Designing optimal observation networks in coastal oceans remains one of the major challenges towards the implementation of future efficient Integrated Ocean Observing Systems to monitor the coastal environment. In the Bay of Biscay and the English Channel, the diversity of involved processes (e.g. tidally-driven circulation, plume dynamics) requires to adapt observing systems to the specific targeted environments. Also important is the requirement for those systems to sustain coastal applications. Two observational network design experiments have been implemented for the spring season in two regions: the Loire River plume (northern part of the Bay of Biscay) and the Western English Channel. The method used to perform these experiments is based on the ArM (Array Modes) formalism using an ensemble-based approach without data assimilation. The first experiment in the Loire River plume aims to explore different possible glider endurance lines combined with a fixed mooring to monitor temperature and salinity. Main results show an expected improvement when combining glider and mooring observations. The experiment also highlights that the chosen transect (along-shore and North-South, cross-shore) does not significantly impact the efficiency of the network. Nevertheless, the classification from the method results in slightly better performances for along-shore and North-South sections. In the Western English Channel, a tidally-driven circulation system, added value of using a glider below FerryBox temperature and salinity measurements has been assessed. FerryBox systems are characterised by a high frequency sampling rate crossing the region 2 to 3 times a day. This efficient sampling, as well as the specific vertical hydrological structure (which is homogeneous in many sub-regions of the domain), explains the fact that the added value of an associated glider transect is not significant. These experiments combining existing and future observing systems, as well as numerical ensemble simulations, highlight the key issue of monitoring the whole water column in and close to river plumes (using gliders for example) and the efficiency of the surface high frequency sampling from FerryBoxes in macrotidal regions.