Bridge Shoreline Engg Bridges A bridge has a

Bridge, Shoreline Engg

Bridges • A bridge has a deck, and supports • Supports are what holds the bridge up – Forces exerted on a support are called reactions • Loads are the forces acting on the bridge

Bridges • A bridge is held up by the reactions exerted by its supports and the loads are the forces exerted by the weight of the object plus the bridge itself.

Basic Concepts Span - the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. Force - any action that tends to maintain or alter the position of a structure Compression - a force which acts to compress or shorten the thing it is acting on. Tension - a force which acts to expand or lengthen the thing it is acting on. Compression Tension

Basic Concepts Beam - a rigid, usually horizontal, structural element Beam Pier - a vertical supporting structure, such as a pillar Cantilever - a projecting structure supported only at one end, like a shelf bracket or a diving board Load - weight distribution throughout a structure

Types of Bridges Basic Types: • Beam Bridge • Arch Bridge • Suspension Bridge The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use. The biggest difference between the three is the distances they can each cross in a single span.

Beam Bridge • The beam bridge – One of the simplest bridges

Forces on a beam bridge Reaction of the supports Weight of The bridge ■ Assuming the weight is in the center, then the supports will each have the same reaction

There is compression at the top of the bridge and there is tension at the bottom of the bridge The top portion ends up being shorter and the lower portion longer A stiffer material will resist these forces and thus can support larger loads

How can we deal with these new forces? • If we were to dissipate the forces out, no one spot has to bear the brunt of the concentrated force. • In addition we can transfer the force from an area of weakness to an area of strength, or an area that is capable of handling the force

A natural form of dissipation • The arch bridge is one of the most natural bridges. • It is also the best example of dissipation

In a arch bridge, everything is under compression It is the compression that actually holds the bridge up See how the compression is being dissipated all the way to the end of the bridge where eventually all the force gets transferred to the ground

Compression in a Arch • Here is another look at the compression • The blue arrow here represents the weight of the section of the arch, as well as the weight above • The red arrows represent the compression Compression Wt of arch + above Compression

Bridge : classification • Bridge consists of : • 1. Superstructure 2. Substructure • Loads imposed on the bridge is taken by supports and transmitted to the foundation • Classified based on the manner in which forces are transmitted

classification • 1. vertical forces transmitted vertically • Simple beam type • Cantilever type

• 2. vertical forces transmitted to the foundation by the supports, the horizontal thrust pushes the supports outwards • Arch type • Rigid frame

• 3. vertical forces transmitted to the foundation vertically, but for stability the structure has to be anchored to rock or a large concrete mass.

Suspension Bridge ■ A suspension bridge can withstand long spans as well as a fairly decent load.

How Suspension Bridge Works ■ A suspension bridge uses the tension of cables to hold up a load. The cables are kept under tension with the use of anchorages that are held firmly to the Earth.

Suspension Bridge ■ The deck is suspended from the cables and the compression forces from the weight of the deck are transferred the towers. Because the towers are firmly in the Earth, the force gets dissipated into the ground.

Suspension Bridge ■ The supporting cables that are connected to the anchorages experience tension forces. The cables stretch due to the weight of the bridge as well as the load it carries.

Investigations • Geol data, direct measurement of waterways, basin of the stream, stage of stream, estimated discharge, frequency of flood, meandering tendency, scouring effect

Shoreline, Coast • Wave action dominant mechanism

SHORELINES a) Waves b) Longshore transport c) Erosional shores d) Depositional shores e) Emergent and submergent shores

Shorelines a) Waves Ocean WAVES = orbital wave (waves of oscillation) Energy advances But: Water does not! Water moves in circular orbits

Shorelines a) Waves Breaking WAVES Waves ‘feel’ bottom when it comes to within half of the wavelength Wave length decreases, wave height increases At critical point the wave becomes too steep and breaks Surf sloshes onshore

a) Waves Wave refraction strongly influences erosion and sediment transport Waves travel more slowly in shallow water so they refract towards the beach. Waves refract around headlands, increasing wave impact on headlands, decreasing it on beaches.

Longshore currents and rip currents

Shorelines c) Erosional Shores Wave refraction along an irregular shoreline • Wave energy is concentrated at headlands and dispersed in bays • Causes erosion of headlands and creation of erosional features Figure 10 -14 b

Shorelines d) Depositional Shores BEACHES • Source of beach sediments - rivers - cliff erosion - marine life Beaches

e) Emergent and submergent coasts Emergent coasts Develop because of uplift of an area or a drop in sea level Features of an emergent coast – Wave-cut cliffs – Wave-cut platforms

e) Emergent and submergent coasts Uplifted, ancient wave-cut benches exposed in southern California

e) Emergent and submergent coasts Submergent coast Caused by subsidence of land adjacent to the sea or a rise in sea level Features of a submergent coast – Highly irregular shoreline – Estuaries – drown river mouths

Evidence of emerging and submerging shorelines • Emergent features: – Marine terraces – Stranded beach deposits • Submergent features: – Drowned beaches – Submerged dune topography – Drowned river valleys

Movement of sand on the beach • Movement perpendicular (↕) to shoreline – Caused by breaking waves – Light wave activity moves sand up the beach face – Heavy wave activity moves sand down the beach face to the longshore bars – Produces seasonal changes in the beach

Movement of sand on the beach • Movement parallel (↔) to shoreline – Caused by wave refraction (bending) – Each wave transports sand either upcoast or downcoast – Huge volumes of sand are moved within the surf zone – The beach resembles a “river of sand”

Longshore current and longshore drift • Longshore current = zigzag movement of water in the surf zone • Longshore drift = movement of sediment caused by longshore current

Types of hard stabilization • Hard stabilization perpendicular to the coast within the surf zone: – Jetties—protect harbor entrances – Groins—designed to trap sand • Hard stabilization parallel to the coast: – Breakwaters—built beyond the surf zone – Seawalls—built to armor the coast

Protective barriers (Littoral barriers) • To prevent or to stop the destruction of a shoreline by waves and currents, protective works (litt barr) are built. • Man made litt barri, termed protective structures, water-front struc are : jetties, groins and breakwaters

Purpose of jettis, groin, breakwater • To protect inlets or estuaries to river and bay • Groin is a litt barri starting at perpendicular or at an angle to shoreline. • The more acute the angle made by the groin with the shoreline, the less is its capacity to form an adequate beach. • Breakwater = barriers constructed to break up and disperse the waves of heavy seas, to provide shelter for ships.

Jetties and Groins • Jetties are always in pairs • Groins can be singular or many (groin field) • Both trap sand upstream and cause erosion downstream

Breakwater • Provides a boat anchorage • Causes deposition in harbor and erosion downstream • Sand must be dredged regularly

Seawalls and beaches • Seawalls are built to reduce erosion on beaches • Seawalls can destroy recreational beaches • Seawalls are costly and eventually fail