Dhaka flooding: Why are there 'rivers of blood' at Eid?

One particular part of Dhaka, Shantinagar, seemed to be the worst effected

As Muslims around the world celebrate Eid al-Adha, people in Dhaka have been sharing pictures showing streets flooded with blood-stained water.
The blood comes from animals sacrificed for the festival, and had mixed with floodwater in parts of the Bangladeshi capital.