I have been working on a hybrid dimensional model using the mixed FEM formulation, in which 3D elements and 2D elements are combined by certain relationships between the degrees of freedom (DOFs) between face flux for 3D elements and pressure DOFs for 2D elements. I'm doing this for Raviart Thomas elements of various orders, i.e., RT0-RT1 etc. However, I'm struggling to find the 1D equivalent of the RT elements. I have not found any reference about 1D mixed formulation elements, let alone 1D RT elements.
I understand that the lowest 1D mixed element would be quite simple: 1 DOF for the constant pressure and 2 DOFs for the velocity field, one for each end node of the element. But what about higher order elements? Pressure DOFs will surely be point evaluations along the interval, but how can the DOFs for the velocity be defined?
Any help or hint will be greatly appreciated.