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Analysis of Raleigh’s Drought Triggers. May 8, 2013. and. Background.

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Analysis of raleigh s drought triggers

Analysis of Raleigh’s Drought Triggers

May 8, 2013

and


Background
Background

  • Previous assessments of current drought plan showed high frequency (as much as 1 out of every 3 years) of plan activation at current demands. Drought plan often triggered at times when it’s implementation is unnecessary and ineffective.

    • Modeling parameters of previous assessments were more conservative;

      lower Swift WD, steeper demand pattern, higher triggers


Previous drought plan
Previous Drought Plan

  • Stage 1

    • Activation : Falls Water Supply Storage Account <= 50%

    • Lift restrictions at 90% WS Storage

    • Primary restriction is limiting spray irrigation to 1 day/week

  • Stage 2

    • Activation : Falls Water Supply Storage Account <= 30%

    • Lift restrictions at 70% WS Storage

    • Residential spray irrigation prohibited. Golf course 50% reduction, carwashes only on weekends, some indoor reduction expected.

  • Stage 3

    • Activation : Falls Water Supply Storage Account <= 10%

    • Lift restrictions at 50% WS Storage

    • Many non-essential uses banned and almost all outdoor use is banned. Numerous indoor/commercial reductions mandated.


Frequency of trigger activation with current drought plan last 30 years of record
Frequency of Trigger Activation with Current Drought Plan, Last 30 Years of Record

1 in

3 yrs

1 in

4 yrs

1 in

8 yrs


Questions to consider
Questions to Consider Last 30 Years of Record

  • Are you comfortable with the frequency of drought plan activation?

  • What is the appropriate waiting period between stages?

  • Is a more rigorous climate change analysis warranted?


Drawdown refill cycles
Drawdown-Refill Cycles Last 30 Years of Record

  • Falls Lake Water Supply has regular refill / drawdown cycle

Drawdown Phase

Refill Phase


Water shortage response plan
Water Shortage Response Plan Last 30 Years of Record

Response by Stage

Estimate contained in WSRP is a flat reduction, but in reality reductions will be greater during the irrigation season and lesser during winter months.

7


Seasonal reductions in drought stages
Seasonal Reductions in Drought Stages Last 30 Years of Record

  • Based on Hazen & Sawyer’s analysis of 2010 demand trends (indoor, outdoor, etc.)


Water shortage response plan1
Water Shortage Response Plan Last 30 Years of Record

Annual Average Response by Stage

Draft

* - Response dependent on season. Estimate presented here is simply an average of the estimates made for each of the 12 months.

9


Modeling assumptions all scenarios
Modeling Assumptions – All Scenarios Last 30 Years of Record

  • Use 2010 monthly demand pattern

  • Total Average Annual Daily Demand = 60 mgd

  • Benton plant operations

    • 8 mgd constant withdrawal

      • Increase to 9.5 mgd when entering Stage 1 of drought plan

    • Never drop EMJ/Falls WD below 36 mgd

  • Assume 30 day waiting period between triggers


Seasonal drought plan
Seasonal Drought Plan Last 30 Years of Record

  • Each stage’s storage trigger varies by month

  • Rescission triggers at same levels as current drought plan

  • Assume same demand reductions as current drought plan


Seasonal drought plan option 2
Seasonal Drought Plan, Option 2 Last 30 Years of Record

  • Higher Stage 1 & 2 spring triggers

  • Rescission triggers at same levels as current drought plan

  • Assume same demand reductions as current drought plan


Frequency of alert trigger activation last 30 years of record
Frequency of Alert Trigger Activation, Last 30 Years of RecordLast 30 Years of Record

1 in

3 yrs

1 in

5 yrs

1 in

6 yrs


Effect of maximizing swift creek withdrawals
Effect of Maximizing Swift Creek Withdrawals Last 30 Years of Record

  • Maximum Swift withdrawal

    • 14 mgd when Benson/Wheeler storage >= 60%

    • 12.6 mgd when Benson/Wheeler storage >= 30%

    • 11.2 mgd when Benson/Wheeler storage < 30%

  • Never drop EMJ/Falls WD below 36 mgd


Max swift creek scenarios last 30 years of record
Max Swift Creek Scenarios, Last 30 Years of RecordLast 30 Years of Record

1 in

4 yrs

1 in

8 yrs

1 in

10 yrs


Max vs 8 mgd swift creek scenarios last 30 years of record
Max vs. 8 mgd Swift Creek Scenarios, Last 30 Years of RecordLast 30 Years of Record

1 in

3 yrs

1 in

4 yrs

1 in

5 yrs

1 in

6 yrs

1 in

8 yrs

1 in

10 yrs


Days in drought restrictions current plan 60 mgd total demand 8 mgd swift wd
Days in Drought Restrictions, Current Plan, Last 30 Years of Record60 mgd total demand, 8 mgd Swift WD

Statistics for last 30 years

Median Start Date = Oct. 2

Median End Date = Feb. 10

Median Duration = 130 days

Max Duration = 210 days


Days in drought restrictions seasonal plan 60 mgd total demand 8 mgd swift wd
Days in Drought Restrictions, Seasonal Plan, Last 30 Years of Record60 mgd total demand, 8 mgd Swift WD

Statistics for last 30 years

Median Start Date = Oct. 10

Median End Date = Feb. 25

Median Duration = 119 days

Max Duration = 196 days


Days in drought restrictions seasonal plan 2 60 mgd total demand 8 mgd swift wd
Days in Drought Restrictions, Seasonal Plan 2, Last 30 Years of Record60 mgd total demand, 8 mgd Swift WD

Statistics for last 30 years

Median Start Date = Oct. 20

Median End Date = Mar. 17

Median Duration = 122 days

Max Duration = 196 days


Days in drought restrictions current plan 60 mgd total demand max swift wd
Days in Drought Restrictions, Current Plan, Last 30 Years of Record60 mgd total demand, Max Swift WD

Statistics for last 30 years

Median Start Date = Oct. 23

Median End Date = Jan. 30

Median Duration = 82 days

Max Duration = 195 days


Days in drought restrictions seasonal plan 60 mgd total demand max swift wd
Days in Drought Restrictions, Seasonal Plan, Last 30 Years of Record60 mgd total demand, Max Swift WD

Statistics for last 30 years

Median Start Date = Oct. 14

Median End Date = Feb. 1

Median Duration = 101 days

Max Duration = 175 days


Days in drought restrictions seasonal plan 2 60 mgd total demand max swift wd
Days in Drought Restrictions, Seasonal Plan 2, Last 30 Years of Record60 mgd total demand, Max Swift WD

Statistics for last 30 years

Median Start Date = Oct. 18

Median End Date = Mar. 4

Median Duration = 82 days

Max Duration = 175 days


Worst case drought scenario
‘Worst Case’ Drought Scenario Last 30 Years of Record

  • Start in April at 80% WS Storage

    • 249.7 ft starting elevation for Falls/Beaverdam

  • Use 2002 hydrology for April & May

  • Use 2007-2008 hydrology for rest of the trace

  • 60 mgd total demand

  • 8 mg withdrawal from Swift

    • 9.5 mgd when in drought plan

    • No less than 36 mgd from Falls/EMJ


No drought plan
No Drought Plan Last 30 Years of Record


No drought plan falls lake elevation
No Drought Plan – Falls Lake Elevation Last 30 Years of Record

Bottom of

Conservation Pool

Sedimentation Pool

Note: For this run demand / min. releases are allowed to be met even when WS/WQ accounts are depleted.


Current drought plan
Current Drought Plan Last 30 Years of Record


Seasonal drought plan1
Seasonal Drought Plan Last 30 Years of Record


Seasonal drought plan option 21
Seasonal Drought Plan Option 2 Last 30 Years of Record


Water not sold during mandatory conservation 60 mgd demand scenario 8 mgd swift wd
Water Not Sold During Mandatory Conservation Last 30 Years of Record60 MGD demand scenario, 8 mgd Swift WD

* - Mandatory conservation triggered, but no reduction in consumption expected


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