Hydrology for Planners LA 222 Spring 2009 EXERCISE 1. THE DRAINAGE BASIN The objectives of this exercise are to gain experience in drawing drainage networks, delineating drainage basins, using a planimeter to measure drainage areas, estimating average precipitation over the basin, and determining stream order for each segment of the drainage network. After observing the procedure en masse, we'll divide into groups of three-four to outline and measure a basin on a USGS 7.5-minute quadrangle sheet. Each group should put the names of its members on their copy of the topo map and a copy of the summary table to turn in. We will outline the drainage network for two basins: a larger basin with multiple tributaries and a smaller one. For each, we determine the stream order of each segment and measure the drainage area. OUTLINING AND MEASURING THE DRAINAGE BASIN I. Delineating the Stream Network and Drainage Basin 1. First, highlight the stream network in blue, beginning with the principal stream channel, and moving up into tributaries, following the solid blue lines (denoting perennial) and dashed/dotted lines (denoting intermittent). Next outline the drainage networks for adjacent basins, because this helps you visualize where the drainage divides must lie. Note that the determinations of perennial and intermittent, and even whether a blue line of any sort is mapped for the channel, is an arbitrary decision of the cartographer making the map. You can also continue upstream of the mapped streams with dashed blue highlighting. This further helps you visualize runoff pathways, and thus helps to locate drainage divides. 2. Once you have outlined all these drainage networks, you know roughly where the drainage divide between each network will go. To locate the divide more precisely, follow the ridge lines separating the basins. If you are not sure about an area, it may help to visualize how a parcel of water would run off. Follow the water downhill to the next channel. [It is difficult to follow the drainage in flat, downstream urban areas because of engineering modifications.] Sketch the drainage divides initially in pencil, so you can correct mistakes (which frequently occur as you follow ridge lines). After you have checked your lines, highlight the divide in yellow. 3. Taking turns to use a planimeter, determine the drainage area of your basin. (The drainage basin is simply the area drained above a given point, commonly defined above a gauging station or point to estimate flood magnitude, so we can define a drainage basin above any point along a channel.) Test the planimeter first by defining a known area (such as a square, one or two inches on the side) and measuring that area. The planimeter reads in square inches or cm. Remember to start and end on precisely the same point, proceeding clockwise around the area to be measured. If your entire area is too large to measure with the planimeter, break it up into smaller portions, measure, and add the areas to derive the total basin area. Be sure to remeasure at least twice to insure that your numbers are accurate. It's easy to slip and err, so repeat the measurements a few times and discard any aberrant values. 1
4. Using the map wheel or estimating with a ruler, measure the length of the channel. (The map wheel reads in cm and in inches.) Record the total length of the channel from the gauge (or other measuring point) up to the drainage divide, following the longest drainage line. As you are measuring length, you can record the distance at which you cross various contour lines and plot a longitudinal profile of the streambed; this is not required in this lab but is a useful tool in geomorphic analysis.
II. The Drainage Network and Stream Order To express the size of a stream, we can specify its width, average flow, or drainage area, to mention some commonly-used variables to express the scale of river channels. Stream order is another such measure, and refers to the ranking of the reach within the larger drainage basin. Rules for calculating stream order: 1. A reach with no tributaries is a first-order stream. 2. A reach below the confluence of two (or more) first order streams is a second-order stream. 3. A reach below the confluence of two (or more) second order streams is a third order stream. 4. A reach below the confluence of two (or more) third-order stream is a fourth-order stream, and so on. Two channels of equal order must join to produce the next highest order. A thousand first order tributaries could flow into a second order channel without changing its order. Many smaller streams depicted on the 7.5-minute, 1:24,000 map are not depicted on smaller scale maps. Thus, what appear as first-order streams depend in part on the scale of the map. Following common practice, we will base our stream order on the appearance of streams on the 7.5minute quad sheet. However, be aware that the USGS uses no consistent criteria for determining when a stream is mapped as a stream on the topo map. In Berkeley, there are many channels that clearly are "streams" yet do not appear on the USGS map, probably because they were obscured by a vegetative canopy and were not obvious on the aerial photographs from which the map was made. 5. Compute the stream order of all stream sections in your drainage network. Label these sections by stream order. In class, we will also trace the drainage network for some larger streams and calculate the orders for segments of those streams.
III. Estimation of Average Precipitation 6. Refer to the map of annual average precipitation by Saah (1989) and transfer isohyetals to your basin. Estimate (or use the planimeter to measure if the patterns are complex) the percentage of the basin in each precipitation band and compute a weighted average for the basin. When undertaking an analysis like this, it is important to recognize the crudeness of the estimate. Orographic influences are pronounced in this region, and long-term rain gauges are few. The resulting isohyetal maps are not gospel. In other words, don't report the results of this analysis to three decimal places. 2
Exercise 1 Drainage Basin Measurement Hydrologic Data for A. Mount Diablo Creek at the Clayton Canal Crossing, and B. Unnamed drainage flowing into farm pond (“res”) just east of the “y” in Ygnacio Valley. ______________________________________________________________________ DA Length Relief Gradient Order Avg Precip (mi2) (ft) (ft) (%) (in) ____________________________________________________________________________ A.
B. ____________________________________________________________________________ Length is distance from measuring point (gauge) upstream along the main channel (and its projection) to the drainage divide. Gradient is elevation change from divide to measuring point (gauge) divided by the total distance between the points. Relief is computed as the difference between the elevation of the lowest point (i.e. your gauge or other point of measurement) and the elevation of the highest point in the basin.
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EXERCISE 2. RUNOFF OBSERVATIONS The purpose of this exercise is simply to get you outside when it's raining, with your eyes open and mind inquisitive. Your assignment is to look at sources of runoff and erosion, follow flowing waters back to surfaces from which they were generated, and draw inferences about runoff and sediment generation from different soils and surfaces. There are no specific requirements for this exercise, but you can take advantage of whatever you see in the field. For example, if you observe muddy water, follow it upstream to the sediment source(s). (Construction sites are good potential sources of erosion.) Wherever possible, measure or estimate quantities of runoff, and compare with the area producing the runoff, its landcover, slope, etc. You can measure the filling rate of a 5-gallon bucket, or estimate the velocity of runoff using a float and stopwatch (and multiply by cross sectional area of flow). Immediately after the rainstorm, look for sites of surface erosion and deposition. Measure the cavities left by erosion and the size of the deposits, and note particle size of deposits and source areas. You can also document high water marks in stream channels immediately after a rainstorm. Take photographs if possible. Try to make observations on more than one land cover type (e.g., woodland, grassland, and pavement). Your report should not exceed 5-pages of text, double-spaced (10-point type). State your objectives, methods (exactly what observations and measurements you made), results, and conclusions. Your results should include tables of any data you collected, and sketch diagrams or maps displaying runoff or erosion processes observed. Consider your observations in light of the runoff processes discussed in lecture.
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