24RuralDrainage.pptx

1Surface DrainageSurface DrainageCE 453 Lecture 24CE 453 Lecture 242ObjectivesIdentify rural drainage requirements and design Ref:AASHTO Highway Drainage Guidelines(1999),Iowa DOT Design Manual Chapter 4 and Model Drainage Manual(2005)3Surface DrainageSurface water removed from pavement and ROWRedirects water into appropriately designed channelsEventually discharges into natural water systemsGarber&Hoel,20024Surface DrainageTwo types of waterSurface water rain and snowGround water can be a problem when a water table is near surfaceGarber&Hoel,20025Inadequate DrainageDamage to highway structures Loss of capacityVisibility problems with spray and loss of retroreflectivitySafety problems,reduced friction and hydroplaningGarber&Hoel,20026DrainageTransverse slopesRemoves water from pavement surfaceFacilitated by cross-section elements(cross-slope,shoulder slope)Longitudinal slopesMinimum gradient of alignment to maintain adequate slope in longitudinal channelsLongitudinal channelsDitches along side of road to collect surface water after run-off7Transverse slope8Longitudinal slope9Longitudinal channel10Surface Drainage System DesignTradeoffs:Steep slopes provide good hydraulic capacity and lower ROW costs,but reduce safety and increase erosion and maintenance costs11Surface Drainage System DesignThree phases1.Estimate of the quantity of water to reach the system2.Hydraulic design of system elements3.Comparison of different materials that serve same purpose12Hydrologic Analysis:Rational MethodUseful for small,usually urban,watersheds(10acres,but DOT says=TcUsed as storm durationIowa DOT says dont use Tc5 minutes26Time of Concentration(Tc)Depends on:Size and shape of drainage areaType of surfaceSlope of drainage areaRainfall intensityWhether flow is entirely overland or whether some is channelized27Tc:Equation from Iowa DOT ManualSee nomograph,next page28Nomograph MethodTrial and error method:Known:surface,size(length),slopeLook up“n”Estimate I(intensity)Determine TcCheck I and Tc against values in Table 5(Iowa DOT,Chapter 4)Repeat until Tc(table)Tc(nomograph)Peak storm event occurs when duration at least=Tc29Example(Iowa DOT Method)Iterate finding I and TcL=150 feetAverage slope,S=0.02(2%)GrassRecurrence interval,T=10 yearsLocation:KeokukFind IFrom Iowa DOT Design Manual30Grass Surface,Mannings roughness coefficient=0.431First guess I=5 in/hrknownsTc=1832Example(continued)Tc with first iteration is 18 minCheck against tables in DOT manualKeokuk is in SE:code=933Convert intensity to inches/hour 34For intensity of 5 inch/hr,duration is 15 minTc from nomograph was 18 min 15 minTc DurationNext iteration,try intensity=4.0 inch/hr35Slope=0.02I=4.0 inches/hrTc=20 minFor second iteration,tc=20 min36Example(continued)I=4.0 inches/hour is somewhere between 30 min and 15 min,Interpolate OK!37What does this mean?It means that for a ten-year storm,the greatest intensity to be expected for a storm lasting at least the Tc(18 min.)is 4.0 inches per hour that is the design intensity38Can also use equation,an example is provided in Chapter 4-4 of the Iowa DOT manual39Rational Methodused for mostly urban applicationslimited to about 10 acres in size(Garber and Hoel suggest 200-acre limit)Q=CIACalculate Q once C,I,and A have been found40AreaArea of watershedDefined by topographyUse GIS contours in lab4142Lab-type Example60-acre watershed50-year stormMixed coverRolling terrain43180Qdesign=180 x 1.0 x 0.6=108CFS44What would the flow have been had we used the rational method?Q=CIASay,c=0.2(slightly pervious soils)I=?Assume round watershed of 60 acres=60/640=0.093 sq mi L=D1800,assume slope=4%(rolling?)Tc for I=6in/h=41 min vs.60 min I=4.8in/h=45 min vs.30 min call it 5.5in/hA=60 Q=.25.560=66 CFS vs.108 cfs