Storms and Coastal Change in the Chesapeake Bay

Storm surges and large storm waves are the great movers of beach sand. When storms reach the Bay, the abnormal rise of water generated by a storm – the storm surge – enables larger waves on top of the higher water level to move sand up and over the beach. Some of that sand also moves offshore. Over time, successive storms can produce a wholesale relocation of the beach in a landward direction by a process called “overwash.” Storm overwash is the mechanism by which beaches “rollover” the landscape through deposition of washover fans. [FIGURE 1] This rollover process can occur without eroding if all the sand in the beach stays in the beach as it moves landward. However, this process can also result in erosion if beach sand is “lost” as it moves. In this case, the sand usually ends up offshore, unable to be carried back to the beach by non-storm waves and/or there isn’t a source of sand available from other places (like updrift of the beach) to resupply the beach.

Scientific data and analysis have shown that storms have eroded and narrowed most of the Bay’s beaches.  It’s not a coincidence that east and northeast facing beaches on the western shore of the Chesapeake Bay have the highest erosion rates (about 1.5 ± 0.6 feet per year) and the highest density of shoreline engineering projects and structures in the Bay [FIGURE 2].  Those beaches have the greatest exposure to extratropical storms, also called “nor’easters.”

Two kinds of storms impact the Bay’s shores – the more frequent, slow moving extratropical (nor’easters) that typically occur from October to April and the less frequent, fast moving, powerful tropical storms (like hurricanes) that can occur from June to November. The storms differ in how they form, their intensity and travel speed, and how they impact beaches and coasts. Scientific evidence suggests that the cumulative effect of successive and large extratropical storms (nor’easters) and tropical storms over time have been the culprits of the observed erosion in the Bay and the reason for growing number of shore protection structures [FIGURE 3].

Big storms, like Isabel in 2003 and Gaston in 2004, also have wreaked havoc on the Bay’s shores. During Isabel, maximum water elevation reached 3.2 - 8.2 feet with maximum wave heights reaching 13 feet along the western shore. It’s no wonder that the engineering structures installed along the Bay’s beaches increased from about 2.2 sites per year before 2004 to 3.8 sites per year after 2004 [FIGURE 4]  There is also scientific evidence that big storms are increasing in intensity along this part of the mid-Atlantic coast.  On top of that, the Bay faces an evolving storm threat as sea levels rise, making resilience planning and environmental stewardship more critical than ever. The Chesapeake's storms are both a challenge and a reminder of nature's enduring influence on this iconic estuary.