Where I can find spatial dataset about Flash floods with a set of parameters highly contributing to flash floods occurrence such as precipitation and DEM ?
Perhaps you can add a geotag to your question or update it with details like continent, country, or region.
For data collected in the US, you can download river and waterbody related information from the US Geological Survey (USGS). Look at https://waterdata.usgs.gov/nwis/uv. This will give you data from various levels (river/creek, lake, watershed etc.) and many details - flow rates (discharge levels, gage levels etc.), water quality (O2 levels, N levels, Ammonia etc.), physical properties (evaporation rates, temperature, wind etc.) and others. You choose what parameters you'd want to model and download the data. In the past, I have downloaded data as CSV files and aggregated them in a database for further processing and ingesting into predictive models.
You can get historical US precipitation data from The National Oceanic and Atmospheric Administration (NOAA). See https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.ncdc:C00313 for download options. As these two organizations may or may not share the same sensor or group of sensors, you'll have to find an optimal blending model. Do you use a gridded dataset (large number of points) or a watershed average model? These questions will drive how you map discharge information to precipitation models.
Also, note that the links I share here are for generic discharge and precipitation measures. https://www.usgs.gov/mission-areas/water-resources/science/usgs-flood-information will have more information of floods and flash floods.