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Lake and river floodplain tutorial

If a river or lake may flood over its borders and causes a large area of standing water beside the normal area of the river/lake this may be simulated with the HYPE floodplain functionality. A HYPE floodplain can be adjacent to a main river of a subbasin or to an outlet lake. Thus local streams and internal lakes is not allowed to have a floodplain. The floodplain and its water body is one slc-class, and the class area is the maximum extent of the floodplain. Simply put, a water body with high water level floods a surrounding area. The area flooded increases linearly with water level up to the maximum extent. Water levels (and thresholds) of the water body and the floodplain determine if water flows to the floodplain or back from the floodplain to the water body. Read more about how the floodplains work in the model description section Floodplains.

The modelled floodplains are described by information given in the FloodData.txt file and by parameters in the par.txt file.

The input data values needed can be estimated in different ways and also be calibrated in HYPE. Here we describe one method to determine input data. For each subbasin that have a floodplain repeat these steps:

  1. Determine how much of the floodplain area in the subbasin that shall belong to the main river and to the outlet lake (if any). Calculate the slc-fractions for main river and olake classes accordingly (e.g. floodplain area of the river + river area together make up the area of the main river class). You may have to reduce the other classes’ slc-fractions so that the sum over the subbasin is still 1. Put the new slc-fractions in GeoData.txt.
  2. Calculate the fraction of the main river and olake classes area that is floodplain. These fractions are entered into FloodData.txt variables fpfmr and fpfol. The rest of the classes' area is river or lake surface area.
  3. The slope of the floodplain areas is determined from elevation data. The floodplain is approximated with a linear relation to area (see Figure 8 in model description). The difference between the lowest elevation and the highest elevation of the floodplain (approximated as linear) are set as water level at maximum areal extent. These are entered into FloodData.txt variables fymmr and fymol.
  4. The threshold for flow from water body to the floodplain is given a value. For an outlet lake this should be normally larger than the lake depth (if outflow occurs before the lake floods over to the floodplain). For a river the threshold can start with a value of zero if nothing better is known. These values are set to the FloodData.txt variables flmrr and floll.
  5. The threshold for flow from floodplain to water body is given a value. If nothing better is known start with zero. This is set to the FloodData.txt variables flmrp and flolp.
  6. The water levels of the river/lake and floodplain will try to reach equilibrium. The speed of equalization is determined by recession coefficients that say how far to equilibrium the water level will reach on one time step. There are separate recession coefficients for flow to and from the floodplain and for main river and outlet lake, in total 4 possible parameters (rcrfp, rcfpr, rclfp, rcfpl). The recession coefficients must be between zero and one. A recession coefficient of one gives a fast response.
start/hype_tutorials/floodplain_tutorial.txt · Last modified: 2017/07/03 16:17 by cpers