SAND ACCRETION ON DYNAMIC LIVING SHORELINES
On an unusually balmy October day a few members of the Underwood & Associates team gathered around a wooden post at the Kyle Point Dynamic Living Shoreline. This 4x4 has a metal yardstick attached to it that was installed in Summer 2020 at the end of the shoreline’s construction in order to measure sand accretion. Over a year later, our small group observed that the sand now measured 13” up the yardstick. This sand accretion was no accident. Dynamic living shorelines are designed and built to not only prevent erosion, but to harness the supply of sand in the system. Vegetated headlands disrupt steep, choppy waves and transform them into more gentle wave energy, which transports sediment onshore and builds up the beach rather than moving sediment offshore and eroding the beach. The gentle slope of U&A shorelines enables wave energy to be dissipated over a longer distance which decreases erosion and enables sand/sediment deposition.
Along Kyle Point the evidence of accretion was visible not only at the measuring stick. Fresh sand being brought to shore was apparent in the wrack line as well as in the vegetation where pillows of sand bulged around the Spartina patens. Sand and sediment accretion is particularly valuable when it comes to a shoreline’s resilience to sea level rise. Accretion provides the opportunity for native vegetation to grow vertically to keep up with the rising water level. In addition to the sand being captured from the Severn River, the wind is also responsible for dispersing sand around the project and may have contributed to some of the 13” of build up around the yardstick. Seasonal wind patterns may cause the shoreline to assume different shapes based on the season. For instance, summer winds typically originate in the southwest and therefore the north east side of the embayments would accrete more in the summer season. Come winter, material from this side of the embayment may swap sides when winds begin to drive more predominately from the north east. Although all sites are different and the amount of sand in a stream, river, bay, or other waterway may vary, the unique design of a dynamic living shoreline will catch and incorporate available sand into the project. Some shorelines are sediment starved, meaning upstream sources of sediment have been hardened with bulkheads or riprap so the downstream shoreline does not receive annual littoral drift of sands and sediments. In those cases, we must import 100% of the sand material in order to supply the shoreline with the sand material it needs. Those shorelines will still be resilient over time; however, they do not grow as quickly. Harnessing the sand in a natural system is the epitome of working with nature and ensuring our projects are self-sustaining and equipped to adapt to climate change.
Small sand dune accretes at Drever Park living shoreline (completed 2003)