Purpose
An assessment tool was developed to simplify the process of determining minimum waste storage pond (WSP) dimensions in conjunction with assuring adequate evaporation of stored runoff over time. In contrast to typical WSP designs that require intermittent pumping and handling of waste water, evaporation ponds (EPs), as waste storage facilities used to contain storm runoff from feedlots, offer long-term passive management and may be acceptable alternatives where annual evaporation exceeds rainfall and where landowners deem typical WSP designs as impractical.
What did we do?
EPs are passive management structures that allow landowners to avoid having to periodically evacuate accumulated water from typical WSPs via pumping or other means (active management). Both EP and WSP systems are designed to collect and store contaminated runoff and sediment from feedlots, but it is assumed that the use or disposal of the contaminated water is practical in the case of WSPs. It is not always practical. Some ranchers operating in arid areas who manage small feedlots during the winter have expressed interest in EPs over WSPs stating that the passive management method will keep their costs low while fitting better with their current operating systems.
At the onset of our first EP project, we found no NRCS tool specific to the design of EPs. The principles outlined in our Agricultural Waste Management Handbook (United States Department of Agriculture, NRCS, 1999) regarding waste storage facility design and our guidance for runoff produced during a 25-year, 24-hour precipitation event using the NRCS Curve Number Method (United States Department of Agriculture, NRCS, 2004) still apply. Next, we chose to build a spreadsheet that allows the designer to perform the multitude of computations needed to consider distinct EP geometric configurations, time periods, and hydrology in the design of EPs. In a nutshell, the model consists of estimating amounts of water and sediment that the EP retains while accounting for water losses due to evaporation and any periodic removal of sediment (Figure 1) over time. For any given EP geometry, the design is considered viable so long as the water level does not supersede the elevation of the embankment top minus freeboard.
The mathematical functions applied are deterministic and follow Natural Resource Conservation Service (NRCS) guidelines (United States Department of Agriculture, NRCS, 1999). Mean monthly evaporation and precipitation data are available via the National Oceanic and Atmospheric Administration website, and runoff from the 25-year, 24-hour precipitation event is estimated using the NRCS Curve Number Method (United States Department of Agriculture, NRCS, 2004).
What have we learned?
1) The design of evaporation ponds, to be used as waste storage facilities, is complex, entails many variables, and typically requires multi-year tests.
2) Simulation of waste storage pond storage levels over time provides visual results that allow the designer to observe expected behavior of these systems and judge whether or not evaporation ponds are viable alternative systems to typical waste storage facilities in which frequent evacuation of storage materials is required.
3) Calculations using a spreadsheet are simplified to the extent that dozens of scenarios can be assembled and executed within a reasonable timeframe. This allows designers to study the results from such sensitivity analyses where one or more variables must be examined at numerous levels/values due to uncertain data at particular sites.
Future Plans
1) Train NRCS field staff regarding the potential pros and cons pertinent to these systems in comparison to others.
2) Encourage landowners to consider evaporation ponds as viable alternatives in areas with low precipitation/runoff.
3) Monitor site input variable values and the evaporation pond sediment and water levels over time to assess whether or not the models applied forecast overall evaporation pond behavior.
Author
Donald Hanson, Design Engineer, Natural Resources Conservation Service donald.hanson@wa.usda.gov
Additional information
Author’s contact via telephone: (509) 323-2949.
References
United States Department of Agriculture, NRCS (1999). National Engineering Handbook, Part 651, Agricultural Waste Management Field Handbook. Washington D.C.: Author. Retrieved 3/14/2014 at: http://directives.sc.egov.usda.gov/RollupViewer.aspx?hid=17092.
United States Department of Agriculture, NRCS (2004). National Engineering Handbook, Part 630, Estimation of Direct Runoff from Storm Rainfall. Washington D.C.: Author. Retrieved 1/6/2015 at: http://directives.sc.egov.usda.gov/RollupViewer.aspx?hid=17092.
Acknowledgements
I extend thanks to these NRCS cohorts for their cooperation and assistance during development of the tool and their comments and suggestions during reviews of this technical paper: Larry Johnson (State Conservation Engineer, Washington State), Joe Gasperi (State Geologist, Washington State), Sally Bredeweg (Environmental Engineer, West National Technical Service Center, Oregon State), Leigh Nelson (Water Management Engineer, National Water Management Center, Arkansas), and Noller Herbert (Director, Conservation Engineering Division, Washington, D.C.).
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