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

Chatfield Reservoir Hydrologic Scenario Development

Jim SaundersWQCD Standards Unit13 March 2008

Roadmap for Technical Review

Month Topic

Sep-07 Technical comparison of existing control regulations

Oct-07 Existing chlorophyll target, incl magnitude, frequency, duration

Nov-07 Evaluation and discussion of concentration translator

Dec-07 Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads

Jan-08 Phosphorus load estimates; produce common set by source

Feb-08 Evaluation and discussion of load translator

Mar-08 Hydrologic considerations for TMAL

Apr-08 Discuss chlorophyll-phosphorus-load linkages as basis for proposal

Jun-08 WQCD to finalize proposal and circulate

Jul-08 Notice due

Nov-08 WQCC RMH

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

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

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

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

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

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?

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?

0.00

0.05

0.10

0.15

0.20

0.25

1000 10000 100000 1000000

Inflow, AF/y

Ph

osp

ho

rus,

mg

/L

South Platte Plum Creek

Flows Largely Independent

0

20000

40000

60000

80000

100000

120000

0 100000 200000 300000 400000

South Platte Flow, AF/y

Plu

m C

ree

k F

low

, AF

/y

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

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 100000 200000 300000 400000

South Platte Flow, AF/y

Plu

m C

ree

k, %

of

Co

mp

ute

d In

flo

w

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

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

0.00

0.05

0.10

0.15

0.20

0.25

0 10000 20000 30000 40000 50000 60000

Flow, AF/y

Ph

os

ph

oru

s, m

g/L

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

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0% 5% 10% 15% 20% 25% 30% 35%

Plum Creek as % of Computed Inflow

Infl

ow

TP

, mg

/L

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

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

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

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