DWR_EFlow_Planning_Scenario_Big_River

A Brief Consideration of Some
Ecological Flow Regimes,
Water Supply Systems, and
Planning:
The “Big River” Watershed
Scenario
Fred Tarver, N.C. Division of Water Resources
EFlow Science Advisory Board Meeting
27 November 2012
FIGURE 1
FIGURE 2
Scenario “Tarheelia” “Wolftown” “Devilville”
Present Demand
(Vol. Equivalent 
to 20% 7Q10)
3.6 cfs 
(2.3 mgd)
2.8 cfs 
(1.8 mgd)
2.0 cfs 
(1.2 mgd)
50‐YR Demand 15.5 cfs 
(10 mgd)
12.3 cfs 
(7.9 mgd)
6.0 cfs 
(3.8 mgd)
Present
Wastewater 
Return  (86%)
3.0 cfs 
(1.9 mgd) 0.0 
1.7 cfs 
(1.0 mgd)
Future
Wasterwater
Return  (86%)
13.3 cfs 
(8.5 mgd) 0.0
5.1 cfs 
(3.2 mgd)
TABLE 1
NODE
DA 
(mi2) “EFLOW” Flow‐By Regime
140Monthly Median or Inflow, whichever’s less
29260Percent Annual Average or Inflow, whichever’s less
3 109 75 Percent Inflow
4 189 80 Percent Inflow
5 158 70 Percent Inflow
6 356 50 Percent Annual Average or Inflow, whichever’s less
7 366 85 Percent Inflow
TABLE 2
FIGURE 3
FIGURE 4
FIGURE 5
FIGURE 6
FIGURE 7
FIGURE 8
FIGURE 9
FIGURE 10
FIGURE 11
FIGURE 12
FIGURE 13
FIGURE 14
FIGURE 15
FIGURE 16
POINTS TO  PONDER :
Nodes on “unaltered” streams and protected watersheds?
Alterations in the upper watershed are attenuated in the lower watershed by intervening 
drainage and discharges. Nodes in lower watershed may not raise a “red flag”.
When is the “red flag” raised? Any time EFlow is breached? Frequency? Duration? %Q?     
Seasonal?
Capturing Secondary & Cumulative Impacts?:
NC(S)EPA Minimum Criteria: “Improvements to water treatment plants that involve less 
than 1,000,000 gallons per day added capacity and total design withdrawal less than one‐
fifth of the 7‐day, 10‐year low flow of the contributing stream;”
Unregulated withdrawals;
Land use; etc.;
High‐flow skimming. There’s a lot of available water during high‐flow events.
Tidal waters?