are they frogs or toads? is this a septic/leach or public? try pouring a box of rock salt down the toilet. water frogs aren't big on salt. chances are they are living in your drainage system full time but only come up during rain. if you run some salt through there about once a month it will make the environment undesirable for them and they'll look for another place to live. It's not toxic so it won't hurt the frogs or anything else that is probably living there as well. more than likely you have breaks in your piping caused by either age or tree roots. the salt will help keep the tree roots from moving in there in search of water and doing more damage.