## 2 Materials and Methods  
### Direct Sampling of Suspended Load  
Until recent times, riverine suspended sediment data have been produced by gravimetric analyses performed on water sediment samples collected most of the time manually or, in the U.S., in some sites by automatic samplers. These methods tend to be expensive, difficult, labor intensive, and, under some conditions, e.g., during high floods, hazardous. Specialized equipment and considerable training are prerequisites for obtaining reliable samples and results [8]. However, in Hungary, direct sampling is still an integral part of the sediment monitoring standardization, and it stipulates sampling by a bottle (which is very inexact) or with a pump [16]. During pump sampling, the sample is brought to the surface (on the Danube River, aboard a measuring ship anchored in the vertical) with a pump. In international literature, pump sampling can as well be executed in a different way, when the sample at a monitoring station is taken automatically through a built-in pump. In Hungarian practice, the end of the pump suction tube is lowered into the sampling point using a suitable weight, and the sample of the required volume is sucked up. The disadvantage of the pumping method is that if the sampling velocity does not match the water velocity at the point, the sediment concentration of the sample will not, either. Depending on the relative sampling rate (sampling rate/water velocity), the difference in concentration can be \(-20\)\(\rightarrow\)\(60\%\). A deviation is also caused if the end of the intake is not parallel to the flow direction. The effects of divergence in various parameters were studied and published at the University of Iowa, U.S. [17]. Direct sampling methods can be further subdivided according to the number of sampling verticals, and the literature distinguishes between single- and multi-vertical methods. The location of the verticals in the case of the single-vertical method is determined based on different technical considerations (in the middle, at the maximum depth). In the selected vertical, the sampler is lowered and lifted up during the continuous sampling. Among the multi-vertical methods, the international literature recommends three: sampling in lamellae with the same water flow, lamellae of the same width, and lamellae of the same area. Sampling is carried out in a similar way for multi-vertical methods as for the single-vertical method (moving at the same speed); the difference is in the number of verticals. For the lamellae method with the same water flow, a vertical average sample should be taken in between 4 and 9 verticals, and for the method of lamellae of the same width, at least 10 verticals [18]. The Hungarian standard on suspended sediment sampling provides for a multi-vertical method where the lamellae are of equal width, but the average sample for a given vertical is to be prepared by taking 1 L of sample in the vertical at each of the 10 points of different depths, evenly distributed, and then pouring these samples together into one canister to form the vertical average sample with a volume of 10 L. In the case of the Danube, the standard prescribes for seven sampling verticals [16]. As stipulated by the Hungarian standard [16], _\"during sampling it is very important to ensure that the sampling nozzle faces the flow, the pipe is not bent and to let enough time before taking samples to flush the pipe. Sampling needs to be carried out with care to adjust the revolutions per minute value (RPM) or the discharge of the pump for the velocity through the nozzle \(V_{\text{in}}\) should not differ much from the velocity of the flow \(v\) at the given point_  
\[0.8\,v\leq V_{\text{in}}\leq 1.5\,v \tag{1}\]  
_In case the velocities are outside this range, the RPM of the pump should be accordingly adjusted, or a tap should be installed at the end of the pipe to ensure that intake velocities match.