Optimal Storage Volumes forRainwater Catchment Systems in           Alaska                     ByGreta Myerchin, Amy Tidwe...
TABLE OF CONTENTSINTRODUCTION................................................................................................
INTRODUCTIONRainwater Catchment System and Storage RequirementRainwater catchment systems are designed to capture and stor...
STORAGE RESERVOIR DESIGNA variety of factors influence storage requirements. In this study, factors relevant toAlaskan rai...
Roof Collection AreaRainwater catchment area was an additional model input parameter. In order toeffectively interpret the...
PROCEDURECommunity SelectionSelection of the modeled communities was based upon population, climaticcharacteristics, and l...
RESULTSAs described above, optimal storage reservoir volumes were estimated for twelveAlaskan communities. The resultant e...
SUMMARYTwo-thirds of Alaska’s population is serviced by a public water supply, while theremaining third utilize independen...
REFERENCES    1. Greer Tank and Welding, Inc.; http://www.greertank.com/PolyTanks.htm (2007)    2. Hinzman, L.D.; The Inte...
Appendix A        Design Year Climate Data for Selected Communities                                                       ...
Appendix B                 Calculation of Current Storage ConditionThe current storage on any given day was calculated acc...
Appendix C    Graphs of Optimal Rainwater Storage Volume for Twelve                      Alaskan LocationsOptimal Storage ...
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   11ATTAC: Alaska Training/Technical Assistance Center
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   12ATTAC: Alaska Training/Technical Assistance Center
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   13ATTAC: Alaska Training/Technical Assistance Center
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   14ATTAC: Alaska Training/Technical Assistance Center
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   15ATTAC: Alaska Training/Technical Assistance Center
Optimal Storage Volumes for Rainwater Catchment Systems in Alaska   16ATTAC: Alaska Training/Technical Assistance Center
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Alaska; Optimal Storage Volumes For Rainwater Catchment Systems In Alaska

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Alaska; Optimal Storage Volumes For Rainwater Catchment Systems In Alaska

  1. 1. Optimal Storage Volumes forRainwater Catchment Systems in Alaska ByGreta Myerchin, Amy Tidwell, Bill Schnabel and Daniel White March 2008 ATTAC Research Report # 200801
  2. 2. TABLE OF CONTENTSINTRODUCTION............................................................................................................. 1 Rainwater Catchment System and Storage Requirement ............................................... 1 Objective ......................................................................................................................... 1STORAGE RESERVOIR DESIGN................................................................................ 2 Storage Reservoir Capacity............................................................................................. 2 Water Usage.................................................................................................................... 2 Roof Collection Area ...................................................................................................... 3 Climate and Threshold Temperature .............................................................................. 3 Precipitation Design Year ............................................................................................... 3PROCEDURE ................................................................................................................... 4 Community Selection...................................................................................................... 4 Optimization Protocol..................................................................................................... 4RESULTS .......................................................................................................................... 5SUMMARY ....................................................................................................................... 6REFERENCES.................................................................................................................. 7APPENDIX A: Design Year Climate Data for Selected Communities............................ 8APPENDIX B: Calculation of Current Storage Condition............................................... 9APPENDIX C: Graphs of Optimal Rainwater Storage Volume for Twelve AlaskanLocations........................................................................................................................... 10Optimal Storage Volumes for Rainwater Catchment Systems in Alaska iATTAC: Alaska Training/Technical Assistance Center
  3. 3. INTRODUCTIONRainwater Catchment System and Storage RequirementRainwater catchment systems are designed to capture and store rainwater for use as aprimary or supplemental water source. In residential homes, stored rainwater can be usedfor landscape irrigation or household services such as toilets and laundry. Collectedrainwater may also be used as a source of potable water, although it is recommended thatthe stored water be properly tested and/or treated prior to this use.Design of rainwater catchment systems requires careful consideration of storage capacity.Inappropriate sizing of a storage tank or reservoir can negatively affect a homeowner inmany ways. Overestimation of the storage requirement can result in an oversized andneedlessly expensive storage system. Undersized storage tanks, on the other hand, maynot fulfill the homeowner’s water use needs.ObjectiveThe objective of this report is to present computer modeling results intended to estimateoptimal rainwater catchment system storage requirements. The resultant optimal storagevolumes were calculated as a function of water demand, catchment area and localclimatic conditions for twelve Alaskan localities. The data are intended to serve asstorage basin design guidelines to be used during system planning. The guidelines areintended to supplement, but not replace, the professional judgment of experienced systemdesigners with site-specific knowledge.This reports describes the optimal storage capacity for systems designed to collect andstore rainwater during the operational season only (i.e., during non-frozen conditions).Optimal storage volume is defined here as the minimum tank volume necessary to meetwater use demand through the operational season while maintaining a reserve capacity ofat least 5%.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 1ATTAC: Alaska Training/Technical Assistance Center
  4. 4. STORAGE RESERVOIR DESIGNA variety of factors influence storage requirements. In this study, factors relevant toAlaskan rainwater catchment systems were considered. Such factors included catchmentsurface area, collection efficiency, water use rates, and climatic characteristics.Storage Reservoir CapacityThis study evaluated storage reservoirs with capacities ranging from 100 to 20,000gallons. Storage tanks fabricated to hold volumes of 100 gallons or less are often usedfor transport or as break tanks, but are not often used as primary rainwater storage tanks.In regions of Alaska where freezing conditions persist, residential exterior tank sizes donot commonly exceed 2,500 gallons, based on physical and insulation restrictions as wellas product availability (Greer, 2007). In circumstances requiring storage of largevolumes (i.e., greater than 2,500 gallons), interior or underground cisterns may beutilized. Alternatively, captured rainwater can be routed to natural ponds or engineeredimpound structures for storage.Water UsageAn estimate of water demand was required in order to estimate the appropriate rainwaterreservoir volume. In practice, water demand varies from person to person and accordingto lifestyle considerations. While water usage in most Alaskan households is similar toaverage US household water use rates, a relatively large proportion of Alaskans reside inlimited water use households. Examples of limited use households include those withoutflush toilets, clothes washers, or shower facilities. An estimate of twenty gallons percapita per day (gpcd) is commonly used when households contain no flush toilet. If ahousehold contains a flush toilet, water use can increase to 70 gpcd (Johanson, 2002).Other authors estimate the household water use for a typical family of four to range from150 to 200 gpd (Siefert, 2004). This report presents results based upon assumedhousehold water use rates ranging from 5 gpd to 250 gpd.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 2ATTAC: Alaska Training/Technical Assistance Center
  5. 5. Roof Collection AreaRainwater catchment area was an additional model input parameter. In order toeffectively interpret the model results, a system designer must estimate the catchmentsurface area of the system being designed. If the entire home roof surface is used forrainwater collection, this can be accomplished by calculating the horizontal surface areaunder the roof, including the eaves. Reducing the estimated area by 10% will account forloss by flushing, overflow, and other loss factors (Johanson, 2002).Climate and Threshold TemperatureIn this study, it was assumed that liquid precipitation was available for rainwatercatchment, but frozen precipitation (i.e., snow, sleet) was only available as springsnowmelt. Additionally, it was assumed that the total volume of accumulated snow in thecatchment system was sufficient enough to fully recharge the storage reservoir uponmelting. Consequently, each modeled operational season began in the spring with a fullreservoir.The modeled operational seasons in this report were bound by threshold periods, definedas 15-day periods during which the average daily air temperature was above or belowspecified threshold temperatures. As a result, the operational periods were considered tobegin on Day 1 of the thaw threshold period, and end on Day 15 of the freeze-upthreshold period. In this study, the mean threshold temperature used for calculating thawwas 0.1 oC, while the mean threshold temperature used for calculating freeze-up was -1.9o C, based upon values recommended by Hinzman (1990).Precipitation Design YearThe model output presented in this study was based upon typical precipitation patternsexperienced in specific Alaskan localities, as represented by the site-specific design year.The design years represented actual climate data (e.g., precipitation patterns and thawseason length) considered to most closely match average conditions for each site studied.A table located in Appendix A provides further information with respect to the designyear characteristics of each modeled location.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 3ATTAC: Alaska Training/Technical Assistance Center
  6. 6. PROCEDURECommunity SelectionSelection of the modeled communities was based upon population, climaticcharacteristics, and location. Twelve relatively large regional hubs were selected in orderto represent a range of climatic characteristics experienced in various regions throughoutAlaska. The modeled communities included Anchorage, Barrow, Bethel, Delta Junction,Dutch Harbor, Fairbanks, Juneau, Ketchikan, Kotzebue, Nome, Tok, and Valdez.Descriptions of the climatic characteristics for each listed community are presented inAppendix A.Optimization ProtocolAn automated evaluation process was devised to estimate the optimal storage volumerequired for each community. The procedure was written in Visual Basic forApplications and was implemented within Microsoft Excel software. The program firstdetermined a representative year for design purposes. This design year was selected froma record of site-specific climate data, and represented the year most indicative of averagesite conditions. Next, the program calculated individual tank profiles for a combinationof collection areas, usage rates, and storage capacities. The resultant tank profiles weregraphic representations of the daily water balance, as summarized in the followingequation: Current Storage = Initial Storage + Precipitation Inflow – Water ConsumedPlease see Appendix B for a more detailed description of the water balance calculations.Storage optimization graphs, located in Appendix C, were prepared for each communitybased upon the results of the water balance algorithm. From these graphs, users candetermine the estimated optimal rainwater storage volume under a given set ofconditions. As described previously, optimal storage capacity is defined as the volume ofstorage necessary to meet the water use requirements at all times during the operationalseason while maintaining a 5% reserve volume.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 4ATTAC: Alaska Training/Technical Assistance Center
  7. 7. RESULTSAs described above, optimal storage reservoir volumes were estimated for twelveAlaskan communities. The resultant estimated optimal volumes are presented in thegraphs in Appendix C. In order to obtain the optimal storage volume for a system underconsideration, the reader is asked to perform the following steps: 1) Identify the listed community with a climate most similar to that at the location of the system being designed. Use the table presented in Appendix A and/or other climate resources for this step. Use this community’s graph (Appendix C) for the remainder of this procedure. 2) Estimate the anticipated water demand (gpd) and catchment surface area (ft2) for the system under consideration, using recommendations provided in this report or elsewhere. 3) On the appropriate community-specific graph, select the surface area curve corresponding to the catchment surface area of the system under design. Identify the point on that curve corresponding to the anticipated water demand listed on the vertical axis. 4) The optimal reservoir volume is represented by the point on the horizontal axis underlying the point at which the selected surface area curve meets the level of anticipated water demand.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 5ATTAC: Alaska Training/Technical Assistance Center
  8. 8. SUMMARYTwo-thirds of Alaska’s population is serviced by a public water supply, while theremaining third utilize independent sources such as private wells or commercial delivery(USGS, 2004). Rainwater collection is an alternative independent water supply practicethat can not only benefit households with little or no available water, but can also reducethe demand on municipally-managed water resources. In order to aid in the design ofrainwater catchment systems statewide, this study was conducted to estimate optimalrainwater storage requirements for a group of Alaskan communities experiencing a rangeof climate conditions. Optimal rainwater storage volumes for twelve Alaskan locationscan be determined from the graphs in Appendix C.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 6ATTAC: Alaska Training/Technical Assistance Center
  9. 9. REFERENCES 1. Greer Tank and Welding, Inc.; http://www.greertank.com/PolyTanks.htm (2007) 2. Hinzman, L.D.; The Interdependence of the Thermal and Hydrologic Processes of an Arctic Watershed and their Response to Climate Change; (1990) University of Alaska, Fairbanks. 3. Siefert, R. D.; Suggestions for Installing Domestic Water Storage Tanks; (2004) University of Alaska, Fairbanks Cooperative Extension Service; Building in Alaska Series; #HCM-04950. 4. Alaska Division of Community Advocacy (ADCA) website: http://www.commerce.state.ak.us/dca/commdb/CF_COMDB.htm. Demographic information for Ketchikan and Fairbanks. 5. Translators: Johanson N., Seifert R.D. Water Cistern Construction for Small House; (2004) University of Alaska, Fairbanks Cooperative Extension Service; Alaska Building Research Series; #HCM-01557. From a Norwegian publication: BYGGFORSK, NBI A515.161Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 7ATTAC: Alaska Training/Technical Assistance Center
  10. 10. Appendix A Design Year Climate Data for Selected Communities Operational Operational Operatioal Season End Design Available Season Length Season Begin DateCommunity Year Rainfall (inches) (days) Date (Thaw) (Freezeup)Anchorage 1977 11.3 212 26-Mar 24-OctBarrow 1986 2.6 107 5-Jun 20-SepBethel 1978 11.0 171 11-Apr 29-SepDelta Junction 1960 8.8 172 14-Apr 3-OctDutch Harbor 1997 54.7 338 1-Jan 5-DecFairbanks 1978 6.9 179 4-Apr 30-SepJuneau 2001 45.0 270 20-Feb 17-NovKetchikan 1952 147.4 333 31-Jan 29-DecKotzebue 1977 6.1 153 4-May 4-OctNome 1997 10.4 167 21-Apr 5-OctTok 1985 6.7 172 17-Apr 6-OctValdez 1976 46.3 243 27-Mar 25-NovOptimal Storage Volumes for Rainwater Catchment Systems in Alaska 8ATTAC: Alaska Training/Technical Assistance Center
  11. 11. Appendix B Calculation of Current Storage ConditionThe current storage on any given day was calculated according to the following equation: S (k ) = S (k − 1) + P(k − 1) ∗ SA − U (k − 1) , whereS(k) = Current day stored water volumeS(k-1) = Previous day stored water volumeP(k-1) = Previous day precipitationSA = Rainwater catchment surface areaU(k-1) = Previous day water comsumption.The equation quantified the available water at the start of any given day as a function ofthe existing stored volume, the tank recharge, and water consumption. Two physicalconstraints imposed on the modeled scenarios dictated that the total storage term couldneither be negative nor could it exceed a specified tank volume.Please direct further inquiries regarding the modeling methods or results to the authors ofthis study. They can be located at the University of Alaska Fairbanks Institute ofNorthern Engineering, http://www.alaska.edu/uaf/cem/ine/.Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 9ATTAC: Alaska Training/Technical Assistance Center
  12. 12. Appendix C Graphs of Optimal Rainwater Storage Volume for Twelve Alaskan LocationsOptimal Storage Volumes for Rainwater Catchment Systems in Alaska 10ATTAC: Alaska Training/Technical Assistance Center
  13. 13. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 11ATTAC: Alaska Training/Technical Assistance Center
  14. 14. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 12ATTAC: Alaska Training/Technical Assistance Center
  15. 15. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 13ATTAC: Alaska Training/Technical Assistance Center
  16. 16. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 14ATTAC: Alaska Training/Technical Assistance Center
  17. 17. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 15ATTAC: Alaska Training/Technical Assistance Center
  18. 18. Optimal Storage Volumes for Rainwater Catchment Systems in Alaska 16ATTAC: Alaska Training/Technical Assistance Center

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