Inland waterways: controlling discharge and regulating water levels
In impounded federal waterways it is essential that the contractually agreed water levels are adhered to precisely. Automation helps by providing a standardised course of action and ensuring smooth operation.
Efficient and sustainable: water moves things along! Germany has an economically efficient network of waterways linking the maritime ports at the North Sea and Baltic coasts with the inland ports. The 7350 kilometres of inland waterways are made up of canals (25%), free flowing waterways (35%) and impounded river sections (40%). On the larger rivers numerous run-of-river power plants have been installed in connection with the construction of weirs, generating electricity with the sustainable resource water. The impounded federal waterways with water-powered electricity production include the rivers Weser, Upper Rhine, Neckar, Main, Moselle, Saar and Danube, with an installed power capacity of currently about 750 megawatts. In this way run-of-river power plants generate about as much energy as is consumed by all ship transportation on the water (cf. Report on Traffic Investments, 2008).
Minimum fairway depth guarantees cost-effective operation of the waterways
The reference water levels of the federal waterways – that is, the water levels requested upstream of a barrage – are planned for navigation and ensure an all year round minimum water depth for shipping. The fairway depth determines the maximum permissible load of a transport and is decisive for economic operation of the waterway. In order to prevent ships running aground or colliding with bridges, the reference water levels must be adhered to very exactly. For example, the tolerance for the reference water level on the Moselle is only ± 5 cm; exact observance is therefore needed. At more than 300 barrages the Federal Waterways and Shipping Administration (WSV) is responsible for discharge and water level control which must ensure safety and ease of shipping traffic in the face of competing usage interests.
The standardisation and automation of processes
For an automated operation of a barrage, inflow and water levels are continually measured at relevant waterway sections by gauges; this data is then transferred to control technology and processed, for example, by filtering out the ship induced waves from the water levels.
The water discharge is calculated using a control algorithm: from the comparison of the measured value (concession level) with a predefined reference water level, the discharge over the weir or power plant can be calculated with a control algorithm. This controller is a component of the control technology. Automatic control of discharge and water level has been implemented in a pilot project on the River Moselle weirs and continues to be used there successfully. At present, automated discharge and water level control is being adapted for use on the Neckar and Saar rivers and on some smaller waterways.
The automated and coordinated management of several barrages is also being studied and further developed by the BAW. The goal is to provide in each case an overall view and to regulate the corresponding sections in an integrated manner: along a chain of barrages, small changes should be allowed for in the local discharge and water level control so that the reference water level can be maintained as precisely as possible in spite of large fluctuations in inflow or – expressed in technical terms – the retention capacity of the impoundments should be exploited as much as possible to ensure constant discharge rates.
The need for interdisciplinary thinking
Every impounded river section, a so-called controlled system, has its own individual regulation behaviour which is linked to its barrage devices consisting of weir and power plant (actuators). Nonetheless, the automation of a barrage or a chain of barrages should be standardised as far as possible. Other challenges in the transition to automation are the transmission technology, the standards required from IT security and technical infrastructure (with regard to machine safety, for example) and the adherence to legal and statutory constraints. These include the way in which responsibility for the flow conditions is coordinated between the WSV and the energy providers. Briefly: automation requires intensive and interdisciplinary agreement of all disciplines involved – i.e. from the fields of hydraulic steel structures and mechanical engineering to measurement technology, electro technology and control technology or river and structural hydraulics.
The description of the actuator “weir” is based on characteristic curves of weirs which describe the specific relationship between gate position, discharge, headwater and tailwater levels. The BAW normally produces these in laboratory models. Hydro-numerical models are produced by the BAW to describe the impoundments.
Computer simulation: reciprocal effects, hydraulics and control technology
Every control describes a feedback system behaviour: an incorrectly adjusted controller can cause instable conditions in the reach of a river. This becomes apparent in water level fluctuations of increasing amplitude. The control of an impoundment must therefore be simulated in advance by computer and shown to be satisfactory. Using mathematical models, the BAW depicts the hydraulics of the impoundment and the actuators as well as the control technology. The joint simulation of hydraulics and control technology then reveals how safely, quickly and robustly the control functions. By varying the decisive parameters, the controller can be optimally adjusted.
Implementation shows first successes and a significant improvement in quality
Then it becomes "real": the control technology developed by the BAW - consisting of a controller, digital filters and signal transformations - must be transferred from the development environment to a target platform capable of functioning in real time, such as, for example, a programmable logic controller (PLC). The transfer to another programme environment is, however, very time-consuming and liable to error. The BAW is therefore trying to find methods for transferring a workable code directly to a target platform. And in this respect, there are already promising results which lead us to expect a definite improvement in quality.
Continual convergence between mathematical models and control operation
The mathematical models for impoundments, the measurements of the decisive parameters discharge and water level and the implementation of the control technology may always contain some errors and thus lead to undesirable deviations between model and reality. For this reason, the BAW has for many years been supervising the live operation and consistently cooperating with the WSV and the power plant operators to achieve constant improvement of automatic operation.