Tidal Currents in Casco Bay

[Dan Foster]

Taken from Modeling Wind and Tidal Circulation in Casco Bay, Maine: a preliminary study

https://www.cascobayestuary.org/wp-content/uploads/2018/02/Modeling-Wind-and-Tidal-Circulation-True-2005.pdf

Western Casco Bay Currents at Flood and Ebb

 

Eastern Casco Bay Currents at Flood and Ebb

 

On the flood:

The vector field in Figures 8a,b show the vertically averaged flow three hours after the beginning of the flood tide for the western and eastern half of the Bay, respectively. The currents entering the Bay through the southwestern portion of the outer boundary are the sources for flooding Portland Harbor, Back Cove, and other coves and estuaries in the western part of the inner bay. These currents enter along the shoreline of Cape Elizabeth and through Hussey Sound, veer to the right and continue along the Falmouth/Yarmouth shoreline to the northern side of Cousins Island, where they converge with currents entering through Luckse and Broad Sounds. Consequently, the waters approaching the Royal River estuary may come from two sources. In Portland Channel, the vertically averaged currents at this stage of the flood tide are on the order of 40-­50 cm/sec. The surface drogues used by Parker (1982) show surface currents in Portland Channel and Hussey Sound to be the highest that were measured in Casco Bay during that survey. During the middle two hours of flood tide, the surface drogues averaged 55-­75 cm/sec through Portland Channel, and 43­-58 cm/sec through Hussey Sound.

On the ebb:

The vertically averaged flow field during the third hour after ebb tide is shown in Figures 8c,d for the west and east halves of the inner bay, respectively. The water west of Cousins Island drains from the inner bay through Portland Channel and Hussey Sound with speeds in the 40-­50 cm/sec range. An equally strong current of lesser volume also flows through Chandler Cove, where it meets with a southwesterly flow from Luckse Sound. After leaving the inner bay, the current from Portland Channel moves along the Cape Elizabeth shore and continues south to exit the computational domain. The flow out of Hussey Sound and Chandler Cove also move due south. On the eastern side of the Bay, the ebb flow out of the Royal River, Maquoit and Middle Bays moves into Luckse Sound and Broad Sound. The water from the Royal River outlet area flows east of Great Chebeague Island, where it continues along the island's south shore, enters Luckse Sound, and continues seaward in a southerly direction. Also, the flow out of Broad Sound continues seaward in a southerly direction.

With regard to tidal flows only, the currents of Casco Bay might be viewed as having a western and eastern circulation, with minor volume exchanges around Great Chebeague Island and Cousins Island.

[Daniel Carr]

Very nice images and report.   Thanks for sharing it. 

 

 

[gyork]

On 5/9/2019 at 6:47 PM, Dan Foster said:

Taken from Modeling Wind and Tidal Circulation in Casco Bay, Maine: a preliminary study

https://www.cascobayestuary.org/wp-content/uploads/2018/02/Modeling-Wind-and-Tidal-Circulation-True-2005.pdf

Western Casco Bay Currents at Flood and Ebb

 

Eastern Casco Bay Currents at Flood and Ebb

 

On the flood:

During the middle two hours of flood tide, the surface drogues averaged 55-­75 cm/sec through Portland Channel, and 43­-58 cm/sec through Hussey Sound.

On the ebb:

 speeds in the 40-­50 cm/sec range.

Thanks for digging this up Dan. This confirms my suspicions/experience that currents, in general, about Casco Bay are, in most locations, 1kn or less. If my calculations are right (cm/sec X 0.0194=knots), conversion results in max floods of 1.07-1.46kn for Portland Channel and 0.83-1.13kn for the Hussey; 0.78-0.97kn for max ebb.

Many times I've felt a pronounced change of conditions (as in washing machine) when rounding SE Cliff, heading for Cocktail Cove, and the clustered vectors on Ernest and James' excellent study are supportive. I suspect that Brown Cow plays a role in funneling water there on the flood.

g

[mhabich]

Washing machine...sounds more like clapotis than current. 

[gyork]

Exactly-large water volume (aided by current) bouncing off the steep cliffs of "Cliff"

Edited May 11, 2019 by gyork

[Dan Foster]

Gary, thanks for converting m/sec into knots for us - much more useful that way!

This site has animated vectors that show the currents in parts of Casco Bay throughout one tide cycle. I may try to create a video clip from this at some point - the Google Earth animation brought my computer to its digital knees.

The original paper I cited has additional info on the contribution of wind-driven surface currents to the overall situation in Casco Bay. That entire website is full of PDFs of studies that may be of interest.