Wed Lunch Paddle: 9/15/2021@ Cohasset

[rylevine]

9_15_2021: Cohasset Harbor to Little Harbor. NSPN WLP. 10:00am – 2:30pm. HT 6:52am 8:3ft, LT 12:45pm 1ft, tidal range 7.3ft, neaps - day past half moon. Participants: Bob L, Prudence B, Ricardo C, Jody H, Mike H, Jane C. 85F, 15kt SW winds, 1ft waves. This was a paddle to practice in the ebb current at Little Harbor. The ground track is shown in Figure 1. The plan to Little Harbor was to exploit the coastline shape and topography to stay as much as possible in the wind-lee. It was expected that because of the relatively small tidal range, the ebb current at the mouth of Little Harbor would be less than what Prudence and I experienced on an earlier springs WLP. The June 23^rd trip started on the flood in the Little Harbor basin, and continued to the ebb with a tidal range of 9.2ft (Wed Lunch Paddle: 6/23/2021@ Cohasset - Trip Reports - NSPN Message Board). We carefully noted then that the Little Harbor ebb started an hour after HT in the outer ocean, and it was expected that today’s paddle would demonstrate the timing of slack before flood and ocean LT. I arrived at 8:30am to the Little Harbor bridge, and the ebb was already flowing weakly. I had hoped to make it by 8am to recheck the hour delay between slack before ebb and HT, but was caught in traffic. We launched at 10am and arrived to Little Harbor at about 10:30am to a surprisingly strong and still building ebb, as expected for a current that started at about 8:00am. We spent a solid hour crossing eddylines and paddling up to a standing wave formed by a center rock ledge. The eddylines were somewhat diffuse with cross currents and boils formed by the openings on either side of a center ledge that was slowly emerging from the falling water depth. To cool off, we did some rescue practice; and also had a genuine capsize. At this location, the wash is into a wide cove so the only consequence of capsizing was a bit of a paddle back in against the wind. A couple of us attempted eddy hops on the right side to get to the upstream eddy but were forced back in the powerful current. Also, as the water level fell, the “dug-in” paddle for the eddy hop sweep hit rocks. As the center rock emerged the overflow caused increasing standing waves and turbulence. All-in-all quite impressive. I think everyone in the group was able to cross the eddylines and straighten the boat; and then ferry across before we broke for lunch on the nearby rock ledges. This was followed by one of the participants running the rapids – sans kayak… - by landing and walking upstream to the bridge. He did this twice, and reported that on the second run he was partially submerged by a whirlpool that quickly released him.

 

Figure 1: Ground track for 9_15_2021 WLP paddle.

     Watching from the shore during lunch, we saw that as the water dropped the dynamic changed. The overflow on the central rock ledge diminished so that there was just turbulence in front of the rock, and the standing waves were located on either side. I regaled the lunch crowd about an experience ten years ago of getting pinned upside-down on the left side flow at a similar water level. As the water level dropped, the offending pinning ledge emerged just as it had in the earlier episode. We made a solemn vow to be cautious in moving water upstream from an obstruction like a rock ledge or bridge abutment. We returned to the flow around 12:30pm – still very strong but with less diffuse eddylines formed from the separated openings on either side. Between the two eddylines, paddlers were able to move directly in front of the central rock in turbulent but less directed water. Some participants got on the peripheral standing waves and a couple of us again attempted to eddy hop on the right side. I considered the left side but recalled my experience from ten years ago; which started from a similar attempt. Importantly, you could discern the water level slowly rising against the central rock even as the flow had hardly diminished. The ocean water level would have to rise up to the level of the peripheral rock ledges to impact the Little Harbor drain. A rough estimate of the exposed drop on either side was around a foot at the 12:45 LT. From the Rule of Twelves, a tidal range of 7.3ft implied about 0.6ft rise by 1:45pm and another 1.2ft rise by 2:45pm. We therefore expected it would take ninety minutes or so before the ocean could start slowing the drain – but of course the water level in Little Harbor was also falling even as the ocean was rising. That was clearly happening as the top of the central rock had less flow; but the peripheral flows were still significant.

     We decided to take a break with a quick run around the nearby Brush Island. From these off-shore rocks we encountered the SW winds on return to Little Harbor. It was about 2:00pm and the flow was still moving over an hour past the 12:45pm LT. I attempted the eddy hop again on the right side, but the flow was still too strong. I decided to try the left side. Here instead of an overfall, the water was bursting out against the pinning rock face we discussed during lunch. On the first attempt, the force pushed my kayak into the rock face exactly as it had 10 years ago – unbelievable! – the boat flipped over and I came down the wash upside-down. After rolling up, I approached it again, and realized that if sufficiently beyond the flow, the subsequent “pin” was actually into calmer water against the rock face. The capsize occurred because my stern was captured by the main flow. I needed to mind the stern and penetrate further up into the flow to purposely catch the pin. It worked. I punched through the main flow, and then pinned upright against the rock face in a turbulent wash which was not that bad. Then I gingerly proceeded close to the rock to a proper upstream eddy. I watched from there as another participant managed the same maneuver and joined me in the eddy. It was very satisfying to run the rapids back down in a 17.5 foot Explorer not known to be a whitewater boat!

     It was past 2:00pm and there still was a strong ebb current through the peripheral openings even as water was rising up the central rock. The lateral “blasting” through the left opening suggested that the drop was now less on that side, and the force was purely due to upstream Little Harbor drain. We decided to head back to Cohasset along the shore. As expected, we lost the wind-lee turning back up into the harbor and it became a bit of a slog against a SW 15kts headwind with gusts. We arrived back to the put-in about 2:30pm after a four nautical mile trip. On the drive back home, I took a detour to visit the Little Harbor bridge. It was now 3:15pm, a full 2.5 hours after outer ocean LT and a nearly imperceptible ebb was still occurring under the bridge. The outside water had probably risen about 2.7ft which seemed sufficient to cut most of the peripheral opening streams. It was effectively at slack.  Little Harbor is a highly controlled example of how upstream geographical "water catchment" (eg rivers, salt marshes, tidal lakes) impact tidal flows to the ocean. Slacks before ebb and flood were clearly offset from the outer ocean tide cycle. Because of a narrow inlet, the outer ocean pressure head required an extra hour to fill the Little Harbor basin on June 23rd, and the Little Harbor pressure head required an extra 2.5 hours to drain the basin today. The forces that determine timing are the outer ocean water level, the basin water level, and the drop over which water has to flow to the sea. Thanks to all the participants today! 

 

Edited September 18, 2021 by rylevine
insert figure

[prudenceb]

Thanks to you, Bob, for a fun day…and for your detailed analysis of features and tidal flow at the bridge.  And to everyone else as well!

But…I didn't realize your capsize was unintentional!  I thought you were just practicing a "Bob maneuver" - capsizing to practice a roll in current.  It was interesting to see your stern briefly pinned against the rock.

[Ricardo Caivano]

Another great WLP. Thank you Bob and all the participants.  A very detailed analysis. I learned a lot yesterday and it was a lot of fun, even the paddle against the wind on the way back!

[prudenceb]

Bob, my Mac can't open Figure 1!

 

[rylevine]

Prudence,

I copied the figure in. Thanks.

Bob

[prudenceb]

Thx.