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AHMED MUSTAFA ID NO. 116670

MOHIB ULLAH ALI ID No. 116679

BY

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a) A river has annual peak of Q100 m3

/sec for a return periodof 100 years. Give a definition of the return period of 100years. What is the probability of occurrence of annual floodpeak with a magnitude equals or larger than Q100 ?

b)The suspended load sediment sampler US D-49 is designedfor field measurement of suspended sediment concentrationin a river of depth not more than 4.5m.However in manycircumstances we have to use US D-49 in a river of depthmore than 4.5m.Under this situation there will be errors in

the measured concentration. Explain two errors and theircauses.

c)Explain the differences between bed material load and bedload. What type of sediment load the US D-49 sampler

measures?

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Return Period

A return period, also known asa recurrence interval (sometimes it is alsocalled repeat interval) is an estimate of thelikelihood of an unknown event, such asan earthquake, flood or a river discharge flowto occur. It is a statistical measurement

typically based on historic data.

http://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Floodhttp://en.wikipedia.org/wiki/Discharge_(hydrology)http://en.wikipedia.org/wiki/Discharge_(hydrology)http://en.wikipedia.org/wiki/Floodhttp://en.wikipedia.org/wiki/Earthquake

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A return period of one hundred year flood is a floodevent that has a 1% probability of exceedence

in any given year. The 100-year flood is also referredto as the 1% flood, since its annualoccurrence probability is 1%

Probability of event / peak flood in next coming 100years

This does not mean that a 100 year flood will happenregularly every 100 years, or only once in 100 years.Despite the connotations of the name "return period".In any given100 year period, a 100 year event mayoccur once, twice, more, or not at all, and eachoutcome has a probability that can be computed

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Gumbel Method:

Recurrence interval Tr = (n +1) / m n is number of years on record; m is the number of recorded occurrences of the event

being considered

Probability distribution P = 1 / T P = m / (n + 1)

Probability percentage of non-occurrence p % iscomputed by the equation

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For better understanding we should considera case study

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A US D-49 sampler will not operate properly if used at too large a depth. For example, if thevolume of the water-sediment mixture is to betwo-thirds of the total container volume for adepth-integrated sample, the sampler should be

one-third full as it reaches the bottom of theriver so that it will have room for the watercollected on the return trip. At the bottom of theriver, therefore, the volume of air in the samplermust be at least two- thirds of the total volume.

At sea level the pressure of the air in the bottlebefore it enters the water is about 34 feet of water (atmospheric pressure).

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If the sampler is lowered below the compressionlimit it will intake too much water and not sampleisokinetically. The compression rate, which isrelated to the compression limit, may restrict the

vertical transit rate to less than 0.4 times the meanstream velocity. If the sampler is lowered toorapidly, however, the incoming water does notcompress the air within the sampler fast enough sothe pressure on the inside of the sampler is lessthan the hydrostatic pressure outside the sampler.

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When this occurs the intake velocityincreases above the stream velocity. Watermay even enter the sampler through the airexhaust vent. If the sampler is raised toorapidly, the compressed air inside the bottlewill not escape fast enough through theexhaust vent and the intake velocity will be

less than the mean stream velocity. Thecompression-rate limit is a function of thesize of the nozzle and sample container.

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The approach angle is determined as the ratioof vertical velocity of the sampler (rate atwhich it is lowered or raised) to the meanstream velocity. If the sampler is lowered or

raised at a rate exceeding 0.4 times the meanflow velocity, the intake velocity will be lessthan the stream velocity. The maximumvertical transit rate for any depth integratingsampler, therefore, should not exceed 0.4times the mean stream velocity of the section

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Bed load : Bed load is the transport ofsediment particles sliding ,rolling or hopping.In the hopping process flow turbulence picksup the sediment particle from the bed and

then drops it back to stream bed.

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Suspended load is the transport of bedparticles when the gravity force is more orless counter balanced by the upward forcedue to turbulence of flowing water.

Wash Load:

It is transfer of small particles D less than50micro meter, finer than the bed material

and rarely found in the river bed. Bed material is defined as bed load

+suspended load(wash load excluded)

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Designed for stream depths not more than4.5 m

Nozzle sizes: , 3/16 and 1/8

Weight: 32 kgs

Volume of sampling container: 395 cm3

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Suspended Load is measured by D-49sampler.