Geology

Channel Restoration Techniques

* Cleaning urban waste from the channel allowing the stream to flow freely * Protecting the existing channel bank is by not removing existing trees * Where necessary, planting additional native trees and other vegetation Trees are important because they provide shade for a stream and their root systems protect the banks from erosion. The objective of channel restoration is to create a more natural channel by allowing the stream to meander and to reconstruct variable water-flow conditions with fast, shallow riffles alternating with slow, deep pools.

Total sediment load of stream includes

* Dissolved load * Suspended load * Bed load * Movement of sediment determined by velocity

Bed load

Coarse-grained sediment moved along bed of stream

Downstream Floods

Downstream floods cover a wide area and are usually produced by storms of long duration that saturate the soil and produce increased runoff. Although flooding in small tributary basins is generally limited, the combined runoff from thousands of tributary basins produces a large flood downstream. A flood of this kind is characterized by the downstream movement of the floodwaters with a large rise and fall of discharge at a particular location.

Suspended load

Fine-grained sediment carried in suspension

What is the most frequent natural hazard

Flooding. 90% of all natural disasters include flooding

Drainage Basin

Geologists commonly use the term stream for any body of water that flows in a channel. The region drained by a single stream or river variously called a drainage basin, watershed, river basin, or catchment. Thus, each stream has its own drainage basin or watershed that collects rain and other precipitation. A large river basin, such as the Susquehanna River basin in the northeastern U.S., is made up of hundreds of small drainage basins, commonly called watersheds, which catch the precipitation that drains into the smaller tributary streams and rivers.

What are the effects of dams and urbanization on streams and floodiing

Historically, humans have responded to floods by attempting to prevent them by modifying streams and rivers. Physical barriers such as dams and levees have been created or the shape of the stream has been changed. The channel shape is changed so that a stream will drain the land more efficiently. However we have yet to build something capable of controlling the heaviest runoff. Dams like levees are prone to failure if they are not properly maintained. Most dams in the US are on private land. Although often constructed with government subsidies, they are poorly maintained and are rarely inspected.

Discharge is usually reported in ft3/second.

Q = W X D X V Where Q = discharge, W = width, D = depth, V = velocity Constantly changing

Watershed

Same as Drainage Basin: Except for the very few who live right next to the river, most people who live on land that drains into the Susquehanna live in a watershed of one of the tributaries. In fact, virtually everyone lives in a watershed that is a part of a larger drainage basin.

Channelization

Straightening, deepening, widening, clearing, and/or lining existing stream channels are all methods of channelization. Objective in this engineering

Alluvial Fans and Deltas

Stream flow expands and slows down as a river goes from mountains into plains or from a channel into an ocean, lake, or pond. At these points, the river commonly forms a fan-shaped deposit on land known as an alluvial fan, or a triangular or irregular shaped deposit known as a delta if it extends into a larger body of water.

How does base level control River Dynamics

The base level of a river is essentially the lowest elevation to which a river may erode. Most often, this elevation is at or close to sea level, although a river may have a temporary base level such as a lake. Rivers, thus, flow downhill to their base level, and a graph showing the downstream changes in a river's elevation is called a longitudinal profile. A river usually has a steeper-sided and deeper valley at high elevations near its origin, or headwaters, than closer to its base level where a wide floodplain may be present. This happens because, at higher elevations, the steeper river gradient increases flow velocity, which, in turn, increases erosion.

How do Alluvial Fans or Deltas Work

The flood hazard associated with alluvial fans and deltas is different from hazards in a river valley and its associated floodplain. Rivers entering an alluvial fan or delta often split into a system of distributary channels. That is, the main river divides into several channels that carry floodwaters to different parts of the fan or delta. Furthermore, these channels may change position rapidly during a single flood or from one flood to the next, creating a hazard that is difficult to predict.

Cutbank

a near-vertical slope created by the fast moving water erosion of the riverbank on the outside of a meander bend. Water moves the fastest here

Recurrence Interval

average time between flood events that are of equal or greater magnitude.

Levees

earthen embankments constructed parallel to the river to contain floodwaters and reduce flooding. Levees completely circle some areas on the floodplains. Levees made by farmers and homeowners failed while levees built by the federal government survived flooding and saved lives and property. There is no uniform building code for levees, so some areas have levees that are higher or lower than others. Failures occurred as a result of overtopping and breaching, or rupturing, resulting in massive flooding of farmlands and towns.

How to calculate Stream Gradient

elevation drop / distance

Braided Channel Pattern

have numerous sand and gravel bars and islands that divide and reunite the main channel, especially during low flow. Overall, braided channels tend to be wide and shallow, compared to meandering channels.

Mitigation efforts

levees channelization channel restoration

Dissolved Load

minerals dissolved in water

Flash Floods

occur in the upper part of a drainage basin and in some small drainage basins of tributaries to a larger river. They are generally produced by intense rainfall of short duration over a relatively small area. If these floods are sudden and of relatively great volume, peak discharge can be reached in less than 10 minutes. Flash flooding is most common in arid and semiarid environments; in areas with steep topography or little vegetation; and following breaks of dams, levees, and ice jams.

Meandering Channels

often contain a series of regularly spaced pools and riffles

Point Bar

the buildup of sand and gravel from the slower water on the inside of a meander