Design Engineering Report: Avon Exhibit 1

Home Browse Search

X. Stability Analysis The structures are designed to withstand the forces of the 100-year flood from potential scour, incipient motion and over-turning moment. Scour Undermining, scour or general bed degradation are the most common methods for failure of large structures in an alluvial channel. Typically a large obstruction in the channel causes localized secondary currents that scour the bed from around and under the structure creating a hole that eventually the structure falls into under the force of gravity. Potential scour depths are dependant on the mobility of the channel bed material. Both Bob Sr. and Bob Jr. are to be set on excavated bedrock; furthermore, both structures have exposed bedrock control immediately downstream. The bedrock has minimal scour potential and the downstream control of bedrock would prevent general degradation of the bed. Bob Sr. is set a minimum of 4.0 feet below the downstream bedrock elevation. Bob Jr. is set a minimum of 2.5 feet below the downstream bedrock elevation. Baby Bob is in an armored reach of boulders and cobbles that also has a minimal scour potential. Baby Bob has a footer rock or is keyed into the bed a minimum of 2.0 feet below the downstream scour pool and below the downstream bedrock elevation. Potential for scour failure is minimal for this project and the designed key down of the structures provides a margin of safety. Incipient Motion Individual particles in the channel bed were analyzed for potentially moving in the channel flow. Sediment transport predictions based on a 100-year flood scenario were performed using incipient motion analysis based on Shields' criteria (Gessler, 1971). This analysis was used to evaluate relative bed mobility from channel shear stress and bed material size distributions. The "critical size" is the size of a particle that is predicted to begin moving under the forces at a given flow. The critical size was determined at each of the three structures using the maximum calculated shear stress (at the cross section) as the critical shear stress in the Shield's Equation. A shear stress was evaluated for the 100-year discharge of 5,430 cfs; however, the stresses are most severe at 2,500 cfs at Baby Bob and 1,400 cfs at Bob Jr. Therefore, the maximum stress up to the 100-year flood event was used to calculate the critical particle size. This procedure was then evaluated for bed slopes and channel slopes as well as a factor for secondary cunents as presented by (Fischenich et. al, 2000). A Factor of Safety (FOS) was then calculated for the designed particles (or boulders) at each structure. The FOS calculation shows how much proportionately larger the proposed boulders are than the boulders that would withstand 100-year floor forces. In other words, a FOS of 1.0 means the boulder may mobilize under the flow rate studied; a FOS greater than 1.0 indicates a stable design. The design boulder size at: Baby Bob = 3.0 feet with a FOS of 2.1, at Bob Jr. = 3.5 feet with an FOS 1.6, and at Bob Sr. = 4.0 feet with an FOS of 1.5. The designed particles associated at all three structures are sufficient in size to not mobilize with the 100-year flood stresses. 9 of 11