North Carolina Administrative Code › Title 15A - Environmental Quality › Chapter 02 - Environmental Management › Subchapter K - DAM SAFETY › Section .0200 - OBTAINING APPROVAL FOR DAM CONSTRUCTION: REPAIR:OR REMOVAL › Section 02K .0205 - SPILLWAY DESIGN

Current through Register Vol. 39, No. 2, July 15, 2024
(a) All dams shall
				  have a spillway system with capacity to pass a flow resulting from a design
				  storm indicated in (e) of this Rule for a hazard classification appropriate for
				  the dam, unless the applicant provides calculations, designs, and plans to show
				  that the design flow can be stored, passed through, or passed over the dam
				  without failure occurring.
(b) A
				  vegetated earth or unlined emergency spillway will be approved when
				  computations indicate that it will pass the design storm without jeopardizing
				  the safety of the structure. The risk of recurring storms, excessive erosion,
				  and inadequate vegetative cover will be considered acceptable in such a
				  spillway when its average frequency of use is predicted to be no more frequent
				  than once in 25 years for existing class B and for class A dams except for
				  small class A dams designed in accordance with all design criteria established
				  by the U.S.D.A, Soil Conservation Service, and as contained in Engineering
				  Standard 378 of the U.S.D.A., Soil Conservation Service; once in 50 years for
				  new class B, small and medium new class C, and existing class C dams; and once
				  in 100 years for large and very large new class C dams. The dam sizes referred
				  to in this Subsection are defined in (e) of this Rule.
(c) Lined Spillways and Channels. The design
				  report shall include design data criteria for open channel, drop, ogee, and
				  chute spillways and other spillway types that include crest structures, walls,
				  channel lining, and miscellaneous details. All masonry or concrete structures
				  shall have joints that are relatively water-tight and shall be placed on
				  foundations capable of sustaining applied loads without undue deformation.
				  Provisions must be made for handling leakage from the channel or underseepage
				  from the foundation which might cause saturation of underlying materials or
				  uplift against the undersurfaces.
(d) Within 15 days following passage of the
				  design storm peak, the spillway system shall be capable of removing from the
				  reservoir at least 80 percent of the water temporarily detained in the
				  reservoir above the elevation of the primary spillway.
(e) It is recognized that the relationships
				  between valley slope and width, total reservoir storage, drainage area, other
				  hydrologic factors, and specific cultural features have a critical bearing on
				  determining the safe spillway design flood. Rational selection of a safe
				  spillway design flood for specific site conditions based on quantitative
				  analysis is acceptable. The spillway should be sized so that the increased
				  downstream damage resulting from overtopping failure of the dam would not be
				  significant as compared with the damage caused by the flood in the absence of
				  dam overtopping failure. A design storm more frequent than once in 100 years
				  will not be acceptable for any class C dam. In lieu of quantitative analysis,
				  the following tables shall be used as criteria for spillway design storms and
				  permissible velocities for vegetated earth spillways:
CRITERIA FOR SPILLWAY DESIGN STORM
SIZE CLASSIFICATION
Size
Total Storage
								(Ac-Ft)1
Height (ft)1
Small
less than 750
less than 35
Medium
equal to or greater than 750 and less than
								7,500
equal to or greater than 35 and less than 50
Large
equal to or greater than 7,500 and less than
								50,000
equal to or greater than 50 and less than
								100
Very Large
equal to or greater than 50,000
equal to or greater than 100
1 The factor determining the
					 largest size shall govern
MINIMUM SPILLWAY DESIGN STORMS
Hazard
Size
Spillway Design Flood (SDF)
Low (Class A)
Small
Medium
Large
Very Large
50 year
100 year
1/3 PMP
1/2 PMP
Intermediate (Class B)
Small
Medium
Large
Very Large
100 year
1/3 PMP
1/2 PMP
3/4 PMP
High (Class C)
Small
Medium
Large
Very Large
1/3 PMP
1/2 PMP
3/4 PMP
PMP
PERMISSIBLE VELOCITIES FOR VEGETATED EARTH
								SPILLWAYS
Permissible velocity1
								feet per second
Erosion resistant soils
Easily erodible soils
Slope of exit channel
Percent
Slope of exit channel
Percent
Vegetation
0 to 5
5 thru 10
0 to 5
5 thru 10
Bermuda Grass
Bahia grass
8
7
6
5
Tall fescue
Kentucky bluegrass
Reed canary
7
6
5
4
Sod forming
grass mixture
5
4
4
3
Lespedeza sericea
Weeping lovegrass
Alfalfa
Crabgrass
3.5
Do not use
2.5
Do not use
2 Increase values 10 percent
					 when the anticipated average use of the spillway is not more frequent than once
					 in 50 years or 25 percent when the anticipated average use is not more frequent
					 than once in 100 years.
Authority
					 G.S.
					 143-215.26;
					 143-215.27;
					 143-215.31;
Eff. June
					 15, 1980;
Pursuant to
					 G.S.
					 150B-21.3A, rule is necessary without
					 substantive public interest Eff. December 23,
					 2017.