Managing Freshwater Resources in the Tropics – A New Perspective
Water is a critical resource. Growing population, coupled with urbanization, is causing serious concerns in ensuring long term sustainability of this resource in India. Developing countries such as India are facing a potential crisis situation- water resource management efforts are in complete disarray on one hand, and the demand for water is growing at an alarming rate on the other. Further, India faces a unique challenge as the annual monsoon rains are concentrated within the narrow time period of three months, causing high flow rates of rivers and consequent high volume of silt getting carried to storage reservoirs.
Heavy siltation of our freshwater systems causes flooding of the plains downstream during wet spells, and water shortages during dry spells. Freshwater management consists of two major factors – firstly, storage systems that hold water to prevent flooding and for use during dry spells, and secondly, their alternative, which is recharge areas that facilitate recharging of the ground water. The type of the surface layer determines the effectiveness of the storage or recharge system. Any change in the surface layer due to siltation or improper desilting will render these systems ineffective or even counterproductive. For example, if the top later is dredged incorrectly, then a recharge system may start behaving like a storage system and vice versa.
Our own city of Pune is fed through a system of dams that store water collected in the monsoons for the long dry spell.
Thus, there is a critical need to urgently desilt these storage dams and reservoirs. This has to be done carefully as well; unlike recharge water bodies, storage systems need to ensure minimal seepage to the ground water systems. Thus, desilting efforts have to ensure that we do not disturb the sediment layer that retains the water within the storage system. The precise identification of the sediment layers is thus very critical to plan desilting efforts.
Sediment classification in reservoirs, lakes, rivers and many other water systems is a critical component of the resource management of these freshwater bodies. Multiple stakeholders need precise sediment classification for various purposes. For instance, fresh water management authorities require sediment classification to desilt water bodies and enhance their storage capacity; flood management agencies, to minimise spillages; underwater archaeology researchers, to study the evolution of the layers; instrumentation engineers, to understand sensor behaviour and the impact of the underwater medium; and many more.
While classifying sediment, sonar technology is used to identify the silt type. When sediment classification is done precisely using effective sonars, it can optimise the dredging cost, and enable the use of appropriate dredging equipment which, in turn, can minimise the damages. There are multiple dredging techniques available, but each has its own type and the costs associated vary from type to type. By classifying sediment precisely, one can manage the removed silt effectively, and thus offset these costs. The classical sediment analysis method of coring is highly resource intensive and localised, whereas using the acoustic technique can be highly resource efficient. This is because the acoustic technique deploys sonars which facilitate large coverage of underwater area.
However, in tropical shallow water conditions, sonars are also likely to perform less optimally for any credible underwater application due to random fluctuation of surface parameters during the day as well as across seasons. Further, import of sonar technology in the absence of local scientific inputs through field experiments has not been very beneficial. Extensive field efforts are required to first understand the unique medium characteristics of our tropical underwater regions so that the sonar signal distortions occurring due to the random fluctuations during the day and across seasons can be mitigated effectively.
The issues do not stop here. Removal of silt leads to a bigger issue of managing the dredged material. The use and value of the silt is determined by its type. It is often recommended that silt, which is very fertile, be dumped in agricultural fields. However, fields have a very narrow window available for accepting the silt, which is not enough for the entire process to be workable. The volume of silt is too massive to be dumped on the fields. The transport cost would be prohibitive and would not make any economic sense in a developing country like ours.
Instead, this material can be used for making bricks; there are now technologies available that can take the silt as input and generate very high number of bricks for construction purposes. This will effectively address our infrastructure requirements while offsetting transportation costs.
Desilting requires precise information about the type of silt deposited in these systems. Conventional desilting efforts are highly resource-intensive and developing nations with limited resources have to walk a tight rope balancing socio-economic and political imperatives with such huge budgetary allocations. That is probably the reason why hardly any desilting efforts have taken place in our country yet. There is no doubt or debate regarding the importance of sediment analysis and the urgency of undertaking desilting.
The entire desilting plan with costing and possible offsets will have to be brought out with advantages over the conventional methods.
Dr. (Cdr.) Arnab Das
Director, Maritime Research Center, Pune