Nature-Based Coastal Protection by Large Woody Debris as Compared to Seawalls: A Physical Model Study of Beach Morphology and Wave Reflection

Anchored Large Woody Debris (LWD) is increasingly being used as one of several nature-based coastal protection strategies along the north-western coasts of Canada and the US. As an alternative to conventional hard armoring (e.g., seawalls), its usage is widely considered to be less harmful to the coastal ecosystem while maintaining the ability to protect the beaches against wave attack and erosion. The effects of seawalls on beaches have been extensively studied; however, the performance and efficacy of LWD and its potential as a suitable alternative to seawalls (and other shoreline protection structures) are still understudied in current research. This paper presents and compares the effects of a conventional vertical seawall with two different LWD structures on beach morphology and wave reflection through large-scale physical modeling in a wave flume at a 1:5 scale. An assessment of techniques used to measure beach morphology and an assessment of model effects were included in the study. It was found that the wave reflection could be reduced by using a single log instead of a wall structure, while changes in the beach morphology response largely depended on the type of the LWD structure. A stacked log wall showed near-identical behavior as a conventional seawall. Visible model effects from the experiments, including the effect of the flume sidewalls on the beach morphology, were quantified and analyzed to inform future research.

Wadeable stream habitat monitoring at Congaree National Park: 2018 baseline report

The Southeast Coast Network (SECN) Wadeable Stream Habitat Monitoring Protocol collects data to give park resource managers insight into the status of and trends in stream and near-channel habitat conditions (McDonald et al. 2018a). Wadeable stream monitoring is currently implemented at the five SECN inland parks with wadeable streams. These parks include Horseshoe Bend National Military Park (HOBE), Kennesaw Mountain National Battlefield Park (KEMO), Ocmulgee Mounds National Historical Park (OCMU), Chattahoochee River National Recreation Area (CHAT), and Congaree National Park (CONG). Streams at Congaree National Park chosen for monitoring were specifically targeted for management interest (e.g., upstream development and land use change, visitor use of streams as canoe trails, and potential social walking trail erosion) or to provide a context for similar-sized stream(s) within the park or network (McDonald and Starkey 2018a). The objectives of the SECN wadeable stream habitat monitoring protocol are to: Determine status of upstream watershed characteristics (basin morphology) and trends in land cover that may affect stream habitat, Determine the status of and trends in benthic and near-channel habitat in selected wadeable stream reaches (e.g., bed sediment, geomorphic channel units, and large woody debris), Determine the status of and trends in cross-sectional morphology, longitudinal gradient, and sinuosity of selected wadeable stream reaches. Between June 11 and 14, 2018, data were collected at Congaree National Park to characterize the in-stream and near-channel habitat within stream reaches on Cedar Creek (CONG001, CONG002, and CONG003) and McKenzie Creek (CONG004). These data, along with the analysis of remotely sensed geographic information system (GIS) data, are presented in this report to describe and compare the watershed-, reach-, and transect-scale characteristics of these four stream reaches to each other and to selected similar-sized stream reaches at Ocmulgee Mounds National Historical Park, Kennesaw Mountain National Battlefield Park, and Chattahoochee National Recreation Area. Surveyed stream reaches at Congaree NP were compared to those previously surveyed in other parks in order to provide regional context and aid in interpretation of results. edar Creek’s watershed (CONG001, CONG002, and CONG003) drains nearly 200 square kilometers (77.22 square miles [mi2]) of the Congaree River Valley Terrace complex and upper Coastal Plain to the north of the park (Shelley 2007a, 2007b). Cedar Creek’s watershed has low slope and is covered mainly by forests and grasslands. Cedar Creek is designated an “Outstanding Resource Water” by the state of South Carolina (S.C. Code Regs. 61–68 [2014] and S.C. Code Regs. 61–69 [2012]) from the boundary of the park downstream to Wise Lake. Cedar Creek ‘upstream’ (CONG001) is located just downstream (south) of the park’s Bannister Bridge canoe landing, which is located off Old Bluff Road and south of the confluence with Meyers Creek. Cedar Creek ‘middle’ and Cedar Creek ‘downstream’ (CONG002 and CONG003, respectively) are located downstream of Cedar Creek ‘upstream’ where Cedar Creek flows into the relatively flat backswamp of the Congaree River flood plain. Based on the geomorphic and land cover characteristics of the watershed, monitored reaches on Cedar Creek are likely to flood often and drain slowly. Flooding is more likely at Cedar Creek ‘middle’ and Cedar Creek ‘downstream’ than at Cedar Creek ‘upstream.’ This is due to the higher (relative to CONG001) connectivity between the channels of the lower reaches and their out-of-channel areas. Based on bed sediment characteristics, the heterogeneity of geomorphic channel units (GCUs) within each reach, and the abundance of large woody debris (LWD), in-stream habitat within each of the surveyed reaches on Cedar Creek (CONG001–003) was classified as ‘fair to good.’ Although, there is extensive evidence of animal activity...

NATURE-BASED COASTAL PROTECTION USING LARGE WOODY DEBRIS

In British Columbia (BC), Canada, and Washington State, USA, anchored Large Woody Debris (LWD) have been extensively used with the specific aim of reducing erosion and limiting wave run-up. Despite its frequent usage, there is currently limited peer-reviewed literature on the design or efficacy of coastal protection using LWD. This paper presents the results of the first systematic research project on this topic, which involved (1) extensive field investigations of existing anchored LWD projects, and (2) large-scale experimental wave modeling of simulated LWD on a gravel beach. The full paper will present an overview of the study methodology, field investigation and experimental modeling results, and provide initial design guidance for the use of coastal protection using anchored LWD.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/ktjVWGfXylk

Hydraulic Aspects of Logjam Formation during Extreme High Floods

The article discusses the mechanisms of formation of congestion of trees in front of the bridge during high floods that can significantly increase the water level in the pre-bridge zone. The analysis of foreign studies is carried out, since in the domestic literature there are no proven schemes for numerically assessing the effect of congestion of trees in front of the bridge on increasing the level of water in passing floods. It is shown that for the formation of congestion of trees, the greatest danger is the presence in the stream of Large woody debris – trees with a developed crown and root system. Linear objects – floating logs, even with their considerable size, pose significantly less danger due to the features of the hydrodynamics of channel flows. The proofs are carried out theoretically based on the generally accepted equations of hydraulics.Relationships are proposed for assessing a possible increase in water level during the formation of congestion of trees in front of the bridge. The results of numerical calculations are presented in the form of corresponding calculation graphs, since the developed relations do not have analytical solutions.Recommendations are given on reducing both the risks of formation of congestion of trees in front of the bridge and their negative consequences. It is shown that this task is not only hydrological, technical, but also, first, economic. Therefore, in each case, it is necessary to consider specially a set of measures to minimize the risks associated with the formation of congestion of trees in front of the bridge. The range of measures can be very wide: from stripping of coastal strips to the engineering and technical preparation of the territory, primarily within the boundaries of settlements, to the passage of floods with an increased level of flooding.