The Cachí reservoir in Costa Rica

For reservoirs with small storage to inflow ratios the cost effective way of maintaining storage is by sluicing, that is by discharging water through low level outlets in order to flush sediment through the reservoir. The effectiveness of the sluicing is dependent upon the duration and degree to which the reservoir is drawn down and on the discharge capacity of the sluices. Examples where sluicing has been effective include the Cachí reservoir.

In 1990 fifteen sediment cores were sampled in the Cachí reservoir and X-rayed at ICE in San José. The sampling also included nine grab samples. For location, see the map below.

Location of cores and grab samples. Median particle sizes of surface sediment. Upper storage level = 990 m a.s.l. From Axelsson 1992 (UNGI Rapport Nr 81).

Localización de los sitios de las muestras de núcleo (C1 - C15) y de sedimentos superficiales (19i - 35d). Tamãno mediano de las partículas del sedimento superficial (the old river channel = antiguo cance del rio). Nivel superior del embalse = 990 m s.n.m.

The grain-size of the newly deposited sediments decreased as expected in a downstream direction with fine sand and coarse silt dominating on the delta plain, clayey silt and silty clay on the lower terraces and in the downstream part of the old river channel.

The deposits on the terraces and in the old river channel are partly bioturbated, and a dense population of tubificids was found in the topmost part of several sediment cores. Some of the softer, organic layers were gas-rich and contained debris of water hyacinths (Eichhornia crassipes).

The silty and clayey deposits on the old river terraces show a rhytmic variation between hard mud crusts and softer layers in the sedimentary sequence. The induration of mud crusts during dehydration is a result of natural cohesion of the silts and clays, and binding by included algal filaments. Cracks, formed as sediment volume is reduced during subaerial exposure, also characterize the mud crusts.

The sediment cores from the old river channel contained harder, more minerogenic and softer, more organic layers, reflecting variations in stream power. The downstream distribution of these layers could partly be followed by inter-core correlation.

Partly overlapping radiographs of the upper part of core C10 from the central part of the Cachí reservoir. The hard (light) sediment parts are probably formed during given flushing periods and other low water periods with subaerial exposure of the sediment surface.

Radiografías de la parte superior del núcleo de sedimentos C10 tomado en la parte central del embalse. Las capas duras (blancas) están probablemente formadas durante los desembalses indicados en la Figura y durante otros períodos con agua baja con exposición subaérea de los sedimentos superficiales.

The radiographs of core C10 document the formation of mud crusts during subaerial exposure of the sediment surface due to flushing. See also the sedimentation-compression curve below.

Sedimentation-compression curve down to a depth of 45 cm in core C10. F = probable flushing periods with subaerial exposure of the sediment surface.

Curva de sedimentación-compresión, calculada radiograficamente, hasta una profundidad de 45 cm en el núcleo de sedimentos C10. F = probables períodos de desembalse. (Effective overburden pressure = Presión efectiva de la sobrecarga. Void ratio = Razón de vacíos.)

According to the radiographic calculations, the newly deposited silty and clayey sediments in the Cachí reservoir often have void ratios higher than 5. The highest void ratios are found in the gas-rich, organic layers with values higher than 20. In the hard crusts, formed during subaerial exposure of the mud flats on the old river terraces, the void ratio may be lower than 2.

Due to the formation of mud crusts during subaerial exposure, the thickness of the dehydrated layers decreased to about one-half and sometimes to one-third of its former value. Apart from the reduction in sediment volume, a very important result of the subaerial exposure of the sediment surface during flushing and other periods with low water levels is a substantial decrease in erodibility of the sediment surface. The stability of the sediments also increases due to the presence of epipelic algae and to the accumulation of mucilage at the surface of the exposed mud flats. However, this effect is partly counteracted by the mud cracks, simultaneously formed as volume is reduced during dewatering.

The downstream part of the Cachí reservoir during high and low water stage respectively. (La parte aguas abajo del embalse de Cachí en alto y bajo nivel de agua.)

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