SECTION 299.15. Hydrologic and Hydraulic Criteria for Dams

(a) Hydrologic criteria.

(1) Minimum hydrologic criteria for proposed dams. The following minimum hydrologic criteria includes those proposed dams to be constructed according to Texas Water Code, §11.142.

(A) A proposed dam design must meet the minimum design flood hydrograph criteria.

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(B) The minimum design flood hydrograph must be based on the size and hazard classification of a proposed dam at the time of the design and calculated using the criteria in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines of Dams in Texas.

(C) Proposed dams and spillways or dams and spillway to be reconstructed, modified, enlarged, rehabilitated, or altered using hydrologic procedures of the Natural Resources Conservation Service will be acceptable, provided that the procedures are shown to be equal to or more conservative than the procedures provided in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines for Dams in Texas.

(2) Exemptions to minimum hydrologic criteria for proposed dams. Any dam designed to withstand overtopping without failure of the dam, including the foundation and abutments, as demonstrated by studies prepared by the owner's professional engineer will be exempt from the minimum hydrologic criteria.

(3) Minimum hydrologic criteria for existing dams. The following criteria applies to dams that existed before the effective date of this subchapter.

(A) An owner of a large- or high-hazard existing dam that was required to meet 100% of the probable maximum flood (PMF) before the effective date of these rules and that is shown by an evaluation by a professional engineer to meet 75% or more of the PMF will not be required to upgrade the dam to meet minimum hydrologic criteria in paragraph (1)(A) of this subsection. The dam will be considered adequate to meet the minimum hydrologic criteria, provided the owner:

(i) has an emergency action plan that meets the requirements in §299.61 of this title (relating to Emergency Action Plans);

(ii) has an operation and maintenance plan for the dam as described in §299.43 of this title (relating to Operation and Maintenance);

(iii) has an inspection program that has been implemented as described in §299.42 of this title (relating to Inspections); and

(iv) submits an annual report to the executive director documenting compliance with the requirements in clauses (ii) and (iii) of this subparagraph, beginning 12 months after the effective date of this section.

(B) An owner of a dam not specified in paragraph (3)(A) of this subsection that was required to meet the minimum hydrologic criteria before the effective date of these rules, but is shown by an evaluation by a professional engineer to meet the minimum hydrologic criteria in paragraph (1)(A) of this subsection, will not be required to be upgraded and the dam will be considered adequate to meet the minimum hydrologic criteria.

(C) An owner of an existing dam that does not meet the minimum hydrologic criteria in paragraph (1)(A) of this subsection or the size or hazard classification has been raised and the dam does not meet the minimum hydrologic criteria in paragraph (1)(A) of this subsection for the new size or hazard classification may be required to submit to the executive director any of the following, prepared by a professional engineer:

(i) final construction plans and specifications as described in §299.22 of this title (relating to Review and Approval of Construction Plans and Specifications) for modifying, enlarging, or altering the dam or spillways to meet the minimum hydrologic criteria as described in paragraph (1)(A) of this subsection, provided the minimum hydrologic criteria at least meets 75% of the PMF and the owner addresses the requirements in paragraph (3)(A) of this subsection;

(ii) an analysis or other option to request a reduction in the minimum hydrologic criteria as described in paragraph (4) of this subsection; or

(iii) a plan for alternatives to upgrading as described in §299.17 of this title (relating to Alternatives to Upgrading Dams).

(D) An owner of an existing dam that meets the requirements of subparagraph (A) of this paragraph and that is required to be modified due to structural deficiencies shall be required to submit to the executive director final construction plans and specifications for the structural modifications as described in §299.22 of this title. The dam will not be required to be upgraded to meet the minimum design criteria in paragraph (1)(A) of this subsection.

(E) An owner of a dam that has been evaluated under this paragraph shall be advised of the requirements for the owner's dam by letter. The owner shall be required to submit a written plan of action to address the requirements and a time frame to complete the requirements.

(4) Reduction of minimum hydrologic criteria. The minimum hydrologic criteria may be reduced as follows.

(A) The owner may request that the executive director reduce the minimum hydrologic criteria if the owner submits:

(i) dam breach analysis, prepared by a professional engineer and using the normal storage capacity non-flood event, the barely overtopping flood event, and the design flood event, if applicable, that demonstrate existing downstream improvements would not be adversely affected, which is defined as the downstream flooding differentials being less than or equal to one foot between breach and non-breach simulations in the affected area;

(ii) one or more technical options included in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines of Dams in Texas, demonstrating that existing downstream improvements would not be adversely affected;

(iii) documentation of the purchase, or an easement for, the property downstream of the dam that would be impacted by a dam failure and showing that it has been dedicated to non-residential and non-commercial use; or

(iv) documentation that the property downstream has been dedicated by the property owner to non-residential and non-commercial use.

(B) The executive director shall evaluate the owner's request for reduction in the minimum hydrologic criteria to determine if the request is appropriate. If the executive director agrees with the analysis, the executive director shall approve the request in writing.

(C) If the executive director does not agree with the owner's request for reduction in the minimum hydrologic criteria, the executive director shall deny the request in writing.

(b) Hydraulic criteria for proposed dams or dams proposed to be reconstructed, modified, enlarged, rehabilitated, or altered.

(1) The owner shall have a professional engineer evaluate the hydraulic adequacy of the dam and spillways using the guidelines in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines of Dams in Texas.

(2) The owner shall have a professional engineer address the stability of the spillways to determine if the spillways will adequately meet the minimum design storm without being significantly damaged.

(3) The owner shall have a professional engineer determine a minimum freeboard for a proposed large size dam as defined in §299.13 of this title (relating to Size Classification Criteria) as outlined in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines for Dams in Texas.

(c) Hydraulic criteria for existing dams. If it becomes necessary for an owner of an existing dam to reevaluate the hydraulic adequacy of the dam and spillways, the owner shall have a professional engineer evaluate the hydraulic adequacy of the dam and spillways using the guidelines in the most current version, at the time of the analysis, of the agency's Hydrologic and Hydraulic Guidelines of Dams in Texas.

Source Note: The provisions of this §299.15 adopted to be effective January 1, 2009, 33 TexReg 10465