Reference no: EM132994378

EA5018 Field Studies in Tropical Water and Soil Science - James Cook University

Dam Trapping exercise

One of the most important tasks that a hydrologist/sedimentologist/engineer (geo-consultant) needs to consider when evaluating the construction of a new dam is to estimate its lifespan. In other words, how long will it take for sediment to infill the dam so that it can no longer be used for its primary purpose? The two critical factors that determine the amount of time it takes to fill a dam with sediment are: (1) The sediment volume (using the density and load of the sediment) entering the dam; and (2) The proportion of sediment that is trapped within the reservoir (i.e. the sediment that does not pass through the dam spillway). A number of methods have been developed to estimate the trapping efficiency of dams that range from relatively simple empirical estimations to more complicated hydrological and sedimentological models (e.g. Verstraeten and Poesen, 2000). However, most of the equations to predict trapping efficiency have been developed in the USA and UK and do not work particularly well in tropical locations which have more pronounced wet and dry seasons. This is largely because the traditional empirical equations only consider annual-scale input data, which is more suited to temperate locations where rainfall is spread more evenly throughout the year. Dam construction has increased greatly in the tropical and sub-tropical regions over the past 50 years (Syvitski et al., 2005) and there is an increasing need to better estimate sediment trapping efficiency of these tropical reservoirs to estimate their useful life. For example, in some countries such as China and India, around 1-2% of dam capacity is lost each year and hence many of these dams only have a life of 50 to 100 years. A modified method has recently been produced to better predict sediment trapping for reservoirs in tropical locations (Lewis et al., 2013).

In this exercise we will use the new dam trapping calculator to estimate the useful life of the Tinaroo Falls Dam. The Tinaroo Dam was completed in 1958 predominately to provide water for the Mareeba-Dimbulah Irrigation Area to support the tobacco industry. You have been provided with the ‘Dam trapping calculator' on LearnJCU to use as well as the daily inflow data into the dam (Barron River at Picnic Crossing gauge).

TASK- Questions you need to answer and submit

Question 1. How much volume of sediment has accumulated in the Tinaroo Falls Dam from the 1958/59 to the 2011/12 water years? Show your calculation steps clearly.

Question 2. Over this 54 year period, what percentage of the capacity of the Tinaroo Falls Dam has been lost due to sedimentation? Show your calculation steps clearly.

Question 3. Calculate the average annual percentage capacity loss over this period. From this annual percentage loss, how many years will it take for the capacity of the Tinaroo Falls Dam to be reduced by 50%? Show your calculation steps clearly.

Question 4. You have now discovered that the flow data (m3 per day) provided was a factor of 10 too high (note that the annual inflow data (in ML) in Table 1 is correct). When the correct data are re-entered into the ‘dam trapping calculator', the new trapping efficiency calculations suggest that 100% trapping has occurred in each water year since dam construction. How much does this change the estimation of the volume reduction in the Tinaroo Falls Dam by, and how long will it now take to reduce the dam capacity by 50%?
Show your calculation steps clearly.

Question 5. In 1972/73, a Government incentive was provided to the farmers upstream of the Tinaroo Falls Dam to widely adopt new management practices (such as to cover paddocks with trash organic matter), which reduced the amount of sediment eroded and delivered to the Tinaroo Falls Dam. This practice reduced the EMCs of the suspended sediment delivered to the dam from 1000 mg/L to 300 mg/L after this time. Assuming 100% trapping of this sediment (as in question 4), how much longer can the Tinaroo Falls Dam operate before reducing to 50% capacity with this change in management practice?
Show your calculation steps clearly.

Question 6. Many of recently built dams in China, India and Africa are losing 1 to 2% capacity per year due to sedimentation/siltation. How do these numbers compare to the Tinaroo Falls Dam and discuss what factors could be driving these sedimentation rates (list & discuss at least 2 factors)?

Attachment:- Dam Trapping exercise.rar