Ognjen Bonacci

Ground water behaviour in karst: example of the Ombla Spring (Croatia)

Abstract The hydro-electric power plant (HEPP) which will exclusively use water from a karst underground storage basin will be built in the vicinity of the abundant karst spring Ombla in Croatia. This paper presents the results obtained by hydrogeologic, hydrologic and hydraulic investigations related to the principles of ground water circulation in the karst. The analyses included the determination of the effective porosity ne of the karst aquifer and the definition of the volume of large conduits and small fractures in the karst which form the aquifer volume. The position and dimensions of large karst conduits have also been defined. It was established that in three small springs, Zaton, Zavrelje and Slavljan, water overflows from the Ombla Spring in periods of high ground water levels. It was also discovered that at certain periods the Dupuit expression for steady-state flow in an unconfined aquifer can be used. In accordance with this, it was possible to determine the values of hydraulic conductivity, K (in m s−1), for the Ombla aquifer. They range from 2 × 10−3 to 5 × 10−3 m s−1), and are inversely proportional to the Ombla Spring discharge. Continuous measurements of the ground water level by several piezometers located in the karst hinterland of the Ombla Spring and simultaneous measurement of the discharge made it possible to define discharge curves of the Ombla Spring dependent upon the ground water levels at various locations. Characteristic features of the discharge curves made the identification of the position and dimensions of the main karst conduits possible.

Water circulation in karst and determination of catchment areas : example of the River Zrmanja

Abstract Karst hydrological investigation of the sinking stream problem of the River Zrmanja is presented. The aim of this analysis is to assess the feasibility of constructing three hydroelectric power plants (HEPP) along the River Zrmanja course. This paper presents a suitable and simple hydrological methodology that can be applied to scarce available data obtained on complex karst terranes. The paper presents a complex but common case of water circulation in a karst system. The primary objectives of the investigations were: (a) to analyse the underground karst connections, (b) to analyse discharge conditions along the River Zrmanja, and (c) to define variations in the catchment area along the River Zrmanja. The fact that the hydrological regime of the River Zrmanja is highly variable, due to the water losses along the open streamflow, strongly influenced the selection of the locations and heights of the HEPP dams. In spite of many hydrological, meteorological and hydrogeological measurements, the River...

Sea water intrusion in coastal karst springs: example of the Blaz Spring (Croatia)

Abstract Brackish karst springs are common along every karstic sea shore consisting of limestone and dolomite. On the Croatian sea coast there are more than 300 permanent or temporary brackish karst springs. From the standpoint of water supply, the problem of karst spring water salinization is quite significant because large quantities of high quality fresh water are not available to be used either as drinking water or for industrial and agricultural purposes. The salinity of brackish karst springs situated along the Adriatic coast varies from 10 to more than 18 000 mg C1 1−1 with an unfavourable distribution during the year. In the wet winter period, when water quantities in the region are abundant, the salinity is exceedingly low. In the warm and dry summer period the chloride concentration is high. At that season, when a shortage of fresh water in the region occurs, especially due to tourism, karst spring water is so salty that it cannot be used at all. The mechanism of sea water intrusion is relativel...

Monthly and annual effective infiltration coefficients in Dinaric karst: example of the Gradole karst spring catchment

Abstract The problems and present methods for the calculation of monthly and annual effective infiltration coefficients in Dinaric karst catchments which cover areas smaller than 200 km2 are discussed. An example is given of the catchment of the Gradole karst spring, which covers 114 km2. The climate on the catchment is North Mediterranean, with average annual rainfall of 986 mm, average air temperature of 11.4°C, and average discharge from the Gradole Spring of 1.98 m3 s−1. All the data refer to a period from 1987 to 1998. An auto-correlation analysis is carried out of daily and monthly discharges from the spring and monthly rainfall on the catchment. Special attention has been given to determination of monthly effective infiltration coefficients. It was found that the time-scale effect makes it infeasible to use the equation for effective infiltration coefficient, because it gives values greater than 1 in 25% of cases, which is theoretically impossible. Therefore, two different procedures for calculation of monthly effective infiltration coefficients are given, out of which one uses the master depletion curve. This procedure gives acceptable and physically well-established values of mean long-term monthly effective infiltration coefficients. Obtained values can be used for regional analyses as well as for water resources management in karst regions. There was also a strong relationship between mean monthly air temperatures of the catchment and mean monthly effective infiltration coefficients. It is found that rainfall distribution during the year significantly influences the annual effective infiltration coefficients, which for the Gradole catchment ranges between 0.356 and 0.763 with the mean value of 0.57.

Definition of catchment area in karst: case of the rivers Krčić and Krka, Croatia

Abstract The study area consists of the spring zones of the Krčić, Krka and Cetina river catchments located in the Dinaric karst, Croatia. Classical hydrological approaches and some newer time and frequency domain methods are used in order to validate the existing hypotheses both qualitatively and quantitatively, and these contribute to factual information about the hydrological behaviour of the catchments. The groundwater recharge rates are calculated by a mathematical model based on Palmers soil-moisture balance method. The values of parameters of the groundwater recharge model are estimated by the spectral method. The calculated monthly and annual groundwater recharge rates form the basis for estimating the hydrological catchment areas of the spring zones and also for the determina-tion of quantitative relationships between the catchments.

Interpretation of groundwater level monitoring results in karst aquifers : examples from the Dinaric karst

The paper presents an attempt to determine the characteristics of karst aquifers using information on groundwater level (GWL) in natural holes and boreholes with different data quantity and time resolution of GWL measurements. In this paper the particulars of karst aquifers were analysed for four examples from the Dinaric karst. In all four study areas, aquifers are formed in bare, deep and well-developed Dinaric karst consisting of Cretaceous limestones. The first example represents a wide area of Imotsko polje in the karst. The aquifer was analysed on the basis of infrequent water level monitoring in natural karst water features (jamas, lakes, wells) and discharges of springs and rivers. The karst aquifer in this example is complex, non-homogenous and variable in space and time, which is frequent in the Dinaric karst. Regardless of the aforementioned it was possible to determine its elementary characteristics. The second example represents 10 wells used for the water supply for the city of Pula. The GWL and salinity were measured once a week in the period between 1981 and 1996. Even though these measurements were relatively infrequent in space and time, they served as bases for assessment of average and maximum aquifer conditions as well as boundaries of saltwater intrusion. In the third example only a portion of aquifer of the karst spring Blaz, which is in the contact with the Adriatic Seas, has been analyzed. It is a spring with an intrusion of salt water. For purposes of study of saltwater intrusion, 26 piezometers were drilled in its vicinity in which GWL, salinity and temperature were measured once a day during 168 days, a period comprising one complete cycle of seawater intrusion and retreat. These measurements proved the existence of dispersed discharge from the aquifer into the sea and its non-homogeneity in space. In the fourth example GWL was measured continuously in 10 deep (up to 300 m) piezometers in the hinterland of the Ombla Spring catchment. The measurement period lasted 2 years (January 1988 to December 1989). The analyses are made with hourly data. The results made it possible to determine numerous characteristics of the karst aquifer and a significant non-homogeneity of groundwater distribution in karst aquifers, depending more on the underground karst phenomena than the surface karst forms. Copyright

Rhythmic karst springs

Abstract Rhythmic springs (ebb and flow springs, intermittent springs, potajnice) belong to the group of springs which appear exclusively in karstified terrains. The paper describes various types of rhythmic springs and gives their classification. It also develops a mathematical model for the functioning of this type of springs based on the principle of recharge and emptying of the underground reservoir through siphon action. Applying this model, according to the observed hydrographs of some rythmic springs in Yugoslavia, the paper explains in detail the structure of the underground reservoir located in the karst.

Identification of a karst hydrological system in the Dinaric karst (Yugoslavia)

Abstract The construction of a reservoir on the Neretva River resulted in the flooding of three springs which now function as ponors (swallow-holes). In the first phase, they discharged 28 m3 s−1 of water from the full reservoir. All the water that was lost through these ponors appeared some 1 km downstream from the dam in a group of springs. Most of these springs were, in the natural state, only temporary, and during the dry period their discharge was approximately 1 m3 s−1. In order to reduce the losses from the reservoir, some reclamation works were carried out resulting in a decrease of the losses by only 6 m3 s−1. Detailed hydrogeological and hydrological analyses were performed with the objective of identifying the karst system, i.e. to determine the position of the karst channels. The definition of the position and dimensions of the karst channels through which the greatest quantities of water are lost from the reservoir was carried out according to the definition of the hydrograph recession curves...

Hydrological explanation of the flow in karst: example of the Crnojevića spring

Abstract This paper deals with various methods of solving the complex problems of the hydrological transformation of rainfall into runoff in karst terrains. As an example of a typical karst catchment, the Crnojevica spring, located in deep Dinaric karst, is used to illustrate, explain and solve several hydrological problems in karst. The introduction deals with the geographical, geological and meteorological factors which conditioned a specific system of surface and underground flows, typical for karst terrains. The paper also explains some basic activities related to the identification of such a system. Special attention has been paid to the karst terrain of the Cetinje polje and its flooding, which occurred in February 1986. This flood initiated numerous intensive investigations which made it possible to define the catchment area of Crnojevica spring and the volume of the underground karst reservoir.

Spring discharge hydrograph

Publisher Summary The spring discharge hydrograph is the final result of various processes that govern the transformation of precipitation and other water inputs in the springs drainage area into the single output at the spring. In many cases, the discharge hydrograph of a spring closely resembles hydrographs of surface streams, particularly if the aquifer is unconfined and reacts relatively quickly to water input. Springs draining karst and intensely fractured aquifers are typical examples—their discharge can increase several times or even orders of magnitude in a matter of hours after heavy rains. At the opposite spectrum are deep ascending springs isolated from a direct influence of surficial processes, such as infiltration of precipitation and showing only slight, delayed seasonal changes in discharge characteristics. The complexity of the water budget determination depends on many natural and anthropogenic factors present in the general area of interest, such as climate, hydrography and hydrology, geologic and geomorphologic characteristics, hydrogeologic characteristics of the surface soil and subsurface porous media, land cover and land use, presence and operations of artificial surface water reservoirs, surface water and groundwater withdrawals for consumptive use and irrigation, and wastewater management. The shape of a discharge hydrograph depends on the size and shape of the drainage area, as well as the precipitation intensity. Analysis of spring hydrographs always includes determination of the general statistical parameters of the time series, such as average, minimum, and maximum flows for the period of record; standard deviation of the flows; coefficient of variation; flow duration curves; and frequency of characteristic flows at the minimum.