2. Design Criteria. 
Early studies of the discharge coefficient C relation of C to the ratio He/Hd. These studies indicated ranged from 3.90 to 4.10 at design head and decreased to 3.10 at zero head. An approximation of the upper value can be derived by transferring the sharp-crested weir coefficient to a rounded weir crest that fits the lower nappe. The head on the rounded crest is known to be 0.888 times the head on the sharp crest. Using a discharge coefficient of 3.33 for a sharp-crested weir and assuming the velocity of the approach flow to be negligible, the coefficient for design head is derived as 3.93. The lower limit of C = 3.10 closely approximates the theoretical broad-crested weir coefficient of 3.087. The theory, which is based on critical depth in rectangular channels, is given by King(1). Friction can be expected to reduce the coefficient at low heads. New, smooth concrete crests should have a high coefficient at low heads compared to crests that have been roughened by weathering or other causes.

3. Test Data. 
The curve in Chart 111-3 is based primarily on data obtained from model tests conducted under Corps of Engineers Engineering Studies Item 801, General Spillway Investigation, at the U. S. Army Engineer Waterways Experiment Station (wES). Only those tests in which a deep approach channel and negligible velocity of approach existed were used in developing the curve. The plotted points from ES 801,are the basis for the curve above the He/Hd ratio of 0.4. Prototype test results are plotted for the low head range, and that portion of the curve is based on the field tests indicated in the legend. More prototype observations are needed for the newer design shapes that approximate the spillway crest defined in Hydraulic Design Criteria 111-1 and 111-2/1.

4. The open-circle data points are from tests on the originally published crest shape (Charts 111-1 and 111-2, dated 4-1-52). The opentriangle points are from recent laboratory tests(2) in which a third shortradius curve (R = 0.04Hd , Chart 111-2/1) was added to the upstream quadrant shape to eliminate the surface discontinuity in the original design where the curve intersected the vertical face of the spillway.

5. References.

(1) King, H. W., Handbook of Hydraulics; For the Solution of Hydraulic Problems, 3d ed. (1939), Pp 379-380 and 4th ed. (1954, revised by E. F. Brater), pp 8-8 and 8-9, McGraw-Hill, New York.

(2) U. S. Army Engineer Waterways Experiment Station, CE, Investigations of Various Shapes of the Upstream Quadrant of the Crest of a High Spillway; Hydraulic Laboratory Investigation, by E. S. Melsheimer and T. E. Murphy. Research Report H-70-1, Vicksburg, Miss., January 1970.

Changing cfs unit to cms