Hydrology, sediment yield, erosion and sedimentation rates in the estuarine environment of the Ria de Vigo, Galicia, Spain

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Abstract

The aim of this study is to provide a budget study with calculated erosion rates. Three methods have been used to calculate sediment yield and denudation rates in the Ria de Vigo: (1) measurements of sediment loads, (2) measurements of sediment accumulation rates at the coast, (3) theoretical calculations of potential denudation. Sediment loads and water discharge were measured over a period of 14 months from May 1997 to July 1998. Two of the tributaries entering the Ria de Vigo were monitored for 12 more months, from May 2000 to May 2001, to observe changes in discharge and sediment loads. This period corresponded with atypical precipitation, with peak monthly values (600 mm) three times higher than those on record.

Water rating curves are typically exponential. Suspended and dissolved loads vary for different rivers, showing values of 1.5 to 130 mg/l during 1997/1998. For 2000/2001, these values are twice as high. Suspended load versus discharge relationships for 1997/1998 were logarithmic, but data from 2000/2001 does not fit the same equation. Dissolved loads are several times higher than suspended loads in almost all cases. Dissolved load concentrations vary more widely with discharge than suspended loads. This is probably due to local pollution and contamination from marine spray in areas closer to the sea.

Second, erosion rates and bed load sediment yields were calculated from accumulation rates at the Ramallosa Complex. Well-preserved estuarine and tidal sediments, associated with the Minor River, have accumulated in this area during the Holocene. 14C ages allow calculation of sedimentation rates (SR) for two intervals. The lower interval extends from 2001 to 484 years BP and yields an SR of 1.12 mm/a. The upper interval extends from 484 years BP to the present and has an SR of 3.3–4.4 mm/a. These differences may be explained by basin dynamics as the beach progressively encloses the area and also by human interference. From sedimentary facies analysis it is concluded that 90% to 95% of the accumulated deposits were transferred to the basin as bed load. Muddy deposits (mostly marshes) are better developed at the upper part of the sediment pile, and inner areas, indicating a progressive shallowing and filling up of the basin. Most of suspended load is exported to the ria, whereas the Ramallosa Complex acts as a sediment sink for bed load derived material.

Calculated potential erosion rates using Ahnert's [Am. J. Sci. 268 (1970) 243] equation show lower values than those estimated from river load concentrations. Potential erosion rates for the Minor River are higher than for the Lagares River which contrast with mechanical denudation rate values from river loads during 1997/1998 which are higher for the Lagares River. During 2000/2001 MDR values were higher than those of the potential erosion rates for both rivers, in line with the extremely high precipitation. Higher values in the Lagares could be in part due to human interference.

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

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