If you absolutely cannot find the Das PDF, do not panic. Other standard texts are more widely available in PDF form legally (older editions out of copyright or open access):
Limitations and considerations
In the realm of civil engineering and hydraulic engineering, few subjects are as critical—or as challenging—as . Whether you are designing irrigation canals, sewer systems, or stormwater drainage networks, understanding the principles of fluid motion under a free surface is non-negotiable. Among the pantheon of textbooks available on the subject, one name frequently surfaces in academic discussions and university reading lists: Dr. Madan Mohan Das . Open Channel Flow Madan Mohan Das Pdf
Known for its "easy to understand" language and concise explanations. If you absolutely cannot find the Das PDF, do not panic