Location: Watershed Physical Processes Research2013 Annual Report
1a. Objectives (from AD-416):
Use the results of the project entitled “Development of Wave-Energy Absorbers for Cost-Effective Protection of Levees in Irrigation Reservoirs” to update Technical Release No. 56 (TR56) “A Guide for Design and Layout of Vegetative Wave Protection for Earth Dam Embankments.” The update material will be geared towards application by NRCS field engineers.
1b. Approach (from AD-416):
Summarize the findings of “Development of Wave-Energy Absorbers for Cost-Effective Protection of Levees in Irrigation Reservoirs” by using appropriate narrative, plots, and lookup tables so that the information can be readily used by a field engineer. The TR56 update will include a review of relevant literature describing research conducted since its publication in 1974. This will result in a more contemporary work that describes the current methodology for using floating wave barriers and vegetation for levee protection. The wave barrier work will be summarized with only information relevant to an engineer seeking to design a wave barrier system included. This will include a technique specific to small reservoirs for estimating wave characteristics from wind speed and fetch length. Example calculations for wave barrier design will include wind/wave prediction, barrier sizing, and estimated performance of the wave barrier. It is not anticipated that this update will include comprehensive specific recommendations on materials, connections, etc. The focus will be on the necessary dimensions and buoyancy needed for wave energy reduction. A limited amount of logistical testing may be performed to help with guidance on floating wave barrier assembly.
3. Progress Report:
NRCS Technical Release No. 56 (TR56) “A Guide for Design and Layout of Vegetative Wave Protection for Earth Dam Embankments” (TR56) is focused on vegetative wave reduction and provides useful information. However, in the case of irrigation reservoirs, the wide variation in water levels imposed by their use as supplemental irrigation creates an environment where vegetation cannot survive to provide adequate protection. This situation shifts the focus to mechanical means of protection such as levee armoring or the use of breakwaters. Levee armoring has been shown to be effective, but the cost is prohibitive. Wave barriers require much less material, but their deployment technique is more critical. In order to achieve maximum wave reduction at a minimum cost, the design should be specific to reservoir dimensions, taking fetch length and wind speeds into account. Activities for FY 13 included: (1) beginning the writing of the guidance document, and (2) monitoring levee erosion at Maddux Reservoir in Fisher, Arkansas. A detailed outline for the guidance document has been written, and analysis that will lead to look-up tables and charts for estimating waves from wind data and for sizing a floating wave barrier were begun. It is anticipated that the guidance document will be completed in September 2013. At the field site, significant erosion was observed, both from overland flow during precipitation events and from waves that occurred when the reservoir was filled during the winter months early in 2013. A researcher from the University of Mississippi National Center for Computational Hydroscience and Engineering participated directly in the establishment of the field site and in the analysis of wind a wave data collected at the site.