This is very helpful for understanding the vaulted vs. slabbed sections of the drain channel, which I had not grasped before (the latter is what is seen in the now-open section in Silwan). So, it seems the Herodian vaulted section is relatively short — a “bypass” around the SW corner of the Temple Mount — and that most of the length of the drain is actually Hasmonean, yes?
I though that my reply may be interesting to others as well.
That’s right, Tom. The Hasmonean section continued to the south, but also to the north of the Herodian “by-pass”. There is, however, another more ancient drain some 20 meters to the east of the present southwest corner of the Temple Mount. On the map below I plotted the two drains together with the respective southwest corners of three phases in the development of the Temple Mount.
The early (possibly First Temple period) eastern drain follows the Tyropoeon Valley and passes the southwest corner of the square Temple Mount (yellow) to the west. This drain appears to be contemporary with the square Temple Mount, which dates from the time of Hezekiah.
The southwest corner of the Hasmonean extension of the Temple Mount (orange) cut this early drain and a new drain with flat covering stones was constructed to the west of this corner. We need to remember that drains usually ran below streets and therefore these drains are important indicators of the city plan in the different periods.
The Herodian southwest corner (green) cut this Hasmonean drain and a vaulted “by-pass” was created to connect the two severed sections of the Hasmonean drain. This by-pass was constructed while Robinson’s Arch was built, for two arch stones fell on the open drain and were left there by the builders.
The vaults were built up against and slightly over the arch stones, which were then covered up quickly, I suppose, before the inspectors discovered what had happened!
Here is a modern picture with the same fallen arch stone in the background.