I would like to check if my reasoning is correct.
Illegal dumping of refuse into drainage can pollute surface water or cause occasional flooding
Dumping is a gerund - nominal gerundive clause where gerund is preceded by an adjective, functioning as a subject. Flooding is pretty much the same case but it is an object.
The difference between gerunds (verbs) and pure nouns is not easy since I am not a native speaker. The same is true for participles and adjectives (the rule of thumb that adjectives can be graded and intensified does not always work because I am not sure if it sounds natural or not.)