Search

Sandbars are ubiquitous morphologic features found in the nearshore environment throughout the world, yet predictive capabilities of their evolution remain limited. In order to provide new insights on the relevant processes controlling sandbar morphodynamics, this study uses a 41‐year record of 637 monthly cross‐shore profiles from Duck, North Carolina, USA, to derive complex empirical orthogonal functions representative of the two dominant modes of sandbar migrations: offshore and onshore propagation. Interference of these two modes produces commonly observed sandbar states. While mild wave energies are traditionally assumed to drive onshore sandbar migration, the offshore mode is repeatedly seen to dominate sandbar migration in mild wave seasons following anomalously high late‐winter wave energy and when the inherited morphology is composed of a single bar or terrace, as opposed to a more common two‐bar state. A data‐derived conceptual model is presented synthesizing the effect of antecedent morphology and wave climate chronology on interannual trends in net offshore sandbar migration. Plain Language Summary: Sandbars commonly exist in shallow water just offshore of sandy coastal beaches. These features are thought to play an important role in protecting the beach from storm waves, but unfortunately there is limited skill in predicting how these sandbars form and move. An unparalleled 41‐year, monthly data set of nearshore sandbar behavior is used in this work to identify a repetitive pattern where emergence of new sandbars in the nearshore occurs when offshore migration occurs during mild wave seasons, which is contradictory to traditional conceptual models for nearshore morphology evolution. A new conceptual map is derived from the data that identifies common sandbar states and the migration direction that produces changes from one state to another, specifically highlighting that sandbar migration direction at the seasonal to yearly scale is strongly dependent on winter wave climates and the sandbar state at the beginning of mild wave seasons each spring. Key Points: Sandbar migrations in on‐ and offshore directions expressed as propagating modes derived from 41 years of surveysOffshore migrations are observed during mild wave conditions when a single‐bar or terraced beach profile is presentThe relative timing of large wave energy prior to extended periods of mild conditions enables offshore migrations [ABSTRACT FROM AUTHOR]

First‐order streams are rarely the focus of research even though they play an important hydrological and ecological role in upland landscapes and may be more susceptible to human impacts than larger streams. We studied 16 first‐order streams in the North Carolina Piedmont to examine relationships between land use, discharge, flashiness and conductivity in the headwaters of the Rocky River and McDowell Creek. Stream gauges were installed on each of the 16 headwater streams to provide constant monitoring of stream depth, temperature and specific conductance. Rating curves were created for each site using models that combined field measured and Manning's calculated discharge. Initial attempts to measure flashiness using the Richards‐Baker Index (RBI) indicated that it is not a good proxy for flashiness in small streams because its use of median daily flows do not capture fluctuations that occur over short time periods. Instead, we developed a new method that focused on the number of days each stream flows above its 99th percentile flow (DMQ99). We also examined individual storm events across multiple seasons. We found that headwater streams in the Piedmont are flashier than more commonly gauged second and third order streams because first‐order streams are more directly connected to the landscape. Streams located in suburban areas tend to be flashier than streams in forested areas as a result of the increase in impervious surfaces and the reduction of evapotranspiration (ET) caused by development. Additionally, we found that all streams become flashier in the winter when ET is lower, suggesting important feedbacks between physical and biological systems. All streams experience periodic extreme conductivity events, indicating significantly impaired ecological conditions. More developed basins generally have higher maximum specific conductance compared with less developed basins although streams have significant natural variability as well. Overall, our results indicate that first‐order streams in the Piedmont are highly impacted by development and that extra caution should be exercised when altering landscapes near small streams. [ABSTRACT FROM AUTHOR]

Gullies are common erosional features in the Piedmont of North Carolina that have often been studied as indicators of poor land-management practices. However, the timing of gully formation, and thus the specific practices that caused their formation, has been largely overlooked in the literature. Here, we use the timing of gully formation to examine the geomorphic and anthropogenic triggers for erosion in the Piedmont. To do this, we mapped 74 gullies in the landscape and found that most contained alluvium within the gully or in alluvial fans. Soil pits were dug at a subset of the gully sites (16 in alluvium, 1 in colluvium, and 1 upland comparison) and examined for soil development, organic content, and particle size. Seven charcoal fragments were sampled from the soil pits and were radiocarbon dated. Radiocarbon ages cluster in the late 1700s, which suggests that early European settlers had an immediate impact on the landscape through deforestation and agriculture. The swift response of the landscape to deforestation and agriculture indicates that the argillic Piedmont soils are pre-disposed to erosion. Our results highlight the importance of erosion prevention as reforested farmland is cleared during urban expansion. [ABSTRACT FROM AUTHOR]