River Engineering
Download ArcGIS 10.8 tutorials:
Changes in in hydraulic conditions are usually taking place quite rapidly in comparison with geological time. The resulting changes in bed topography (bed levels and river width) normally take much longer. In river engineering works the delayed response to changes in hydraulics and morphology must be taken into account. It should be emphasized that in a river system often many transient processes are active at the same time, all of them having been initiated in different periods.
Changes in rivers can be divided into two classes
- Changes in bed-form dimensions
- Steeper bed profiles
- Changes in non-uniform reaches caused by flow variations
- Changes in the river boundary conditions and human activities may result in long-lasting even permanent changes in the river environment.
- They may provide a strictly local improvement which hardly changes the upstream or downstream river reaches; or
- They may be designed to alter completely a river regime, thus affecting the river over almost its entire length
River regulation
- Upstream impact
- Local base-level is raised to a position at which the water surface intersects the original bed
- Maximum rise in height determined by the crest of the dam spillway
o
With
the reduction in transporting ability as a stream enters a reservoir, a depositional
wedge is constructed and the channel gradient locally lowered
- Downstream impact
o Reduction
in the magnitude of flood peaks by as much as 90 per cent
o Marked
decrease in the sediment load, especially in those reaches immediately below
the dam
o Degradation
of the channel bed, typically at rates much higher than in natural rivers, by
sediment-free water
o Since
degradation is usually at a maximum close to the dam and progressively declines
downstream, channel slope tends to become flatter, although the amount of
degradation can vary considerably along a river
River channelization
- Extend often over long stretches of river
- Rivers respond to channelization both within and beyond the modified reach
Straightening
- River is shortened by artificial cut offs, thereby steepening the gradient and increasing flow velocity and transport capacity
- The modified stream attempts to establish a new equilibrium gradient through a combination of upstream progressing degradation and downstream aggradation
Resectioning
- Widening and/or deepening of the river channel to increase its conveying capacity and thereby reduce the incidence of overbank flooding
- Widening has the effect of reducing unit stream power and therefore sediment discharge, so that deposition may occur
- Deepening may increase the susceptibility of the river banks to erosion and may trigger upstream progressing degradation within tributaries
Levee construction
- Channel banks are artificially raised to confine floodwaters
Bank protection
- Use of structures such as gabions and steel piles to control bank erosion
Clearing and snagging
- Removal of obstructions from the watercourse, thereby decreasing resistance and increasing flow velocity
- In general, training structures are primarily designed to:
o
contract
the channel by means of training structure
o preserve
the cut off areas to be utilized for agriculture following their sedimentation
o produce
a new, stable bank at the training structures
- In the unimproved river a balance exists between the energy of water and the resistance
- In the regularized channel, however, the resistances are considerably lower and the river does not require such a slope as it had before to overcome the resistances
- In order to reinstitute the balance between the transporting power and the resisting force the slope must decrease
- Together with the decrease of the slope due to channel deepening, a further purpose of regularization, namely lowering of ground water level, is attained
- Channel deepening can be:
o desired,
taking into account diminution of overflows
o harmful,
taking into account agriculture, if due to the lowering of ground water levels
it causes excessive drying of soils
Longitudinal profile and determination of equalized profile
- River meanders, bed-forms such as the riffle-pool sequence and large-scale dunes cause local, within-reach slope variations
- However, mean reach slopes are averages over this local variability and are mutually adjusted to cross-section and plan-form properties in the medium-term steady-state time scale
- Consecutive reach slopes combine to create the complete long profile, which reflects long-term geological development influenced by tectonic and morphogenetic histories as well as recent channel pattern adjustments
- Rivers tend to develop a concave – upward form of the long profile
- Long profile adjustments are emphasized as being necessary to maintain sediment transport with the available discharge and given channel characteristics
- However, mutual adjustment of slope, plan-form and cross-section properties characterize the true response of alluvial streams to the multivariate environmental controls of run-off, flood magnitude and frequency, sediment yield and size
- The interaction of these controls determines the slope at the reach scale, and their downstream variation determines the spatial adjustments of slopes which create the complete long profile
- The course component of bed material controls channel slope
- Long-profile adjustments include changes of channel bed elevation, channel gradient and overall profile shape
- Bed elevation changes result from aggradation and degradation, which reflect alterations in the river’s transport capacity relative to sediment supply
- Factors affecting changes in stream bed elevation
o Aggradation;
increasing bed elevation
- Upstream control; e.g. glacio-fluvial sediment entering at headwater
- Downstream control; e.g. rising bed level
- Basin-wide
control; e.g. increasing sediment yield: stream flow ratio caused by climatic
change, vegetation clearance, etc.
o degradation;
decreasing bed elevation
- Upstream
control; e.g. bed-load entrapment
- Downstream
control; e.g. falling base level and knick point recession
- Basin-wide
control; e.g. decreasing sediment yield: stream flow ratio caused by climatic
change, conservation measures, etc.
- To compute the designed cross-section we take equalized slope, possibly on the long distance
- The designed alignment usually involves some shortening of a natural river course due to rectification by means of cut offs
- Prior to the adoptation of the equalized slope it is necessary to determine ‘fixed’ points:
- Natural,
e.g. rocky bars, major tributaries, rapid changes of valley width,
- Artificial,
e.g. weirs, sills, bridges, etc.
- Most often carried out regularization is only to make the existing slope even, rarely its reduction or increase
- In the first case the design of slope consists in making it even between the ‘fixed’ points
- straight line, if the slope differences at the beginning and the end of the reach are negligible
- curve, in case the differences are large
- A tributary acts in various ways depending upon the ratio of discharge and quantity of brought in sediment
o If
the tractive force of the river is not sufficient to get rid of the sediment
brought in from the tributaries it can be easily increased by steepening the
slope which is obtained by shortening the river course
o If
conversely, bottom and bank erosion is too great the slope should be decreased
by means of sills, dams, etc.
o A
mountain tributary of the lowland river increases quantity of sediment which
the river is unable to transport and hence the ‘fixed’ point will rise,
steepening continuously the slope below, and decreasing above the tributary
Determination of normal stage and normal discharge
- Variation of discharge over the year is a general characteristic of practically every river
o Main cause of for this is undoubtedly the uneven distribution of rain
o The regularization design should be adjusted to all discharge variations that happen in a given river
- River regulation is associated with a certain water stage, defined with respect to the purpose for which we attempt the works, e.g. flood control, water intake, navigation, bank protection, etc.
- The stage which is taken as the basis of the attempted regularization is the normal stage and the corresponding discharge is the normal discharge
- To this normal stage the training structures should be first of all adopted
- For diverse structures, we assume different normal stages, which result from the following statement
o
The
absolute highest stage or probable stage with low frequency , e.g. 1% or 0.1%
are the basis for computations of levee spacing and capacity of storage
reservoirs
o
Mean
high water is a normal stage for mountain stream engineering
o
Summer
high is a normal stage for spacing of summer (or period of vegetation) levees
protecting land against flooding during vegetation
o
Mean
annual flow is a normal stage for lowland (sand-bed) river engineering
o
Periodic
stages are meaningful in the case of rivers regularized for navigation
o Mean
low stage, computed from a longer period of time is a normal stage of
regularization for low water of the rivers used as waterways requiring a
certain depth during the entire period of navigation
o
The
absolute lowest stage is a normal stage for design intake especially for large
towns
- The choice of a normal stage is decided by different factors:
(a)
character
of a river and its size
(b)
variation
of stages and discharges
(c)
quantity
and character of transported sediment
(d) Aim of regulation: water supply, irrigation and/or drainage, navigation, bank protection, etc.
- The normal stage for the same river may be different in its different reaches and is usually lower downstream
Sediment control at river intakes
- Sediment control starts already with influencing the approaching flow
- Experience shows that it is much easier to prevent the intrusion of sediment moving close to the bed than of sediment in suspension
- The first step towards sediment control at an intake, therefore, should be encouraging the concentration of sediment in the fluid layers close to the bed
- This can be achieved by applying appropriate river training measures capable of decreasing the flow velocity and of suppressing turbulence
Control of sediment moving close to the bed
- Various methods and techniques use the principle of the application of a horizontal diversion separating the upper layers containing mostly pure water from the sediment-laden lower layers
Control of suspended sediment
- Prevention of suspended sediment
from entering the intake is very difficult, particularly when it is rather
uniformly distributed over the water depth
- The settling process can be speeded by adding chemicals
- Sediment rejection by means of sills and bars using spur dykes, training walls, etc as auxiliary devices
- Sediment extraction by means of sediment tunnels and flashing devices
- Sediment ejection by means of vortex tube sediment ejectors
- Creating a settling basin before withdrawing the water by means of diversion dam
- Constructing a settling tank after diverting the water
- The settling tank can be the longitudinal type (straight flow)
- Constructing a settling tank after diverting the water
- The settling tank can be the circular type (vertical vortex)
0 Comments