Section 3. HYDRAULIC DESIGN A. Weirs and Orifices A.1. WEIRS (original) (raw)

NOTE: Some of the graphs contained in this section are copied from the Los Angeles Hydraulics Manual and we wish to give them credit for their efforts. Also, applicable graphs are for 8 " curb heights which may not meet Rio Rancho standards. A weir is a barrier in an open channel, over which water flows. A weir with a sharp upstream corner or edge such that the water springs clear of the crest is a "sharp crested weir". All other weirs are classified as "weirs not sharp crested". Weirs are to be evaluated using the following equation: Q = CLH 3/2 where: Q = Discharge in cfs C = Discharge coefficient from Handbook of Hydraulics, King and Brater, 5th Edition (or comparable) L = Effective length of crest in feet H = Depth of flow above elevation of crest in feet (approach velocity shall be disregarded in most applications) Applications Weirs are generally used as measuring and hydraulic control devices. Emergency spillways in which critical depth occurs and overflow-type roadway crossings of channels are the most common applications of weirs. Channel drop structures and certain storm drain inlets may also be analyzed as weirs. Special care must be exercised when selecting weir coefficients in the following cases: a. Submerged weirs b. Broad crested weirs c. Weirs with obstructions (i.e., guardrails, piers, etc.)