This paper describes a new approach for the development of data-intensive web applications that depend on non-trivial data manipulation. E-Commerce web sites, on-line auction systems and large enterprise web portals fall into this category as they require comprehensive data access, data processing and data manipulation capabilities. However, existing methodologies mainly concentrate on modeling content, navigation and presentation aspects of read-only web sites. Approaches that consider modeling data operations incorporate them into existing models resulting in a less clear design. We argue that existing models are not sufficient to express complex operations that access or modify web application content. Therefore, we propose an additional Operation Model defining operations for data-intensive web applications. We also propose the utilization of a web application generator to create an Operation Layer based on this Operation Model.