Catabrosa aquatica; the Whorl-grass is a plant of moist and wet grounds - often inundated - like pond margins. hence the Latin name "aquatica". It prefers nutrient-rich habitat's. It spreads horizontally with the inflorescence stems growing somewhat upwards. I find the rather short but wide leaves in combination with the broad leaf-sheaths, quite characteristic. It should also be noted that the spikelet's normally have only one single flower.
It is a member of the Grass family (Graminae, also known as Poaceae).