A forecast point is a location where stage forecasts are shown on the basis of a hydraulic model. For the time being forecast points are only shown for the basins with detailed forecasts (if the forecast point box at the bottom of the map is ticked): the Yser, Dender, Dijle/Zenne and Demer basins.
A hydraulic model includes all the topographical data of a watercourse and its valley as well as the structural works on the watercourse such as arches, bridges, flood control basins, dams, etc. This model is fed with flow series from hydrological models. The hydraulic model calculates the progress of flows in the watercourse and in the valley, as well as the accompanying stages and stream velocities, with which the hydraulic model can generate flood maps.
Flanders comprises 11 hydrographic basins or flow areas. From (approximately) West to East these are the basins of the Yser, the Leie, the Bruges Polders, the upper Scheldt, the Ghent Canals, the Dender, the lower Scheldt, the Dijle and Zenne, the Nete, the Demer and the Meuse.
A hydrologic model or precipitation drainage model is a model that describes a relationship between precipitation and evapotranspiration in a basin on the one hand and the flow to the watercourse on the other hand. The hydrologic model is evaluated against a measured flow series and is subsequently used to convert a precipitation series into flow series.
A mode is a code that indicates the status of the forecasting system. The three modes are basic mode, alert mode and alarm mode. If the system is in alert mode, this means that floods will start to occur in non-critical areas. If the system is in alarm mode, critical locations can be flooded. The mode determines the system’s behaviour. In the transition from basic mode to alert or alarm mode, the colours will change to orange or red on the website, and measurement data will also be transmitted more frequently.
OBM is an abbreviation for Operational Basin Model. It is the technical term used by the VMM for the collection of systems that make real-time forecasts of floods possible. They are fed by a large range of measurements and precipitation forecasts and convert these using hydrologic and hydraulic models. OBM-Centrale makes calculations using hydrologic models, OBM-Dender, OBM-Dijle, OBM-IJzer and OBM-Demer make calculations using hydraulic models.
The operator is the person responsible for the evaluation and interpretation of the flood forecasts. Upon each automatic transition to alert or alarm mode in one of the 11 hydrographic basins, the first operator is alerted via text message. They decide whether a flood forecast is of such a nature that emergency services and relevant administrations are to be alerted.
PDM stands for Probability Distributed Moisture model. This is the hydrologic model used in the VMM flood forecasters.
These are precipitation time series measured by precipitation radars. Precipitation radar is based on a radar antenna emitting a pulsating radio signal, which is partially reflected by precipitation, and receiving its echo. From the direction of the antenna and the time elapsed between the emission of the pulse and the reception of the echoes, the position of precipitation areas is derived. The VMM forecasting systems work with a composite image from three radars, one in Zaventem (Belgocontrol), one in Libramont-Wideumont (KMI) and one in Avesnois, Northern France (Météo France).
A pluviograph or rain gauge is a device that measures precipitation. Most rain gauges are equipped with a logger that measures the precipitation distribution over time. The VMM reference pluviographs use weighting scales instead of the tipping bucket system; this is a more expensive, but more accurate measuring method.
Regional precipitation is a precipitation series generated for a specific (measuring) point in a watercourse which approximates as accurately as possible the total precipitation fallen in the area that drains into this point in the watercourse. Regional precipitation is composed in various ways in the flood forecasters (rain gauge measurements, radar images, …), depending on the best available information at any time. Regional precipitation is used in the systems as input for hydrologic models, which convert the precipitation series into a flow series near the measuring point. Regional precipitation is also sometimes referred to as PDM precipitation since the hydrological model applied here is the PDM model.
A return period indicates the probability that a specific event will take place, which is mostly expressed in years. The probability that an event will take place with a return period of 5 years is twice the probability of an event occurring with a return period of 10 years.
The specific flow is calculated by dividing the flow at a specific location by the upstream surface area flowing to this location. It is most often expressed in l/s/ha.
An SRM zone is a subcatchment for which a specific flow is calculated at the OBM-Centrale for the surface runoff. The specific flow is calculated by means of precipitation forecasts, soil features, land use and the topography in the sub-basin area. The 1,346 SRM zones in Flanders are used to detect possible local flooding.
Mathematical comparison that allows to convert stages into discharges and vice versa. This comparison must be set up separately for each measurement station.
A stage gauge or level recorder is a device that measures and records the water level or stage in a river. By means of rate of flow measurements near the level recorder a stage-flow rating is often set up, a relationship between stage and flow. This stage-flow rating allows the conversion of stage time series into flow time series at the location of the level recorder.
The Belgian national tidal reference is a reference for height measurements in Belgium and refers to height indications above sea level. All height values in the measurements of the waterways and their valleys and all stages are expressed in accordance with this reference system. The use of this reference system is indicated by the notation m TAW.
The waterways in Flanders are subdivided into a number of categories. First there are the navigable waterways and canals run by entities within the Mobility and Public Works (MOW) policy area. Secondly there are the unnavigable waterways, which are subdivided into category 1, 2 and 3. The VMM controls the Flemish category 1 unnavigable waterways, which means unnavigable waterways with a source area of at least 5,000 hectares. Category 2 waterways are run by the Provinces and category 3 waterways are run by the municipalities.
The watertoets is an advisory instrument that judges whether an initiative (construction of a house, development of a street) can have damaging effects on the environment as a result of a change in the status of the surface water, the groundwater or water-dependent nature.
Further information can be found at www.watertoets.be.