Friday, March 23, 2018

Dawho River Shallows - An 8 ft MLLW Path

Tonight is a break from my usual Key West posts to focus on one of the shallow areas we will be transiting on our way north in the spring. There are a number of these area but most don't offer an alternative route. It appears that Dawho River does offer a deeper path around the shallows. The article below was posted on the Active Captain Facebook page and is also under "ICW Tips" at the left. As the saying goes, "We'll return to our regularly scheduled programming tomorrow..."

In the analysis below, I proposed an 8 ft MLLW path through the Dawho River east entrance. Note that I have not personally tested the route for depth so proceed with great caution, preferably at high tide. Let us know your results since an 8 ft MLLW route would be a great benefit to all those transiting the ICW in that area.  I will personally be taking this route in the spring trip northward. Now, let’s look at the analysis supporting the new route.


The east entrance to the Dawho River has a very narrow, shallow channel with current depths down to 5 MLLW in places. The charted channel cannot be followed and to get the 5 MLLW you have to avoid G115 by 450 ft. On March of 2017, the Charleston USACE did a depth survey of the entire area around the channel and it showed an 8 ft MLLW path through the area but it’s far outside the charted channel. The survey is shown below:

USACE survey of 3/16/2017 with grid lines drawn in


Let's look at two detail cutouts of the above chart. The first one is the detail on the eastern end (the entrance when going south) showing the 3 ft MLLW spot right in the middle of the channel! Look at the nice, deep water northward.

Here's the 3 ft spot right in the middle of the channel you would like to avoid
Look at the deep water north  of the channel!
Here's the second detail showing 8 ft MLLW crossing from the deep water (in blue) to the channel following the red line shown in the top most chart with the black lines. Just follow along from the deep blue to the channel in a southwest direction, always staying in the survey area.

Just stay south of the edge of the survey for 8 MLLW, according to the survey chart

The full survey by USACE is linked here, In the chart at the top, I’ve drawn black lines to form a grid connecting the xy coordinates of the chart. They are slanted since the chart is not positioned exactly north up. I drew in an ideal route shown as a red line in the chart. Using the grid, I interpolated the xy coordinates and then used a conversion routine from Earthpoint to convert the xy readings to Lat/Long, which I plotted in Aqua Map. I checked the conversions against several Lat/Long readings along the edge of the USACE chart and they matched so they ought to be correct. Feeling brave? Note that it takes you over land according to the chart below. It’s just like how the red side route first appeared at Fernandina. That route also took you over “land” according to the charts until the charts were later updated

The 8 MLLW route plotted in Aqua Map

 Here’s the same Aqua Map view (see below) but with the same route plotted but with a satellite overlay. You can see that there’s no “land” at waypoint 3 and it looks more like the USACE survey.


Same route, same Aqua Map but with satellite view overlay ( really nice feature of Aqua Map

How about SonarChart? The problem is that the readings revert to chart readings if no one has been through an area. The early Fernandina SonarChart had the same problem and it also showed shallow water where we now pass through with 14 MLLW after the USACE survey.

I didn't plot the route since SonarChart does not import routes but you get the idea
So, what to do? Obviously, we need someone to try out the route, preferably at high tide and with a shallow draft vessel just in case the USACE is all messed up in their readings – but I think they are correct and an 8 MLLW path is there just waiting to be confirmed and taking a load off boaters with deep drafts trying to get through at low tide.

The route can be downloaded at BDawhoX. Let us know if you try it out.

2 comments:

Unknown said...

Love your blog, your software research, and the scallops recipe. I’ve shared the link to your iPad nav research with everyone I know.
Thanks

Bob423 said...

Ronald, thanks a bunch - share away! I received a reply from a boater that lives in the area so I expect a confirmation (or otherwise) sometime next week. I also put the post as a permanent page under the “ICW Tips” section at left. That would be a better link to give out since it wouldn’t change.