Hydrology, sediment yield, erosion and sedimentation rates in the estuarine environment of the Ria de Vigo, Galicia, Spain
Introduction
The Ria de Vigo is located at the south of the Galician Rias Baixas. There are 36 individual tributaries draining the Ria de Vigo (Fig. 1). The largest, the Verdugo–Oitaven River, is located at the head of the ria and is oriented approximately parallel to its longitudinal axis. The rest are transversal river systems. In spite of being small, the numerous transverse rivers on both margins contribute substantially (>30%) to the total amount of fresh water and sediment load introduced into the ria. Nonchannellised areas occupy about 12.5% of total drainage area, in which diffusive runoff and mass wasting processes dominate (Perez-Arlucea et al., 2000a).
From preliminary research, conducted between May 1997 and July 98, 19 rivers were selected for systematic monitoring. The research was subsequently extended to a second 12-month period, from May 2000 to May 2001, but only the largest rivers of the transversal system were selected this time: the Minor and the Lagares. The Ramallosa Complex is located in the Minor River mouth, where estuarine and tidal deposits are well preserved. The Minor River is an example of a fairly natural system with little anthropogenic influence. It flows along forest and lowlands with only a small proportion of cultivated areas on the floodplain. Population density is low, and is concentrated mostly near the river mouth. In contrast, the Lagares River traverses the rather big city of Vigo and the catchment shows a much higher population density.
Previous hydrological and budget data on the rias are very scarce. Some hydrologic parameters are published by the Consello da Cultura Galega (1996). Most fluvial discharge data sources for the Ria de Vigo consist of theoretical calculations related to catchment size and meteorological records. Some discharge and sediment load measurements are provided by Nombela, 1989, Rios et al., 1992, Alejo, 1994, Pazos et al., 2000, Perez-Arlucea et al., 2000a, Perez-Arlucea et al., 2000b, Perez-Arlucea et al., 2001. Additionally, there is a total lack of information about directly measured erosion, sediment yield and sedimentation rates in the whole of Galicia. The only available data are centred on specific contexts such as deforestation due to fires, in which the main issue was to evaluate soil damage caused by arson on Galician forests, mostly from the 1960s to 1980s (Bara and Vega, 1983, Benito et al., 1991, Diaz-Fierros et al., 1982, Diaz-Fierros et al., 1983, Diaz-Fierros et al., 1990, Diaz-Fierros et al., 1994, Soto Gonzalez, 1993, Vega, 1983, Vega et al., 1982). Other papers deal with erosion on small experimental erosion plots (Rodriguez Martinez-Conde et al., 1995, Rodriguez Martinez-Conde et al., 1996a, Rodriguez Martinez-Conde et al., 1996b, Rodriguez Martinez-Conde et al., 1998, Rodriguez Martinez-Conde et al., 2001, Valcarcel Armesto, 1998, Vila Garcia, 1996). Most of these references have a very local character and the data have very limited use.
The aim of this study is to provide a budget study with calculated erosion rates based on (1) suspended sediment and dissolved loads, sediment flux and sediment yield; (2) sediment accumulation rates at the coast and (3) theoretical calculations of potential denudation rates based on catchments relief. Our study is thus a broad one, comprising geomorphologic, hydraulic and sedimentologic approaches to the main tributary rivers in the southern slope of the Ria de Vigo and associated sedimentary environments at the river mouths.
Section snippets
Methodology
There are no permanent gauging stations in the Vigo catchments apart from some private ones located at reservoirs. Consequently, stations were set up in both rivers. These stations were positioned to avoid tidal influence and at the same time to sample the maximum flow. Stage fluctuations were registered every week, from May 1997 to July 1998 and from May 2000 to May 2001.
Several discharge values (Q) were measured at different stages (H) over 2 years, using a propeller current meter.
Climatology and local meteorological conditions
Climatic conditions for the Southern Rias of Galicia (Fig. 4) can be described as oceanic with Mediterranean influence (drastic decrease in rainfall during July and August). Seasonal temperatures are fairly homogeneous over the whole area, apart from some local influence of elevation and proximity to the sea. Rainfall conditions respond to regional topography and penetration paths for oceanic humid winds. Dominant winds blow from SSW and NNW. There is rainfall all year, with maximum values from
Catchment areas: composition, geomorphology and hydrology
In order to obtain denudation rates from river loads, several aspects have to be considered: lithology and weathering conditions in the source areas, hydrology, sediment loads, sediment flux and sediment yield, and geomorphological parameters such as slope, catchment areas, etc.
River sediment loads and sediment flux
All tributaries have low sediment loads during normal fair weather conditions. Bed materials consist chiefly of boulders and cobbles at the heads of rivers, grading to very coarse granules, pebbles and medium-coarse sand toward estuarine areas. Quartz and feldspar grains dominate, although metamorphic rock fragments, mainly schists and slates, can be locally quite abundant. The Minor and the Lagares Rivers have the highest proportion of sand size particles of all the transversal tributaries.
Up
Sediment yield and erosion rates from river loads
To estimate short-term erosion rates from river loads, average discharges for the Minor and the Lagares rivers, sediment flux, suspended and dissolved sediment yields were calculated for both study periods. Solid suspended sediment yield (SSY) mean values for the Minor near Baiona Bay were 25.8 ton/km2 a for 1997/1998 and 139.7 ton/km2 a for 2000/2001. Values for dissolved sediment yields (DSY) were 109.3 and 204.0 ton/km2 a, respectively. Considering an average rock density of 2.7 g/cm3,
Sedimentology of the coastal deposits
Coastal deposits are not very prominent in the Ria de Vigo. Beach and aeolian sand dunes are preserved in protected bays along margins between low cliffs (Nombela et al., 1995). Generally, high cliffs dominate the coast line. Several rivers form prominent estuaries such as the Verdugo-Oitaven, the Minor and the Lagares. Tidal flats are well preserved in the San Simon embayment and small areas of tidal flats and marshes can be found elsewhere associated with estuaries and protected by sandy
Sediment budget
Calculated sedimentation rates (SR) are shown in Table 4. SR estimations have been made with depths measured in the cores corrected for compaction (a linear decompaction was made considering penetration depths measured in the field when coring) and sediment ages. Apart from the oldest and deepest sample d which yields 1.1 mm/a, sedimentation rates vary from 3.3 to 4.4 mm/a. Accumulation rates (AR) (sediment weight per area and unit time; Table 5) have been estimated using an SR/AR conversion
Conclusions
The objectives of the proposed budget study with calculated erosion and sedimentation rates were completed. The most important results and conclusions are:
- –
Data obtained for May 2000 to May 2001 show a very atypical meteorological year. Rainfall conditions were the most intense on record and had an important influence on river discharge, sediment loads and yields, which can all be considered as extreme historic values. For both the Lagares and Minor rivers, discharge values obtained in 1997/1998
Acknowledgments
This paper is written in the framework of the Research Group EX1 (number 256) from Vigo University and has been possible thanks to grants from the O.M.A. at the University of Vigo and the following projects: PGIDT00MAR30103PR, PGIDT00PX130105PR, CICYT REN2000-1102MAR and UNESCO IGCP-464. We thank Salvador Rodriguez Muñoz for providing meteorological data at Gondomar station, Leopoldo Pena for 14C data corrections, also Sandra Rua, Montse Martinez and Ivan Leon for laboratory analysis. Our
References (40)
Present, past and potential denudation rates: is there a link? Tentative evidence from fission-track data, river sediment loads and terrain analysis in the South Indian Shield
Geomorphology
(1998)- et al.
Geochemistry of major and trace elements in sediments of the Ria de Vigo (NW Spain): an assessment of metal pollution
Mar. Pollut. Bull.
(2000) Abundance of chemical elements in the continental crust: a new table
Geochim. Cosmochim. Acta
(1964)- Alejo, I., 1994. Estudio dinamico y sedimentario de la Bahia de Baiona. Tesis Doctoral inedita. Universidad de Vigo,...
- et al.
Complejo intermareal de a Ramallosa, (Bahia de Baiona, Pontevedra): I. Ecologia y evolucion
Thalassas
(1990) Functional relationships between denudation, relief, and uplift in large mid-latitude basins
Am. J. Sci.
(1970)- et al.
Effects of wildfire on forest soil in the northwestern Spain
Proc. II Symp. Fire Ecol.
(1983) - et al.
Soil erosion studies in NW Spain
- Consello da Cultura Galega (1996), As augas de Galicia. Concello da Cultura Galega, Santiago de Compostela. 611...
- et al.
Efectos erosivos de los incendios forestales en suelos de Galicia
Solute loss and soil erosion in burnt soil from Galicia (NW Spain)
Incendios forestales en Galicia. Sus causas y efectos sobre el medio ambiente
Action of forest fires on vegetation cover and soil erodibility
Sedimentary basins
Evolution, Facies and Sediment Budget
As rias
Notas climatologicas e hidrologicas de las Rias Bajas
Inf. Téc.-Inst. Esp. Oceanogr.
The TESS-1 suction corer: a new device to extract wet, uncompacted sediments
J. Sediment. Res.
Sedimentation in the mesotidal rias of Galicia (North-Western Spain). Ensenada de San Simon, inner Ria de Vigo
Spec. Publ. Int. Assoc. Sedimentol.
Origen y evolucion geomorfologica de las rias atlanticas de Galicia
Rev. Soc. Geol. Esp.
Cited by (55)
Microplastic accumulation dynamics in two Mediterranean beaches with contrasting inputs
2022, Journal of Sea ResearchDecadal changes in the spatial coverage of Zostera noltei in two seagrass meadows (Ría de Vigo; NW Spain)
2020, Regional Studies in Marine ScienceCitation Excerpt :A possible explanation to the expansion of Z. noltei observed in A Ramallosa could be the modification of the structure and chemical composition (a supposed organic matter enrichment due to nutrient discharges) of the sediment, becoming more suitable for the species colonization and development, thus increasing its biomass and length (Cabaço et al., 2008). Finer sediments are supplied through the rivers to the meadow of A Ramallosa due to inland deforestation (Alejo et al., 1990; Pérez-Arlucea et al., 2005; Méndez-Martínez et al., 2011), increasing the mud content of the marsh and favouring the Z. noltei settlement (Vilas, 1981; Mattheus et al., 2010; Valle et al., 2010). Indeed, the expansion here reported in A Ramallosa is consistent with those described for the same area by Alejo et al. (1990), between 1956 and 1988.
Determination of denudation rates by the measurement of meteoric <sup>10</sup>Be in Guadiana river sediment samples (Spain) by low-energy AMS
2018, Journal of Environmental Radioactivity