Section 0400-45-07-.07 - DESIGN STANDARDS FOR NEW DAMS(1) Design of Dams. Designs of dams shall conform to accepted practices and procedures of the engineering profession. Design references developed by the U.S. Army Corps of Engineers, Soil Conservation Service, and the Bureau of Reclamation may be used although the limiting criteria must be in accordance with these rules. (a) All dams will be considered on an individual basis and reviewed in accordance with prevailing practices that are currently accepted by the engineering profession.(b) All structures other than Category 3 dams constructed before 2008 shall be designed to withstand seismic accelerations of the following intensities: Zone 1=0.025g, Zone 2=0.05g, Zone 3=0.15g. Zones refer to "Geologic Hazards Map of Tennessee" by Robert A. Miller, 1978. All dams constructed during or after 2008 shall be designed to withstand the peak ground acceleration for an earthquake with a 10% probability of exceedance in 50 years as determined by the United States Geological Survey at the time the construction permit is issued. A different peak ground acceleration may be used if site specific studies using accepted engineering practices determine that a different value is appropriate.(c) A complete engineering report, plans, and specifications shall be submitted for each dam.(d) A complete geotechnical report shall be submitted as an integral part of the engineering report for all Hazard Potential Category (HPC) 1 and 2 dams (defined in Rule 0400-45-07-.05). For HPC 3 dams, sufficient investigation will have to be made to determine if the site and the fill material to be used are suitable, and this information will have to be included in the engineering report.(e) A hydrologic/hydraulic analysis shall be submitted as an integral part of the engineering report for all dams. A breach analysis shall be submitted for all HPC 1 and HPC 2 dams and, for the former, shall be included in the Emergency Action Plan. The breach analysis must use surveyed cross sections at all stations where homes or other structures may be flooded. A sunny-day breach shall be modeled with the impoundment at the elevation of the emergency spillway invert when the failure begins, or, if there is no emergency spillway, at the elevation of the inlet of the principal spillway. Breach modeling under sunny-day, overtopping, or any other conditions, is site specific.(f) All Category 1 dams shall submit to the Commissioner an Emergency Action Plan. This plan shall include, but not be limited to, the following: 1. Inundation information and an inundation map based on the breach analysis.2. Procedures for notification of people downstream and law enforcement and other government agencies.3. Resources for emergency actions such as contractors, equipment supply businesses, etc.(g) Design calculations for all major components of the structure, i.e., spillways, pipes, etc., shall be included in the engineering report.(2) Principal Spillways. (a) All component parts of the principal spillway except attached gates and trash racks will be of equal durability. The structural design criteria and detailing of such spillways will conform to recognized standards and codes of practice.(b) In requiring the capacity of the principal spillway, the Commissioner may consider: (1) the benefits that accrue to the reduction of the discharge rate, (2) damages that may result from prolonged storage in the reservoir, (3) damages that may result from prolonged outflow, (4) the possibility of occurrence of significant runoff from two or more consecutive storm events within the time required to empty the reservoir, and (5) limitations in water rights or other legal requirements.1. All conduits under a dam shall support the external loads imposed with an adequate factor of safety. They must withstand the internal hydraulic pressures without leakage under full external load and settlement. They must convey water at the design velocity without damage to the interior surface of the conduit.2. Principal spillway conduits under earth dams shall be designed to support fill heights greater than the original constructed height where there is a reasonable possibility that it may become desirable to raise the embankment height at a later date to incorporate additional storage.3. Principal spillway conduits shall be of reinforced concrete pipe, cast-in-place reinforced concrete, ductile iron pipe, or plastic pipe. Fill height and foundation conditions require special considerations for ductile iron pipe and plastic pipe so that each use will be checked on an individual basis; cradling or encasement in concrete may be required. Welded steel pipe is not acceptable for Category 1 and Category 2 dams, and corrugated metal pipe is not acceptable for any class of dam.4. Principal spillway conduits shall be field tested for watertightness before backfilling. This requirement as well as the method of testing shall appear on the plans or in the specifications.5. Rigid principal spillway conduits shall be designed as positive projecting conduits.6. All reinforced concrete water pipe - steel cylinder type - prestressed, all reinforced concrete water pipe - steel cylinder type - not prestressed, and all reinforced concrete water pipe - noncylinder type - not prestressed, shall meet the AWWA specifications effective at the time of application.7. Elliptical or other systems of reinforcement requiring special orientation of pipe sections are not permitted in pipe drop inlet barrels.8. Reinforced concrete pipe, with or without cradles, shall be designed to support at least 12 feet of earth fill above the pipe at all points along the conduit.(c) The minimum inside diameters of pipes shall be as follows: 1. Category 3 dams: The minimum diameter of the principal spillway barrel will be 18 inches for fill heights up to 50 feet and 24 inches for greater heights; or Where the drop inlet is designed hydraulically in such a way that the flow in the barrel under all possible conditions of discharge and foundation consolidation is positively known to be open channel flow with the water surface in the conduit subject to atmospheric pressure only, the minimum diameter shall be 18 inches; or
Where welded steel pipe is used, the principal spillway shall be designed in accordance with conditions presented in subparagraph (e) of this paragraph.
2. Category 2 dams: The minimum diameter of the principal spillway barrel shall be 24 inches.3. Category 1 dams: The minimum diameter of the principal spillway barrel shall be 30 inches.4. Smaller conduits may be used if detailed calculations show them to be hydraulically and structurally adequate and all other requirements of this rule 1200-5-7-.07 are met.(d) Where the barrel and cradle or bedding are to rest directly on firm bedrock thick enough so that there is essentially no foundation consolidation under the barrel, the cradle under the pipe need not be articulated.(e) Principal spillways of welded steel pipe may be used for Category 3 dams under the following conditions, all of which must be met: 1. The minimum diameter of the barrel will be 18 inches.2. The height of fill over the pipe will be less than 35 feet.3. Welded steel pipe conduits are to conform to American Society of Testing Materials (ASTM) specifications A53, A120, A135, A139, or A134 and are to be structurally designed as rigid pipe. A joint extension safety margin of 1.5 inches is to be provided for conduits on yielding foundations. Welded pipe is to be protected by an approved exterior coating.(f) Conduit joints will be designed and constructed to remain water tight under maximum anticipated hydrostatic head and maximum probable conditions of joint opening, including the effects of joint rotation, and must have a margin of safety where required.(g) Trash racks will be designed and built to provide positive protection against clogging of the spillway at any point. The average velocity of flow through a clean trash rack will not exceed 2.5 feet per second with the water elevation in the reservoir five feet above the top of the trash rack or at the crest of the emergency spillway, whichever is lower. Velocity will be computed on the basis of the net area of opening through the rack. For dry dams, a trash rack may be used in lieu of a ported concrete riser. The principal spillway trash rack will extend sufficiently above the anticipated sediment elevation at the inlet to provide full design flow through the spillway with velocities through the net area of the trash rack above the sediment elevation not in excess of two feet per second when the water surface in the reservoir is five feet above the top of the trash rack.
(h) All closed conduit principal spillways designed for pressure flow will have an anti-vortex device.(3) Drawdown Facilities.(a) All new dams shall have a drawdown facility. This facility shall be capable of draining the reservoir in ten (10) days or less. It may be assumed that this requirement has been met if seventy-five (75) percent of the liquid volume from the normal water storage elevation has been evacuated in the ten (10) day period. The use of a longer period must be justified.(b) The necessary drawdown facility for any dam shall be made an integral part of the principal spillway structure if the principal spillway configuration warrants it, but in no case will the drawdown facility be valved on the downstream side of the embankment. Siphon facilities will be accepted after proper engineering justification.(c) Subparagraph (b) of this paragraph does not apply in the case of a water supply line through the dam, but in such cases provision must be made for a positive shutoff on the upstream side of the structure.(d) Drawdown systems shall be maintained in an operable condition. Drawdown valves shall be opened and closed at least annually to ensure operability.(4) Emergency Spillways. (a) An emergency spillway shall be provided for each structure, unless the principal spillway is large enough to pass the routed freeboard hydrograph discharge and the debris that comes to it. A conduit type principal spillway having a barrel with a cross-sectional area of 20 square feet or more, an inlet which will not clog, and an elbow designed to facilitate the passage of debris, is the minimum size and design that may be utilized without an emergency spillway. If a principal spillway of this type and size is not provided, danger from clogging requires the use of an emergency spillway regardless of the volume of storage provided.(b) A single uncontrolled open channel spillway may be used for all purposes provided it is designed to accommodate all discharges, including the freeboard storm, without damage to the structure. However, a positive means to drain the reservoir must also be provided.(c) Emergency spillways shall be proportioned so that they will pass the freeboard hydrograph at the safe velocity determined for the site. They shall have sufficient capacity to pass the freeboard hydrograph with the water surface in the reservoir at or below the maximum storage elevation.(d) Minimum Freeboard Design Storms | Size | Freeboard Design Storm (6 Hour) |
| Small | 1/2 PMP |
| Intermediate | PMP |
| Large | PMP |
(e) All dams shall have an emergency spillway system with capacity to pass a flow resulting from a 6 hour design storm indicated in subparagraph (d) of this paragraph for the size corresponding to the dam. Any new dam constructed between October 3, 1987, and February 19, 2001, shall be required to pass the Freeboard Design Storm specified in subparagraph (3)(b) of Rule 0400-45-07-.06. However, if the applicant's engineer provides calculations, designs, and plans to show that the design flow can be stored, passed through, or passed over the dam without failure occurring, or if he can successfully demonstrate to the Commissioner that the dam is a safe structure and can certify that the dam is sufficient to protect against probable loss of human life downstream, said dam design may be approved by the Commissioner. The establishment of the criteria in subparagraph (d) of this paragraph does not eliminate the need for sound engineering judgment but only establishes the lowest limit of design considered acceptable.(f) The relationship between the water surface elevation in the reservoir and the discharge through the emergency spillway shall be evaluated by computing the head losses in the inlet channel upstream of the control section, or if a control section is not used, by computing the water surface profile through the full length of the spillway. Manning's formula will be used to evaluate friction losses and determine velocities.(g) The freeboard hydrograph shall be routed through the reservoir starting with the water surface at the elevation of the principal spillway inlet.(h) A vegetated earth or unlined emergency spillway shall 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 not more frequent than once in 25 years for Category 3 dams, once in 50 years for Category 2 dams, and once in 100 years for Category 1 dams.1. Vegetated and earth emergency spillways may be open channels and may consist of an inlet channel, a control section and an exit channel. Subcritical flow exists in the inlet channel and the flow may be supercritical in the exit channel.2. Vegetated emergency spillways may be trapezoidal in cross-section and shall be protected from damaging erosion by a grass cover. They shall be used at sites where a vigorous grass growth can be sustained by normal maintenance without irrigation.3. Earth spillways may be used in those areas where vegetative growth cannot be maintained. They are similar to vegetated spillways but are designed for lower permissible velocities and less frequent use. The needed maintenance after a flow occurs is the responsibility of the certificate holder.4. Earth and vegetated emergency spillways are designed on the basis that some erosion or scour is permissible if its occurrence is infrequent, if maintenance facilities are provided, and if damage from a severe storm, as represented by the freeboard inflow hydrograph, will not endanger the structure.5. A Manning's ''n'' of 0.040 may be used for determining the velocity and capacity in vegetated spillways. Permissible velocities in earth spillways may be based on an "n" value of 0.020 but the capacity of earth spillways will be based on an appraisal of the roughness condition at the site.6. When the anticipated average use of a vegetated emergency spillway is more frequent than once in 50 years, the maximum permissible velocity will be in accordance with the values given below. The values may be increased 10 percent when the anticipated average use is not more frequent than once in 50 years or 25 percent when the anticipated average use is not more than once in 100 years. The maximum velocity limitations given below for vegetated or earth emergency spillways apply to the exit channel.7. The values given will be the upper limit for all grasses. Values for grasses or grass mixtures will be determined by comparison with the values shown, with due consideration given to the growth characteristics and density attained in the local area by the species under consideration.8. Where bona fide studies or investigations have been made to determine the permissible velocity for a specific soil and site, these values may be used in lieu of those shown below.9. Maximum Permissible Velocities for Vegetated Earth Spillways. Grasses or Grass Mixtures
| Soil Type | Slope | Maximum Permissible Velocity |
| Erosion Resistant | 0-5 % 5-10% | 8.0 fps 7.0 fps |
| Easily Erodible | 0-5 % 5-10% | 6.0 fps 5.0 fps |
(5) Earth Embankments. (a) Sufficient freeboard shall be provided to prevent overtopping with the passage of the freeboard hydrograph plus the additional freeboard required by the site for wave action.(b) The top width of earth embankments will not be less than the value given by the following equation: Click to view Image
where H=height of embankment in feet.
W=minimum top width of embankment in feet.
(c) The earth embankment will be riprapped or have other wave erosion protection provided over the full range in stage between three feet above and below the normal pool elevation.(d) All dams shall be designed and constructed to prevent the development of instability due to excessive seepage forces, uplift forces, or loss of materials in the embankment, abutments, spillway areas, or foundation. Seepage analysis for design and inspection during construction shall be in sufficient detail to prevent the occurrence of critical seepage gradients. All dams permanently impounding water shall be constructed with an embankment toe drain with drain pipes installed to discharge the seepage.(e) All dams shall have a permanent bench mark monument located near the embankment in undisturbed soil or in bedrock. This bench mark shall be detailed in the plans and specifications.(f) All dams shall be protected from surface erosion by appropriate vegetation or some other type of protective surface such as rip-rap or paving and shall be maintained. Examples of appropriate vegetation include, but are not limited to, Bermuda grass and fescue. All inappropriate vegetation such as honeysuckle, briers, bushes and trees shall be kept off the dam by routine mowing.Tenn. Comp. R. & Regs. 0400-45-07-.07
Original rule filed October 16, 2012; effective January 14, 2013. Rule renumbered from 1200-05-07.Authority: T.C.A. §§ 69-11-101 et seq., and 4-5-201 et seq.