The subject of SrAddha is vast like an ocean.
There are two basic forms of SrAddha. One is known as PArvaNam and the other is called as Ekoddishtam.
SrAddha can be broadly classified into three categories nitya, naimittika and kAmya.
There are 12 types of SrAddhas:
(8) Suddhyartha SrAddha
(12) Pushti SrAddha
PratyAbdika SrAddha especially for one's father or mother has to be necessarily performed as anna SrAddha only. No substitute is allowed in the form of Ama SrAddha or HiraNya SrAddha.
In other cases, if anna SrAddha cannot be performed, it can be done as Ama Sraddha under certain conditions.
If Ama SrAddha cannot be performed, it can be done as HiraNya SrAddha under certain conditions.
If HiraNya SrAddha cannot be performed, under extreme conditions beyond one's control, one can give grass or piNda to cow/perform tila tarpaNam/ undertake upavAsa/chant the SrAddha Mantra.
Kindly note the compassion of dharma SAstras towards all under varied circumstances.