Autogestión del aprendizaje desde la perspectiva de Luis Insuasty (2002)

When a large capacity dam is constructed across a stream, a backwater is caused on the upstream side of the dam, which reduces the energy gradient, and velocity of flow. This effect is felt for several kilometers in the upstream direction. As a result, the ability of the stream to transport sediment load is progressively reduced from beginning of backwater curve towards the dam and the excess sediment gets deposited. If the reservoir capacity to the annual inflow ratio is about or greater than unity, most of the sediment gets deposited upstream of the dam and only a small percent of finer material may pass over the dam and through the sluices. Since such deposition over a period of years reduces the valuable capacity of the reservoir to store water, reservoirs are periodically surveyed to determine the amount of

Table 3.10 Typical values of q_S/ q_T for natural streams in U.S.A.

River Sediment size in mm Average q_S / q_T

Mississippi river at mouth 0.14 0.90

Colorado river at Yuma 0.10 0.80

Niobrara river near Cody 0.30 0.49

Niobrara river near Valentine 0.27 0.47

Snake river near Burge 0.29 0.67

Five mile creek near Riverton 0.24 0.81

Middle Loup river near Dunning 0.33 0.53

Biose river near Twin Springs 0.10 0.65

Moore Creek above Granite Creek 0.25 0.75

sediment deposited in the reservoir. Hence such surveys provide valuable data on sediment load carried by the stream.

There are two general methods of conducting reservoir surveys. These are the range-line survey, and contour survey. The general procedure of carrying out reservoir surveys has changed little in the past four or five decades; however significant advances have taken place in the equipment available for carrying out the surveys. Choice of the method depends on availability and character of previous mapping or survey records, the size of reservoir, degree of accuracy needed, and scope of study objectives. The range-line method is more widely used for medium and large reservoirs. In this method a number of cross sections of the reservoir are surveyed before the reservoir is first filled and then periodically resurveyed. These cross sections are called ranges. From known data for the consecutive surveys at each range line, one can determine area of sediment deposition, from which total volume of sediment deposited on the upstream side of the dam can be determined.

Contour method is used for small reservoirs, which are occasionally empty, or at low stage. The contour method uses essentially the topographic mapping procedures. To apply this method first a good contour map of the reservoir is prepared before its filling. Similar contour map can be obtained periodically many times by aerial survey. The contour interval is 1.5 m to 0.5 m. From such consecutive contour maps the sediment volume deposited during certain period can be ascertained. New techniques of reservoir surveys are being used at present and these are discussed by Bruk (1985).

Now a days reservoir surveys are carried out using Global Positioning System (GPS). GPS (GARMIN 2000 and Chatterjee et al. 2001) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by U.S. Department of Defence; this system is now available for civilian use. It works in any weather condition, 24 hours a day.

These satellites circle the earth twice a day in a precise orbit and transmit signal information to the earth. Using signal information from three or more satellites at the same time, the receivers on the earth use triangulation techniques to calculate the exact location of the reservoir. The GPS receivers have a number of potential errors, but if Differential Global Positioning System (DGPS), is used by having two identical receivers, they provide an accurate means of surveying. The basic principle of DGPS is that errors calculated by two receivers in a local area will have common errors. Here one GPS receiver at the base station is located on the surveyed point, and the second one called the rover station is located on the motorised boat, which collects bathymetric data for reservoirs. The reference station GPS receiver knows the position of its antenna and can determine the errors in satellite signals. The error between measured and calculated is the total error. The range errors for each satellite are formatted into messages and the modular encodes these data. In an amplified form these data are radiated through antenna to roving GPS Station for real time position correction. Hence, when the two receivers are operated concurrently, by comparing and processing of signals of both the stations, the position of roving station can be obtained with adequate accuracy. The depth measuring unit consists of sonic sounding equipment, which comprises recorder, transmitting and recovering transducers and a power supply. This equipment needs frequent calibration. With such equipment depths can be measured with an error less than one percent.

Once the volume of sediment deposited in a given period is known, it can be converted into corresponding weight if the average unit weight of sediment W_av over a period of T years is known.

Miller (1953) has given the following equation for W_av.

W_av = W_o + 0.434K T

T T

-

-L NM O

QP

1ln ( 1) ...(3.17)

where W_av and W_o are the average unit weight and initial unit weight in kN/m³ of the deposited sediment in T years and the coefficient K depends on the sediment size and method of reservoir operation. Values of K as recommended by Lane and Koelzer are given in Table 3.11.

Table 3.11 Recommended values of K in Eq. 3.17 (U.S. Govt. and IIHR 1943)

S. No. Reservoir operation Deposited sediment

Boulders, gravel sand Silt Clays

1. Sediment always submerged or nearly submerged 0 0.90 2.51

2. Normally a moderate reservoir draw down 0 0.42 1.68

3. Normally considerable reservoir 0 0.16 0.94

4. Reservoir normally empty 0 0 0

Knowing the percentages of the individual fractions in the deposited sediment W_o can be determined as W_o = Si

N=1W_oi p_i/100. In the same way weighted K value can be determined and used in Eq. (3.17).

To determine initial unit weight Table 3.12 can be used:

Table 3.12 Initial unit weights of sediment

Material kN/m³

Clays 7.5

Silts 9.5

Sands 16

Gravel 20

Boulders 22

As has been already mentioned, some material flows over and through the dam; therefore sediment deposited in the reservoir during a given time will be less than sediment flowing into the reservoir during the same time. This ratio expressed in percent is commonly known as the trap efficiency Te of the reservoir, which will vary between 0 and 100 percent. If the trap efficiency of the reservoir is known, yearly quantity of sediment deposited in the reservoir can be converted into sediment yield using the relation:

Sediment yield =

Annual quantity of sediment deposited in the reservoir

(Trap efficiency /100)

F HG I

KJ

In general, the trap efficiency of a reservoir depends on the ratio of storage capacity to annual inflow, age of the reservoir, shape of the reservoir, method of reservoir operation, sediment size and its distribution and type and location of outlets. There is no general method available for the determination of trap efficiency, which takes into account all these factors. What is being used at present is the trap efficiency versus (capacity/inflow) ratio curve proposed by Brune (1953) on the basis of record of 44 normally ponded reservoirs in U.S.A., (see Fig. 3.9). On the same figure are also plotted some data from reservoirs in China, India and South Africa. At present this curve is used in most of the countries for normally ponded reservoirs.