Influences and drivers of woody debris movement in urban watercourses

Abstract It is recognised that the blockage of culverts by woody debris can result in an increased risk of infrastructure damage and flooding. To date, debris transport analysis has focused on regional fluvial systems and large woody debris, both in flume and field experiments. Given the social and economic risk associated with urban flooding, and as urban drainage design shifts away from subsurface piped network reliance, there is an increasing need to understand debris movement in urban watercourses. The prediction of urban watercourse small woody debris (SWD) movement, both quantity and risk, has undergone only limited analysis predominantly due to lack of field data. This paper describes the development of a methodology to enable the collection of accurate and meaningful SWD residency and transportation data from watercourses. The presented research examines the limitations and effective function of PIT tag technology to collect SWD transport data in the field appropriate for risk and prediction analysis. Passive integrated transponder (PIT) technology provides a method to collect debris transport data within the urban environment. In this study, the tags are installed within small woody debris and released at known locations into a small urban natural watercourse enabling monitoring of movement and travel time. SWD velocity and detention are collated with solute time of travel, watercourse and point flow characteristics to identify the relationships between these key variables. The work presented tests three hypotheses: firstly, that the potential for unobstructed or un-detained SWD movement increases with flow velocity and water level. Secondly, that SWD travel distance, and the resistance forces along this travel path, influence SWD transport potential. Thirdly, the relationship between SWD and channel dimensions is examined with the aim of advancing representative debris transport prediction modelling.</jats:p

A new method for estimating trash screen blockage extent

This paper presents findings from research that examined the potential for developing equations that predict the extent to which trash screens at culverts are likely to become blocked. This was assessed using multiple regression analysis in which the average area of debris blockage at screen faces calculated from field inspection records obtained from 140 screen sites in Belfast, Northern Ireland, was correlated with pertinent driving variables including channel properties, flow characteristics, landuse types, the degree of social deprivation and the properties of the screens themselves. This analysis enabled the development of simple empirical equations requiring input data that can be extracted from readily available sources. The key variables identified and the properties of the equation coefficients are discussed within the context of their statistical significance. </jats:p

Analysis of the performance of debris screens at culverts

Construction of a culvert will impact ambient hydraulic characteristics such that upstream flow depths may increase due to any constriction at the inlet. Exacerbating this, flow can be further restricted within a culvert due to internal blockage by debris. Screens to prevent debris entering a culvert may also cause blockages, thus heightening the flood risk. In response to this problem, the research reported in this paper made use of a Froude-scaled physical model to investigate how screen blockage by debris is influenced by the geometry and positioning of a screen. Analysis based on 105 000 debris passes is used to show that, as expected, the potential for screen blockage by debris increases as the ratio of debris length to bar spacing increases. Furthermore, screen angle and position is shown to have a significant influence on blockage potential. This research involved the development of a methodology that can be used to assess the efficiency of different trash screen configurations. To build upon the analysis from this initial research and continue working towards the development of a predictive model that can aid future screen design, the research needs to be extended to look at the process of cumulative debris build up on screens. </jats:p

Dammed deltas: Sinking Asian deltas in a warming world

People who live on tropical deltas are making local adaptations to tackle myriad challenges, but should we be looking upstream? Rapid expansion of dams for hydropower is damaging the rivers that supply deltas, increasing the uncertainty about how delta communities should adapt in the future

Quantifying River Channel Stability at the Basin Scale

This paper examines the feasibility of a basin‐scale scheme for characterising and quantifying river reaches in terms of their geomorphological stability status and potential for morphological adjustment based on auditing stream energy. A River Energy Audit Scheme (REAS) is explored, which involves integrating stream power with flow duration to investigate the downstream distribution of Annual Geomorphic Energy (AGE). This measure represents the average annual energy available with which to perform geomorphological work in reshaping the channel boundary. Changes in AGE between successive reaches might indicate whether adjustments are likely to be led by erosion or deposition at the channel perimeter. A case study of the River Kent in Cumbria, UK, demonstrates that basin‐wide application is achievable without excessive field work and data processing. However, in addressing the basin scale, the research found that this is inevitably at the cost of a number of assumptions and limitations, which are discussed herein. Technological advances in remotely sensed data capture, developments in image processing and emerging GIS tools provide the near‐term prospect of fully quantifying river channel stability at the basin scale, although as yet not fully realized. Potential applications of this type of approach include system‐wide assessment of river channel stability and sensitivity to land‐use or climate change, and informing strategic planning for river channel and flood risk management