1. Devils Culvert
1.1. Option A: Concrete Box with Spillway and Stilling basin:
(PREFFERED OPTION)
Reinforced concrete box structure with spillway is designed to accommodate the stream flow
through the rapid change in elevation. The flow is channeled over the spillway and into the stilling
basin 3m below. The stilling basin allows for energy dissipation of the water, with the result, minimal
erosion downstream.
Upstream, a 30 m stretch of stream-bed is provided with grouted rip-rap to consolidate the river bed
and minimizing upstream erosion. Two concrete V-shaped drain ditches are provided on either side
of the concrete box to catch road run-off and channels that into the stilling pool.
Pro’s

The concrete box structure protects the
embankments from 3 sides, (sides of the
streamflow, and 2 sides from the drains.
minimizing erosion.

Concrete box structure is more
inherently stable that the retaining walls
of option 2.

Stilling basin dissipates energy from the
stream. The resultant lower energy
downstream will not erode the surface
bed, and undercutting will be minimized.

The grouted rip-rap located upstream,
channels the water over the spillway

Concrete works are minimized to a single
location
Cons

One large drop into stilling pool to
dissipate the energy
1.2. Option B: Series of Stilling drops
A series of concrete “rapids” that dissipates the energy in smaller steps. The “rapids” are formed by
concrete retaining walls incorporating a spillway which directs the streamflow into a stilling basin at
each step. Upstream of each retaining wall is a streambed of grouted rip-rap which directs the
streamflow into the spillway. Energy dissipation of the streamflow is achieved in smaller steps at
each rapid.
Pro’s

Smaller concrete structures are created,
but a larger number of them. (3 No off)
Cons

Retaining walls of option 2 is not as
stable as the concrete box structure as in
option 1.

The concrete works are spread over a
larger area

Each Concrete retaining wall will be
required to be tied back into each
embankment. Greater disturbance of the
stream.
2. Crossing of Stream with sewer pipe
2.1. Option A: Gravity flow sewer system, with Stream crossing by a pipe bridge(PREFERRED
OPTION)
The sewer system is a gravity flow system. To achieve a gravity flow, the sewer pipe route is as
shown on the attached sketch. The crossing of the stream is achieved by a pipe bridge.
Pro’s





Cons
A gravity flow sewer system is provided.
This system is safer, self-cleaning, and
more reliable. As no part of the system
is under any pressure, bursting of pipes
and associated sewerage spillages are
minimized
The gravity solution presents a more
economical solution, as pumps and
associated infrastructure are not
required. The long-term cost associated
with operations and maintenance are
minimized.
The solution is “environmentally
friendly” - saving of electricity from
carbon sources, as pumps are not
required.
Existing infrastructure is ideally located
upstream to convey the sewerage away.
No modification to existing upstream
works required.
Minimum interference in the water
course

a pipe bridge is required to cross the
stream

A longer pipe route is required
2.2. Option B: Routing of sewer pipe along existing road alignment
The sewer pipe alignment is taken along the existing road alignment to cross the stream using the
existing road culvert. Although this option was considered, this was not a feasible option. A
longitudinal section along the pipe route shows that the lowest point of the route is at the culvert
crossing the stream . A gravity sewerage system is not feasible with this route, as a column of
sewerage will be trapped at the lowest point allowing settlement of solids, and with time, the line
would block up.
If a pumped system is then coupled to this option, the option becomes feasible and the advantages
and disadvantages discussed hereunder.
Pro’s

Cons
a pipe bridge is not required to cross the
stream . no interference in the water
course.

Pumped system is prone to operational
and maintenance problems., high
pressures, and potential burst pipes.

Higher cost of pumps, associated
infrastructure and operations

Not environmentally friendly, as
electricity is used in the process
Some modification to the existing
upstream works is equired

3. Access Road Crossing Stream
The water treatment plant is located adjacent the farm dam. In order to access the water
treatment plant, for operations, maintenance etc, an access road is required to the plant. The
access road will have to cross a watercourse.
3.1. Option A: Access road with culvert across waterway (PREFERRED OPTION)
Paved road from existing road network, crossing a watercourse with a multiple barrel box culverts
leading to the water treatment plant.
Pro’s

Cons
Length of the route optimized, as the
route is directly from the road network
to the water treatment plant. A Longer

The route entails a stream crossing, with
a multiple barrel culvert.
road going up and around the
watercourse will lead to greater land
disturbance n sensitive areas. Footprint
is significantly reduced.

Multiple barrel culvert is an efficient
manner to cross a watercourse. will be
designed for 1:100 year storm event.

Aesthetically positive. The final structure
will be mostly at ground level and below.
This will not intrude on the natural
surrounding landscape.
3.2. Option B: Routing of road to go around the watercourse
The road route entails going significantly upstream along the watercourse and then going through a
series of smaller watercourses .
Pro’s

Cons
Crossings of the streams is done higher
up, small crossings are required, as the
streams are much smaller.

Large disturbed footprint. The road
route will traverse sensitive mountain
slopes, and a wetland. This will make the
land susceptible to erosion and
degradation of the wetland.

Longer road route. Larger disturbed
footprint.

Crossing of may streams.

Higher costs due to longer route