A flowmeter is a measuring instrument that transmits sound waves through the water to measure the flow velocity of the water, expressed in m/s. Together with a measured wet upper surface or cross section of the watercourse (m²), this results in a flow, expressed in m³/s. The flow velocity of the water can be measured in various ways. On the one hand, there are the so-called runtime meters, which measure the time difference between two sound waves transmitted and received on opposite banks. The difference in runtime is an indication of the flow velocity of the water. On the other hand, there are Doppler flow velocity meters, which measure the flow velocity on the basis of the frequency shift between a transmitted high-frequency sound wave and the sound wave reflected back by particles (moving at the speed of the water).
- Forecast point
Point where water level forecasts based on a hydraulic model are shown. In this case, measurements are usually not available.
A freeboard indicates the margin before the dykes of navigable watercourses are flooded. It is the distance between the water level and the dyke. For the short-term forecasts under the flood and tide themes, freeboards appear on the map if the dykes of the watercourse change colour. Depending on the margin and the severity of the situation, they are coloured green, yellow or red for normal situation, heightened water level and flooding, respectively.
The Hydrological Information Centre belongs to the Hydraulics Research Laboratory and falls under the Department of Mobility and Public Works of the Flemish Government. The HIC has measurement infrastructure and forecasting systems for the navigable watercourses in Flanders.
- Hydraulic model
A hydraulic model contains all topographic data of a watercourse and its valley, as well as of the civil engineering structures on the watercourse, such as spans, bridges, retention basins, weirs, etc. The flow series from hydrological models are fed into this model. The hydraulic model calculates the progression of the flow in the watercourse and in the valley, as well as the associated water levels and flow velocities. The hydraulic model uses this information to generate flood maps.
- Hydraulics Research Laboratory
The Hydraulics Research Laboratory is a department of the Technical Support Services of the Mobility and Public Works department of the Flemish Government. An expertise centre, it carries out research into the effects of water in motion, and is one of the partners of waterinfo.be.
- Hydrographic basins
Flanders has 11 hydrographic basins or catchment areas. From west to east, these are the basins of the IJzer, the Bruges Polders, the Leie, the Ghent Canals, the Upper Scheldt, the Dender, the Lower Scheldt, the Dijle and Zenne, the Nete, the Demer and the Meuse.
- Hydrological model
A hydrological model or rainfall-runoff model is a model that describes the relationship between rainfall and evoptranspiration in a catchment area, on the one hand, and the runoff to the watercourse, on the other hand. The hydrological model is tested against a measured flow series and subsequently used to convert rainfall series into flow series.
- Second General Water Level (Tweede Algemene Waterpassing, TAW)
The second general water level is a reference for height measurements in Belgium and refers to height indications above sea level. All height values in the measurements of the watercourses and their valleys and all water levels are expressed according to this reference system. The use of this reference system is indicated by the notation "m TAW".
- Short-term forecast
Computer models use measurements and weather forecasts to calculate water levels and flows in the watercourses up to 48 hours ahead. These forecast levels and flows can be viewed under the “flooding” and “tides” themes in the graphs on waterinfo.be, which are generated around the clock. For short-term forecasts on the navigable watercourses, the uncertainty of the forecast is also indicated. The map for these themes shows flood maps and freeboard maps. On flood maps, the blue areas indicate how far the flooding is expected to extend. Freeboard maps use the changing colours of the dykes of navigable watercourses to indicate the distance between the forecast water level and the dyke. Under the theme “rainfall”, you can find short-term rainfall forecasts up to 48 hours ahead.
The status of a measurement or forecast point indicates the condition of the water system, with a distinction being made between normal, pre-alert, alert and alarm. Each status is assigned a colour (green, yellow, orange, red). The thresholds for transition to another status were pre-defined for measurement points and forecast points, on the navigable and non-navigable watercourses, and for the various themes (flooding, tides, rainfall, drought). For the theme 'flooding' the meanings of the terms are given below:
||Heightened vigilance, no flooding
||Particularly heightened vigilance, non-critical flooding possible
||Maximum vigilance, critical flooding possible
On the graphs of the forecasts, the abbreviation 'STV' stands for the start time of the forecast (starttijd van de voorspelling). This is the time at which the computer model started computing, using the most recent measurements and rainfall forecasts available at that time.