THE RIVERS KEEP
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Low embankments may be sufficient where only exceptional summer floods have to be excluded from meadows. Occasionally the embankments are raised high enough to retain the floods during most years, while provision is made for the escape of the rare, exceptionally high floods at special places in the embankments, where the scour of the issuing current is guarded against, and the inundation of the neighboring land is least injurious. In this manner, the increased cost of embankments raised above the highest flood-level of rare occurrence is avoided, as is the danger of breaches in the banks from an unusually high flood-rise and rapid flow, with their disastrous effects.
A most serious objection to the formation of continuous, high embankments along rivers bringing down considerable quantities of detritus, especially near a place where their fall has been abruptly reduced by descending from mountain slopes onto alluvial plains, is the danger of their bed being raised by deposit, producing a rise in the flood-level, and necessitating a raising of the embankments if inundations are to be prevented.
Longitudinal sections of the Po River, taken in and , show that its bed was materially raised during this period from the confluence of the Ticino to below Caranella , despite the clearance of sediment effected by the rush through breaches. In the UK, problems of flooding of domestic properties around the turn of the 21st century have been blamed [ by whom? This exposes the properties on the floodplain to flood, and the substitution of concrete for natural strata speeds the run-off of water, which increases the danger of flooding downstream.
Much of it was done under the auspices or overall direction of the United States Army Corps of Engineers. One of the most heavily channelized areas in the United States is West Tennessee , where every major stream with one exception the Hatchie River has been partially or completely channelized. Channelization of a stream may be undertaken for several reasons. One is to make a stream more suitable for navigation or for navigation by larger vessels with deep draughts. Another is to restrict water to a certain area of a stream's natural bottom lands so that the bulk of such lands can be made available for agriculture.
A third reason is flood control, with the idea of giving a stream a sufficiently large and deep channel so that flooding beyond those limits will be minimal or nonexistent, at least on a routine basis. One major reason is to reduce natural erosion ; as a natural waterway curves back and forth, it usually deposits sand and gravel on the inside of the corners where the water flows slowly, and cuts sand, gravel, subsoil , and precious topsoil from the outside corners where it flows rapidly due to a change in direction.
Unlike sand and gravel, the topsoil that is eroded does not get deposited on the inside of the next corner of the river. It simply washes away. Channelization has several predictable and negative effects. One of them is loss of wetlands. Wetlands are an excellent habitat for many forms of wildlife, and additionally serve as a "filter" for much of the world's surface fresh water. Another is the fact that channelized streams are almost invariably straightened. For example, the channelization of Florida's Kissimmee River has been cited as a cause contributing to the loss of wetlands.
It can also increase flooding downstream from the channelized area, as larger volumes of water traveling more rapidly than normal can reach choke points over a shorter period of time than they otherwise would, with a net effect of flood control in one area coming at the expense of greatly aggravated flooding in another. In addition, studies have shown that stream channelization results in declines of river fish populations.
A study of the Chariton River in northern Missouri , United States, found that the channelized section of the river contained only 13 species of fish, whereas the natural segment of the stream was home to 21 species of fish. This loss of fish diversity and abundance is thought to occur because of reduction in habitat, elimination of riffles and pools, greater fluctuation of stream levels and water temperature, and shifting substrates.
The rate of recovery for a stream once it has been dredged is extremely slow, with many streams showing no significant recovery 30 to 40 years after the date of channelization. For the reasons cited above, in recent years stream channelization has been greatly curtailed in the U. The United States Government now has in place a " no net loss of wetlands" policy that means that stream channelization in one place has to be offset by the creation of new wetlands in another, a process known as "mitigation. The major agency involved in the enforcement of this policy is the same Army Corps of Engineers which for so long was the primary promoter of wide-scale channelization.
Often, in the instances where channelization is permitted, boulders may be installed in the bed of the new channel so that water velocity is slowed, and channels may be deliberately curved as well. In the U. Congress gave the Army Corps a specific mandate to include environmental protection in its mission, and in it authorized the Corps to undertake restoration projects. Clean Water Act regulates certain aspects of channelization by requiring non-Federal entities i.
Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navigation purely by works which regulate the flow; their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel, while a lock has to be provided alongside the weir, or in a side channel, to provide for the passage of vessels.
A river is thereby converted into a succession of fairly level reaches rising in steps up-stream, providing still-water navigation comparable to a canal; but it differs from a canal in the introduction of weirs for keeping up the water-level, in the provision for the regular discharge of the river at the weirs, and in the two sills of the locks being laid at the same level instead of the upper sill being raised above the lower one to the extent of the rise at the lock, as usual on canals.
Canalization secures a definite available depth for navigation; and the discharge of the river generally is amply sufficient for maintaining the impounded water level, as well as providing the necessary water for locking. Navigation, however, is liable to be stopped during the descent of high floods, which in many cases rise above the locks; and it is necessarily arrested in cold climates on all rivers by long, severe frosts, and especially by ice.
Many small rivers, like the Thames above its tidal limit, have been rendered navigable by canalization, and several fairly large rivers have thereby provided a good depth for vessels for considerable distances inland. As rivers flow onward towards the sea, they experience a considerable diminution in their fall, and a progressive increase in the basin which they drain, owing to the successive influx of their various tributaries.
Thus, their current gradually becomes more gentle and their discharge larger in volume and less subject to abrupt variations; and, consequently, they become more suitable for navigation. Eventually, large rivers, under favorable conditions, often furnish important natural highways for inland navigation in the lower portion of their course, as, for instance, the Rhine , the Danube and the Mississippi. River engineering works are only required to prevent changes in the course of the stream, to regulate its depth, and especially to fix the low-water channel and concentrate the flow in it, so as to increase as far as practicable the navigable depth at the lowest stage of the water level.
Engineering works to increase the navigability of rivers can only be advantageously undertaken in large rivers with a moderate fall and a fair discharge at their lowest stage, for with a large fall the current presents a great impediment to up-stream navigation, and there are generally great variations in water level, and when the discharge becomes very small in the dry season. It is impossible to maintain a sufficient depth of water in the low-water channel.
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The possibility to secure uniformity of depth in a river by lowering the shoals obstructing the channel depends on the nature of the shoals. A soft shoal in the bed of a river is due to deposit from a diminution in velocity of flow, produced by a reduction in fall and by a widening of the channel, or to a loss in concentration of the scour of the main current in passing over from one concave bank to the next on the opposite side.
The lowering of such a shoal by dredging merely effects a temporary deepening, for it soon forms again from the causes which produced it. The removal, moreover, of the rocky obstructions at rapids, though increasing the depth and equalizing the flow at these places, produces a lowering of the river above the rapids by facilitating the efflux, which may result in the appearance of fresh shoals at the low stage of the river.
Where, however, narrow rocky reefs or other hard shoals stretch across the bottom of a river and present obstacles to the erosion by the current of the soft materials forming the bed of the river above and below, their removal may result in permanent improvement by enabling the river to deepen its bed by natural scour.
The capability of a river to provide a waterway for navigation during the summer or throughout the dry season depends on the depth that can be secured in the channel at the lowest stage. The problem in the dry season is the small discharge and deficiency in scour during this period. A typical solution is to restrict the width of the low-water channel, concentrate all of the flow in it, and also to fix its position so that it is scoured out every year by the floods which follow the deepest part of the bed along the line of the strongest current.
This can be effected by closing subsidiary low-water channels with dikes across them, and narrowing the channel at the low stage by low-dipping cross dikes extending from the river banks down the slope and pointing slightly up-stream so as to direct the water flowing over them into a central channel. The needs of navigation may also require that a stable, continuous, navigable channel is prolonged from the navigable river to deep water at the mouth of the estuary. The interaction of river flow and tide needs to be modeled by computer or using scale models, moulded to the configuration of the estuary under consideration and reproducing in miniature the tidal ebb and flow and fresh-water discharge over a bed of very fine sand, in which various lines of training walls can be successively inserted.
The models should be capable of furnishing valuable indications of the respective effects and comparative merits of the different schemes proposed for works. From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
Gov. Cuomo proposes bold ‘Revive Mother Nature’ Initiative
Unsourced material may be challenged and removed. Water portal. The consequences are the loss of groundwater recharge, reduced base flows in streams, increased flooding, and lower water quality. Instead of channeling stormwater into pipes and drains, a more natural method of water management has benefits for the environment and economy. Natural stormwater management refers to management approaches that accomplish one of three things:. These approaches, when used properly, help maintain a more natural watershed, which in turn keeps our rivers clean and our communities healthy.
Green infrastructure solutions such as permeable pavement, green roofs, and rain gardens reduce the stormwater runoff that flows into sewer systems and triggers sewage overflows. Green infrastructure practices make cities act more like forests by capturing rainwater where it falls, filtering out pollutants and reducing large volumes of runoff. From rain gardens to green roofs, green infrastructure practices decrease pollutant loadings into waters, which can reduce illness from recreational contact or polluted drinking water. Green infrastructure solutions can also improve air quality and mitigate the urban heat island effect to lower heat stress related fatalities.
Green infrastructure can mitigate localized flooding.
These practices improve access to healthy and affordable food when combined with urban agriculture strategies and to green space for recreation. These solutions are cost-effective. Local governments can reconfigure traditional stormwater infrastructure to increase infiltration. For example, the City of Seattle reduced runoff by 97 percent the year after converting an open ditch stormwater drain to an attractive roadside swale garden, decreasing the width of the adjacent street and planting native vegetation. One of the biggest challenges of climate change is the uncertainty and expense it is imposing on communities and businesses.
Among the greatest appeals of green infrastructure is the fact that it is cost-effective and reliable — saving money and bringing a much greater degree of certainty and stability to planning processes and investment decisions. The investments we make today are ones we will live with for decades and possibly centuries to come.
Changing energy landscape
American Rivers is ensuring our leaders make the right decisions for rivers, clean water, and communities and that green solutions become the preferred way to ensure the health and well-being of our children and grandchildren. What is stormwater? Stormwater flooding a street in Washington, DC Impervious surfaces such as roads, parking lots, and roofs associated with sprawling urban development significantly change natural river flow patterns and the recharge of underground water supplies. What are the health impacts of pollution?
Stormwater outfall gushing water into the local river The public health and environmental implications of sewage overflows are tremendous. Sewage pollutes our waters with pathogens, excess nutrients, heavy metals, and other toxins.
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How should we manage stormwater? Natural stormwater management refers to management approaches that accomplish one of three things: Use soil and vegetation in a constructed technique, such as rain gardens or green roofs, to mimic natural hydrologic processes like percolation through soil and plant uptake and transpiration.
Preserve natural features, such as floodplains with a natural vegetation buffer along streams that can slow, filter, and store polluted runoff.