OAR 690-020-0042: Minimum Design Requirements for New Dams or to Increase Dam Height: Spillways

(1) Dams on stream channels and all High Hazard Rated Dams must have a Spillway. (2) Spillway(s) design to build a new Dam or to increase Dam Height shall include the following minimum elements: (a) Utilization of Inflow Design Flood. Determination of Inflow Design Flood as described in OAR 690-020-0037 is required to determine the required Spillway capacity; (b) Hydraulic evaluation of flow through control section. Flood flow through the control section must be calculated and the minimum Freeboard at the Inflow Design Flood must be one foot for High Hazard Dams and two feet for Significant and Low Hazard Dams; (c) Optional low elevation Spillway. An interior Spillway connected to the low level Conduit may be used for Low and Significant Hazard Dams, and for High Hazard Dams only with specific approval by the State Engineer. The capacity of the low elevation Spillway may be considered in design of the overflow Spillway; (d) Stable Spillway control section. The Spillway control section must be hydraulically and structurally stable for the Inflow Design Flood and have permanent features so that the control section is identifiable for re-measurement of cross section during routine inspections; (e) Spillway channel stability. Spillways shall be designed to be structurally adequate and stable under all conditions of reservoir operation. Spillway structures of High Hazard Dams shall be designed for earthquake ground motions per OAR 690-020-0036; (f) Reinforced concrete specifications for spillways. Structural elements of reinforced concrete shall be designed for both strength and serviceability. The 28 day strength of structural concrete shall be provided. The strength properties of the reinforcing materials shall also be provided and contract drawings shall clearly indicate the size, location, spacing, and cover requirements. Treatment of construction joints and contraction/expansion joints shall be described and special provisions for strength transfer and leakage prevention identified. Air entrainment shall be provided in cast-in-place concrete if needed for freeze-thaw protection, durability, and workability; (g) Debris booms. For High and Significant Hazard Dams, debris or log booms may be required. Where required, they shall be designed for the Spillway approach where logs and other debris may block or damage the Spillway structure. The design shall specify the necessary anchor capacity, and the design of the anchors; (h) Gates and Flashboards. Detailed drawings and specifications are required for Spillway Gate structures or flashboards, if present on the proposed Dam. Operations and maintenance plans are required for any Dam with a Gated Spillway, or where flashboards or stop-logs are used in the Spillway as per OAR 690-020-0068; and (i) Energy dissipation. The design of stilling basins for High Hazard Dams, and where required by the State Engineer for Significant Hazard Dams, shall be based on calculated hydraulic forces and designed to dissipate energy and minimize scour and erosion from the Inflow Design Flood. (3) Low and Significant Hazard Dams constructed off channel are not required to have a Spillway, if redundant mechanisms to prevent overfilling are included in the design.