Chatfield Reservoir Hydrologic Scenario Development

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Chatfield Reservoir Hydrologic Scenario
Development

• Jim Saunders
• WQCD Standards Unit
• 13 March 2008

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Roadmap for Technical Review
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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

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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

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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

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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

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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

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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?

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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?

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Flows Largely Independent
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Relative Importance of Plum Creek

• TP concentration in Plum Cr gtgt South Platte
• When is Plum highest? not at highest flows
• Median Plum 16

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Expanding the Scenario

• Started with median total inflow
• Set proportion from Plum Creek
• Median (16)
• High end (gt30)
• Return period?
• What determines Plum Creek contribution to inflow
  TP concentration?
• Dependence of concentration on flow
• Relative importance of flow

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Concentration and Flow in Plum

• Annual avg concentration is load/inflow
• Plateau abv 20,000 AF/y (TP0.175 mg/L)

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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

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Defining a Return Period

• Plum Cr gt 20,000 AF/y in 11/31 yrs
• Plum Cr gt 20 of inflow in 11/31 yrs
• Both criteria met in 6/31 yrs (19) return
  period about 5 y

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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

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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

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Whats Next?

• Next month technical review as basis for
  proposal connecting the dots
• Hydrologic scenario
• Load translator
• Concentration translator
• Standards, goals, and attainment
• Tracking memo