7 Del. Admin. Code § 5103-8.0

Current through Reigster Vol. 28, No. 6, December 1, 2024

Section 5103-8.0 - Requirements for Engineering and Design

8.1 Geotechnical Investigation

8.1.1 A geotechnical investigation shall be conducted to characterize the soil, concrete and rock conditions at the dam and spillway site and borrow areas (if appropriate) sufficient for design purposes. The investigations may consist of intrusive and/or non-intrusive techniques that are deemed acceptable to the Department.

8.1.2 The means and methods for conducting the investigation of existing dams shall be selected so as not to jeopardize the integrity of the dam or ancillary facilities.

8.2 Spillway Requirements

8.2.1 The minimum Spillway Design Flood (SDF) used to calculate required spillway capacity shall be determined as follows:

8.2.1.1 Hazard Class I - The SDF shall be the PMF

8.2.1.2 Hazard Class II - the SDF shall be 50% of the PMF

8.2.1.3 Hazard Class III - the SDF shall be the 24-hour, 100-year frequency, Type II storm. Any later technology adopted by the U.S. Department of Agriculture, Natural Resources Conservation Service, may be substituted for the use of the Type II storm with the approval of the Department.

8.2.2 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 SDF. When appropriate, and upon the approval of the Department, an incremental flooding assessment may be used for the rational selection of a reduced spillway design for specific site conditions based on quantitative and relative impact analysis. 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 a dam overtopping failure. The minimum design storm for the dam shall be the 100-year storm.

8.2.3 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.

8.2.4 All dams shall have an adequate storage for the SDF or have a spillway system that will safely pass the SDF without endangering the safety of the dam.

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

8.2.6 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.

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

8.2.7.1 Pipe conduits shall be of such design as to safely support without leakage the total external loads in addition to the total internal hydraulic pressure. 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

8.2.7.1.1 For Class I and II dams, the minimum allowable inside dimension of the pipe conduit is 30 inches to allow for inspection.

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

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

8.2.7.3 The pipe conduit must be designed so that negative pressures will not occur at any point along the primary (principal) spillway system.

8.2.7.4 Filter diaphragms or other seepage control methods approved by the Department must be installed to control seepage along the conduit.

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

8.2.7.6 An anti-vortex device must be included in the design, unless the applicant can demonstrate that one is not necessary.

8.2.7.7 A self-cleaning trash rack, approved by the Department, shall be installed at the intake to prevent clogging of the pipe conduit.

8.2.7.8 An emergency spillway shall be provided.

8.2.7.9 Corrosion protection shall be provided for all steel pipes as necessary.

8.2.7.10 Concrete cradles or encasements shall be provided for conduits through earth dams.

8.2.7.11 All structural concrete and steel structures shall be designed to resist the anticipated design loads using current design methodologies acceptable to the Department.

8.2.8 Should a vegetated or unlined auxiliary spillway, approved by the Department, be installed, it must be able to pass the SDF without jeopardizing the safety of the structure and with a computed average frequency of use less than:

8.2.8.1 Once in 100 years for Class I dams,

8.2.8.2 Once in 50 years for Class II dams, or

8.2.8.3 Once in 25 years for Class III.

8.3 Outlet Works and Drawdown Requirements

8.3.1 Except for excavated impoundments, dams shall include a device to allow draining of the reservoir within a reasonable amount of time. The following factors shall be considered in determining the reasonable time period for drainage:

8.3.1.1 The risk and nature of a potential dam failure

8.3.1.2 The time likely to be available to avert a failure after notice of conditions threatening the safety or stability of the dam

8.3.1.3 The influence of rapid drawdown on the stability of the dam, its appurtenant works, and the natural upstream slopes of the reservoir

8.3.2 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.

8.3.3 All pipe conduits used as drawdown drains for all dam classifications shall meet the requirements of Section 8.2.7, except that the minimum allowable inside dimension may be less than 30 inches.

8.4 Dam Requirements

8.4.1 The dam design shall designate what materials are suitable for the foundation of the dam, estimate the amount of foundation settlement, and describe the necessary foundation treatment for the structure.

8.4.2 The dam design shall designate what materials are to be used to construct the dam structure and provide the necessary design criteria for those materials. For embankment dams this includes, but is not limited to gradation, plasticity, strength, permeability, compaction and moisture criteria, lift thickness, and other properties that may be required by the Department. The source of the materials for embankment dams shall be designated. For concrete dams, the information provided shall include, but not be limited to the type of concrete, type of cement, aggregates, mix design, unconfined compressive strength, slump, and placement and curing procedures.

8.4.3 The applicant must demonstrate that the dam structure is stable under its various loading conditions, which must include at a minimum: during and after construction case, steady state case, maximum pool case, rapid drawdown case, and appropriate earthquake loading case. The engineering analysis and design shall follow current accepted engineering practices such as those of the U.S Army Corps of Engineers, the Natural Resources Conservation Service, the U.S. Bureau of Reclamation, the Federal Energy Regulatory Agency or other recognized standards and methodologies acceptable to the Department.

8.4.4 A seepage analysis shall be conducted to estimate the quantity of seepage that is anticipated to flow through, beneath, and/or around the dam. Unless it can be demonstrated that they are unnecessary, the design should include seepage reduction and/or control features to limit and/or collect and control seepage. Example techniques to reduce seepage include cut-off trenches, low-permeability cores, slurry trenches, or other proven methodologies acceptable to the Department. Example techniques for controlling and collecting seepage generally include drains, filters, and relief wells or other methodologies acceptable to the Department.

8.4.5 The design shall allow for certain freeboard as a minimum, not to be reduced under any circumstances during the operation of the dam and reservoir. The design freeboard is for that reservoir stage that will exist when the pool has reached maximum level during the spillway design flood with the outlet works and overflow spillway operating as planned. The freeboard is to be calculated to prevent overtopping and protect the dam against the destructive forces of waves, frost, settlement, and surface erosion.

8.4.6 The design shall provide for upstream slope protection against the action of waves and ice.

8.5 Other Requirements

8.5.1 Design references used shall be cited in the information submitted to the Department.

8.5.2 The Department may include, either at the time of granting a Permit to Construct a New Dam, or a Permit to Alter or Repair an Existing Dam or at a later date, a requirement that the owner provide and install devices necessary for the future inspection and surveillance of the dam. The number, type, and location of the devices shall be determined as a result of the Department's evaluation of the site and nature of the dam, the complexity of local natural conditions, and the degree of risk resulting from any future deterioration or failure. The requirement may include provisions for the measurement of the settlement of the crest or slopes of dams, the movement of walls in the valley or reservoir, the increases in pore water pressure in earth or increases in flow from drainage systems, and the installation of other devices required to detect serious changes in the structure or the affected area to allow the repair of deficiencies before more serious risk develops.

8.5.3 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.

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

8.5.5 In the case of multiple dams, when they are spaced so that the failure of an upstream dam or structure could endanger the safety of one downstream, the possibility of a multiple failure shall be considered in reviewing the design.

8.5.6 Utilities crossing within dam embankments are prohibited unless demonstrated to the satisfaction of the Department that such utilities will not jeopardize the safety of the dam.

7 Del. Admin. Code § 5103-8.0