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This 9 page Class Notes was uploaded by Ara Hartmann on Tuesday October 13, 2015. The Class Notes belongs to CE421 at Lafayette College taught by DavidBrandes in Fall. Since its upload, it has received 22 views. For similar materials see /class/222308/ce421-lafayette-college in Civil Engineering at Lafayette College.
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Date Created: 10/13/15
The Hydrologic Budget CE 421 Fall 2005 The hydrologic cycle is represented graphically below db 39 Vapors Cool to Form E Clouds and Precipitation JN 7 K J 39139 3 Sun39s heat 3 5 causes quot 39 eva oration 724 Mb 9 39 quot PD 1 i i 11 2 Surface m v 4 Water vapor I g I runoff we d W ft N ma V vw39 Evaporation from precipitation Evaporation from lakes and rivers Evaporation from oceans Transpiration and evaporation from crops trees and other plants where S total storage of water in the watershed I in ows O out ows As you learned in uid mechanics this equation can be applied to any control volume tank lake watershed etc For a watershed the boundaries are typically defined by the land surface top the bedrock surface bottom and topographic diVides sides Now assuming 1 all groundwater in the watershed eventually discharges to the stream might not be true for some geologic settings and 2 there are no anthropogenic transfers of water across watershed boundaries we can write the water budget equation for the watershed as P ET Q a grave equation f where P precipitation ET evapotranspiration Q stream ow base ow runoff at the watershed outlet and S water storage groundwater soil moisture lakes snow glaciers A note about units P and ET are typically given in Lt inday but Q is in L n cfs P and ET are Spmuzl meoxwemerits e mm in d area Lt x Area L1 L n 2 Q E Some important points regarding Watershed Water bugets 1 ate 39 39 39 39 usuallymonthlyor annual 2 It is otten assumed that I a over a year s time that is the total annual precipitation total annual strea 39 39 Rm annual rim quot year they are based on the hydrologic vvater year WY which goes from Oct 1 to Sept 30 of 39 A niaiian in November and December as snow may not melt until the next calendar year 3 In temperate regions like Pennsylvania annual Qis typically about 13 ofPrthis may vary depending on geology soil type land use and land development It also varies from year to year a c wet conditions are when it rains periods of drought etc What s missing that is critically lrnportantw a hydrologlst Delineation of Watershed Contributing Area CE 421 Hydrology One the most fundamental skills in hydrology is to be able to accurately determine the contributing area for any point along a stream or for a site where stormwater management is required The contributing area or drainage area for a particular point of interest is the surrounding land area from which all runoff will ow to the point of interest To be able to delineate one rst needs to know how to read a contour map Typically we are working with a 75Minute Series Topographic Map from the United States Geological Survey USGS They are named by Quadrangle and usually the largest town on the map is the name of the Quadrangle These maps have a scale of l24000 or 1 2000 and a contour interval of 20 feet The contours connect points of equal elevation and are shown in brown Heavy brown lines are the 100 ft contours The gure below is from the Easton PA Quadrangle Based on the contours shown our elevation here at Lafayette is about 350 ft MSL Other features shown on the USGS topographic quad maps are rivers streams municipal boundaries towns roads buildings forested areas etc The following figure illustrates some important features about topographic maps Water flow is perpendicular to contour lines The closer the contours the steeper the slope the farther apart the contours the atter the slope If the V or U shape in the contours poinm uphill the feature is a valley streams often connect these points If the V or U shape in the contours poinm downhill the feature is the nose of a hill 7 you will never see a stream here Wide loops in the contours represent hills narrow loops in contours represent ridges 7 these are often drainage divides Sometimes you may see a loop with tic marks along the inside 7 this is a depression Permanent perennial streams are shown with solid blue lines intermittent seasonal streams are shown with dash dot line but sometimes the maps are wrong Floodplains are not marked on topo maps but they are often evident as at ground adjacent to rivers Sometimes esp in terrain underlain by limestone bedrock small valleys do not have streams in them but they still may be major drainageways for stormwater 0 Never rely on topographic maps as your primary source of land use information many recent land developments are not shown here Sometimes it is not at all obvious which way the water goes Tools for delineation Pencil amp eraser or use AutoCAD with topo map as background raster image Topographic map General method as they say practice makes perfect I Identify the point of interest7 your goal is to enclose all the area upstream of this point from which all runoff will ow to the point of interest I Locate the drainage divides or major topographic features such as hills and ridges that diVide ow between adjacent streams sketch in these boundaries first I Delineate the watershed boundaries such that they 1 are always perpendicular to contour lines because water ows downhill by the steepest path available and 2 do not cross drainage diVides I Check that there are no areas within your delineated watershed where the contours show ow going away from your point of interest The Base ow Component of Stream ow CE 421 Engineering design work typically involves the runoff component overland ow or stormflow of stream ow However most of the ow in a stream consists of base ow fed by slow continuous drainage from groundwater and soil moisture although in losing streams ow may be lost to groundwater The following gure Fetter 2001 shows howa hydrograph can theoretically be divided into components representing different processes Rising Crest Falling lt imb limb gtllt Recessiongt 3 0 Total 3 streamflow D N l I 4 H Duration of gt precipitation 30 Horton a overland quot flow 5 I 4 0 5 Baseflow ST 2 Q I I J Direct 2 T precipitation 15 m lnterflow T Q I A I 12 6 12 6 12 Engineers have started paying more attention to base ow in recent years for two reasons 1 recurring drought has tested the ability of streams to continue owing all summer and people take notice when streams stop owing 2 the idea that land development reduces stream base ow due to reducing infiltration of rainfall some of which becomes recharge to groundwater this is one reason that most new stonnwater ordinances require infiltration Separating base ow from storm ow There are several possible methods for hydrograph separation as shown below 1001000 I I I I l I I r Total strea mrow Base ow 10000 M 1000 L a O O I II I 1 Mi FLOW IN CUBIC FEET PER SECOND at y gt rt I I I I I I I I I I I I I I I I I I 1 1 21 41 e1 81 101 121 141 161 181 201 221 241 261 281 301 321 341 361 TIMEIN DAYS The local minimummethod used for continuous hydrographs USGS 1999 Q I g Constantslope method 1 U 1 a I Concave method q quotquotquotquotquotquotquotquotquotquotquotquotquotquotquotquot 39 395 39 39 39 39 39 39 39 39 39 39 39 T Constantdischarge method 10 I q quotquot S I 39 gtp I qm 39 39 Time Single event hydrograph separation methods McCuen 2004 1b methods described by Mcciien 2004 and shown above are Constantdxschmge 7 start albeginning ofrising limb draw horizontal line until it Constant L 39 39 quot I39 L 12 anlm ml 39 in ection point on the recession limb o N 39 391 39 39 39 i w bins of concentrati on mi siii my is i point do it by eyeball n o e t A where t is days from who in ection point andA is area in mi2 cr u when draw an upward sloping line in die in ection point i o mnoff between me m1 hydrograph and die base ow separation line using discrete datad1e area is approximated by me trapezoidal integration method gnu bun Euslllnll mask m 51 quotMumrm Sen1mm 7m cancm Helium Ease uw n lI mzmn Mianu n i i i i i iim on m i 1 1 I 39 Does chi make smse mink in terms ofDarcy s Law
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