Lesson 6: Well Hydraulics PDF

NAME: _______________________________________________ SCORE: CLASS SECTION & SCHEDULE: __________________________ LESSON 6: WELL HYDRAULICS INTENDED LEARNING OUTCOME After finishing this chapter and practicing the sample problems at the...

NAME: _______________________________________________ SCORE: CLASS SECTION & SCHEDULE: __________________________ LESSON 6: WELL HYDRAULICS INTENDED LEARNING OUTCOME After finishing this chapter and practicing the sample problems at the end of the module, you will have the ability to:  Understand the relationship between wells and aquifers.  Examine the impact of pumping on groundwater levels and flow.  Create and assess wells for best results.  Interpret well test data to determine aquifer properties. DISCUSSION Freshwater is an essential asset, and groundwater plays a pivotal role in fulfilling our needs. Wells act as the main link between us and this concealed gem. Well hydraulics explores the scientific and engineering principles that govern the movement of water within and around wells, allowing us to extract groundwater e ectively and in a sustainable manner. Well Hydraulics is the study of flow of water in and out of wells. It helps us understand how water moves through the ground and how we can manage our groundwater resources. As water drains through the soil, it flows through the root zone and then through a zone referred to as unsaturated zone (also called as vadose zone or zone of aeration). The pores of the geologic material in the unsaturated zone are partially filled with water. The remaining portion is filled with air. The water then continues to migrate vertically down through the soil until it reaches a level at which all the openings or voids in the soils are filled with water. This zone is what we called as zone of saturation, the saturated zone or phreatic zone. The water in the zone of saturation is what we called as groundwater. QUESTION: True or False: Wells can only be drilled straight down. True, sometimes, straight well drilling is not feasible due to surface obstacles or environmental issues. In such cases, angled drilling becomes necessary to reach the water source. Angled drilling can also be beneficial when dealing with sloped aquifers, enabling access to the most productive areas for optimal water output. Moreover, if there is contamination near the desired well site, angled drilling provides a solution to avoid those areas and guarantee access to clean water. SPECIFIC YIELD The percentage of water that is free to drain from the aquifer under the influence of gravity. It is not equal to porosity because the molecular and surface tension forces in the pore spaces keep some of the water in the voids. It reflects the amount of water available for development. STORAGE COEFFICIENT This parameter is akin to specific yield. It is the volume of available water resulting from a decline in the piezometric surface over a unit horizontal cross-sectional area. This parameter has a unit of m3 m-3. Figure 6.1 | Specific Yield Lesson 6: WELL HYDRAULICS Prepared by: MNVillacarlos  Table 6.1 | Specific Yield per material CONE OF DEPRESSION When the well is pumped, the level of the piezometric surface in the vicinity of the well will be lowered. This lowering, or drawdown, causes the piezometric surface to take the shape of an inverted cone called a cone of depression. This coe icient has a unit of m2 s-1. The shape of the cone of depression depends on the characteristics of the water-bearing formation. Shallow and wide cones for coarse sand and gravel. While deeper and narrow cones for fine sand and sandy clay. As the pumping rate increases, the drawdown increases. The slope of the cone also steepens. See Figure 6.4 Two wells should be placed no closer together than two times the thickness of water-bearing strata. For more than two wells, they should be spaced at least 75 meters apart. Figure 6.2 | Cone of Depression Figure 6.3 | Di erence between deep and shallow wells Lesson 6: WELL HYDRAULICS Prepared by: MNVillacarlos STEADY FLOW IN A CONFINED AQUIFER Table 6.2 | Terms Transmissibility T or K m ·s 2 −1 Thickness of artesian aquifer D m Height of the piezometric surface above h1 m confining layer at a distance, r1, from the pumping well Height of the piezometric surface above h2 m confining layer at a distance, r2, from the pumping well Radius from pumping well r1, r2 m Logarithm to base e ln SAMPLE PROBLEM: STEADY FLOW IN UNCONFINED AQUIFER An artesian aquifer 10m thick with a piezometric surface 40m above the bottom confining layer is being pumped by a fully penetrating well. Steady state drawdowns of 5m and 1m were observed at two non-pumping wells located 20m and 200m, respectively, from the pumped well. The pumped well is being pumped at a rate of 0.016 m 3s-1. Determine the hydraulic conductivity of the aquifer. *See Table 6.2 SAMPLE PROBLEM: A 0.50m diameter well fully penetrates an unconfined aquifer, which is 30.0m thick. The drawdown at the pumped well is 10.0m and the hydraulic conductivity of the gravel aquifer is 6.4 x 10-3 m s-1. If the flow is steady and the discharge is 0.014 m s-1, determine the drawdown at a site 100m from well. Lesson 6: WELL HYDRAULICS Prepared by: MNVillacarlos  Although Siquijor does not have any major rivers, it does have 3 minor rivers. These rivers have not been thoroughly evaluated yet but could be considered as potential future water sources. Additionally, the region has several lakes, dams, and reservoirs that hold promise as water sources for irrigation, domestic, and industrial purposes. Some lakes are also being developed for eco-tourism ventures. GROUNDWATER WATER Groundwater is the region's primary water source for domestic, CENTRAL VISAYAS TOTAL WATER RESOURCES agricultural, and industrial purposes. The majority (87%) of groundwater sources in Central Visayas are accessed through shallow and deep wells. There are approximately 28,504 wells in Region VII. Negros Oriental has the highest number of privately-owned shallow wells, while Cebu has the most publicly-owned deep wells. Unfortunately, many groundwater extractions are taking place without the necessary permits, leading to unregulated withdrawal and excessive use of water. According to the National Water Resources Board (NWRB), In addition to groundwater, springs are also widely utilized for Central Visayas has a total water resources potential of 2,939 water supply in the region. Among the provinces, Bohol has the million cubic meters (mcm). highest number of springs being developed as water sources. Out of this, 2,060 mcm (70%) comes from surface water WATER RIGHTS sources, and 879 mcm (30%) comes from groundwater sources. Water rights in the Philippines are governed by the Water Code of the Philippines (Presidential Decree No. 1067) and the National Water Resources Board (NWRB), which is the agency The surface water potential of Central Visayas accounts for just 2% of the total surface water potential in the country. Similarly, responsible for regulating and managing water resources in the the groundwater sources in the region make up only 4% of the country. total groundwater potential in the country. The state holds ownership and control over all water resources, When we consider the water available per person in the region, including surface water and groundwater. including both surface and groundwater sources, it amounts to Water rights are granted through water permits issued by the only 425 cubic meters per capita per year. This is significantly below the standard requirement of 1,000 cubic meters per capita NWRB where it defines the conditions and terms under which set by the World Resources Institute, indicating a scarcity of water can be extracted, used, and managed. water resources. There are di erent types of water permits issued by the NWRB, depending on the purpose of water use: SURFACE WATER Surface water is a significant water source in the region,  domestic, primarily sourced from rivers. Region VII has approximately 30  agricultural, major rivers that have the potential to supply water.  industrial,  commercial, and other specific purposes Lesson 6: WELL HYDRAULICS Prepared by: MNVillacarlos Water rights holders are required to comply with the terms and conditions stated in their permits, including limitations on the quantity and quality of water use. CAN WE REVOKE WATER PERMITS? Water permits are subject to renewal and can be modified or revoked by the NWRB if there are violations or changes in the water management policies and priorities. The NWRB also regulates the collection of water fees and charges to ensure equitable and sustainable water resource management. Overall, water rights in the Philippines aim to balance the needs of various users while ensuring the sustainable and responsible use of water resources. Lesson 6: WELL HYDRAULICS Prepared by: MNVillacarlos