( NOTICE: This is an excerpt from the the rules and regulations intended to carry out the purposes of the Dam Safety Law of 1967. If you wish to view the full text under SubChapter K, please go to an unofficial site for N.C. Administrative Code, or view it at the Division of Land Resources, Dam Safety Program site. )

SECTION .0500 - Minimum Stream Flows To Maintain Aquatic Habitat

.0502 Required Minimum Flow For Dams (Not Small Hydro Projects) 

(a) A dam operated by a small power producer, as defined
in G.S. 62-3(27a), that diverts water from 4,000 feet or less
of the natural streambed, shall be exempt from this Rule.
(b) A dam proposed for a small stream with a mean annual
daily flow less than or equal to 3.0 cubic feet per second
(cfs) shall be subject to the following review process in
determining the required minimum flow: 
(1) If the mean annual daily flow is less than or equal to 3.0
cfs and the 7-day, 10-year low flow (7Q10) is less than or
equal to 0.2 cfs; and if there are no existing point source
discharges of wastewater to the  affected stream reach; then
no minimum release will be required.
(2) If the mean annual daily flow is less than or equal to 3.0
cfs and the 7Q10 is less than or equal to 0.2 cfs; and one or
more existing point source discharges of wastewater enter the 
affected stream reach; then the minimum release shall be
equal to the 7Q10. 
(3) If the mean annual daily flow is less than or equal to 3.0
cfs and the 7Q10 is greater than 0.2 cfs, then the minimum
release shall be equal to the 7Q10. 

(c)   If the mean annual daily flow is greater than 3.0 cfs,
then the following procedures shall be used to determine the
minimum flow requirement: 
(1) The minimum flow for a dam on a stream with poor aquatic 
habitat shall be the 7Q10 flow determined by using U.S. 
Geological Survey procedures.
(2) The minimum flow for a dam on a stream with moderate
aquatic habitat in the piedmont, as defined in Rule .0501(h)
of this Section, shall be determined using regression
equations provided in this Subparagraph. 
(A) All flows used in regression equations shall be measured
in cubic feet per second, all drainage areas shall be
measured in square miles, and all logarithmic expressions
shall refer to base 10 logarithms. 

(B) The regression equation used to determine the minimum
flow for a stream in the piedmont which exhibits moderate
aquatic habitat, and for which no continuous stream gage
record, as defined in Rule .0501(i) of this Section, exists,
shall be as follows:


     LRF  = (3.204 x M) - (2.618 x D) 

     LRF  = LOG of regression flow  
     M  = LOG of mean annual daily flow  
     D  = LOG of drainage area


The regression flow (RF) is calculated by raising 10 to the
power of the LRF.  
If the drainage area is greater than 95 square miles, the 
required minimum flow is 1.4 x RF. 
Otherwise the required minimum flow is equal to RF. 

(C) The regression equation used to determine the minimum
flow for a stream in the piedmont which exhibits moderate
aquatic habitat, and for which a continuous stream gage
record, as defined in Rule .0501(i) of this Section, does
exist, shall be as follows:

     LRF  = (0.812 x M) + (8.111 x E92) 
                          - (4.806 x E85) - (3.275 x E95) 

     LRF  = LOG of regression flow  
     M     = LOG of mean annual daily flow  
     E85  = LOG of 85% annual exceedance flow 
     E92  = LOG of 92.5% annual exceedance flow 
     E95  = LOG of 95% annual exceedance flow

The regression flow (RF) is calculated by raising 10 to the
power of the LRF. 
The required minimum flow is 1.1 x RF. 
  
(3) The minimum flow for a dam on a stream with moderate
aquatic habitat, located in a geographical region for which
regression formulas are not provided, shall be determined by
a site-specific instream flow study, as defined in Rule
.0501(j) of this Section, conducted by the applicant or his
consultants and subject to the approval of the Department.
(4) The minimum flow for a dam on a special case stream,
or on a stream with good aquatic habitat, shall be determined 
by a site-specific instream flow study, as defined in Rule 
.0501(j). 
This study shall be conducted by the applicant or his
consultants, and shall be subject to approval by the
Department.
(5) If the applicant or owner disputes the minimum flow
determined by the procedures described in Subparagraphs
(c)(1) or (c)(2) of this Rule for streams with poor or
moderate aquatic habitat, he may undertake a site-specific
field study, as defined in Rule .0501(j) of this Section,
subject to the review and approval of the Department.  The
final minimum release required will not exceed the amount
determined by the procedures described in this Rule. 

(6) The minimum release schedule for a water supply
reservoir shall include  provisions for reductions in the
minimum flow which coincide with reductions in the usable
water supply storage remaining in the impoundment and
with reductions in the amount of water withdrawn from the
reservoir. 
(A) This system of tiered releases shall apply to new water
supply reservoirs and any existing water supply reservoirs
for which the minimum release is revised.  
(B) The exact percentage of storage which triggers
reductions in minimum flow will depend on several
site-specific factors, including, but not limited to: 
(i) size of the reservoir; 
(ii) rate of the water supply demand; 
(iii) hydrologic characteristics of the impounded stream; and
(iv) the impoundment levels which result in local efforts to
reduce water usage through conservation measures. 
(C) At least three levels of minimum releases shall be
included in the release schedule for a water supply reservoir.
(D) When usable water supply storage has been reduced to a 
level which triggers the first reduction in minimum flow,
then the average daily water withdrawal shall be reduced by
at least 10 percent from the average daily withdrawal for the
60 day period immediately prior to the first reduction in the
minimum flow.  The water supply operator shall accomplish
this reduction in withdrawal within two weeks of the reduction 
in the minimum release. 
(E) When usable water supply storage has been reduced to a
level which triggers the second reduction in minimum flow,
then the average daily water withdrawal shall be reduced by
at least 20 percent from the average daily withdrawal for the
60 day period immediately prior to the first reduction in the
minimum flow.  The water supply operator shall accomplish
this further reduction in withdrawal within two weeks of the
second reduction in the minimum release. 
(F) The water system operator shall document reduction in
water withdrawals by submitting reports of daily water
withdrawals to the Department.  These shall be submitted
every two weeks for as long as the minimum release is
reduced below the amount normally required. 
(G) An example is shown in the table below.  (Note that the
percentages of water supply storage which trigger the changes 
in minimum release are site-specific for this example and may 
vary according to the factors described in Part (B) of this 
Paragraph.) 
       
 Remaining Usable Water    Minimum   Water Use
   Supply Storage          Release   Reduction


 between 70% and 100%         A        ---
 between 40% and 70%          B        10%
 below 40%                    C        20%


A = Normal minimum release determined by a field study,
regression equation, or use of the 7Q10 
B = Intermediate reduction in minimum release  
C = Low minimum release equal to no more than the 7Q10

(7) An existing dam which was built subject to review under
the National or the State Environmental Policy Acts, and for
which a minimum release has been established, will not have
its minimum release changed under this Rule. However, the
Department may review and adjust the minimum flow
released by any other existing dam if there is evidence of
any of the following conditions downstream of that dam:
(A) water quality standards not being maintained; 
(B) water quality classifications which are being only 
partially supported or not being supported; or  
(C) aquatic habitat not being maintained. 
(8) If the minimum release required from an existing water
supply reservoir is reviewed by the Department, any
increase in minimum flow will be determined on a
case-by-case basis in consideration of the following factors,
including, but not limited to: 
(A) availability of water to meet existing demands; 
(B) rate of growth in water demand; 
(C) planned development of alternative sources of water
supply; 
(D) structural difficulties; 
(E) capital costs; and 
(F) anticipated improvements in water quality and aquatic
habitat in the affected reach resulting from the proposed
change in minimum flow. 

The change in minimum release shall be set no higher than
an amount which would reduce the water supply safe yield,
as determined by standard accepted engineering practices,
by more than 10 percent. 
(9) If a new minimum release requirement from an existing
water supply reservoir is being delayed until a new source of
water supply is developed, then this delay shall not exceed a
period of five years from the written notification that a new
minimum release will be required.  This period may be
extended by approval of the Environmental Management
Commission in consideration of the following factors: 
(A) delays in developing a new water supply source; 
(B) changes in water quality and aquatic habitat in the
affected reach; or 
(C) availability of water to meet existing demands. 

History Note: Statutory Authority G.S. 143-215.24;
143-215.25; 143-215.31; 143-215.32; 143-215.33; 143-215.36; 
Eff. December 1, 1994; 
Amended Eff. April 1, 1995. 

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