Living on a barrier island in South Carolina, the importance of currents becomes obvious. There are significant sections of the beach near my house (Sullivan’s Island) that are off-limits to swimming because of the dangers. There is a navigation buoy just off the beach that during ebb tides is almost sideways. I am guessing there is four plus knots of current there sometimes – not quite Woods Hole, but close. Anyway, enough to easily shape sedimentation patterns.

Not surprising the inlets between islands in this part of SC are ebb dominated (see image below). They control sand bypassing events and act as a large subaqueous sand source (Kana et al., 2013). During erosional events the sand is temporarily stored in the ebb deltas and conserved within the littoral zone, i.e., it is not lost from the nearshore system. For the continued stability along these ebb-delta-controlled islands the inlet dynamics are on par with the wave climate (Hayes, 1979). Behind the barrier islands (Sullivan’s Is., Isle of Palms, and Dewees and Capers Islands) are large expanses of marsh, tidal creeks, and the intra-coastal waterway; there is a lot of water that has about a six foot tidal range and needs places to ebb out.

Breach Inlet and ebb delta - Note the expanse of sand extending from the northern island - Isle of Palms.

In recent years the island to the north (Isle of Palms) has had a couple of renourishments adjacent to the ebb delta on the it’s north end. This is concerning for the people who live in the immediate area ($ for renourishment) and for the rest of the people living on these two inhabited islands. What it suggests is that the ebb delta is not bypassing at the same rate as in the past and it needs a costly push to maintain previous levels. I understand the data is not conclusive on the change in bypassing cycles, but it is a red flag in my book.

This takes me back to the currents and all the water tied up behind the islands, much of it in marshes. Unlike a lake, bay or open water the amount of water held in the marshes is not a linear function. So, when SLR is inserted in the equation (say a rise of 30 cm) the net change in flow is not zero in a marsh setting. See Schematic Below:

If you measure the volume between the upper and blue lines, i.e., the “present” tidal envelope, you will have an estimate of the amount of water stored in the marsh that will ‘flush’ out during a tidal cycle. Similarly, the volume between the purple lines would be the hypothetical volume from the marsh under SLR. And, I think you can see that if all the water was in channel, the difference would be a net zero, and this would be a non-issue.

Unfortunately, of fortunately, the additional water must flow back out of the inlets – resulting in an increase in velocities (more water, same time and outlet geometry). An important point here (!!) - I am not assuming any vertical marsh accretion, and also assuming that the inlets are stabilized.

This gets me back to coastal dynamics on the beaches; I wanted to see how much change in ebb delta velocities we are potentially looking at. My unfortunate professional opinion is that increasing the seaward directed velocities will flush more sediment out of the system to deeper water, thus making it ‘less available’ for bypassing.

Practical Example:

I pass over Breach Inlet (between Sullivan’s Is., and Isle of Palms) almost daily on my bike – part of my COVID-19 exercise regime – and decided to look specifically at how much change in water volume could occur here. It is very much a stabilized inlet. So, I did some watershed analysis of the marshes behind the islands with the help of some hydrologic enforcement on the VDATUM surfaces; it is not specifically correct but provides some reasonable boundaries.

Figure 1. Watershed analysis of marshes to the west of the barrier islands. Pink area is ‘theoretical’ drainage to Breach Inlet.

You can see that the drainage to Breach Inlet (pink area) is hypothetically all from behind Isle of Palms. This is not an overly important point as these marshes are fairly ubiquitous. Anyway, I computed the volume in that pink area at present tidal conditions (i.e., 1992 tidal epoch) and with 0.3 m of SLR, which is almost what we are looking at in 2020.

I computed the volume of water in the pink area at MLLW and also at MHHW and then subtracted the MLLW from the MHHW volumes (as was done in the blue lines in the schematic). Likewise I computed the ‘purple’ lines in the schematic, namely the MLLW + 0.3m and MHHW + 0.3m (Table 1). The important figure here is that the increase is 27% more than present. I would expect that percentage to be roughly the same regardless of the specific area feeding Breach Inlet.

Table 1. Volume calculations from “pink area” in Figure 1.

This leads to three big questions:

1) does 27% represent another regime (inlets are no longer functioning as in the past) or is it a modification that will change the specifics but not the system;

2) will overall water volumes be re-routed - and what affects will that have; and

3) what is the rate of marsh accretion (I am guessing it is less that SLR) and do we need to help it out in order to keep the inlet functioning.

I apologize, I don’t know if leaving this off with questions is the right way to go, but feel free to contact me if you have any thoughts. My two cents is that 27% is a big deal and we should start to ask ourselves if SLR is doing more than just causing linear erosion (shoreline retreat); and saving these islands has more to do with the totality of the surrounding environments (like we are finding with COVID-19) than just adding sand to the beach.

Barrier Islands and Ebb Dominated Deltas: Implications of SLR on Shoreline Change