The Water Supply and Distribution System of the Nabataean City of ...

Charles R. Ortloff
Figure 3. Elevated water channel/piping system on west face of El Khubtha
Mountain.
that collected from rainfall runoff for urban housing
needs and rituals at nearby tomb complexes. While
runoff capture probably supplied the Zurraba reservoir,
local spring sources, including Ain Mousa, were
also important in the early phase of city development
and may have also contributed water to the reservoir.
Although reservoir water could be used to supplement
the Siq channel, Ain Mousa water supplies were later
switched to a piping system and the open channel
was abandoned. Rainfall runoff and spring flow still
enabled the Zurraba reservoir to supplement the Siq
pipeline when required. Although the modern town
has obliterated ancient hydraulic connections to and
from the reservoir, there is no topographical constraint
to a channel path which would have directed reservoir
water into the open channel (or into a later pipeline
system) to provide supplementary water supply. The
Jebel el Khubtha pipeline, by contrast, appears to be
the main outflow path for reservoir water in the area
between Jebel el Khubtha and Wadi Mataha (B;2, C;2).
Surplus water, after filling the cistern, was directed to
the main city fountain (Nymphaeum 42, B;2) through
either a pipeline or subterranean channel. Although no
traces of this connection have been excavated, pipeline
fragments in the area suggest this connection. From
a systems point of view, the Zurraba reservoir served
principally to maintain cistern levels in the royal tomb
area by intermittent water release. The Ain Mousa
spring, on the other hand, provided the continuous
supply for the Nymphaeum through piping supported
in a carved channel through the Siq (Figs. 5a & b) that
replaced the earlier open channel. The Zurraba reser-
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voir thus served as a backup system
for rapid delivery of large volumes
of water at short notice to the Jebel el
Khubtha cisterns and served to augment
the continuous, but declining,
water supply to the Nymphaeum
from the Ain Mousa spring during
dry seasons. The ability to provide
an ‘on-demand’ water supply from
this backup source would have
been most useful to large caravans
entering the city that would place
a sudden demand on water supply
and usage.
Pipeline carrying capacity
considerations: the Zurraba-Jebel
el Khubtha system
While a spring produces a given
volumetric flow rate, the limitation
on how much can be transported by pipeline stems
from its technical characteristics (diameter, internal
roughness, slope and supply head). Piping design
requires the spring output flow rate to match (or
be less than) the theoretical carrying capacity of the
pipeline. Examination of Nabataean pipeline designs
yields insights into their ability to understand internal
pipeline flow phenomena, and to construct solutions
to overcome problems limiting maximum throughput.
For an upper estimate of the volumetric flow rate that
the Jebel el Khubtha pipeline system could sustain,
it is assumed that the angle of about 0.005 shown by
the elevated channel supporting the pipeline (Figs. 1
& 3) corresponds to the critical flow angle (Morris &
Wiggert 1972). The maximum flow height within the
piping is at critical depth equal to perhaps 50 per cent
or more of the pipe diameter. This condition means
that water flows in open channel mode through the
piping, with an airspace above the water surface, and
that the flow rate is maximum for given diameter and
slope. This type of hydrostatically unpressurized flow
reduces leakage between socketed pipeline elements,
while providing the theoretical maximum flow rate for
low angle piping functioning in either open channel or
full flow mode. The maximum flow rate permissible
then would be the critical velocity x piping wetted
cross-sectional area. For a fall in channel height of
about 40 m over the 8 km pipeline path around Jebel
el Khubtha, the theoretical maximum volumetric flow
rate is about 90 m 3 /hr for 20 cm diameter piping. In
practice, owing to the many mortared joints between
the 0.3 m terracotta piping segments (perhaps of