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Chatfield Reservoir Hydrologic Scenario Development

This roadmap outlines the purpose, issues, and options for developing a new hydrologic scenario for Chatfield Reservoir. It includes examples, problems with the existing scenario, and recommendations.

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Chatfield Reservoir Hydrologic Scenario Development

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  1. Chatfield Reservoir Hydrologic Scenario Development Jim Saunders WQCD Standards Unit 13 March 2008

  2. Roadmap for Technical Review

  3. For Today… • Explain purpose served by hydrologic scenario • Review examples • Outline issues for Chatfield • Problems with existing scenario • Options for new scenario • Make a recommendation

  4. What Purpose Does the Hydrologic Scenario Serve? • Part of logical basis for linking implementation of controls to attainment of standard • Necessary for defining allowable load in terms of pounds (=flow x concentration) • Control regulations define allocations in pounds

  5. Hydrologic Scenarios in Existing Control Regulations • Dillon • 1982 (212,000 AF); return period ~3y • Index future P loads to base year (1982) • Cherry Creek • 1982 (2245 AF); return period ~1.2y • Index to 1982 base year • Chatfield • Original: 1982 (93,000 AF); return period ~3y • Revised: Q10 (261,000 AF); actual return period ~5y • Bear Creek – not specified

  6. Comments on Chatfield Scenario • Rationale for Q10 is based on exceedance probability for load rather than in-lake concentration • Concentration threshold could be exceeded at any flow if load is high enough • Assumes implicitly that higher load means poorer WQ; not necessarily true

  7. Conceptual Basis for New Scenario • How is the allowable phosphorus load influenced by hydrologic conditions? • Is the chl-TP relationship affected by flow? – depends (in concept); flow may control of TP • Is the TP conc-load relationship affected by flow? – depends (in concept) on P retention • Logical basis: highest inflow concentration is most likely to yield highest in-lake concentration • What determines highest inflow TP concentration? • Not necessarily a low flow scenario • Depends on mix of two sources: SP and Plum

  8. Starting Point for Hydrologic Scenario Development • Select median total inflow • WQCD often uses median flow in TMDL development for streams • Median computed inflow: 100,860 AF • Determine relative importance of the two main tributaries for setting the inflow concentration • Inflow concentration is total load/total inflow • Does each tributary represent a constant proportion of total inflow? • Does concentration vary with flow in either tributary?

  9. Phosphorus Annual Average Concentration and Tributary Flow • South Platte – conc not related to flow • Plum Cr – higher conc at higher flow • Which influence is stronger in mixed flow?

  10. Flows Largely Independent

  11. Relative Importance of Plum Creek • TP concentration in Plum Cr >> South Platte • When is %Plum highest?; not at highest flows • Median %Plum = 16%

  12. Expanding the Scenario • Started with median total inflow • Set proportion from Plum Creek • Median (16%) • High end (>30%) • Return period? • What determines Plum Creek contribution to inflow TP concentration? • Dependence of concentration on flow • Relative importance of flow

  13. Concentration and Flow in Plum • Annual avg concentration is load/inflow • Plateau abv 20,000 AF/y (TP~0.175 mg/L)

  14. Influence of Plum Creek on Inflow TP • Realistic range of inflow % (backdrop of median total inflow) • More Plum Cr flow (as %) means higher inflow phosphorus concentration for reservoir

  15. Defining a Return Period • Plum Cr > 20,000 AF/y in 11/31 yrs • Plum Cr > 20% of inflow in 11/31 yrs • Both criteria met in 6/31 yrs (19%); return period about 5 y

  16. WQCD Recommendation for Hydrologic Scenario • Median total inflow – 100,860 AF/y • Plum Creek; set % contribution • Option 1: median (16%) • About 16,000 AF/y; TP conc below plateau • Option 2: 20% • About 20,000 AF/y; TP conc on plateau • Exceedance frequency about once-in-5 yrs

  17. TMAL Development Issues not included in Technical Review • Partitioning of load between South Platte and Plum Creek basins • Allocations to sources within each basin • Define margin of safety

  18. What’s Next? • Next month – technical review as basis for proposal; connecting the dots • Hydrologic scenario • Load translator • Concentration translator • Standards, goals, and attainment • Tracking memo

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