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Chapter 3 BASIC DATA - DENVER METROPOLITAN WATERSHEDS The basic data for this investigation were obtained from a special network of gaging stations established in the Denver metropolitan region as a cooperative pro- jeet between the U.S. Geological Survey (USGS) and the Urban Drainage and Flood Control District (UDFeD). The USGS provides the technical expertise to collect and process the basic data. Some of the stations have been in operation since June 1968. The USGS has been responsible for design and development of better instrumentation and more effective methods of data reduction. The instrumentation for the Denver network has been developed from the maturity gained from the operation of the Texas networks at Houston. Austin, Bryan, San Antonio, Dallas and Fort Worth. Appendix A contains a detailed list of the gaging stations. Gaging Small ~bdn Stn~.-- Measuring floods on a small urban stream presents its own unique problems. These streams are ephemeral in nature while the large streams in the metropolitan region are pereJlllial streams. The watersheds are small and the catchments usually do not have a recording rain gage nearby. Under conditions of zero discharge, the stream gaging stations tend to develop operational problems. The floats stick or do not respond immediately when the flood hydrograph begins. Stilling well inlets tend to become plugged or closed. Since the response time of the small urban watershed is short, any hesitation in the response of the recorder adds very materially to the uncertainty in the determination of the water- whed response time. The gaging of floods from small urban watersheds requires a high degree of reliability between the rainfall hyetograph and the runoff hydro- graph. To achieve precise synchronism between the recording rain gage record and the stream hydrograph, the USGS has developed a Dual Digital Water Stage Recorder-Recording Rain Gage. The stream stage and the level of water in the rainfall measuring cylinder are recorded by two digital punched-tape recorders. The records of stage sensed by both recorders are simul- taneously punched at five-minute intervals beine acti- vated by the same battery-operated timer. wa.teJL Stage. Rec.oJtdeJL. -- The water stage recorder is mounted on the top of a five-foot length of standard four-inch galvanized iron pipe which functions as a stilling well. The stilling well intakes are six one- quarter-inch holes drilled radially around the peri- phery of a standard cap which is screwed to the bottom of the stilling well. The cap is positioned such that one hole is at the upstream face and one hole is at the downstream face. The inlets tend to be self purg- ing. The stilling well is usually set so that the cap is one or two inches above the bottom of the channel. The water stage record does not respond to a small amount of "base flow." The installations have been described by Gonzales and Ducret (1971). Re.co4ding Rain Gage. -- A recording rain gage is installed at each stream gaging station. The measuring tube for the recording rain gage consists of a 5 1/2 - foot length of standard 3-inch galvanized pipe. The pipe is mounted vertically with a sheet steel metal shelter at the top to house the recorder, timer and a 7.5 volt battery. The rain gage receiver is as-inch by lO-inch rectangle. The rainfall is concentrated by a funnel into a copper tube which leads to the pipe measuring tube. The recorder senses the water level by means of a float. The vertical pipe has the capacity to collect 7.0 inches of rainfall. When the accumula- tion exceeds this amount, a siphon is primed and the entire contents of the measuring tube is evacuated in about 90 seconds. A sufficient amount of water is re- tained in the measuring tube so that the float never rests on the bottom of the pipe. Thus the rain gage will respond immediately to any new rainfall. Rating C~ve..--A number of the stream gaging sta- tions have been installed at the upstream end of a circular culvert pipe. The rating curves have been developed from the head loss relationships of water entering a culvert pipe. Because of the short duration of the runoff, it is usually difficult to obtain field verification of the rating curve. The stream gaging I station at Stapleton Airport is installed in a 6-foot diameter storm sewer. The control consists of a Palmer Bowlus flume which has been fabricated of sheet metal and installed in the conduit by bolting to the sewer wall with small bolts anchored in concrete pipe wall. A sheet metal Parshall flume is used at one of the gaging stations. A broad crested weir is used at ano- ther of the Boulder watersheds. Op~onaf Vi6nicutti~. -- The location of the gaging stations is shown on Figure 1. Some of the gaging stations are operated during the summer season (the flood season) since June 1968. The original plans called for having 30 stations operational by the end of the summer 1972. The station locations were select- ed to 1) provide a variety of types of urban en- vironment, 2) have both old and newly developed loca- tions, 3) have simple and stahle hydrologic configurations. Among the characteristics of an ur- ban watershed are the dynamic changes taking place. These changes militate against the third attribute listed previously. By its very nature, the urban en- vironment is changing and the hydrologic characteristics also change. At a number of the stations the culvert configura- tion has changed or been extended upstream necessi- tating removal of the gaging station. In a number of instances additional runoff has been diverted into the watershed through changes in the culvert drainage sys- tem upstream from the gaging station. In a number of the watersheds the area could change from storm to storm depending upon the direction of gutter flow. A list of the gaging stations is given in Appendix A. The data processing is accomplished by a computer because the two stage tapes are punched according to a binary code. The rainfall is reported in inches to the nearest 0.01 inch although the rain gage can re- solve rainfall to the nearest 0.005 inch. The stream stage is reported to the nearest 0.01 foot. The stage record is converted to discharge using the rating equation and reported to the nearest 0.1 cubic foot per second. In reducing the data, the stage in the precipita- tion gage is recorded at midnight of each date. When a storm commences, the precipitation is not recorded until at least 0.015 inch has been recorded in 5 minutes-- then the precipitation amount is recorded at each 5-minute interval until the precipitation ends. The runoff is recorded whenever there are measurable changes in stream stage. 12