Location: Watershed Physical Processes Research2013 Annual Report
1a. Objectives (from AD-416):
The primary objective of this work will be to update NRCS Technical Release No. 56 (TR56) “A Guide for Design and Layout of Vegetative Wave Protection for Earth Dam Embankments” with the results of the USDA-ARS, National Sedimentation Laboratory (NSL), Technical Report No. 65 “Experimental and Numerical Investigations of Floating Breakwater Performance”. The updated material will include guidelines for field implementation of floating wave barriers. A specific objective is to provide additional field data and improve the existing guidelines for sizing and deployment of the floating breakwaters.
1b. Approach (from AD-416):
The Technical Release No. 56 (TR56) update will include a review of relevant literature describing research conducted since its publication in 1974. The contemporary report will include current methodology for using floating wave barriers and vegetation for levee protection. The new report will summarize the findings of National Sedimentation Laboratory (NSL) Technical Report 65 by using appropriate narrative, plots, and lookup tables. The floating wave barrier work will be summarized and include only information relevant to an engineer seeking to design a wave barrier system. 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 new field station will be constructed in a selected pond, with cooperation from the NSL, to collect long-term field data, which will include synchronous measurements of wave height, water temperature and wind speed as well as soil type and erosion rate. The new data sets will provide more reliable guidelines on wind-wave prediction in small ponds and begin to relate them with levee erosion rates.
3. Progress Report:
Natural Resources Conservation Service (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.