Establishment of the Sediment Monitoring System for the Sava River Basin - Page6

Proposed Sediment Monitoring Program

Basic Considerations

"Sediment monitoring" is focussed only on determination of sediment quantities in SRB. Namely, sediment quality monitoring in SRB countries is already in place or will began soon, as a part of WFD (Water Framework Directive, 2000/60/EC) compliant monitoring of status of water bodies.

Sediment monitoring in SRB should be based on coordinated national monitoring programs, having a common or at least comparable methodology, instruments and techniques.

As suspended sediment plays the dominant role in the morphological processes on the Sava River and in lower courses of its tributaries, system for monitoring of suspended sediment should be established as soon as possible. Two SRB countries (Slovenia and Croatia) already monitor suspended sediment transport in the Sava River and its tributaries, while other countries (Bosnia and Herzegovina and Serbia) will need to re-establish monitoring.

Bed load monitoring is not envisaged for the first phase, as it is very costly and difficult. Also, the first phase is dedicated to preparation of data for the sediment balance of the Sava River itself, where bed load has comparably small role in comparison to suspended load. In the later phases of development, when monitoring will be expanded to large mountainous tributaries, bed load data will be needed also.

 

Methodology of Data Collection

Variations of suspended sediment load in the Sava River and its large tributaries are very frequent, and depend on hydro-meteorological conditions and other circumstances.

As suspended sediment load is a product of water flow and suspended sediment concentration (SSC), measurements of these components should have at least a comparable accuracy. Presently, water flow on almost all gauge stations in the SRB is measured using contemporary devices which provide continuous data. It is expected that in the further enhancement of flow monitoring network all gauges will provide continuous data on water flow.

To obtain more reliable data on suspended sediment load, especially during high flows, SSC data should also be continuously collected. Discrete SSC values (provided by taking sediment sample once a day) are not sufficient for flood events, when the transport is very intensive and variable in time. On the other hand, taking a few samples per day may be very costly and overburden the national hydro-meteorological services.

Instead of traditional data collection methods requiring routine collection and analysis of water samples, there is a number of promising surrogate technologies to continuously monitor suspended sediment. There are commercially available instruments operating on bulk optic (turbidity), laser optic, pressure difference, and acoustic backscatter principles. These instruments are permanently developing, and differ a lot in terms of cost, reliability, robustness, accuracy, sample volume, susceptibility to biological fouling, and suitable range of mass concentration and particle size distribution.

Turbidity meters are proposed for SRB monitoring, as widely used all around the world in sediment monitoring programs, and also in some parts of the SRB (Slovenia). They provide reliable data where the point measurements can be reliably correlated to the river's mean cross section concentration value, effects of biological fouling can be minimized, and concentrations remain below the sensor's upper measurement limit.

Turbidity meters can be installed in gauge stations along the Sava River and lower sections of its tributaries, at reasonable cost (approximately 5000$ per unit). Individual, site specific field calibrations are needed to make a correlation between point SSC and turbidity, and moreover between point SSC and cross-sectional average SSC.

 

This is the reason why point sediment sampling and cross sectional measurements should be (re)established at all sediment monitoring profiles in the SRB. These measurements should continue until the above mentioned correlations became reliable and cover the whole range of flows.

Point or depth integrating samplers should be used following ISO 4365:2005 standards. ISO 4363:2002 standards should be followed for profile mean SSC and mean particle size distribution measurements.

Acoustic Doppler profilers may be used for occasional cross sectional suspended sediment measurements at gauge stations. ADCPs are currently in use in Slovenia, Croatia and Serbia for flow measurements. The same measurements can be used to derive suspended sediment concentrations, as the acoustic backscatter to measure.

Presently, knowledge in the Sava countries on possible use of ADCP in suspended sediment measurements is very limited (available only in Croatia). Common field exercises, where standard techniques and ADCP are used in parallel for suspended sediment transport measurement, and help of the ADCP manufacturers will be needed to enhance it.

Daily data on suspended sediment concentrations and transport should be collected, and printed in the Hydrological yearbooks of ISRBC. Monthly and yearly suspended sediment quantities will be derived also.

Sampling of riverbed material should be obligatory part in all cross sectional measurements. However, it might be of interest for all countries to do the sampling campaign along the Sava River every 6 years, in the time of riverbed survey. Sampling campaigns are also to be performed after large floods. Standard grab samplers should be used, and ISO 4365:2005 standard followed. Bed material sampling should be done, not only on some locations, but all along the Sava river main course.

Survey of river cross sections will be done as a part of flow/sediment measurement in gauge stations. However, a survey campaign along the whole course of the Sava River should be done every 6 years. Navigation and water management agencies should determine the distance between cross sections to be surveyed. Cross sections should be surveyed using single beam echo-sounder, eventually combined with multi beam echo-sounding on some locations.

Data on reservoir sedimentation are very important to balance the sediments in the SRB. In the first phase, dedicated to the Sava River main course, surveys of cross sections within reservoirs should be done only in Slovenia. Eventually, these data will be needed for reservoirs on all Sava tributaries.

Data on cross sections of the Sava riverbed and dredging data should also be collected from navigation and water management agencies

Hydrological Information System of the International Sava River Basin Commission (Sava HIS) provides a tool for collecting storing, analysing and reporting a sufficiently high quality data hydrological and meteorological data, including sediment data.

Hydrometeorological data collected by Sava HIS system are based on and contain position of the gauging stations. Such data with spatial component can be visualized on a map as a point object with related hydrometeorological attribute data. Therefore, the SavaHIS web-based application for processed data is integrated with Sava GIS Geoportal in order to combine datasets and functionalities of Sava GIS platform.

The institutions should prepare the processed datasets for upload into central SavaHIS database under authorized part of application.

 

Fig08
Figure 8: Sava GIS Geoportal with enriched SavaHIS layer group.