Small Coastal Catchment Research Articles

An open-source and QGIS-integrated physically based model for Spatial Prediction of Rainfall-Induced Shallow Landslides (SPRIn-SL)

This paper presents a new open-source and physically based model for Spatial Prediction of Rainfall-Induced Shallow Landslides (SPRIn-SL) through the Quantum GIS (QGIS) software. SPRIn-SL consists of a set of shell scripts developed using the Python language that can be directly run from the QGIS processing toolbox through a user-friendly graphical interface. The tool implements the infinite slope method by incorporating the TOPOG and the Green-Ampt models to consider groundwater flow and transient rainfall infiltration, respectively. Furthermore, DEM pre-processing procedures to extract reliable terrain morphometric features, a new statistical method for modelling soil depth and a procedure for predictive accuracy evaluation, were implemented. By using a 1-m resolution DEM, the developed model was tested in a small coastal catchment of Cinque Terre (Liguria, Italy), providing accurate outcomes, and proving to be an easy-to-use tool for landslide susceptibility zoning which can have useful implications on the risk reduction.

Geomorphic and vegetative river recovery in a small coastal catchment of New South Wales, Australia: Implications for flow hydrology and river management

Throughout eastern Australia, and much of the New World, river systems have undergone a series of hydro-geomorphic and vegetative changes following colonisation. Recent research has shown that river recovery is occurring, as demonstrated by improved geomorphic and vegetative condition, but there is limited information on the extent of returning riparian vegetation and its effect on roughness and flow hydrology. Using the Allyn River, New South Wales, Australia, as a case study, historical aerial photographs and satellite imagery were used to characterize the structural changes brought about by geomorphic recovery and quantify the amount of riparian vegetation cover that has returned to the system since the 1940s. Along the Allyn River, total riparian vegetation canopy cover has increased from 30% in 1940 to 38% in 1967 and 63% in 2016. Using space-for-time substitution and retrospective analysis, riparian vegetation roughness, calculated using a terrestrial laser scanner (TLS) gap fraction method, showed an increase in average Manning's n from 0.0156 in 1940 to 0.0194 in 1967 to 0.0372 in 2016 for flows that reach a 2 m stage height (approximately 1 in 2 yr flood average recurrence interval). As a result, flood wave travel time has extended by over 8 h (or 220%) for small to moderate within-channel flows, but further river recovery is required to affect major flow hydrology. This study highlights the outcomes of catchment-scale passive river recovery and provides improved methods for quantifying site and stage height specific vegetative roughness. The demonstrated influence of increased vegetation cover on flow hydrology and geomorphic condition holds significant promise for agencies, practitioners and land managers to achieve substantial river recovery outcomes using active and passive vegetation rehabilitation and river management.

Disastrous Flash Floods Triggered by Moderate to Minor Rainfall Events. Recent Cases in Coastal Benguela (Angola)

The present work focuses on two recent flash floods in coastal Benguela (Angola), both triggered by moderate rainfall but which had disastrous consequences for local populations (namely 71 deaths in 2015 and 17 in 2019). The research involved a regional survey to establish the effects of these floods combined with a geomorphological and socio-economic analysis of the most affected areas to understand the main forcing factors. The two flash floods produced major damage in restricted sectors within very small coastal catchments (<16 km2). The prevalence of fine-grained sedimentary rocks, relatively steep hills, thin soil cover, and vegetation scarcity are natural factors that promote surface runoff. However, socio-economic conditions are most likely the main reasons of flood damage. Namely, rapid population growth with poor planning and making use of low-quality construction materials, the high waste yields that are not properly managed and the absence of flood risk awareness. In the small valleys around the fast-growing cities of coastal Benguela, hazardous flash floods occur recurrently, even after moderate precipitation. Most affected areas are determined by local conditions that compromise drainage at the time of the rainfall event, being very difficult to predict.

Salmon‐derived nutrient and organic matter fluxes from a coastal catchment in southeast Alaska

Abstract Salmon are important vectors for biogeochemical transport across ecosystem boundaries. Here we quantified salmon contributions to annual catchment fluxes of nutrients (N and P) and organic matter (C, N, and P) from a forested catchment in coastal southeast Alaska. Concentrations of ammonium and soluble reactive phosphorus increased by several orders of magnitude during spawning and were significantly correlated with spawning salmon densities. Nitrate concentrations increased modestly during spawning and were not significantly correlated with salmon densities. Salmon had a modest legacy effect on inorganic N and P as evidenced by elevated streamwater concentrations past the end of the spawning period. Dissolved organic carbon concentrations did not respond to the presence of salmon; however, concentrations of dissolved organic nitrogen and phosphorus showed a significant positive relationship to salmon densities. Changes in spectroscopic properties of the bulk streamwater dissolved organic matter pool indicated that streamwater dissolved organic matter became less aromatic and biolabile during spawning. On an annual basis, salmon were the dominant source of streamwater fluxes of inorganic nutrients, accounting for 92%, 65%, and 74% of annual streamwater fluxes of ammonium, nitrate, and soluble reactive phosphorus, respectively. In contrast, fluxes of organic matter were dominated by catchment sources with salmon accounting for <1% of the annual catchment flux of dissolved organic carbon and 12% and 15% of the annual fluxes of dissolved organic nitrogen and phosphorous respectively. These findings indicate that, in small coastal catchments, salmon can be a quantitatively important source of dissolved streamwater nutrients with implications for productivity in downstream estuarine ecosystems.

Summer rainfall dissolved organic carbon, solute, and sediment fluxes in a small Arctic coastal catchment on Herschel Island (Yukon Territory, Canada)

Coastal ecosystems in the Arctic are affected by climate change. As summer rainfall frequency and intensity are projected to increase in the future, more organic matter, nutrients and sediment could be mobilized and transported into the coastal nearshore zones. However, knowledge of current processes and future changes is limited. We investigated streamflow dynamics and the impacts of summer rainfall on lateral fluxes in a small coastal catchment on Herschel Island in the western Canadian Arctic. For the summer monitoring periods of 2014–2016, mean dissolved organic matter flux over 17 days amounted to 82.7 ± 30.7 kg km−2 and mean total dissolved solids flux to 5252 ± 1224 kg km−2. Flux of suspended sediment was 7245 kg km−2 in 2015, and 369 kg km−2 in 2016. We found that 2.0% of suspended sediment was composed of particulate organic carbon. Data and hysteresis analysis suggest a limited supply of sediments; their interannual variability is most likely caused by short-lived localized disturbances. In contrast, our results imply that dissolved organic carbon is widely available throughout the catchment and exhibits positive linear relationship with runoff. We hypothesize that increased projected rainfall in the future will result in a similar increase of dissolved organic carbon fluxes.

Identification of Priority Areas for the Conservation of Stream Fish Assemblages: Implications for River Management in France

AbstractFinancial and human resources allocated to biodiversity conservation are often limited, making it impossible to protect all natural places, and priority areas for protection must be identified. In this study, we applied ecological niche models to predict fish assemblages in the stream network of France. Four non‐correlated conservation objectives were derived from these species assemblages: taxonomic diversity, functional diversity, natural heritage importance and socio‐economic value. We proposed a multi‐objective prioritization method based on the Pareto optimality principle to rank the planning units (i.e. 6097 subcatchments) according to their inherent trade‐offs between the four conservation objectives. Four types of hydrosystems of great conservation importance presenting specific fish assemblages were identified: (i) the most upstream areas of large catchments; (ii) the most downstream areas of large catchments; (iii) the small coastal catchments of the English Channel and the Atlantic Ocean; and (iv) the Mediterranean streams of medium altitude. The fish assemblages characterizing these hydrosystems were complementary and representative of the entire fish fauna of France. Most of these priority subcatchments were found to be practically suitable for the implementation of conservation actions, which is very promising for the protection of river biodiversity. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of climate change on shallow landslides in a small coastal catchment in southern Italy

In different areas of the world, shallow landslides represent a remarkable hazard inducing fatalities and economic damages. Then, the evaluation about potential variation in frequency of such hazard under the effect of climate changes should be a priority for defining reliable adaptation measurements. Unfortunately, current performances of climate models on sub-daily scales, relevant for heavy rainfall events triggering shallow landslides, are not reliable enough to be used directly for performing slope stability analysis. In an attempt to overcome the constrains by gap in time resolution between climate and hazard models, the paper presents an integrated suitable approach for estimating future variations in shallow landslide hazard and managing the uncertainties associated with climate and sub-daily downscaling models. The approach is tested on a small basin on Amalfi coast (southern Italy). Basing on available basin scale critical rainfall thresholds, the paper outlines how the projected changes in precipitation patterns could affect local slope stability magnitude scenarios with different relevances as effect of investigated time horizon and concentration scenario. The paper concludes with qualitative evaluations on the future effectiveness of the local operative warning system in a climate change framework.

Evolving flood patterns in a Mediterranean region (1301–2012) and climatic factors – the case of Catalonia

Abstract. Data on flood occurrence and flood impacts for the last seven centuries in the northeastern Iberian Peninsula have been analysed in order to characterise long-term trends, anomalous periods and their relationship with different climatic factors such as precipitation, general circulation and solar activity. Catastrophic floods (those that produce complete or partial destruction of infrastructure close to the river, and major damages in the overflowed area, including some zones away from the channels) do not present a statistically significant trend, whereas extraordinary floods (the channel is overflowed and some punctual severe damages can be produced in the infrastructures placed in the rivercourse or near it, but usually damages are slight) have seen a significant rise, especially from 1850 on, and were responsible for the total increase in flooding in the region. This rise can be mainly attributed to small coastal catchments, which have experienced a marked increase in developed land and population, resulting in changes in land use and greater vulnerability. Changes in precipitation alone cannot explain the variation in flood patterns, although a certain increase was shown in late summer–early autumn, when extraordinary floods are most frequently recorded. The relationship between the North Atlantic circulation and floods is not as strong, due to the important role of mesoscale factors in heavy precipitation in the northwest of the Mediterranean region. However, it can explain the variance to some extent, mainly in relation to the catastrophic floods experienced during the autumn. Solar activity has some impact on changes in catastrophic floods, with cycles related to the quasi-biennial oscillation (QBO) and the Gleissberg solar cycle. In addition, anomalous periods of high flood frequency in autumn generally occurred during periods of increased solar activity. The physical influence of the latter in general circulation patterns, the high troposphere and the stratosphere, has been analysed in order to ascertain its role in causing floods.

Habitat carrying capacity is reached for the European eel in a small coastal catchment: evidence and implications for managing eel stocks

1. Despite carrying capacity being one of the most important parameters in population management and modelling, we lack substantial evidence for habitat limitations on freshwater species. Here we tested the ideal free distribution (IFD) hypothesis using an indirect behaviour-based method for small closed populations assuming that animals can effectively estimate habitat suitability and distribute themselves accordingly in time and space. 2. We analysed spatiotemporal variations in the density of the European eel Anguilla, a catadromous species with good colonisation abilities in a small coastal catchment in France. The general linear model used enabled us to test simultaneously the effect of temporal, macro- and meso-scale habitat factors on the presence and abundance of eels at 30 sites over an 8-year period. 3. Almost every site sampled had eels, whatever its location on the catchment and its habitat characteristics. Density estimates (overall mean ± SD of 0.40 ± 0.48 m−2) were at the upper range of other values for European catchments. Moreover, eel densities were mainly influenced by the availability of suitable habitats (rocky substratum and instream cover), which suggests that their distribution reflects an IFD. 4. Despite marked variability in recruitment, the density of the oldest size-class remained stable over the study, suggesting that density-dependent mortality occurred, probably due to intraspecific competition for space and food and to predation. 5. These findings suggest that eel habitats are saturated in the Frémur. Therefore, we suggest that the mean abundance of eels observed could serve as a threshold value for other male-dominated river stocks (provided they have a similar overall percentage of suitable habitats) that are common in small, low gradient streams on the north-Atlantic coast of Europe.

Environmental determinism of year‐to‐year recruitment variability of European eel Anguilla anguilla in a small coastal catchment, the Frémur River, north‐west France

The influence of environmental factors (mainly the river flow) on the year-to-year variability of European eel Anguilla anguilla fluvial recruitment in a small coastal catchment, the Frémur River (north-west France) was examined. A comprehensive survey of catches from fixed traps at two weirs located at 4.5 km (Pont es Omnes Dam) and 6.0 km (Bois Joli Dam) above the river mouth was carried out between 1997 and 2004. Young pigmented elvers (mean +/-s.d. total length, 133.7 +/- 29.6 mm) were recruited in eel-passes from February to October, but the main runs followed a modal curve from April to September with a peak centred in May to June. Catches varied greatly between years, from 381 to 26 765 elvers. For each trap, a positive linear relationship between monthly mean river flow that preceded the maximal intensity of captures and annual total catches was observed. These relationships explained 73.1% (P < 0.01) and 89.0% (P < 0.001) of the year-to-year variability of the recruitment observed in the Pont es Omnes and Bois Joli traps respectively. A significant increase in river flow at the beginning of the migration peak would thus trigger a greater proportion of A. anguilla settled in the estuary and in the downstream zone of the Fremur River to begin their freshwater colonization. The physicochemical roles of changes in river discharge in stimulating upstream migration are discussed. It is concluded that fluvial recruitment in the Fremur River is mainly determined by environmental factors.

Prioritising sewerage maintenance using serviceability criteria

In the UK and elsewhere, there has been a concerted move away from primarily assessing sewer system capacity to focus on ‘serviceability’ as a key performance indicator in sewerage provision. Increasingly, water regulators are recognising that a prime concern of customers is avoiding flooding of their property, as environmental improvements can often be intangible. However, regulators insist that environmental factors are also given strenuous attention. Based on these considerations, it is evident that sewerage providers will increasingly consider undertaking proactive maintenance on sub-critical and non-critical sewers in order to reduce the risk of customers experiencing reduced levels of serviceability. This paper documents the development of a methodology to manage capital maintenance expenditure within the context of ‘serviceability’ to customers, the public and the environment. The methodology has been applied alongside Infoworks-CS software to determine critical points in a small coastal catchment in Edinburgh, Scotland. The paper also discusses how these results are linked to customer complaints held in a geographic information system (GIS) database, and how the approach developed was able to recommend which assets should be proactively maintained to minimise the risk of serviceability loss. For the catchment considered, key serviceability performance indicators were found to be combined sewer overflow (CSO) spills, flooding in residential areas, flooding in commercial zones and disruption to traffic.

Inadmissible evidence: knowledge and prediction in land and riverscapes

Analyses of daily water quality data from two streams draining a pair of small coastal catchments in South Devon, England reveal that what conventionally would be thought to show random noise, has a discernable structure and is clear evidence of process. Catchment and aquatic systems are non-equilibrium systems and climate drivers cause fluctuations in water quality both in terms of the concentrations of individual parameters and in the correlations between parameters. The data reveal non-linear coupling at small scales and show evidence of fractal properties both of which may be evidence of self-organised phenomena at small scales in catchments and streams. These data show that: (a) water quality and catchment nutrient export data may be strongly aliased, and (b) there is a fundamental degree of indeterminacy underlying the data we can collect and the knowledge we can generate from the data. New techniques of data based modelling that use the data itself to define more parsimonious predictive models are needed because such an approach recognises the partial nature of our knowledge and requires adequate monitoring and adaptive management programs.

Spatial distribution of an eel population (Anguilla anguilla L.) in a small coastal catchment of northern Brittany (France). Consequences of hydraulic works

Le Fremur est un bassin versant cotier de Bretagne de 60 km 2 comportant de nombreux amenagements hydrauliques (barrages, chaussees de moulin, stations de jaugeage, etc.) ayant fortement reduit l'accessibilite par les civelles depuis au moins cinquante ans. En 1992, un ascenseur a anguilles a ete installe sur le principal barrage (14 m de haut), puis en 1996 deux autres obstacles (5 m de haut) ont ete amenages plus en aval pour restaurer la libre circulation des anguilles dans le bassin versant. En 1995, avant l'amenagement des passes, l'abondance et la structure de la population sont estimees a l'echelle du bassin versant par peche electrique dans 33 stations (methode d'epuisement des stocks). La biomasse moyenne apparait elevee (19 g/m 2 ) malgre la presence des obstacles a la migration. Toutefois, on note une accumulation d'anguilles en aval de nombreux amenagements hydrauliques et de tres faibles abondances en amont de ces derniers. De plus, a l'aval du premier obstacle, a 2 km de l'estuaire, la population est dominee par les individus de moins de 100 mm, alors que cette classe de taille est quasiment absente en amont. Ces resultats suggerent que le potentiel d'accueil du bassin versant n'est sans doute pas atteint et que les biomasses moyennes pourraient augmenter avec l'amelioration des conditions de migration grâce aux passes a poissons.