Integrated and Sustainable Water Management Plan for a Growing City - A Case Study (Part 3)
This three-part blog series is derived from an accepted paper that was published as part of the proceedings of the 2020 World Environmental and Water Resources Congress, originally scheduled for May 17-21 in Henderson, Nevada, sponsored by the Environmental and Water Resources Institute (EWRI) of the American Society of Civil Engineers (ASCE). The event was cancelled due to the COVID-19 pandemic.
Part 1 of this series provided background information on the discussions and decisions that were made leading up to this unique and progressive water management plan.
Part 2 provided location information, river hydrology and water rights information, and a detailed description of the proposed water supply plan.
This week’s blog, the final part of our three-part blog describes the workings of the point-flow/exchange model created by Lytle Water Solutions (LWS).
POINT-FLOW AND EXCHANGE MODEL
Rueter-Hess Reservoir - future capacity
Because of the complexity of the issues related to understanding the physical and legal water availability of water, the diversion flow rates needed from the river at various locations, the exchange potential to move water upstream without the need for approximately 50 miles of pipeline, the storage volumes needed, the locations of storage, the ability to firm yields for delivery into a 100-mile pipeline, the pipeline flow rates and sizings, and estimates of the total municipal and agricultural yield of the project, a model necessarily had to be constructed to assess all of these variables.
LWS has constructed a point-flow and exchange model, using a base model initially developed by Mr. Ken Fritzler, a Lower South Platte Water Conservancy District (LSPWCD) Board member. The model has a daily time step so river call data, as well as changing flow and diversion records can be evaluated. The period of the model is 1999 through 2014, which results in a model that covers multiple hydrologic cycles, including wet, dry, and average flow years. As such, the effect of flow variations has been incorporated into the model.
The LWS model was built in Excel and contains daily data related to the following parameters:
Daily South Platte River flows at gages from Denver to Julesburg (at the Nebraska state line (a distance of 220 river miles);
Daily diversions by ditch and reservoirs in the study reach (Figure 4, shown in Part 2);
Daily water right call records;
Elevation-area-capacity (EAC) tables for each reservoir;
Evaporation rates from free water surfaces; and
Precipitation records.
By integrating the factors described above with assumptions related to operations of the project, the LWS model can then evaluate the numerous variables that need to be quantified for both Parker Water & Sanitation District (PWSD) and LSPWCD, related to expected yields as well as costs. In this way the model has been used to assess the technical and economic feasibility of the proposed water supply project. The most significant of the more than 50 variables assessed in the model include the following:
Diversion rates from the South Platte River to Iliff and Prewitt Reservoirs;
Storage volumes in the all three Reservoirs;
Seepage rates in all three reservoirs;
Minimum flow left in the South Platte River during exchanges to satisfy downstream water rights, as well as the interstate Compact;
Ditch losses in all ditches in the exchange reach;
Consideration of Denver 1940 Agreement future changes in river flows from measured historic flows;
Exchange rate from Iliff to Prewitt Inlet Canal;
Percentages of free river flow which is diverted during the irrigation season and non-irrigation season;
RO Plant losses; and
Flow rates for all pipelines.
To facilitate the functionality of the model, a dashboard was created that shows the flow chart of all of the variables described above. A screenshot of this dashboard is shown in Figure 8: Any variable can be modified on this dashboard and the model recalculates based on that change in approximately one second. Therefore, extensive sensitivity analyses can be completed almost instantaneously. In fact, the model has been used in meetings to test variables on the spot so that decisions can be made with the contemporaneous input by all of the parties.
Figure 8: Screenshot of flow chart dashboard for point-flow model.
The success and speed of the model is largely attributable to the model’s design. Since it was always envisioned that extensive changes and project design evolution would be a factor, the model was made using many small step calculations, as opposed to few complex formulas. This allows for faster calculations for sensitivity testing of hundreds of automated runs, faster and easier quality assurance/quality control of the model, and makes the model much easier to modify. This individual cell simplicity allows the more than 650,000 cells in the core of the model to be run with a new set of variables in less than one second.
While the individual calculations are restricted in their complexity, the model has become extremely complex. For example, the amount of water released from the downstream reservoir is dependent upon not less than fifteen values in the model, ranging from simple factors like the volume of water available to exchange, to evaporation rates in the third and final reservoir in the system being considered as a loss that can be offset in real-time by increasing the exchange potential by a like amount. Every calculation in the model is similarly dependent on other values in the model to ensure that the model is operating at maximum efficiency. This model has provided an invaluable tool in developing the preliminary design of a very large and complex water supply project. The model has provided the initial design for:
Iliff Reservoir = 4,000 ac-ft
Combined river diversion rate to Iliff Reservoir = 80 cfs
Yield from Iliff Reservoir to LSPWCD downstream users = 1,500 ac-ft/yr
Maximum exchange rate = 150 cfs
Average annual exchange volume = 9,000 ac-ft/yr
Exchange efficiency = 81%
Additional Prewitt Inlet Canal capacity = 305 cfs
Junior water right average annual diversion = 60,000 ac-ft
Maximum Prewitt Reservoir forebay capacity = 15,000 ac-ft
Maximum pipeline capacity from Prewitt to Fremont Butte Reservoir = 200 cfs
Maximum storage at Fremont Butte Reservoir = 72,000 ac-ft
Pipeline size to Rueter-Hess Reservoir = 36 inches
Maximum capacity of PWSD water delivery line = 35 cfs
Average annual yield to Parker = 20,500 ac-ft
Maximum annual yield to Parker = 25,409 ac-ft
Average annual yield to LSPWCD users from Fremont Butte =11,100 ac-ft
Figure 9: Preliminary design features.
As these data show, the LWS model can provide statistics on every facet of the water supply project so preliminary design work can be initiated. The preliminary design features of the project are shown in Figure 9.
SUMMARY
The PWSD and LSPWCD have been working collaboratively to develop a plan that provides significant benefits to both entities, while also being an environmentally-sound project. This innovative water supply plan helps meet a portion of the water supply “gap” identified in the 2015 Colorado Water Plan and does so without requiring the traditional permanent buy and dry concept where agricultural lands are taken out of production, which potentially can cause environmental damage to the river system. In this way fertile agricultural lands are not retired, as that not only reduces agricultural productivity that is a vital part of Colorado’s economy, but retirement of irrigated land can also have a domino effect by negatively affecting many aspects of rural economies, including implement dealers, seed dealers, car dealers, businesses in small communities, etc.
This water supply project can serve as a template for other projects being considered by the South Platte Regional Opportunities Working Group (SPROWG), a current effort funded by the Colorado Water Conservation Board, to conduct feasibility studies of comparable projects along varying reaches of the South Platte River downstream of the Denver metropolitan area. SPROWG is designed to (1) maximize the use and effectiveness of available South Platte River water, (2) minimize buy and dry of agricultural lands, and (3) evaluate cooperative efforts to fill the municipal and agricultural water supply gap. These objectives are all being met as part of this water supply project.
CREDITS
The foresight of the PWSD and LSPWCD Board of Directors and their managers, Mr. Ron Redd, P.E. of PWSD and Mr. Joe Frank of LSPWCD cannot be overstated, as this project is a non-traditional approach that requires expansive thinking to develop such a multi-faceted, complex project. Significant contributions have also been provided to the development of this project concept by Mr. Jim Yahn, General Manager of the Prewitt Reservoir and the North Sterling Reservoir, as well as LSPWCD Board members Messrs. Ken Fritzler and John Rush.
Lytle Water Solutions, LLC has worked on behalf of the PWSD, but has collaboratively worked with LSPWCD representatives, as well as consultants for the NECWC and LSPWCD, Mr. Matt Lindberg, P.E. with Brown & Caldwell and Mr. Mark McLean, P.E. of Deere & Ault, Inc., respectively.
If you have questions or would like additional information on creative solutions to water issues, please give us a call (303-350-4090) or send us an email at lws@lytlewater.com.
Bruce Lytle, P.E. bruce@lytlewater.com
Chris Fehn, P.E., P.G. chris@lytlewater.com
