When anti-seep collars are used in lieu of a drainage diaphragm, they shall have a watertight connection to the pipe. Maximum spacing shall be approximately 14 times the minimum projection of the collar measured perpendicular to the pipe. Collar material shall be compatible with pipe materials. The anti-seep collar(s) shall increase by 15% the seepage path along the pipe.
Drop inlet spillways must all have adequate anti-vortex devices.
Low stage releases shall have a non-clogging trash rack.
To prevent clogging of the conduit, an appropriate trash guard shall be installed at the inlet or riser.
Supply pipes through the dam to watering troughs and other appurtenances shall have an inside diameter of not less than 1-1/4 in.
Table 4 - Minimum spillway capacities
Emergency Spillways.
Emergency spillways convey large flood flows safely past earth embankments.
An emergency spillway must be provided for each dam, unless the principal spillway is large enough to pass the peak discharge from the routed design hydrograph and the trash that comes to it without overtopping the dam. The following are minimum criteria for acceptable use of a closed conduit principal spillway without an emergency spillway: a conduit with a cross- sectional area of 3 ft 2 or more, an inlet that will not clog, and an elbow designed to facilitate the passage of trash.
The minimum capacity of a natural or constructed emergency spillway shall be that required to pass the peak flow expected from a design storm of the frequency and duration shown in Table 4, less any reduction creditable to conduit discharge and detention storage.
The emergency spillway shall safely pass the peak flow, or the storm runoff shall be routed through the reservoir. The routing shall start either with the water surface at the elevation of the crest of the principal spillway or at the water surface after 10 days drawdown, whichever is higher. The 10-day drawdown shall be computed from the crest of the emergency spillway or from the elevation that would be attained if the entire design storm were impounded, whichever is lower. Emergency spillways shall provide for passing the design flow at a safe velocity to a point downstream where the dam will not be endangered.
Constructed emergency spillways are open channels that usually consist of an inlet channel, a control section, and an exit channel. They shall be trapezoidal and shall be located in undisturbed Emergency spillways on stormwater management ponds shall not be located on fill material. The side slopes shall be stable for the material in which the spillway is to be constructed. For dams having an effective height exceeding 20 ft, the emergency spillway shall have a bottom width of not less than 10 ft.
Upstream from the control section, the inlet channel shall be level for the distance needed to protect and maintain the crest elevation of the spillway. The inlet channel may be curved to fit existing topography. The grade of the exit channel of a constructed emergency spillway shall fail within the range established by discharge requirements and permissible velocities.
Structural-emergency-spilllways.
If chutes or drops are used for principal spillways or principal emergency or emergency spillways, they shall be designed according to the principles set forth in the Engineering Field Manual for Conservation Practices and the National Engineering Handbook-Section 5, Hydraulics; Section 11, Drop Spillways; and Section 14, Chute Spillways. The minimum capacity of a structural spillway shall be that required to pass the peak flow expected from a design storm of the frequency and duration shown in Table 4, less any reduction creditable to conduit discharge and detention storage.
Visual-resource-design.
The visual design of ponds shall be carefully considered in areas of high public visibility and those associated with recreation. The underlying criterion for all visual design is appropriateness. The shape and form of ponds, excavated material, and plantings are to relate visually to their surroundings and to their function.
The embankment may be shaped to blend with the natural topography. The edge of the pond may be shaped so that it is generally curvilinear rather than rectangular. Excavated material can be shaped so that the final form is smooth, flowing, and lifting to the adjacent landscape rather than angular geometric mounds. If feasible, islands may be added for visual interest and to attract wildlife .
Design Criteria for excavated ponds
Runoff.
Provisions shall be made for a pipe and emergency spillway if necessary. Runoff flow patterns shall be considered when locating the pit and placing the spoil (see table 4).
Side slopes.
Side slopes of excavated ponds shall be stable and shall not be steeper than three horizontal to one vertical. A 10 foot level bench shall be provided one foot below the normal pool elevation. Pond slopes above the normal pool elevation shall be no steeper than three horizontal to one vertical and a reverse slope bench, at least ten feet wide, shall be located one foot above the normal pool elevation.
Perimeter form
If the structures are to be used for recreation or are highly visible to the public, the perimeter or edge should be curvilinear.
Inlet protection.
If surface water enters the pond in a natural or excavated channel, the side slope of the pond shall be protected against erosion.
Excavated-material.
The material excavated from the pond shall be placed so that its weight will not endanger the stability of the pond side slopes and so that it will not be washed back into the pond by rainfall. It shall be disposed of in one of the following ways:
1. Uniformily spread to a height that does not exceed 3 ft, with the top graded to a continuous slope away from the pond.
2. Uniformly placed or shaped reasonably well, with side slopes assuming a natural angle of repose. The excavated material will be placed at a distance equal to the depth of the pond but not less than 12 ft from the edge of the pond.
3. Shaped to a designed form that blends visually with the landscape.
4. Used for low embankment and leveling.
5. Hauled away.
Plans and specifications
Plans and specifications for installing ponds shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose. See page S- 378-1 for items to be considered in development of the construction drawings and specifications.
Substantiating Data for Documentation
Installation and construction check notes shall be kept in sufficient detail to determine if the practice meets standards and specifications. minimum requirements as applicable are:
1. Profile notes along centerline of top of completed embankment.
2. Cross section notes at on( or more locations on the completed embankment.
3. Profile notes along centerline of earth spillway.
4. Cross section notes of earth spillways as needed to determine whether planned grade and dimensions have been met.
5. Location, size, type, grade, and/or pertinent elevations of the principal spillway, trickle tube, influent and/or effluent lines, and all other required appurtenances.
6. Depth and area of normal pool if these minimums are a part of specifications.
7. Maximum and minimum operational elevations.
8. Statement on condition or adequacy of clearing of the site and reservoir if this is a part of the plan.
9. Statement as to the condition or adequacy of vegetation on the embankment, spillway, and other disturbed areas.
10. Information on whether pond has been stocked.
11. Information on whether design features for fish management were incorporated.
12. Type and adequacy of sealing treatment.
13. Type and location of fencing and safety features where appropriate.
Sufficient survey will be made to determine quantities if required by the drawings and specifications or to determine number of practice units as applicable.
Measurements and computations for quantities will be recorded and filed to the extent that they are required to determine the number of practice units performed, or as requested by the land user.
These construction specifications apply to all ponds designed to meet the requirements of practice standard Pond(378)
Embankment Ponds
Foundation Preparation
The foundation area shall be cleared of trees, logs, stumps, roots, brush, boulders, sod, and rubbish. Where needed to establish vegetation, the topsoil and sod are to be stockpiled and spread on the completed dam. and spillways. Foundation surfaces shall be sloped to no steeper than 1: 1. The foundation area shall be thoroughly scarified before placement of the fill material. The surface shall, if necessary, have water added and be compacted so that the first layer of fill material may be compacted and bonded to the foundation.
The cutoff trench and any other required excavations shall be excavated to the lines and grades shown on the drawings or as staked in the field. To the extent they are suitable, excavated materials are to be used in the embankment.
Existing stream channels in the foundation area shall be sloped no steeper than 1: I and deepened and widened as necessary to remove all stones, gravel, sand, stumps, roots, and other objectionable material and to accommodate compaction equipment.
Foundation areas shall be kept free of standing water when fill is being placed on them.
Fill Placement
The material placed in the fill shall be free of detrimental amounts of sod, roots, frozen soil, stones over six (6) inches in diameter (except for rock fills), and other objectionable material.
Backfill material shall be placed around structures, pipe conduits, and antiseep, collars at approximately the same rate on all sides to prevent damage from unequal loading,
The placing and spreading of fill material shall be s tatted at the lowest point of the foundation and the fill brought up in horizontal layers of such thickness that the required compaction can be obtained. The fill shall be constructed in continuous horizontal layers except where openings or sectionalized fills are called for. In those cases, the slope of the bonding surfaces between embankment in-place and embankment to be placed will not be steeper than three (3) horizontal to one (1) vertical. The bonding surface is to be treated the same as that specified for the foundation so as to ensure a good bond with the new fill.
The distribution and gradation of materials shall be such that there will be no lenses, pockets, streaks, or layers of material differing substantially in texture or gradation from the surrounding material. Where it is necessary to use materials of varying texture and gradation, the more impervious material shall be placed in the center and upstream portions of the fill. Where zoned fills of substantially differing materials are specified, the zones shall be placed according to lines and grades shown on the drawings.
Moisture-Control
The moisture content of the fill material shall be such that the required compaction can be obtained. Material that is too wet shall be dried to meet this requirement. Material that is too dry shall have water added and mixed until the requirement is met.
Compaction
The construction equipment shall be operated over the areas of each layer of fill in a way that will result in the required compaction. Special equipment shall be used when the required compaction cannot be obtained without it.
When a minimum required density is specified, each layer of fill shall be compacted as necessary to achieve a density not less than that specified,
Fill adjacent to structures, pipe conduits, and antiseep collars shall be compacted to a density equivalent to that of the surrounding fill by means of hand tamping or manually-directed power tampers or plate vibrators Compaction adjacent to concrete structures will not be started until the concrete has attained sufficient strength to support the load.
Erosion-Protection
Vegetation shall be established on all exposed surfaces of the embankment, spillway, and borrow area where soil and climatic conditions permit. Where soil or climatic conditions preclude the use of vegetation, and erosion protection is needed, non- Vegetative means such as mulches or gravel may be used. In some cases, temporary vegetation may be used until conditions are right for establishment of permanent vegetation. The embankment and spillway shall be fenced where necessary to protect the vegetation.
Seedbed preparation, seeding, fertilizing, and mulching shall comply with the SOS Standards and Specifications for Critical Area Planting, Practice Code 342.
Principal Spillways
Corrugated metal pipe and its fittings shall conform to the standards set forth in the appropriate American Society of Testing Materials Designation(s) suitable for the intended purpose. 'These may include, but not be limited to, ASTM A-760, A- 762, A-849, A885, B-745, and B-790. Antiseep collars are to be of materials compatible with the pipe and installed so at to be watertight. The pipe shall be installed in accordance with the manufacturer's instructions. The pipe shall be firmly and uniformly bedded throughout its length and shall be installed to line and grade as shown on the drawings.
Concrete
Concrete shall receive the detail in mix design and testing consistent with the size and requirements of the job. Mix requirements or necessary strength should be specified. Type of cement, air entrainment, slump, aggregate or other properties are to be specified where necessary. All concrete is to consist of a workable mix that can be placed and finished in an acceptable manner. Necessary curing should be specified. Reinforcing steel is to be placed as indicated in the drawings and held securely in place during concrete placement. Subgrades and forms are to be installed to line and grade and the forms are to be mortar tight and unyielding as the concrete is placed.
Drainage-Diaphragm, Foundation, and Embankment Drains
Drainage diaphragm, foundation, and embankment drains, when required, will be placed to line and grade as shown on the drawings. Detailed requirements for drain material and any required pipe will be shown in the drawings and specifications for the job.
Embankment and Excavated-Ponds
All completed construction shall conform to the lines, grades, and elevations shown on the drawings or as staked in the field.
Construction operations shall be carried out in such a manner and sequence that erosion and air and water pollution will be minimized and held within legal limits. All work will be conducted in a skillful and professional manner.
The completed job shall present a professional appearance.
Measures and construction methods that enhance fish and wildlife values shall be incorporated as needed and practical.
Fencing and cover to control erosion and pollution shall be established as needed.
Appropriate safety measures, such as warning signs, rescue facilities, fencing, etc., shall be included in the construction drawings and specifications.
An operation and maintenance plan shall be agreed to by the owner and operator.
Structure-Classification
All structures (dams) will be reviewed and classed according to factors and procedures outlined in 520.21 of Subchapter C , Subpart C - Dams, SCS National Engineering Manual as supplemented herein. The class ("a," "b," and "c") as contained in this document is related to the damage that might result from a sudden major breach of the earth embankment. Structure classification and land use for runoff determination must take into consideration the anticipated changes in land use throughout the expected life of the structure, The classification of a dam is the responsibility of the designer, and subject to review and concurrence of the approving authority.
The classification of a dam is determined only by the potential hazard from failure not by the criteria. Classification factors in the National Engineering Manual, as supplemented, are given below:
Class "a" - Structures located in rural, agricultural, or urban areas dedicated to remain in flood tolerant usages where failure may damage non-resident buildings, agricultural land, floodplains, or county roads.
Class "b" - Structures located in predominantly rural or agricultural areas where failure may damage isolated homes, main highways or minor railroads, or cause interruption of use or service of relatively important public utilities.
Class "c" - Structures located where failure may cause loss of life or serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or railroads.
Where breach analysis has indicated that overtopping of downstream roads will occur, the following guidelines will be used:
