Environmental Engineering Reference
In-Depth Information
a shortening of their life, blockage of navigation channels, and even the infilling of harbors
and estuaries.
A lowland stream choked with debris is illustrated in
Figure 8.8.
The origin of the debris
is slope failures along a new highway some 20 km upslope in the Andes Mountains of
Ecuador. Prior to highway construction, the stream was essentially pristine and was a
tourist location. Flooding eventually destroyed most of the homes in the adjacent town.
The gabion wall was constructed when debris deposition began but it eventually failed
during floods.
Protection and Prevention
River banks
and
channels
are provided with protection by retaining structures, concrete lin-
ings, or riprap. The river in
Figure 8.7,
located on the lower slopes of the Andes Mountains
of Peru, frequently carries high-velocity flows causing substantial bank erosion.
Unreinforced, and unanchored concrete gravity walls failed by undermining all along this
section of river. The solution proposed was to cut the slope to about 26
, place a geosyn-
thetic layer, and relocate the roadway. Either large concrete blocks weighing 2 tons each or
large rock blocks were to be placed over the geotextile to armor the slope. Design weight
of armor vs. the maximum stream velocity is discussed by Blake (1975).
Foundations for bridges must be placed at adequate depths to provide protection
against scour, the most frequent cause of bridge failures. A rule of thumb is to place the
foundations at a depth equal to four times the distance between the flood and dry-weather
levels unless hard rock is at a shallower depth (Smith, 1977).
Gullies
on slopes above and below roadways, or other construction, usually require pro-
tection from erosion. Exceptions may be where flow velocities are low and flow is over
strong materials. Traditionally, concrete lined ditches were the solution, but where flows
are high and soils are relatively weak, the concrete lining often breaks up at the joints.
Geosynthetics are becoming a common treatment, such as illustrated in
Figure 8.9.
A low
cost treatment using grain bags soaked in cement that conforms to an irregular surface is
shown in
Figure 8.10.
After the bags are placed the ditch is then brushed with a layer of
cement. Other relatively low-cost treatments are used to reduce flow velocities. Examples
are “split-board check dams” (Gray and Leiser, 1982) or low gabion walls.
Slopes
receive protection by planting fast-growing vegetation and installing surface
drainage control. For example, in excavation of benches for a highway cut slope, when the
first bench is prepared it should be seeded immediately, even before work proceeds to the
next levels. Cutting numerous benches along a slope also retards runoff and washing. In
°
FIGURE 8.8
Slope failures along new highway (
Figure
8.6),
5 km upslope, clogged stream with
debris. Resulting floods during El Nino
destroyed most of the homes in the
adjacent village.
