Coastal Sediment Transport

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1 Coastal & Marine Coastal Sediment Transport Mazen Abualtayef Assistant Prof., IUG, Palestine

2 Sediment Transport Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. The generic categories of sediments is as follows: Gravel Sand Silt Clay

3 Introduction Sediment, moved by waves and wind, may be academically divided into cross-shore and alongshore sediment transport. Sediment movement can result in erosion or accretion (removal or addition of volumes of sand). Erosion normally results in shoreline recession (movement of the shoreline inland); accretion causes the shoreline to move out to sea.

4 Dynamic beach profile The shape of a beach is called the beach profile. It responds to the environmental conditions (waves and water levels) imposed upon it and if such environmental conditions are constant, the beach profile should remain relatively constant (equilibrium profile). In some model tests at Queen's University, however, the beach profile still changed after 1500 hours (2 months) of continuous testing with a single wave and water level in a hydraulic model.

5 Dynamic beach profile Comminution: solid materials (Shingle, cobble and sand) are reduced in size by grinding process. Bird (1993) identifies 20 causes of long-term beach erosion. The causes may be combined into four important groups: - Decrease in sediment supply, - Comminution, - Submergence, - Human interference. erosion Natural causes of

6 Cross-shore transport If Wave E = High sediments carried offshore Stored as Sandbar If Wave E = Low sediments carried onshore From Sandbar buildup of sand

7 Alongshore transport

8 Cross-shore transport تكد س Stockpiling: تعو ض Compensating: The beach profiles respond to storm-calm cycles by shifting sand in the cross-shore direction, forming a dynamic equilibrium. Nature has provided for such storms by stockpiling large quantities of sand in dunes. The dunes are a long-term protection against coastal erosion, because they provide adequate elevation of the land contours to prevent flooding and form emergency reservoirs of sand. During storms and storm surges, sand on upper beach is moved offshore. This allows waves to come further into shore and they will attack foot of dunes causing them to become unstable and deposit large amounts of sand close to shore, compensating for sand moved offshore by storm waves. When storm has passed, most or all of sediment moved offshore will normally return onshore. In practice, the situation is complicated by alongshore transport, offshore bar formations, etc., which may prevent some of sediment from moving back onshore after a storm.

9 الوحدات السكن ة Condominiums: Dune-beach disturbance Ignorance have disturbed many dune-beach systems. Hotels have been built on the most seaward dunes, or on sites that were first cleared and graded, thus destroying the dunes. The hotel guests can now see the sea from their room and can walk straight onto the beach without having to climb over the dunes first. Roads have been built on or seaward of the dunes so people can drive along the sea. Developers sell building lots and condominiums in dune areas so that people can live very near the water. Dunes are also regularly paved into parking lots.

10 طب ع / فطري Indigenous: Dune-beach encouragement To encourage dune-beach system, indigenous vegetation covers dunes should be ensured. Vegetation cover decreases wind velocity above sand, thus preventing loose sand from being blown inland. Dune vegetation is fragile and traffic (walking, riding and motor vehicles) can easily destroy such cover. For that reason, all-terrain vehicles, automobiles and bikes do not belong in the dunes. Lighter traffic such as pedestrians and horses can be channeled on specially reinforced paths. Any removal of fragile vegetation cover leaves exposed sand. When this sand is blown away, a depression is formed and adjacent plants will be undermined, increasing the size of damaged area and allowing further removal of sand to form a blowout. Special paths can be designed that are reinforced by larger material that is not removed by the wind, to prevent blowouts. Dune growth can be encouraged by placing fences parallel to the shore in strategic locations, (CERC, 194). The fences will slow down wind velocity, so that blowing sand is deposited behind the fences.

11 ضاهى Emulate: Soft protection If the dune-beach system is really a utopia, then our protection designs would do well to emulate it. Instead of building a seawall that reflects wave action and provides no emergency sand reservoir, we can protect the coasts by an artificially placed dune-beach system. Soft protection has many advantages, but the most important is that it can be used as a recreational space. Blowing sand can be a problem because of a sudden abundance of dry sand, immediately after placement. Careful planning and immediate covering of the dune by vegetation are important to keep the sand in place and prevent sandblasting facilities landward of the nourishment.

12 Soft protection To put the concept of artificial beach nourishment into perspective; most major tourist destination beach resorts are regularly replenished. When the hotels replace the dunes, the beautiful beaches quickly erode to the point that no beach is left. The hotels and other structures are then endangered and are often protected by seawalls and groins. Fewer tourists will now come to sunbathe (on the seawalls). The hotels need both protection and recreational beach and this can be provided by artificial beach nourishment. Nourishments can also be reinforced by structures and combinations of artificial nourishment with groins, artificial headlands and offshore breakwaters are required.

13 Alongshore transport When waves approach a shoreline at an angle, alongshore transport (also often called littoral transport) takes place. It is often the most important design consideration.

14 Alongshore transport Waves approaching the shore at an angle will move sediment along the shore in the direction of wave propagation. There are two mechanisms: beach drifting in the swash zone, transport in the breaking zone.

15 Alongshore transport Beach drifting in the swash zone: The wave action pushes sand up the beach in the wave direction. When the wave retreats, the water and sediment particles are accelerated by gravity and travel down the steepest incline, perpendicular to the beach. Transport in the breaking zone: The turbulence in the breaking zone stirs the material into suspension and it is carried by alongshore current, generated by the momentum of the breaking waves. The same turbulence and current also transport sand as bedload along the bottom.

16 Alongshore transport Sediment transport due to various incident waves from left can then be added up to yield a sediment transport rate to right (Q + ). Similarly we can define sediment transport rate to left (Q - ). The sum of these two is called the gross sediment transport rate and the difference is the net sediment transport rate. This net rate has a direction and terms updrift and downdrift are relative to direction of net sediment transport.

17 Computation of littoral transport A detailed sediment transport calculation incorporates many carefully measured wave, current, beach and sediment parameters into a numerical model to determine actual detailed (in time and space) sediment transport rates. Several efforts have been made worldwide to collect the necessary field data to formulate such models properly. The ultimate success with a detailed computation depends on data. Field data are required, along with data from hydraulic models to provide controlled and repeatable data sets.

Computation of littoral transport For the present, our limited stock of good, sufficiently detailed calibration data and our inadequate understanding

The bulk sediment transport method relates total alongshore sediment transport rates to a few simple wave and beach parameters.

18 Computation of littoral transport For the present, our limited stock of good, sufficiently detailed calibration data and our inadequate understanding of detailed sediment transport processes normally leads us to simpler, bulk volume computations discussed below. The bulk sediment transport method relates total alongshore sediment transport rates to a few simple wave and beach parameters. The calibration data for this method are simpler to obtain but the answers are less sophisticated. Examples of bulk sediment transport expressions are the CERC formula (CERC, 194) and the Kamphuis (1991) expression.

19 Complications Throughout the previous discussions it was assumed that infinite amounts of beach material are available for sediment transport, alongshore sediment transport is essentially the net transport, which takes place in one direction, the effects of individual storms can easily be averaged into long term littoral drift quantities.

20 Complications Limited Amounts of Beach Material Many coastal areas exhibit wide beaches backed by substantial dunes and thus have virtually unlimited amounts of sandy beach material available, and all the usual expressions were developed for such areas. But many coastal areas do not have unlimited quantities of sand, because erosion processes meet either man-made or natural formations that do not contain sufficient sand to supply the cross-shore or alongshore sediment transport potentials.

$The actual rate is normally viewed as a long-term average rate and it is most often considered to be a simple fraction of$

21 Complications Limited Amounts of Beach Material Potential sediment transport rate (calculated from formulas) Actual rate is less than the potential rate and can only be determined by a sediment budget calculation, which takes account of all the sediment inflows and outflows, and all the sediment sources and sinks of a system. The actual rate is normally viewed as a long-term average rate and it is most often considered to be a simple fraction of the long-term average potential rate.

22 Complications Limited Amounts of Beach Material Potential sediment transport rate (short term) will be reached during single storms and short-term erosion and accretion rates, even in areas with relatively little beach material, are closely related to potential rate. For this reason sediment transport rate should be expressed over short time spans of hours or days rather than years. In areas of short sand supply, potential sediment transport rate is large, relative to available sand. This leaves the impression that erosion and damage is always very rapid. One storm can remove material (at the potential rate) that took years to build up (at the actual rate). Short term actual sediment transport rates can approach potential rates while the long-term average actual rates are smaller and a function of supply and loss of sand.

23 Complications Sediment Transport in Two Directions Building groins will interrupt littoral transport and that sand will build up (accrete) on the updrift side and be taken away (erode) from the downdrift side.

24 Complications Sediment Transport in Two Directions The gross transport (the sum) was large and the net transport (the difference) was small. The contours around the groin, which had been placed on an initially straight, eroding shoreline. They quite clearly show accretion with radically differing beach profiles on both sides of the groin.

25 Complications Sediment Transport in Two Directions We cannot simplify designs so that they only take net longshore sediment transport into account. One example of a design difference is shown below where it may be seen that same length groins will collect more sand (and damage adjacent areas more) if alongshore transport is in two directions.

26 Complications Short Term Littoral Transport Even when sand supply is not limited, a few days of storm will normally move as much or more material than the relatively small waves move during the remainder of the year. That is, why accretion around groin in Fig is different on the left side and the right side. The left side, where the slopes are gentle, accretes most of the year as a result of small waves. The right side, with steeper slopes, accretes during short periods of high wave energy. Thus, we cannot only think in terms of annual littoral transport but must be careful to consider shortterm storms, etc. For example, the groins in Fig b appear quite safe. However, one storm could remove all the sand, flank the groins and destroy them if they are located in an area where single storms account for a large proportion of the total sediment transport or are capable of moving most of the available material.

SHORELINE AND BEACH PROCESSES: PART 2. Implications for Coastal Engineering

SHORELINE AND BEACH PROCESSES: PART 2 Implications for Coastal Engineering Objectives of the lecture: Part 2 Show examples of coastal engineering Discuss the practical difficulties of ocean engineering