Connecticut Administrative Code › Title 25 - Water Resources, Flood and Erosion Control › 32d - Source Water Protection Measures › Section 25-32d-4 - Calculation of safe yield

Current through July 10, 2024
(a)
Surface
			 water sources.
Safe yield shall be developed using a mathematical mass
			 balance methodology and shall be based on a ninety-nine percent dry year or a
			 critical dry period with a 1 in 100 occurrence frequency and shall be based on
			 the usable storage capacity of a reservoir which can be used without additional
			 equipment or treatment, except that the safe yield may be less due to
			 requirements for the passing of minimum stream flows or other release
			 requirements. The statistical frequency analysis shall be performed by
			 developing a low flow duration curve using the adjusted stream gaging data for
			 the critical drawdown duration. All surface water safe yield analyses shall be
			 performed by an individual with a minimum of five years experience in surface
			 water analysis and a bachelor's or advanced degree from an accredited college
			 or university in hydrology or related engineering field, or a professional
			 engineer licensed in accordance with Chapter 391 of the Connecticut General
			 Statutes with a minimum of five years experience in surface water analysis. For
			 cases where a mass balance analysis cannot be performed due to insufficient
			 usable storage volume, such as run of the river type situations or diversions,
			 the safe yield shall be determined based upon an analysis of the streamflow for
			 a ninety nine percent dry year assuming a seven day average flow duration.
			 Information developed for other sections of the water supply plan may be
			 referenced, if appropriate. The methodology for determining the safe yield of
			 surface water supplies shall include the following:
(1) Inflow into the reservoir shall be based
			 on gaged streamflow data collected from within the watershed or calculated from
			 measured historical reservoir levels. Where such data is not available,
			 unregulated stream gaged data from another watershed (external) which closely
			 approximates the watershed of interest shall be used as determined by a
			 verification analysis of historic inflows or reservoir levels versus the
			 selected gage. Factors to consider when selecting the external gaging station
			 shall include amount of stratified drift, land uses, slope, stream length,
			 length of record, vegetation and geomorphology. The selected stream gage flow
			 record or historic inflow record shall be of sufficient length and period of
			 record as necessary to perform the required frequency analysis in subdivision
			 (10) of this subsection. In cases where historic reservoir data is insufficient
			 or unavailable for a verification procedure, then the selected gage shall have
			 similar watershed characteristics and worst case low flows.
(2) Operating rules. The operating rules for
			 the movement of water, reservoir conditions, and operation of the reservoir or
			 reservoir system shall be listed and described. Reservoir conditions shall
			 include the total and usable reservoir storage capacity; top and bottom
			 elevation of the reservoir dam; spillway elevation, length and type; elevations
			 and diameters of water supply intakes; and use of flashboards. Operating rules
			 shall address conjunctive use of multiple reservoirs or wells, diversions,
			 alternate release patterns, and operation of reservoirs in series or parallel.
			 Operating rules shall be utilized in performing safe yield
			 calculations.
(3) Computational
			 interval. The mass balance analysis shall utilize a computational interval of
			 no more than one month. Daily flow analysis may be required to appropriately
			 model flood skimming diversions or low flow diversions unless truncated flow
			 hydrographs are developed.
(4)
			 Diversions. The safe yield analysis model shall include any diversions of water
			 into or out of the watershed. The operating characteristics, flow capacity of
			 the diversions and the runoff to the point of diversion shall be provided. Both
			 existing and proposed diversions shall be analyzed, provided such proposed
			 diversions are identified as needed within the five year planning
			 period.
(5) Withdrawal rates. The
			 reservoir outflow due to water withdrawal shall be varied on a monthly basis,
			 based upon historic withdrawals for the last five year period of record. All
			 supportive data shall be provided.
(6) The safe yield analysis shall be extended
			 to determine the time to refill after the critical dry period assuming normal
			 system operation, annual withdrawal rates equal to the calculated safe yield
			 and inflow from the period immediately following the critical dry
			 period.
(7) The safe yield of
			 surface water sources shall be analyzed as a combined multiple reservoir system
			 based upon a flow routing analysis and specified operating rules, unless
			 previously approved by the department.
(8) Safe yield model inflow.
(A) Developing inflow record. The flow record
			 for the chosen streamflow gage shall be adjusted to the watershed being
			 analyzed by a ratio of the watershed area being analyzed to the watershed area
			 of the selected streamflow gage. Further adjustment may be necessary to
			 calibrate the safe yield model based upon verification procedures.
(B) Verification of safe yield model. In
			 cases where an external stream gage is utilized, the inflow data shall be
			 verified by comparing the end of period storage levels predicted from the
			 chosen streamflow gage record against the actual measured historical reservoir
			 levels from a representative dry period. Operating rules indicated to be in use
			 during the chosen dry period shall be used for the verification
			 procedure.
(C) Period of record.
			 The entire period of record using mass balance methodology shall be analyzed to
			 determine the critical dry period.
(D) Usable storage. The reservoir yield shall
			 be developed using usable storage capacity based on bathymetric or topographic
			 surveys and shall factor in sediment deposition. The calculation of usable
			 storage excludes storage based on flashboards and water that cannot be accessed
			 without special use of pumps or other emergency techniques.
(E) Direct precipitation. Direct
			 precipitation on the surface area of the reservoir shall be calculated using
			 the closest representative precipitation gage for the historic critical dry
			 period or the ninety nine percent exceedance. Published data shall be used
			 where possible. If unpublished data is used the data shall be submitted in
			 support of the analysis. Water companies may choose to use the net impact of
			 the direct precipitation minus the evaporation. The precipitation data shall be
			 based on an interval no greater than one month.
(9) Safe yield model outflow
(A) Evaporation rates. The safe yield
			 analysis shall incorporate monthly evaporation rates computed over the surface
			 area of the reservoir either as calculated at the end of each computational
			 interval or, assuming a constant surface area based upon two-thirds of usable
			 storage capacity. Monthly evaporation rates as listed in this sub-paragraph
			 shall be used in the safe yield analysis:
Evaporation rates (inches per
						  month)
January
0.85
February
0.93
March
1.51
April
2.15
May
4.15
June
5.10
July
5.61
August
5.25
September
3.64
October
2.60
November
1.66
December
1.34
(B)
			 Consumptive losses to the watershed shall be evaluated.
(C) Dam leakage. Leakage rates shall be based
			 upon field measurements or data obtained from the Department of Environmental
			 Protection. If data is not available, then use of an estimated value is
			 acceptable.
(D) Minimum streamflow
			 releases. The minimum streamflow release shall be determined in accordance with
			 Sections
			 26-141a-1
			 through 26-141a-26, inclusive, of the Regulations of Connecticut State Agencies
			 and, where applicable, Sections
			 22a-365
			 through
			 22a-378,
			 inclusive, of the Connecticut General Statutes, and the regulations adopted
			 pursuant to Section
			 22a-377
			 of the Connecticut General Statutes. This requirement may be met by dam leakage
			 and required riparian releases which equal or exceed the required minimum
			 releases.
(10) 1 in 100
			 occurrence frequency. A statistical frequency analysis shall be performed using
			 a Log-Pearson Type III distribution analysis to confirm that the average
			 inflows over the critical drawdown duration equal or exceed a 1 in 100
			 occurrence frequency. A minimum of thirty years of streamflow record is
			 required, unless otherwise approved by the department. The computed 1 in 100
			 occurrence frequency flow for the specified critical drawdown duration shall
			 then be compared to the average flows for the same historic period. If
			 necessary to meet or exceed the 1 in 100 occurrence frequency requirement, the
			 inflow record shall be modified by a ratio adjustment and the mass balance
			 analysis shall be rerun accordingly.
(A) All
			 low-flow data used in computing Log-Pearson Type III frequencies shall be
			 non-zero values. If zero values have occurred, then the statistical parameters,
			 such as mean, standard deviation, and skew, shall be adjusted as recommended by
			 the United States Geological Survey in technical memorandum number 89.11,
			 available from the United States Geological Survey.
(B) For critical drawdown durations exceeding
			 three hundred and sixty five days, the data to be used in the frequency
			 analysis shall be non-independent values based upon flow periods equal to the
			 critical drawdown duration within consecutive overlapping years.
(C) If the inflow record utilized in the safe
			 yield analysis exceeds the 1 in 100 occurrence frequency, then, at the water
			 company's option, the inflow record may be modified by a ratio adjustment to
			 exactly meet but not be under the 1 in 100 occurrence frequency requirement and
			 the mass balance analysis rerun accordingly.
(11) Submittal requirements. The water
			 companies required to submit plans shall submit information on the dam leakage
			 quantities, precipitation, riparian releases, minimum streamflow releases or an
			 indication of exemption to such releases, critical drawdown duration, drought
			 duration, 1 in 100 year low flow value, frequency analysis, safe yield
			 computations including input and output, schematic of the reservoir system and
			 stage or storage tables and curves, and the stage or area tables and curves,
			 for approval. All sources of data used in the safe yield analysis shall be
			 referenced. A summary graph of reservoir storage versus time for the critical
			 dry period and extended to refill shall be submitted.