|
<br />Designing for Dynamic Equilibrium in Streams
<br />
<br />Missouri increased its meander processes after dam installation.
<br />The farmland had to be protected, and streambank controls
<br />were installed. This work proved to be successful in main-
<br />taining the farmland, but the river had no other means of
<br />satisfying its required sediment load than to pick it up frbm
<br />the bed.
<br />Robbins and Simon (1982) studied modified western
<br />Tennessee streams in reaches with bridge structures. They
<br />concluded that stresses imposed on the channels by past
<br />channel modifications led to down cutting, headward erosion,
<br />downstream aggradation, accelerated scour, bank instabilities
<br />and, in some cases, were still affecting bridges more than 50
<br />years later.
<br />A very sad example from a different part of the world is
<br />the developments on the Nile River since closure of the $1-
<br />billion High Aswan Dam about 21 years ago in Egypt; sad,
<br />because a nation's dream was not fulfilled. Besides numerous
<br />morphologic changes of the Nile, resulting in physical and
<br />economic problems, the fertile and very important Egyptian
<br />Nile delta lands are being eroded and transported into the
<br />Mediterranean. The "hungry" Nile River picks up an easily
<br />obtainable sediment load. Problems appear to be of such mag-
<br />nitude that Egyptian engineers are contemplating plans to
<br />construct a canal system, bypassing most of Lake Nasser to
<br />transport the up-river sediment load into the river below the
<br />dam (Kashef, 1981). Others propose to slowly dismantle the
<br />High Aswan Dam, because this would be less costly than the
<br />maintenance of the present system (Ibrahim, 1984).
<br />
<br />MAKING STRUCTURAL MEASURES EFFECTIVE
<br />
<br />Having read the impressive examples of human interference
<br />with large stream systems, one may ask what happens in small
<br />streams if the bed-material load is withheld. After all, land
<br />managers are more often involved in problems of small streams
<br />than in large river projects.
<br />Also, small streams have been controlled by relatively
<br />large dams that formed reservoirs. Generally, the design ob-
<br />jective was sediment retention - and not water storage - to
<br />protect downstream values. Yet certainly, as stated earlier,
<br />the consequential scenario was the same as in rivers.
<br />More often, however, small dams (check dams) are used
<br />. in small streams for purposes of erosion control, fishery en-
<br />hancement and other land management objectives. Check
<br />,- dams are gradient control structures, because if installed
<br />correctly, Le., in relatively great numbers at given spacings,
<br />decreases in water surface slope (energy gradient) are intro-
<br />duced. The decrease occurs due to tail water formation up-
<br />stream from the check dams. Downstream from the dams,
<br />the water overfall over the dams leads to energy dissipation. \
<br />Thus, the lost momentum of the flow must be regained. The
<br />overall effect of these processes is the transformation of high
<br />turbulent flows into more tranquil ones with less available
<br />energies. For an effective control as described, however, it, is
<br />imperative that design criteria for planning, construction and
<br />spacing be followed (Heede, 1966). If not - as an analogy -
<br />
<br />it may take only one missing brick to lead to the destruction
<br />of a wall.
<br />Check dams withhold some water and, with time, also
<br />sediment. After the relatively small storage capacity for both
<br />is fulfilled, both continue to be discharged into the stream
<br />reaches. In reality, there is not a serious disruption of water
<br />flow or sediment discharge. If dam spacing was selected in
<br />accordance with expected sediment wedges upstream from the
<br />structures (Heede, 1966), decreased channel gradients and the
<br />transformation of the flow to less turbulent, or tranquil,
<br />regimen will prevent the occurrence of a "hungry" stream.
<br />Equations for spacing calculation were published elsewhere
<br />(Heede, 1976). An existing computer model for gully control
<br />could easily be modified for perennial small streams to give
<br />design, cost and spacing for a treatment of small streams by
<br />check dams (Beede and Weatherred, 1981).
<br />Besides sediment withdrawal, other hydraulic-geomorphic
<br />aspects should be considered for the prevention of new critical
<br />sites. One of these aspects is flow separation into high and
<br />low velocity segments in the cross-sectional area of flow,
<br />caused by structures such as check dams, flumes, or bank
<br />protection works. Flow separations, as described, lead to the
<br />formation of vortexes that often create bed and bank scour.
<br />To prevent these problems, check dams require an apron,
<br />end sill, and bank protection below the structure. The apron
<br />protects against bed scour, dissipating flow energies on the
<br />apron but also leading to lateral vortexes (eddies), aimed at the
<br />banks at each side of the apron. Bank. protection measures
<br />are, therefore, required to prevent scour and possible destruc-
<br />tion of the dam. The end sill, placed at the downstream edge
<br />of the apron, is intended to force the hydraulic jump down-
<br />stream and away from the apron. This jump is required for
<br />the transformation of the supercritical into subcritical flow
<br />below the structure. Where the jump impinges on the bed, a
<br />vortex with a horizontal axis is created that rotates toward
<br />the structure and scours the bed. If the place of impinge-
<br />ment is too close to the structure, the apron could be under-
<br />mined and destroyed. The jump requires additional energy
<br />dissipation and hence a more tranquil flow occurs, as com-
<br />pared with the flow approaching the check dam. The effect
<br />of the described combined processes and measures is effective
<br />flow energy dissipation and prevention of flow separation.
<br />Often where flumes or similar structures are installed, the
<br />cross-sectional area of flow within the structure is narrowed
<br />as compared with the channel. Ifa smooth transition of
<br />width into the wider channel does not exist, high velocity
<br />flows will occur in the center of the channel and relatively
<br />calm water at each bank.. The action of the high velocity
<br />flows on the calm water is similar to the action of a fast-
<br />running man hitting the shoulder of a standing man. If the
<br />standing man is not tluown to the ground, he will rotate into
<br />the direction of the rUlmer. The same happens to the cahn
<br />water when it rotates into an eddy (vortex with a vertical
<br />axis), fust downstream, then into the bank., followed by up-
<br />stream and again downstream movement. Back scour results
<br />at each side of the channel downstream from the structure.
<br />
<br />355
<br />
<br />WATER RESOURCES BULLETIN
<br />
|