of Big Tub Harbour has been established. The question that remains is “what is causing the damage?”. This post will dig into the existing research that exists to address that question.

Twenty years ago, the tour boats operating in Big Tub Harbour included the True North II, the Seaview III, and the Blue Heron V (shown below).

                                                     

Now, the True North II and the Seaview III are gone. The Blue Heron V remains, and in addition the Blue Heron Zodiacs, Bruce Anchor Zodiacs, and Great Blue Heron (shown below). All jet drive boats which allow them to have the benefit of shallow hulls and powerful engines.

    

Ok, I'll stop distracting you with pretty pictures and get to the point. Twenty years ago, is when the National Water Research Institute (NWRI) completed the report entitled “Water Movements at the West End of Big Tub Harbor, Ontario:  What is Causing the Scouring of Sediments in the Vicinity of the Wreck of the Sweepstakes?”. It is this report that is repeatedly referenced and cited in support of the current tour boat operations when environmental concerns are voiced.

I am sure it is apparent from the start of this post that I suspect that the conditions twenty years ago, when the study was conducted, are greatly different from the conditions today. Further, if we delve into the report there are additional causes for skepticism that this report is at all relevant to the current discussions.

First, a quick summary of the report; Parks Canada identified scouring of sediment around the shipwreck of the Sweepstakes in Big Tub Harbour and because this wreck is of economic importance as a tourist attraction they engaged NWRI to study the cause of the scouring which was felt to be hastening disintegration of the wreck. Water currents and sediment were measured in the vicinity of the Sweepstakes over the summer and fall of 1993 and 1994. The study found that "tour boat propeller wash can produce transient currents capable of stirring up bottom sediments but only if the boat is handled aggressively using full-power bursts of thrusts" the study further concluded that "the surf beat phenomenon seems the most likely source of erosional energy".

The study's conclusions were with regard to stern-drive tour boat propeller wash . There is no reference to the impacts of jet-drive tour boats (because none were operating at the time the study was conducted) which comprise the majority of those operating today.

The definition and impact of 'aggressive full-power bursts' was determined based on a single-event experiment with the Blue Heron V in which the tour boat operator was to mimic 'normal' and 'aggressive' operations. The influence of the tour boat operators potential bias towards the study outcome was not considered.

The correlation of surface effects to the sediment was based on weather data from Wiarton; a town about 80 km south. While the study concluded that water movement associated with surface waves seemed to be the main agents for sediment movement, the confidence in the findings appears to be weak. Additionally, the study also recognized that boat traffic was capable of stirring up sediment.

A simple study was completed by Ralph Suke on July 29, 2014 that demonstrated the jet wash from tour boats could generate enough current in the deep area adjacent to the Sweepstakes to move objects placed on the bottom.  The objects chosen were a pot brush and soup ladle sank gently to the bottom when released. The objects moved in a predictable pattern when large tour boats passed over. Photographs were taken (see below).

It is important to note that these findings occurred only for tour boats with jet drives. The largest of the new Bruce Anchor propeller driven boats, which produce a much smaller wake, did not move the same objects on the same day.  

Before

After 

Note the brush and the pink plastic pieces displaced away from the wreck with the brush handle turned away from the wreck, indicating that jet wash is being deflected down the side of the wreck and out. This is consistent with the silt patterns being seen.

After shot taken from a different angle

The NWRI study is being used as evidence by the current tour boat operators to argue that scouring of sediment around the Sweepstakes is not caused by their operations. However, as discussed in this post, that argument is weak at best. The study is out of date, was completed under conditions that no longer exist, and needs to be updated by an independent third party. We need to determine what actions are conclusively causing the scouring so these effects can be mitigated. Without a clear cause it is impossible to mitigate the effects. Simply continuing on with a disregard for the impacts of our actions (or inaction as the case may be) is irresponsible and negligent.