Abstract
Coastal zones and their connected freshwater and marine systems are among the environments most acutely exposed to accelerating climate hazards. European policy has responded by placing Digital Twin (DT) infrastructures at the centre of its climate adaptation and ocean governance agenda: Destination Earth for the Earth system, and the European Digital Twin of the Ocean (EDITO) for the marine and coastal domain. This scoping review defines and evaluates Hydro Digital Twins (HDTs), encompassing Digital Twins of the Ocean (DTOs), coastal-zone DTs, and freshwater-system DTs, as instruments for coastal management, water territory governance, and climate adaptation decision support. Conducted in accordance with PRISMA-ScR, the review screened 231 records (January 2020 to September 2025) and retained 21 studies, each coded against a thirteen-parameter framework organised on a governance axis and a data-centric axis. Governance objectives consistently outweigh technical motivations: risk reduction dominates (7/21 studies, 33%), followed by environmental monitoring (5/21 studies, 24%) and coastal management (4/21 studies, 19%). Remote sensing is used in 19/21 studies (90%) as the near-universal data backbone; artificial intelligence, coded as an analytical method rather than a data source, appears in 10/21 studies (48%) and concentrates in the River/Lakes domain (6/7 studies, 86%), which also holds three of the four predictive outputs; citizen science data, though a stated European policy priority, appears operationally in only 2/21 studies (10%). The modal maturity category is TRL 4–5 (6/21 studies, 29%), only one study reaches TRL 6, and 6/21 studies (29%) report no formal validation. The review clarifies the architectural distinction between HDTs and Web GIS and situates these findings within the European coastal governance agendas of Destination Earth and EDITO. All proportions above are properties of the reviewed corpus and are not extrapolated to the HDT field as a whole.