Simply converting the eastern portion of the SE Freeway to a boulevard increases vehicular-pedestrian conflict, and is totally unnecessary, as evident from the cross sections in the Middle Anacostia Transportation Study, seen here [warning- PDF] at page 35 "Boulevard Typical Section #2 - Looking East towards Barney Circle".
Evidently the topography favors covering over the existing freeway, with the new boulevard atop; not shown is the idea of covering the RR nor extending the cover towards the Anacostia River with a terrace/staircase, as already done at the Lincoln Memorial. As this involves covering the existing freeway -- IOW no new depressing -- this is a comparatively cheap freeway under-grounding, unlike say this freeway to the west between the 11th Street and 1th Street Bridges.
The idea of covering the segment of this freeway between 14th and 15th Streets SE appeared in this late 1960s illustration seen here.
Continuing this cover westerly requires removing the elevated approaches to the 11th Street Bridges, something that I propose with a replacement with below grade approaches that would conflict with the existing heavy RR, though compatible if converted to vehicular traffic or lighter rail which can tolerate grade changes upwards of 5% whereas heavy RR can only tolerate a 1% grade. As with the highway, this railroad right of way is coverable.
Continuing this cover easterly to Barney Circle would be feasible with allowances for the ramps.
Likewise, the topography allows such a relatively easy under-grounding for this freeway's needed extension to RFK Stadium- East Capitol Street, and is fully compatible with planning for a slow speed waterfront boulevard atop, including the idea of a traffic circle at Massachusetts Avenue SE. Here to the railroad should also be covered.
Only those promoting a mindless ideology against urban freeways -- alas those traditionally in control of the governments -- would rather place all of this vehicular traffic upon the surface waterfront boulevard.