Darrel M. Temple

Headcut development in vegetated earth spillways

Vegetated earth (soil and rock) auxiliary or emergency spillways have been used extensively on flood control reservoirs within the United States. Despite their widespread use, the processes by which these spillways erode during extreme events are only imperfectly understood, and there is a need for procedure to better predict spillway performance and safety. Therefore, research utilizing both laboratory and field data was undertaken to improve criteria for design and analysis of these spillways. For computational purposes, it was found that erosion of vegetated earth spillways could be divided into three phases. These phases are vegetal cover failure, concentrated flow erosion, and headcut advance. A computational procedure is developed for predicting the time associated with the first two of these phases. The procedure combines simplified flow and detachment rate relations in a form intended to minimize data requirements while allowing application to a broad range of conditions. Results of applying the procedure to predict headcut formation are shown to be generally consistent with available field data. This procedure may, therefore, be used to estimate the time of headcut formation for given flow and channel surface conditions.

Tractive Force Design of Vegetated Channels

ABSTRACT A method is presented for applying tractive force con-cepts to the design of vegetated channels. The method is developed from previously published data on flow resistance and allowable velocities in grassed water-ways. It is intended for use in its present form as a design tool, but also provides a framework in which to place refinements as the mechanics of this type of flow become more fully understood. The protective value of vegetal channel lining is con-sidered to be derived from two related, but distinct, in-teractions of the vegetation with the flow field. The first is the generation of turbulent eddies at a significant distance from the soil boundary resulting in an increase in flow resistance, and the second is a change in the structure of the turbulent eddies in immediate proximity to the boundary. The vegetation is therefore classified in terms of two indices believed to relate directly to these ac-tions. Guidelines are provided for the selection of these indices according to type and quality of cover. This approach eliminates the need for tabulating allowable velocities for each possible combination of channel slope, soil, and vegetal cover by considering the properties of the soil and those of the vegetation separately. An additional advantage is that the existing flow resistance curves are put in equational form as a single curve family.

Estimating Flood Damage to Vegetated Deep Soil Spillways

Although design guides are available, there is presently no procedure available for predicting damage and/or failure of a vegetated earth spillway subjected to flood flow conditions. The limited data available for vegetated spillways constructed in deep soil deposits are examined and shown to have sufficient consistency to allow calibration of simplified relations for use in damage estimation for spillways having similar geologic and hydraulic conditions. The nature of the data and the assumptions underlying these simplifications are discussed. Application of the relations with appropriate engineering judgment provides a means of damage and/or failure prediction until more analytic procedures become available.

Design of Grass-Lined Open Channels

ABSTRACT TRACTIVE force design of grass-lined channels is discussed briefly in terms of its advantages and its limitations. A step by step computational procedure for stability design is presented with example computations and a discussion of the required parameters.

Stability of Grass Lined Channels Following Mowing

ABSTRACT THE slope-partitioning method of stress separation is applied to erosion data taken from grass-lined open channels. The major trends of the data behavior are found to be reasonably explained by the slope-partitioning approach combined with a linear erosion-rate model. The large difference between the gross boundary stress and the stress found to be effective in soil particle detachment underscores the importance of stress partitioning in the analysis of erosion problems. An analysis of comparable channels tested in mown and unmown conditions suggests that cover removal by mowing has only a minor effect on the vegetal cover factor used in grass-lined channel design.

Design of Grass-Lined Channels: Procedure and Software Update

The allowable erosionally effective stress design procedures of Agricultural Handbook #667 are widely used for design of grassed waterways and other grass-lined channels. However, the design aids, including computational software, provided in that publication have become outdated. It has also been observed that for trapezoidal channels with flat bank slopes, direct application of the traditional n-VR curves results in over-sensitivity of flow resistance to changes in bank slope at flow depths near vegetal overtopping. The nature of the transition from unsubmerged to submerged flow conditions in these channels is discussed and software using a computational procedure that appropriately accounts for this action is presented.

Discharge Coefficients for Vegetated Earth Embankments

A direct means of estimating unit discharge for an overtopped embankment is presented. Although applicable to all embankments within the specified parameter range, the presented table of discharge coefficients was developed specifically for application to vegetated earth dams. The coefficients are based on backwater computations using flow-dependent flow resistance. These computations demonstrate that the assumptions of negligible energy loss across the embankment will lead to significant errors for vegetated embankments when ratio of overtopping head to embankment width is small.

Allowable Stress Design of Stable Channels in Noncohesive Material: A Review and Update

THE relationship between the Shields diagram criterion for initiation of sediment motion and current stable channel design criteria is discussed. It is shown that the laboratory data represented by the Shields diagram is in better agreement with the empirically developed channel design criteria than is often assumed. A stress-partitioning approach to allowable stress design is discussed which applies to both coarse and fine material and is consistent with our present knowledge of flow-boundary interaction. Application of the approach is straightforward. No new data is introduced, and the discussion is simplified through consideration of only dominant processes and parameters. Emphasis is on application to design problems.