The good kind of backwater valve requires a drop from inlet to output. In one case it is 3/4 inch. There may be others with a little less. It may be too late to put one of those in now with everything already in concrete.
Regarding the notch on your flapper, I don't know. You might try contacting the manufacturer and ask the purpose of the notch. I would be interesting in hearing the answer.
The usual failure mode on a flapper valve is failure to seal after time. Your primary failure was failure to open.
Regarding the notch on your flapper, I don't know. You might try contacting the manufacturer and ask the purpose of the notch. I would be interesting in hearing the answer.
The usual failure mode on a flapper valve is failure to seal after time. Your primary failure was failure to open.