I guess the first question is how are you testing for residual chlorine?  There could be inherent, slight positive interferences that have emerged in the light of your recent heavy stormwater.  One obvious one is mentioned in your post, suspended solids.  That is mostly going to be a factor when doing a colorimetric determination.

Another thing to consider is that fact that most chlorine tests aren't actually testing specifically for chlorine.  Rather, they test for compounds that behave like chlorine does, specifically its oxidizing properties.  Witness the fact that potassium permanganate can be used as a standard for your residual chlorine test.  This is not because it is naturally the pink color of the DPD result, but because it reacts like chlorine does with the DPD reagents.

Is there any particular reason you have a limit of 0.05 mg/L chlorine as your limit?  Standard Methods does not specify a lower limit of chlorine.  The only guidance it gives is in how to neutralize the chlorine detected.  Since you neutralize to the removal of color from the starch-iodine reaction, that has always been the detection method I utilized for the presence of absence of chlorine when doing BOD.

I don't have a hard and fast number for what qualifies as needing treatment.  We always would check for oxidizers in our BOD samples by slightly acidifying the sample, adding the potassium iodide solution, and then adding a few drops of starch indicator.  If there was any blue color we would titrate with sodium sulfite to remove the blue and add that proportionately to the sample to be incubated.

If we have any chlorine present we must dechlor.  We check by the method stipulated in SM 5210 B.  Any color presents and we remove it with the sulphite solution and then proportionate to remainder of sample volume.

Chlorine will reduce the number of bugs in your samples and give you a lower result which is a false value.  

~Eric