1. Summary of Chesdin Reservoir Reliability
January 25, 2018
Casey Caldwell
Steve Nebiker

2. Summary
2017 assessment
Overall reliability assessment

3. 2017 Assessment
Use OASIS model of the reservoir; model was initially developed in 2012 to support development of the new permit
For this analysis used updated raw water demand and process water return patterns
Use model to develop drawdown percentiles for the entire hydrologic record (1926 – 2017)
Compare 2017 drawdown to period-of-record percentiles

4. New Demand & Return Patterns
Update monthly patterns in model based on average of last 5 year (2013 – 2017) of historic data
For average annual withdrawal use average of 31.8 mgd
Year
Avg. Annual Raw Water Demand (mgd)
2013
30.2
2014
33.5
2015
31.6
2016
31.2
2017
32.6
Average
31.8

5. New Raw Water Demand Pattern

6. Demand pattern in model compared to actual recent withdrawals

7. 2017 Summary
2017 was a dry year – see plot below comparing inflow to historical percentiles. Note that flows from August-December are from USGS provisional gage flows.

8. 2017 Chesdin Drawdown (inches) vs. Percentiles from Simulated Record

9. Overall Reliability Assessment
With record updated through 2017, show the reliability metrics that were used in 2013 to support the approval of the current minimum release permit
Metrics were developed with ARWA and VA DEQ
These metrics assess water supply reliability (including how often the drought plan would be activated), and also environmental (fish habitat) and recreational goals

10. System Performance Goals
Drought plan activation frequency
Voluntary – 1 in 5 years
Mandatory – 1 in 25 years
Emergency – 1 in 91 years (hydrologic period of record)
Drawdown frequency
Recreation > 2 ft (May 15 – Sep 30) – 1 in 5 years
Recreation > 4 ft (May 15 – Sep 30) – 1 in 10 years
Out-Migration > 3.5 ft, 45+ consecutive days (Sep 1 – Nov 30) – 1 in 17 years
Maintain minimum storage – 60 days of supply
Preservation of downstream habitat (determined by DEQ)
DEQ proposed release protocol protects habitat. Question is can we make things better for other uses without unacceptably affecting habitat.
Reserve of 60 days of supply is a contingency for droughts worse than those in the record.

11. Demand Projections
Use linear interpolation of raw water demand to estimate future demands

12. Highest demand where all goals are met: 43.5 MGD (~2080 demand)*
Drought Plan
Trigger
Current Conditions
(31.8 MGD)
2030
Conditions
(34.5 MGD)
~2080 Conditions
(43.5 MGD)
Frequency of Trigger Activation
Stage 1 (Voluntary)
1 in 18 yrs
1 in 6 yrs
Stage 2 (Mandatory)
< 1 in 91 yrs
1 in 46 yrs
1 in 30 yrs
Stage 3 (Emergency)
1 in 91 yrs
Frequency of Drawdown Event
Rec > 2 ft
1 in 8 yrs
1 in 4 yrs**
Rec > 4 ft
1 in 10 yrs
Mig > 3.5 ft, 45+ days
Preserves 60-day supply?
Yes (137 days)
Yes (122 days)
Yes (72 days)
Red = goal not met
Yellow = within 1 event of goal Green = goal met
Recreation statistics are for May 15 – Sep 30
Out-Migration statistics are for Sep 1 – Nov 30
* Increased sedimentation (dead storage) proportionally w/ demand
** The only limiting metric at 43.5 mgd is the Recreation drawdown > 2 ft; other than that could potentially go above 43.5 mgd and meet the rest of the goals

13. Conclusions
Chesdin Reservoir is very reliable at current system demand levels
2017 was a dry year, but still not close to hitting levels of major concern
Results are likely conservative
The new permit for minimum releases added a significant amount of reliability over the old release requirement (previously presented)
With demand growth projections, the reservoir will still be able to maintain all performance objectives for the next 50+ years
After that, the key metrics pertaining to water supply reliability will still be met
For a reservoir operated for water supply first, it is reasonable to expect some degree of drawdown during dry periods