Appendix B: Rainwater Catchment

Transcription

1 Appendix B: Rainwater Catchment

2 Introduction Rainwater catchment can be a viable option for providing potable water to a community, such as the rural Honduran village of La Fortuna. In a typical rainwater catchment system, a gutter attached to the roof transfers rain from the roof to a cistern located next to the home. In contrast to common centralized water distribution systems, rainwater catchment systems are decentralized systems. Decentralized systems work well for scattered villages like La Fortuna. However, four requirements must be met for optimal usage of the system: 1. There must be enough rainfall all year to either use immediately or store in a cistern. 2. Roofs used for rainwater collection must be made of metal or another smooth material to facilitate maximum collection. 3. Roofs must provide adequate surface area to collect enough water for consumption. 4. The system must provide at least 20 liters of water per person per day for a sustainable level of development, as specified by the World Health Organization. Objectives 1. To determine if rainwater catchment is an adequate method to provide La Fortuna with potable water. 2. To design a rainwater catchment system that could be easily adapted to other villages. Design Attachment B-1 shows the location of various sized houses, and thus various sized roofs, in La Fortuna. Roof sizes are categorized as small, medium, and large. The larger a roof, the more rainwater it can collect. Calculations indicate that the typical small roof cannot provide 20 L water/person/day, a medium roof meets this requirement for 2 months of the year, and a large roof meets this requirement for 9 months of the year. If this standard is lowered to 15 L water/person/day, the small houses still cannot provide enough water, the medium houses can provide enough water for 5months of the year, and the large houses can provide enough water for 10 months of the year. Given these results, rainwater catchment at individual homes is not a viable option for La Fortuna. Limitations Short-term collection of rainfall data reduces accuracy. Roof sizes are not necessarily sufficient for consistent supply. Roof conditions are not necessarily capable of or safe for use as catchment surface. Availability and transportability of materials can be difficult. Treatment of water stored in cistern for an extended time is as of yet undetermined. Results Using the design parameters specified by the Christian water development organization Lifewater International, a capacity of 140 liters is appropriate for a small house to supply each

3 member of the household with 15 liters per person per day (Lifewater 1999). However, a cistern this large does not cost significantly less than the 560-liter cistern design chosen for a typical medium-sized house. With a 560-liter cistern, a typical household of 6 people residing in a house with approximately 680 square feet of roof could have 25 liters of water/person/day throughout the year. This cistern would be a reinforced concrete rectangular prism with dimensions of 1.5 meters by 1.5 meters by 2.25 meters. The plans for such a cistern are shown in Attachment B-7 (Lifewater 1999). At a large house, a larger cistern, about 5000 liters, is a typical design choice, but due to its excessive cost, the same 560-liter cistern is appropriate for large houses as well. It would provide a sufficient amount of water per person and allow for much easier and less expensive fabrication. Therefore, all houses in La Fortuna would use the medium-sized cistern. While these results are reasonably accurate, they do not account for all the contributing factors of rainwater catchment. Further analyses should be made to analyze drought conditions, the effects of long-term storage, and the possibility of water rationing during months with less precipitation. Materials and Costs Before a rainwater catchment system can be installed, the roof must be able to properly catch and distribute the rain. In La Fortuna, this requires replacing some existing roofs with better materials. A total of 25 houses need their roofs replaced or repaired before a rainwater catchment system could be implemented. The distribution of these houses based on size was shown previously as Attachment B-2. Roof replacement costs range from $50 to $1,400, depending on the size of the roof. For ranges of cost for small, medium, and large roofs, refer to Attachment B-6. The materials needed to construct a cistern include cement, steel reinforcing rods, sand, gravel, and water. Refer to Attachment B-7 for unit costs. Each medium-sized cistern costs $50. This excludes the cost of sand, gravel, and water because the village supplies these items at no cost. Conclusion Rainwater catchment is a viable option for some houses in La Fortuna. More than half the houses can collect 15 liters/person/day, while the rest of the houses can collect 25 liters/person/day or more. However, these estimates are only valid for the rainy season. During the dry season, few households could collect enough water through rainwater catchment to survive. Because of this shortage and inconsistency of water supply, rainwater catchment in La Fortuna does not meet the objectives defined in this study, and therefore is not recommended for use in La Fortuna. However, the methods of data interpretation and the design of the rainwater catchment system can be applied to other villages under different conditions.

4 Attachments Attachment B-1. Small, Medium, and Large Houses

5 Attachment B-2. Roof Materials. Clay tile and corrugated iron conducive to rainwater catchment.

6 Attachment B-3. House Size Distribution and Averages

7 House Size Roof Area (ft 2 ) Number of Average Roof Roofs to be People per Average Number Houses Area (ft 2 ) Replaced Household of People Small < Medium 500-1, Large > 1, Attachment B-4. Water Supply for Typical Small, Medium, and Large Houses Typical Small House (336 ft 2 roof, 5 people) Month Collectable Usage/month Rainfall/month (L) Delta/month (L) January February March April May June July August September October November December Water surplus or storage: -10,127 Typical Medium House (680 ft 2, 6 people) Collectable Usage/month Delta/month L water/ person/day Rainfall/month (L) (L) (x > 20 L?) Water surplus or storage: 7,618 2 Typical Large House (2000 ft 2, 4 people) Collectable Usage/month Delta/month L water/ person/day Rainfall/month (L) (L) (x > 20 L?) Water surplus

8 Attachment B-5. Cistern Plans (Lifewater 1999) Attachment B-6. Ranges of Project Costs Cost Range for Replacing Roofs Cistern Cost Area (m 2 ) Zinc cost ($) # Houses Cost Min Max Min Max Small $ $ $ 1, Medium $ $ $ Large $ $ 1, $ Total Needed Roof Replacement 945 $ Total Cistern Cost $ 1, Total Cost $ 2,084.62

9 Attachment B-7. Unit Costs of Materials Material Unit amount Unit cost Cement 50 kg $ 6.00 Zinc sheeting for roofs m 2 $ 0.46 Steel reinforcing rods kg $ 0.44 Sand Gravel Village provides for free. Water

10 Works Cited Lifewater International, Designing a household cistern.