15A NCAC 02K .0205       SPILLWAY DESIGN

(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.

 

History Note:        Authority G.S. 143‑215.26; 143‑215.27; 143‑215.31;

Eff. June 15, 1980.