WWW.WATERPOWERMAGAZINE.COM FEBRUARY 2010 43
FLOOD MANAGEMENT
flood risk from the Burn of Mosset at the 1 in 100 year return period.
Without investment in flood alleviation the flood damages in Forres
would exceed £43M (US$70M) over the next 50 years.
Moray flood alleviation was established in 2001 for the purpose
of delivering flood alleviation to a number of communities in Moray,
including Forres. It is an integrated team comprising The Moray
Council, Royal Haskoning, Morrison Construction and EC Harris.
The construction engineer under the Reservoirs Act for this scheme
was Ian Gowans.
THE SOLUTION
The Forres (Burn of Mosset) flood alleviation scheme comprises a flood
storage reservoir, situated on the Burn of Mosset at Chapelton, which
lies some 1.5km south of the town, combined with some minor flood
defences within the town of Forres and two small earthfill dams on the
periphery of the reservoir/storage area to avoid flooding of other prop-
erties. It is estimated to provide a standard of protection of at least 1 in
100 years, including an allowance for climate change up to 2080.
Work commenced on the scheme in 2002, with construction taking
place between November 2007 and November 2009.The flood stor-
age reservoir comprises:
U Earthfill embankment dam with a maximum height of 6.9m and a
crest length of about 200m.
U A 160m wide spillway, the upper part surfaced with open stone asphalt
and a permeable concrete slab construction at the toe of the spillway.
U A stilling basin, with the surface protected by two layers of
rock armour.
U A flow control structure incorporating a baffled crump weir, which
discharges up to 8.5m
3
/sec together with two penstock controlled
fixed orifices, for maintenance purposes (both closed under normal
operating conditions).
U A 3.8Mm
3
capacity flood storage reservoir.
Flows in excess of the capacity of the baffled crump weir are stored in the
reservoir until the inflow rate reduces and the retained water level reduces.
The spillway will allow the more extreme floods to be safely discharged
when the reservoir is full. The spillway is designed to pass the extreme
flood events required for this category A reservoir (PMF summer inflow
= 313m
3
/sec, 1 in 10,000 inflow = 163m
3
/sec) without damage.
BAFFLED CRUMP WEIR
The scheme was required to operate without user intervention, without
power and allow passage of migratory fish. A range of discharge control
mechanisms were reviewed during design, including a simple orifice, a
flume, and other control systems. However, to achieve a discharge of no
more than 8.5m
3
/sec, and to avoid flooding downstream over the range
of heads, it was decided to develop a baffled crump weir (BCW) device.
This would be along the lines of that described in a paper at the 13th
British Dams Society Biennial Conference in 2004, by Ackers et al [1].
The concept and optimised arrangement is diagrammatically illus-
trated in Figure 2. There are three main flow regimes: normal (low
flow), upstream baffle control and downstream baffle control. Physical
modelling [2] was used to determine the optimised geometry of the
baffled crump weir and the associated stage v discharge relationship.
The model tests also allowed the observation and measurement of flow
conditions in the culvert downstream of the control structure itself and to
assess the need for energy dissipation/tail water control measures. The per-
formance of the bypass sluices, which may be used if the control structure
were to block, or be shut off for maintenance purposes, was also tested.
These are controlled penstocks which remain closed under normal operat-
ing conditions. Should a blockage occur to the BCW during a flood event,
then one or other of the bypass controls may be operated.
Measurements and observations relating to the performance of the
crump weir for assessing the ability of migratory fish to pass over the
weir were also made.
The optimised baffle arrangement, in the 1m wide channel, was
identified from the tests by achieving an acceptable stage/discharge
relationship. It consists of two vertical baffles, the downstream baffle
having an angled lip on its lower edge to ensure clean separation of
the flow from the structure. This is the first baffled crump weir flow
control structure constructed in a UK dam.
THE SPILLWAY
The spillway has a long curved crest that fits in well with the local topog-
raphy of the valley. This allows the PMF to discharge over the dam whilst
minimising the flood surcharge. During such events, water velocities on
the spillway will reach up to about 7m/sec for a period of up to 30 hrs.
It was deemed highly desirable to maintain the ‘green’ appearance
of the dam to blend in with the surrounding landscape. Therefore, the
design of the spillway and stilling basin combines hard engineering
beneath an aesthetically pleasing grass covering.
A number of options to reinforce the face of the spillway and still-
ing basin were considered. Open stone asphalt was considered the
best option for the upper slope of the spillway because it is relatively
flexible, accommodating minor settlement of the earth fill and can
easily be vegetated.
The factor of safety against sliding between the earth fill and the
spillway construction materials was critical. The most important fac-
tors being: the slope angle, the flow depth/velocity and the angle of
internal friction. In the final design the earth fill surface was benched
and covered with a needle punch geotextile. A 600mm layer of coarse
gravel was placed on the benched profile, covered with a 400mm
layer of open stone asphalt and topsoil [3].
A robust solution was required for the lower slope of the spillway
because of the higher water velocities and turbulence associated with
the hydraulic jump. A system involving a permeable concrete solu-
tion, with a ‘voided’ concrete slab on the surface was developed for
this area. This was constructed first along the toe and up the lower
part of the spillway slope. Then two layers of rock armour, infilled
with sand and gravel together with top soiling, was utilised to armour
the remainder of the horizontal and sloping areas around the periph-
ery of the stilling basin.
Open stone asphalt was utilised on the upper spillway slope. This is
the first large-scale use of open stone asphalt on a UK dam spillway.
Open stone asphalt is an inert material that does not deleteriously
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Discharge (m^3/s)
Reservoir level above crump weir crest (m)
Max design discharge rate 8.5 cumecs
Dam spillway crest level +30.7m ODN
Transition from upstream to downstream baffle control
Transition from crump weir to upstream control
Best fit line through test data
Upstream baffle control
Downstream baffle control
Void concrete slab with topsoil infill Grass
No-fines
concrete
“Normal”
concrete
Above, top: Figure 3 – Baffled crump weir stage vs discharge relationship
Above, bottom: Figure 4 – Construction of permeable concrete slab to spillway