I would like to extend a warm welcome to the August issue of Water Management, which presents an interesting and diverse selection of papers covering the broad themes of flood risk assessment, river engineering and hydraulic modelling. In the paper by Woods (2015), a new catchment-based method is developed to better integrate UK climate change data into design flood estimation and flood risk modelling. The other three papers focus on reporting results from physical modelling studies investigating new methods of scour control at bridge piers (Pagliara et al., 2015), the operating conditions of a new baffle–brush fish pass design (Kucukali and Hassinger, 2015), and the characteristics of air frequency distributions in self-aerated flows (Deng et al., 2015).
In the first briefing paper of the issue, Deng et al. (2015) present results from an experimental study on the development of self-aerated open channel flows. The process of self-aeration within high-velocity open channel flows has particular relevance to spillway and chute design where the volume of entrained air can have a significant influence on the depth of the flowing water layer generated. Furthermore, it is known that the presence of entrained air in self-aerated flows can prevent or reduce damage due to cavitation. In their experiments, Deng et al. study the distribution of air-phase frequency in the developing self-aerated region of the open channel flows under different initial flow velocities. Their results demonstrate that the distribution of the air-water flow structure within the flow layers is a developing process involving the coexistence of entrapped and entrained air. Observed distributions of air frequency within these flow layers are also shown to be dependent on the air-water flow structure within the developing self-aerated region and to have a parametric dependence on the initial flow velocity.
Improved understanding of linkages between future climate change scenarios, physical catchment responses and flood risk is essential to enable developers to design more sustainable flood-resistant and resilient infrastructure. In the paper by Woods (2015), the author explores current limitations that exist in UK planning policy regulation, which adopt UK-wide average weightings to account for future climate change impacts on the magnitude of 1-in-100 year design flood events. It is noted that this deterministic approach cannot take account of more localised climate change impacts on the physical characteristics in the actual river catchments themselves. Within the paper, the author develops a novel approach to incorporate land-use data and additional climate change data obtained from UK climate projections 2009 (UKCP09) into current flood estimation and modelling frameworks. This combined methodology is then trialled in a case study at a development site on the River Cam near Cambridge, UK, with a series of scenarios tested for 1-in-100 year design flood events that reflect plausible changes to the physical catchment characteristics in the 2080s. When compared to baseline predictions using current UK flood risk assessment frameworks, this new methodology is shown to predict a significantly higher magnitude 1-in-100 year flood event with more rapidly rising and receding flood hydrograph limbs and greater inundation depths on floodplains at the development site.
Bed scour at bridge piers has received a significant amount of research attention over many years, mainly as it continues to be a major cause of structural failure. These scour problems can be exacerbated further by the partial obstruction of two-phase, water-sediment flows around the pier by large accumulations of floating debris, which can result in localised flow accelerations. Within the paper by Pagliara et al. (2015), scaled flume studies are conducted to test the efficiency of a novel scour countermeasure involving the placement of regular grid arrays of macro-roughness elements downstream of a circular bridge pier with a large debris accumulation. The results show that the macro-roughness elements arrays have a significant effect in reducing the maximum scour depth and/or the overall scour volume and planar surficial area. Investigations of the temporal evolution in bed scour patterns also reveal three distinct stages in the erosion process that describe the development of the scour hole and the confining influences offered by the macro-roughness elements. The authors conclude that, from a practical and economic view point, the use of macro-roughness elements is comparable in effectiveness with other scour reduction methods, such as sills and gabions, and offers a simple approach to scour control in the presence of debris accumulations. They suggest that further investigations are needed to delineate the influence of different macro-roughness element configurations on scour evolution downstream of bridge piers.
The selection and hydraulic design of fish passes present significant challenges to ensure that flow conditions can be optimised for different fish species and ages that may be present at a particular river site. In their paper, Kucukali and Hassinger (2015) conduct a series of hydraulic tests on a novel baffle–brush fish pass design over a range of flow conditions and submergence depths. The composite design proposed incorporates a side-by-side arrangement of super-active (Larinier) baffle and brush-type fish passes with a transitional flow region in between. Detailed flow velocity and turbulence measurements for this hybrid configuration indicate two distinct characteristic velocity regions, with high intensity turbulent flows generated above the baffle region transitioning smoothly to calmer, less turbulent flow patterns in the brush region. The authors conclude that the new fish pass design offers a wide range of favourable flow conditions and submergence depths that permit different migratory corridors and in-stream habitats for larger salmonid species and smaller, younger or weaker fish alike. Their results are therefore of considerable interest to both the river engineering and aquatic biology/ecology communities involved in the future design of fish passes.
