this depends on the case. fill areas, not advisable. dense to very dense, admissible. Very stiff to hard plastic materials, admissible. The case depends on the flow type and shear stress.
it has no single absolute solution. 45 degrees... stable should stability concerns are questioned.
scour on the other hand should be addressed. shear stress is high, can be addressed by grout type. i.e. i approximate the range of shear stress at 250-300KPa (this is high and theoretically dangerous at a state of supercritical flow). however, 3000psi grout may be utilized. 3000psi has a tensile strength of 8-15% of compressive strength. making it a basis for the shear strength assuming a tresca model, the material has around 2.5-5 times the shear stress, therefore admissible.
Detailing is another matter, structural aspect may provide a minimum reinforcement to maintain the geometry and may utilize a gabion mattress.
im sure, as the case may be that the flow is intermittent. furthermore, not all areas are dry. so use a mixed approach when necessary. gabions when applicable, gabion mattress when applicable, grouted riprap when applicable.
in the end, its easier to design utilizing sheet piles and reinforced concrete for protection. however, it may also be easier to widen the river if the value of the land is low.
as i have said.. there is no fixed solution.