68 MISC. PUBLICATION 43 4, U. S. DEPARTMENT OF AGRICULTURE 



is necessary to insure that the weir notch is rigid, the crest level, and 

 the sides vertical at a 90° angle to the crest and exactly 2 feet apart. 

 The weir must be made sharp-crested by beveling the downstream 

 edges of the crest about 45°. 



The weir pond is made by so widening and deepening the ditch for 

 some distance upstream from the notch as to assure practically a still- 

 water condition before the water flows over the weir. The procedure 

 is as follows: 



Commencing at approximately 50 feet upstream from the weir, widen the ditch 

 by tapering gradually to the full width of the weir wall or bulkhead. The water 

 must approach the weir in straight lines without swirling, eddy, or current. 



Deepen the ditch gradually to the depth of the weir bulkhead. 



Raise the banks starting at the weir and continuing upstream 50 to 100 feex, 

 depending upon the grade of the ditch and how close to the head of the ditch the 

 weir is placed. This should not be closer than about 50 feet. Experience has 

 shown that a distance less than 25 feet between the weir and the supply discharge 

 point usually causes too great a surface disturbance of the water to permit of an 

 accurate measurement of the head h and is liable to result in an increased velocity 

 of waterflow over the weir. The amount of raise should be only enough to give a 

 free fall of water from the weir crest to the water level below. 



The bottom of the ditch for a short distance downstream from the weir should 

 be protected by an apron of rock or plank to prevent scouring from the water 

 falling over the weir notch. 



The pond must not be allowed to fill with silt and sediment or other debris. 



After the weir pond has been constructed, the weir is installed in 

 accordance with the standard weir plan (fig. 19). Care should be used 

 in the installation of the weir to make certain of the following 

 conditions: 



The wall is set true and vertical. 



The stakes are driven solid and to the required depth. 



The joints are tight. 



The notch is properly centered over the center line of the ditch. 



The crest is perfectly level and set at the proper elevation above the ditch- 

 bottom level. 



All dimensions of the notch conform accurately to the construction plan. 



Every precaution is taken to prevent water from washing under the bottom 

 or around the sides of the weir. 



The soil is properly filled in and tamped around the weir and the weir-pond bank 

 brought flush with the top of the weir. 



The ditch just below the weir is adequately lined with rock, plank, or sandbags 

 to prevent scouring due to the water falling over the weir notch. 



The weir gage may be an ordinary ruler or a hardwood stick grad- 

 uated to feet and inches, but it is preferable to have it graduated to 

 feet and tenths and hundredths of a foot. It should be set upstream 

 from the weir notch a distance of 6 feet on a solid post driven into the 

 ground just far enough from the ditchbank to assure that it will 

 always be in the water. The reason for placing the gage as described 

 is that the depth of water h (fig. 19) from which the flow over the weir 

 is computed is measured vertically from the crest to the horizontal 

 plane or the still-water surface in the pond upstream from the weir. 

 There is a decided curving downward of the water surface near the 

 weir notch and it is necessary to get beyond the effect of this curvature 

 in order to get the correct depth. The zero of the gage should be 

 accurately set, level with the weir crest. This may be done with a 

 straightedge and a good carpenter's level. The crest and gage levels 

 should be checked carefully for settling or frost action. 



Table 17 shows the discharges in cubic feet per second and gallons 

 per minute for different heights (H) of water (as measured by the weir 





