News letter Dam edition

Ingeokring Newsletter
scribed as a fertile land. The land was cultivated by controlling
floodwater in the ‘wadis’, and had fertile gardens and
fields with fruits and spices.
The ancient dam was not intended to create a reservoir that
could be drawn all over the year round. The main object was
flood control, i.e. to catch and divert the unpredictable
floods resulting from rainfall in the mountainous catchment
area and secondly to raise the water to the level of the agricultural
land at both sides of the wadi known as the two
gardens. Raising this water level made it possible to distribute
the water over distant fields, so extending the irrigated
agricultural area.
Dam construction and irrigation scheme
Prior to the construction of the ancient Great Marib Dam, a
smaller diversion dam existed in the Wadi As Sudd. The remnants
are located approx. 300 m to the east of the ancient
Great Marib Dam in the centre of the valley. It is estimated
that this dam was built 1 000-2 000 years before the Great
Marib Dam. It was 55 m long and 30 m wide, built with
stones as big as 2 m which were cemented with lead. However,
the attempts to maintain this structure for irrigation
were not successful due to the accumulation of sediment
and deposits.
The initial Great Marib Dam was built around 750 B.C. The
dam was a simple earth structure, 580 m in length and is
estimated to have been 4 m high. It ran straight across the
wadi between high rocks on the southern side to a rock
shelf on the northern bank belonging to the Jabal al Balaq
mountains. The dam was built slightly downstream of the
narrowest point in the Wadi Dhana to allow space for a natural
spillway and sluices between the northern end of the
dam and the high rocky cliff. The initial storage capacity was
about 55 million m 3 .
Around 500 B.C. the dam was heightened. The second structure
was a 7 m high earth dam. The cross-section was triangular
with both faces sloping at 45 degrees. The water face
(i.e. the upstream slope) was covered with stones set in mortar
to make the dam watertight and to resist the erosive
effects of waves. The final form of the Marib dam was
reached after the end of the rule of the Kingdom of Sheba.
From 115 B.C. onwards the ruling people in Southern Arabia
were the Himyarites and the next major reconstruction appears
to be a Himyarite work. This reconstruction led to a
new 14 m dam with elaborate water works at both ends.
Like any irrigation scheme, Marib faced an ever increasing
threat from silt. The seasonal floods brought each year
about 2.5 million m 3 of silt which was either held behind the
dam or spread over the irrigated land. As a result, the level
of the irrigated land rose about 1 cm a year or about 1 m
each century. The great dam functioned for about a thousand
years, during which time the level of the two gardens
must have risen by about 10 metres. Upstream of the dam a
mass of sediment piled up as well. A considerable amount is
nowadays still found at the northern sluice system. The Sabaeans
and their successors raised the levels of the sluice
gates, north and south, they built up the overflow weir on
the north sluice and they heightened the main wall, but it
was a struggle they could not win. It is estimated that the
fields could be flooded to a depth of 60 cm and the total
area irrigated amounted to around 9600 hectares. The total
population was probably between 30 000 and 50 000. The
fields were situated at a short distance from the two sluices.
At the north side the fields extended at a distance of 3 km
from the wadi. The pattern of those fields was still clearly
visible on the aerial photographs of the area taken in 1974.
Southern sluice system
The southern sluice system was known as ‘Marbat el-Dimm’.
It is a smaller and less complicated structure than the north
but was in much better condition in 1987. The spillway was
an overfall located about 7 metres below the top of the
dam, the spillway width was 3.5 m.
Northern sluice system
The northern system of sluices included an overfall spillway
and a channel outlet. The channel outlet was located between
the spillway and the earth dam, and it contained one
great wall over 140 m long, about 9 m thick and from 5 to 9
m high. This system conducted water from the northern end
of the dam through a single 1 000 m long and 12 m wide
canal to a rectangular structure that divided the water entering
into twelve different streams.
At the dam, the discharge flowing into the canal was controlled
by two gates. The overfall gates were 2.5 m wide, and
the maximum gate openings were 1 m and 4 m below the
dam crest. In the initial section of the canal, the canal walls
widened to form a basin of about 23 m wide and 65 m long
(essentially the length of the northern wall) which acted as a
settling basin for heavier material carried by the wadi. The
1000 m long canal had earth walls that were covered with a
cemented stone lining on the inside (i.e. water side). The
cross-sections of the canal and of the settling basin were
approximately rectangular.
The spillway was a 40 m wide overfall with a broad unlined
rock crest. The crest elevation of the overflow spillway was
located 3 m below the height of the dam and the spillway
was not gated.
Dam edition | Double Issue 2007/2008 | 70