N.J. Admin. Code § 7:20-1.9

Current through Register Vol. 56, No. 21, November 4, 2024

Section 7:20-1.9 - Design criteria

(a) The minimum design storm used to calculate required spillway capacity must be determined according to the following table:

Hazard	Spillway Design Storm (SDS)
Class I	PMP
Class II	One-half PMP
Class III	24 hour 100 year frequency,
Type III storm[1]
Class IV	24 hour 100 year frequency,
Type III storm plus 50%[1]

[1] Any later technology adopted by the U.S. Department of Agriculture, Natural Resources Conservation Service may be substituted for the use of the Type III storm.

(b) For existing dams, it is recognized that the relationships between valley slope and width, total reservoir storage, drainage area, and other hydrologic factors have a critical bearing on determining the safe spillway design flood. When appropriate, based on the design of the dam, rational selection of a safe, reduced spillway design for specific site conditions based on quantitative and relative impact analysis is acceptable. The spillway should be sized so that the increased downstream damage resulting from overtopping failure of a dam would not be significant as compared with the damage caused by the flood in the absence of a dam overtopping failure. The minimum design storm for the dam shall be the 100 year storm.

(c) All Class II and III dams shall, where practicable, incorporate in the proposed design, the ability to make modifications necessary to increase the spillway capacity of the facility or other alternative measures if the downstream hazard potential increases.

(d) All dams shall have adequate storage for the design storm or have a spillway system which will safely pass the design storm without endangering the safety of the dam.

(e) Each spillway shall include a satisfactory means of dissipating the energy of flow at its outlet without endangering the safety of the dam.

(f) The capacity of the spillway system shall be equal to the peak inflow of the design flood unless the applicant demonstrates by flood routing procedures that the spillway system has the capacity to safely pass the resulting water flow.

(g) Pipe conduits may be used for the primary (principal) spillway. When so used, the following requirements shall be met:

1. Pipe conduits shall be of such design as to safely support the total external loads in addition to the total internal hydraulic pressure without leakage. The type of construction material used shall be consistent with the anticipated life of the structure. Corrugated metal pipe shall not be used in the construction of new dams.

i. For Class I and II dams, the minimum allowable inside dimension of the pipe conduit is 30 inches.

ii. For Class III dams, the minimum allowable inside diameter of the pipe conduit is 18 inches.

iii. For Class IV dams, the minimum allowable inside diameter of the pipe conduit is 12 inches.

2. All pipe conduits shall convey water at the maximum design velocity without damage to the interior surface;

3. The pipe conduits shall be designed so that negative pressure will not occur at any point along the primary (principal) spillway system;

4. Drainage filters or other methods approved in writing by the Department must be installed to control seepage along the conduit;

5. Adequate allowances shall be incorporated in the design to compensate for differential settlement and possible elongation of the pipe conduit;

6. An anti-vortex device shall be included in the design unless the applicant has demonstrated to satisfaction of the Department that one is not necessary;

7. A trash rack, approved by the Department, shall be installed at the intake to prevent clogging of the pipe conduit;

8. An emergency spillway shall be provided; and

9. Cathodic protection is required for all metal pipes.

(h) Should a vegetated or unlined auxiliary spillway, approved by the Department, be installed, it must be able to pass the design storm without jeopardizing the safety of the structure and that has a predicted average frequency of use less than:

1. Once in 100 years for Class I dams;

2. Once in 50 years for Class II dams; or

3. Once in 25 years for Class III and IV dams.

(i) Drawdown requirements are as follows:

1. Except for excavated impoundments, all dams shall include a device to permit draining the reservoir, as approved in writing by the Department. Computations for the minimum time required to drain the reservoir shall be required for new and existing dams.

2. Unless the applicant demonstrates to the satisfaction of the Department that there is a need to locate a valve downstream from the dam and that the areas downstream of the dam will remain protected, all valves or sluice gates in pipe conduit drains must be installed upstream of the dam.

3. All pipe conduits used as drawdown drains for all dam classifications shall meet the requirements of (g) above, except that the minimum allowable inside dimension may be less than 30 inches.

4. Dams which impound water on a permanent basis shall include a means to allow the reduction of the reservoir water surface elevation five feet in 10 days at a rate not to exceed one foot per day. This requirement shall not apply to dams whose intended purpose requires and whose design allows faster drawdown times. For existing dams which satisfactorily meet Department safety and operating criteria, the applicant may, with prior approval of the Department, present alternative reservoir drawdown plans.

(j) Design references used shall be cited in the information submitted to the Department.

(k) Monitoring devices to permit inspection and assessment of the dam's condition may be required by the Department for use in the inspections during and after completion of construction.

(l) The applicant shall demonstrate to the Department that the riparian rights of downstream property owners will be protected during construction, during the period when the reservoir is being filled and during the life of the dam and reservoir.

(m) Unless the applicant can demonstrate that an alternative slope is acceptable, upstream slopes of an earth dam may be no steeper than three horizontal to one vertical ratio, and downstream slopes may be no steeper than two horizontal to one vertical ratio. Measures are required for protection of upstream slopes against wave action or rapid draw-down and for protection of the downstream slope against scour or erosion due to high tailwater.

(n) Freeboard requirements are as follows:

1. Sufficient freeboard shall be provided to prevent overtopping of the dam or any dike or levee due to passage of the design flood or due to frost damage, ice damage or wave action.

2. For all dams the minimum elevation of the top of the dam must be that necessary to pass the design storm with at least one foot of freeboard to the top of the dam.

3. Where special conditions of severe frost damage, ice damage or wave action may occur, higher elevations than required in (n)2 above may be required and should be considered by the applicant.

(o) The Department may require the design and installation of any additional or modified measures by any applicant for a dam permit where appropriate to insure the protection of human health or safety.

N.J. Admin. Code § 7:20-1.9

Amended by R.1990 d.276, effective 6/4/1990.
See: 22 N.J.R. 279(a), 22 N.J.R. 1760(a).
In (a), Type III was Type II. Added new (b) and redesignated existing (b) through (n) as (c) through ( o). In (g), clarified design criteria of pipe conduits and prohibited use of corrugated metal pipe in new construction. In (i)1, added draining time computation requirement, and in (i)2, deleted "For earth dams" and added downstream valve exception. In (n)2, deleted requirement regarding crest elevation of spillways.
Amended by R.1995 d.266, effective 6/5/1995.
See: 27 N.J.R. 795(a), 27 N.J.R. 2210(a).
Added (i)4.
Amended by R.2005 d.338, effective 10/3/2005.
See: 37 N.J.R. 1591(a), 37 N.J.R. 3814(a).
In (a), substituted "Natural Resources" for "Soil" in the table footnote 1.
Amended by R.2008 d.167, effective 6/16/2008.
See: 39 N.J.R. 4893(b), 40 N.J.R. 3715(a).
In (g)4, substituted "Drainage filters" for "Anti-seep collars".