Complex sentence: The terrain of the area is generally plain with an average altitude of about 5 m above mean sea level, receiving an annual rainfall of 1300 mm mostly during the northeast monsoon (September--November) and partially during the southwest monsoon (June--August).
Split into simpler sentences: The terrain of the area is generally plain with an average altitude of about above mean sea level. It receives an annual rainfall of, mostly during the northeast monsoon (September--November), but also during the southwest monsoon (June--August).