FURTHER THOUGHTS ON TROPICAL CYCLONE DELUGES

This post is the third in a trilogy pondering the very real possibility that we have entered a “new normal” with respect to tropical storms and hurricanes, focusing in particular on relatively small storms on the Saffir-Simpson scale producing prodigious amounts of rain in the eastern Carolinas (part 1; part 2). Here I offer some additional thoughts.

Flooding from Hurricane Matthew along U.S. 421 in Wilmington, NC (near the USS North Carolina Battleship Memorial)(Associated Press)

Compound flooding

In my previous post I suggested, as have others, that we may be entering a “new normal” with respect to increasing precipitation associated with tropical cyclones. In eastern North Carolina and in south Texas we have empirical evidence in the form of Hurricanes Matthew, Harvey, and Florence in 2016-2018. Basic climatological principles suggest that the ongoing global warming will produce this result, and climate models bear this out.

The 100 largest area-averaged, multiple day precipitation events in the U.S. record from 1949-2018 were examined by Kunkel and Champion (2019).  Hurricane Harvey was the single largest event for an area sized 50,000 km² and a duration of 4 days. Rainfall associated with Hurricane Florence ranked seventh. Almost all of the top 100 events occurred in the southeastern United States or along the Pacific coast. Hurricane Matthew (2016) resulted in 1-day rainfall records at Tarboro, Fayetteville, Lumberton, and Raleigh, NC and at Florence and Dillon, SC (Weaver et al., 2016). Hurricane Florence resulted in new peak streamflow records at 28 gaging stations in the Carolinas (Feaster et al., 2018).

I am looking out on my rain-soaked yard in Craven County, NC, where it sure seems wetter than normal. Indeed, data from the nearby weather station in New Bern shows 90 mm of rain so far this month, and 1648 so far this year—the averages for Dec. 20 are 55 mm since Dec. 1, and 1309 for the year.

But this ain’t nothin’, really. The real story in these parts is the increased precipitation from tropical cyclones. The largest floods in memory in many locations in eastern NC occurred in conjunction with Hurricanes Florence in 2018, Matthew in 2016, and Floyd in 1999. At many locations these three represent, in on order or another, the 3 largest floods ever recorded. The key question being asked is whether this is the “new normal;” whether more frequent and/or more powerful storms and rainfall events (relative to say, the 20^th century, are what we are going to get from now on. As one who suffered >$35K worth of uninsured water damage from Florence, I hope to hell not. But the evidence is not on my side.

U.S. Geological Survey Flood inundation map for Kinston, NC (Neuse River) for hurricane Matthew in 2016.

Large dams trap a great deal of river sediment. But in many cases this does not result in a significant reduction in sediment delivery by rivers to the coast. This is due largely to the fact that the lower reaches of many coastal plain rivers were sediment bottlenecks long before the dams were built, and did not deliver much sediment to the coast to start with, and to the long under-appreciated importance of sediment sources in the lower coastal plain and within the coastal zone.

This has been known, at least in some case studies, for 30 years. However, these case studies have done little to offset the conventional wisdom that because (A) dams trap sediment (100 percent of bedload and often >90 percent of suspended load), and (B) rivers are an important source of coastal sediments, then (C) sediment delivery to the coast has been reduced to the coastal zone since a proliferation of dam-building in the 1950s and 1960s, leading to problems such as beach erosion and wetland loss.

Each account of landscape evolution, development, or history--whether narrative, chronology, model, or otherwise--is considered a story. Each story implies a beginning (starting point, initial condition, genesis), a middle, and an end. The middle includes the processes, transformations, or pathways connecting the beginning to the end. The end may be a final state, culmination, or conclusion per se, or the contemporary or observed state at a given point in time.

History, wrote Tony Horwitz (2008), is an arbitrary collection of facts and observations. Myths are created and perpetuated. To expand a bit in the context of historical Earth and environmental sciences, history is an arbitrary collection of facts and observations, filtered by aspects of historical preservation, and limitations of perception and interpretation. Historical narratives are created, negotiated, and perpetuated. Historical narratives—explanations, chronologies, historical descriptions, chronicles, and, yes, myths—are forms of stories. The key point is that while historical science is (at least at its best) grounded in facts and data, however censored and variably perceived, the reporting and dissemination thereof is in the form of created, negotiated, and perpetuated stories.

Earth’s climate is changing. Always has, always will; so that statement would’ve been true a thousand years ago, and will be so a thousand years hence. However, evidence is accumulating that climate is now changing faster and more radically than ever before in human history, faster than ever before in the recent geologic past, and in some respects faster than in Earth history, period. 

Villagers cluster on Polder 32, an artificial island in southwest Bangladesh with an uncertain future (Tanmoy Bhaduri, Sciencemag.org)

In addition to sea-level rise, global warming puts Bangladesh at greater risk for stronger and more frequence tropical cyclones.

Geomorphic and pedologic systems and ecosystems may sometimes experience mode shifts from dynamically unstable, divergent development to dynamically stable and convergent (or vice versa)(Phillips, 2014).  Here I explore the idea of how this can occur in the evolution of soil, regolith, and weathering profiles. 

Weathering profile, NSW, Australia

In a 2018 article, I analyzed the model below, based on epikarst soils.

From Phillips, 2018. 

In a spatial adjacency graph (SAG) the graph nodes or vertices are nominal or categorical spatial entities—for example soil types, landform types, geological formations, or vegetation communities. Any two nodes are connected (i.e., there exists link between them) if they are spatially contiguous. Thus, if  types A and B at least sometimes occur adjacent to each other, they are connected, and if they never occur spatially adjacent to each other, there is no edge connecting A, B. In the attached note I address a spatially explicit form of SAGs, based on raster representation of categorical spatial units. In particular, it presents a method for assessing the complexity of these spatial patterns. 

Raster soil map of Essex County, Vermont. The colors indicate the raster soil types; these are overlaid with additional data. Source: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/geo/?cid=stelprdb1254424

It has been 21 months since I posted to this blog. Partly that can be attributed to laziness; partly to not having anything new to say (at least about Earth and environmental sciences and geography) that I did not have another outlet for. I'm not sure anyone really noticed the blog was gone, but now it is back. 

Much of that no-blog time was spent writing a book, to be published by Elsevier, on landscape evolution. This will integrate geomorphological, pedological, ecological, and hydrological theories on the evolution of landscapes, ecosystems, and other Earth surface systems. It is grounded in an approach based on the inseparability of landform, soil, and ecosystem development, vs. the traditional semi-independent treatment of geomorphic, ecological, pedological, and hydrological phenomena. Key themes are the coevolution of biotic and abiotic components of the environment; selection whereby more efficient and/or durable structures, forms, & patterns are preferentially formed and preserved; and the interconnected role of laws, place factors, and history.