The 6 NM stretch of the ICW behind Isle of Palms has about 9 channels between the marshes and the ocean which cross the ICW. Each of these crossing has the potential to create shoals. If you look at the attached screen shots, you can see that each of these inlet crossings has developed the classic pattern of shoals on the ocean side which results in the M shaped route you must follow as you pass many inlet crossings along the ICW.
Although it was dredged in early 2015, we have noticed that this shoal stretch is getting worse with each passing trip since then according to Navionics Sonar Charts. We watched one boat pass us and get parked hard on the shoal on the spring of 2018. In the fall of 2018, we noted (again using Sonar Charts) that the shoaling was getting more severe, but there was heavy equipment working on the inshore island building a berm for dredge spoils. This spring we noticed that the shoal was even more pronounced than last fall, and the equipment was now building a berm on a second island. Hopefully this shoal will be dredged soon.
We transited here March 31 about half an hour past low. We draw 4.5 feet. This trip we were following Navionics Sonar Charts and Aqua Map Master with the USACE surveys. The surveys indicated that we might see some 5-foot spots. NOAA was showing that we would have +.75 feet of water at low tide. So, in theory we could probably have run straight down the middle. But being a bit cautious we opted to try and find deeper water just north of the M shaped route suggested by the USACE surveys.
Picture 1 Shows an over view of this stretch of the ICW.
Picture 2 Shows the shoals near Breach Inlet Crossing and at G117A. These is the classic shoaling pattern at an ICW inlet crossing.
Picture 3 Close up of the survey and our track at the inlet crossing at G177. This is an even more graphic example of ICW inlet crossing shoaling. This close up also demonstrates one important feature of Aqua Map Master. There are two sets of survey data. The brightly colored data is from November 2018. But there is also slightly grayed out survey data outside of the November data. This is the data collected in September 2013. Having transited this type of inlet crossing dozens of times and having multiple tracks through here from previous trips, we looked at the available survey data and concluded that the 2013 survey indicated that we might find more water just outside the limit of the November survey. Knowing that “in theory” we had more than enough water, we took a track outside the November survey. Had we gone straight up the middle we would have expected to see 5’+0.75’ = 5.75 feet of water. On our track the least we noted was 7.5 feet. (NOTE: In these areas of ICW shoaling, just being on a track 25 to 50 feet to the side of someone else’s track can put you in very different depths.)
Picture 4 This is our track on Navionics Sonar Charts which was running on a second iPad. The Navionics chart bolstered our decision to move over to the north side a bit to find deeper water.
Based upon our experience, a boat with 4-foot draft probably could transit at MLLW. With 5-foot draft I would not suggest making this crossing for 2 hours each side of low water. With 6-draft I’d not transit though here 3 hours before low water to 2 hours after. The NOAA predictions for Charleston Cooper River at the NOAA ODIN page will give you information about whether the tide is running above or below the predicted levels.