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46 TECHNIQUES OF WATER-RESOUJl{)ES INVESTIGATIONS is dependent on (1) inflow, (2) size of pipe, and (3) size of pond. This factor can be determined only by continual reading of a gage or from a recorder chart. Because continuous records of stage usually are not available at culvert sites, an estimate must be made from recorder graphs of nearby streams. Rates of rise in ponds subject to cloudburst runoff have been recorded as high as 10 feet per hour where the ou tflow was through " small pipe. The ponds at these places had sur- face area.s of 10-15 acres. If this same flow were ponded at an ordinary crest-stage site, the rise could be considerably greater. The reduction in peak flow through any.given culvert varies with stage as shown in figure 26. In general, pondage does not exist to any great degree at low stages. However, if the culvert is set high in the fill, then a sizeable pond may form before flow through the pipe begins. For this situation many low peaks may be com- pletely absorbed in storage. Stage-discharge relationships for culverts Many small-area, high-wat.er gaging st.at.ions instrumented with crest-stag-e gng:es are located at culverts because of the convenient means-of measuring peak discharge. At many such in- stallations experience has shown that either type 1, 2, or type 4 flow may be expected through a considerable range of discharge. At high heads, types 1 and 2 flow will usually change to either type 5 or 6 flow, respectively. For example, a steep culvert with free getaway might always support type J flow until the headwater-diam- et,er ratio reaches 1.5, when flow will become oit her type 5 or 6. A flat culvert with Cree get- H-WllY mig-ht ulwnys hll'~e type 2 flow llt head- wnl('I'-ditUlwtel' mtios less thun 1.5. 'Vilh an inh'fllH'diRtc slope tutd free gettlwny, II trunsi- tioll from type I to type 2 flow might always lH'l'\It' 1\1 11 relat.ively fixed upstream stage. AIs'o, 11 culn'rt mny nlwn:v~ he submerged at both ends. so thnt, type 4 flow will oCCllr nt, all high stnges. Gages shollld alw"ys be installed "t the ap- proach section and alon~ the downstream embankment if tailwnter can be significnnt, A st.nge-dischnrge rela.tion, often called !\ rnting curye, can be prepared in ndynnce of actual flood peaks, but several field verificntions oC the accuracy of the crest-stage recordings and of the constancy of flow type should be made before the rating curve is used. For types 1 and 2 flow various critical depths can be assumed, and the corresponding discharges and head- water elevations computed, For type 4 flow, dic;charge can be computed for various ralls, becnllse d\scharge is a function of the difference in water-surface elevation between headwater '"HI tailwnter, Type ~ flow does not lend it.self to a direct computation of the rating curvel bnt n Cairly reliable rating can be developed in the manner discussed below by making nu- merous computations usin~ assumed yalues. Field data are very valuable, however, in deter- mining the usable range of values: There is no need for making many computations in the range where the curves will never be used, In a steep channel the point of zero flow at, the gage mny be higher than the point of zero flow at the culvert entrance. In this case t.he channel is the control "t low st"ges, and t.he theoreticnl curve cnnnot be used until the cIII- 'vert becomes the con tro1. Unless the Frollde number at the approach section is less than about 0.70, there is no assurance that the culvert is the cont,ro1. Even then, n field check m"y be necessary to ensure against a sharp break in channel slope just. above the culvert. The effect of a changing approach section must be considered in drawing ratings for low- head flow. Rating curves should not be used if the approach channel shifts sufficiently to alter the approach velocity head or friction loss, Small changes in area.s will have no effect provided the velocity head and friction loss "re small, bllt they may have considerable effect where these items are large, When the "pproach channel is f"irly stable, Cllrves oC area and con- yeyance are helpful in making computations {Jf discharge. The rating curves shown in the section under t.ransitions are combinat~ons of curves repre- senting certain types of flow, These figures are used to show rating curves for flow types 1-6, tlS well as the transitions between certain com- binations. Current-met<.. measurements shonld be made to help define rating curves, These are especially vnlu"ble at low stages iC there is a possibility of critical flow occurring between the approach e e --e