The culvert measures 18 feet across (5.5m) and the water was 5 feet deep (1.5m) at the center.

As a rough estimate, we figured 18 feet width x 4 feet average depth x 10 feet per second = 720 cfs (20.5 m³/second), pretty amazing for our small watershed of about 4.5 square miles (11.7km²).