There are questions on how much PRE-applied herbicide is left in the field after a foot plus of rain. The short answer to the question is little to none.

A more detailed answer is that it can vary some by herbicide. More water-soluble herbicides like atrazine are long gone.  Others that are broken down with water like dicamba or fomesafen also are no longer present in the field. Herbicides that are less water soluble like Dual Magnum, Valor, Warrant or Leadoff will not readily leach or breakdown with water. However, when the soil particles those herbicides are attached to erode out of the field then of course they will not help with weed control.  The same result will be seen when silt and/or sand washes into a field and buries those herbicides too deeply to provide control.

The other consideration is what effect on the weed seed bank will the flooding have? All weed seeds will float.  Many weeds like Palmer amaranth evolved to fit well into the environmental niche of flood plains where the seed floats in and the subsequent plants grow exceptionally fast to outcompete all other plants. As such, expect an increase in weed pressure in these fields that have flooded with various species of weeds but particularly pigweed, cocklebur and many grass weeds.

Another application of herbicide that will provide residual control of those weeds is warranted.  Keep in mind that all herbicides have season maximum use rates. Those max use rates vary by herbicide so please refer to the label.  In most cases, another application will still be on label. An example would be Dual Magnum which has a max-season use rate of 2.67 pts/Acre.   Most use a pint per acre early so another pint can be applied and be on label.