Modified Drop-Boxweir for Monitoring Flows From Erosion Plots and Smallwatersheds

Sediment-laden flow and chemical-constituent concentration data from erosion plots and small watersheds areoften needed for erosion and water-quality impact evaluations and modeling. The original drop-box weir was modified topass the design flow through the V-notch and box sections of the weir. Approach conditions consisted of gutters andaprons that divert runoff water and sediment to the box at right angles. An evaluation of the effects of these approachconditions on the original rating for the drop-box weir and the effects of not having a false back wall for erosion-plot usewas conducted. A rating for low flows was obtained. The rating for the drop-box weir with the new approach conditionswas different from the original rating. Three rating equations were fit to the data for the three flow ranges. Observationsof weir performance suggested that the reason for the three equations was related to the stepped side weirs. This studyand other published studies led to the conclusion that the drop-box weir should not be modified unless a new study of theeffects of the changes on the rating are evaluated. The gage height for the design flow for sizing the modified drop-boxweir was established. A decision table was developed for appropriate use of the drop-box weir.