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Chatfield Reservoir Hydrologic Scenario Development

Chatfield Reservoir Hydrologic Scenario Development. Jim Saunders WQCD Standards Unit 13 March 2008. Roadmap for Technical Review. For Today…. Explain purpose served by hydrologic scenario Review examples Outline issues for Chatfield Problems with existing scenario

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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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