I have water levels during flood at the center line of the river (100 km long) at every 100 m. I also have the DEM of the area in which the river flows. How to generate the flood inundation extent of the river in the area in which it passes? I know it can be done manually by locating the contour line corresponding to each of the point on the DEM. However, this appears to be quite cumbersome exercise. Can you suggest an alternative?
I have seen this done and in general the answer will depend on flow rates and on volume. But it's not work I've done. I suspect if you drop a note to ESRI folk and ask one of their analysts there they can provide a more precise answer. ESRI has always been very good about sharing knowledge on specific problems. If you're using QGIS their very active member database and info' board could also be hlepful.
Just a couple of comments. DEMs based on most normal topographic maps are not that accurate in elevation, and if river is covered by heavy canopy or braided channel that shifts frequently, not that reliable. DEMs developed from LiDAR are better at georeferencing channel(s) location and often improve ability to estimate elevation and have improved topographic contours into less than a meter Contour interval perhaps. I would ask how were the water levels determined in the center of river in flood stage. If coastal river, tidal stage during flood could also have a bearing. A lot will depend on the accuracy and precision of the various levels of water and land. GIS can absolutely make a pretty map, but the contours associated with river crossings and depth of the channel are seldom that great. If the river has a functional floodplain with abrupt slope defining, determining the lateral extent is less problemmatic. If and how rivers flood depend a lot on the stream type and channel dimensions. A flat area on a GIS map adjacent to a stream could be a floodplain or a terrace (abandoned floodplain). The upper extent of the point bars are usually at the elevation of a floodplain, but if the point bars are up against a steep river bank, it probably a terrace.
If you have the water levels at the centerline, you may create a plane surface according to the heights as a raster. This raster you may "intersect" with your DEM, that is you can use map algebra to calculate the difference of the two rasters. The areas with negative values will be inundated, the other note, You may convert the areas with negative values back to vector polygons.
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