CE 3354 Engineering Hydrology Lecture 9 Rational Equation
- Slides: 40
CE 3354 Engineering Hydrology Lecture 9: Rational Equation Method Introduction to HEC-HMS
Outline l ES-4 Solutions l Rational Equation Method l Introduction to HEC-HMS
ES 4 Solution l Solution sketch on-line l Go through and discuss each part of the sketch, and demonstrate software as necessary http: //www. rtfmps. com/ce 3354 -2015 -3/3 -Homework/ES-4/ce 3354 -es 4 solution/cd 3354 -es 4 -solution-sketch. pdf
Rational Equation Method l The rational method is a tool for estimating peak discharge from relatively small drainage areas. (Mulvaney, 1850; Kuichling, 1889) l CMM pp. 496 -502
Assumptions l Rainfall area. is distributed uniformly over the drainage l Rainfall intensity is uniform throughout the duration of the storm. l Response time for the drainage area is less than the duration of peak rainfall intensity.
Assumptions l The rational method does not account for storage in the drainage area. Available storage is assumed to be filled. l The calculated runoff is directly proportional to the rainfall intensity. l The frequency of occurrence for the peak discharge is the same as the frequency of the rainfall producing that event.
Typical Limitations l Drainage areas less than 200 acres (some jurisdictions allow up to 640 acres) l Minimum duration is prescribed to prevent “infinite intensity” at short times – typically 10 minutes (some jurisdictions allow 5 minutes) l No substantial storage (or all storage filled)
Typical Procedure (1) Area Less than 200 acres? (2) Estimate Tc (3) Design Intensity for ARI EBDLKUP-2015; NOAA-14 (4) Estimate C (5) Compute Qp=Ci. A (6) Validate/verify
Calculation Sheet
Runoff Coefficients l CMM p. 498
Runoff Coefficients l Texas Hydraulic Design Manual
Runoff Coefficients l Oregon Hydraulics Manual – Values similar in most sources
Time of Concentration l The value of Tc is important in rational method for estimating rainfall intensity. l It is also used in other hydrologic models to quantify the watershed response time
Time of Concentration l Time of concentration (Tc) is the time required for an entire watershed to contribute to runoff at the point of interest for hydraulic design l Tcis calculated as the time for runoff to flow from the most hydraulically remote point of the drainage area to the point under investigation.
Time of Concentration l Travel time and Tc are functions of length and velocity for a particular watercourse. l. A long but steep flow path with a high velocity may actually have a shorter travel time than a short but relatively flat flow path. l There may be multiple paths to consider in determining the longest travel time. l The designer must identify the flow path along which the longest travel time is likely to occur.
Time of Concentration l Various l On Methods to Estimate Tc the server in readings: CMM pp. 500 -501 has several formulas l HDS-2 pp. 2 -21 to 2 -31 has formulas and examples l LS pp. 196 -198 has several formulas l
Time of Concentration l Examine l l l 3 Methods to Estimate Tc NRCS Upland Kerby-Kirpich NRCS Velocity Method l Similar in scope; depend on distances, slope, and land surface conditions. Module 3
NRCS Upland Methdod l Specify flow path l Determine cover on flow path l Determine slope(s) along path l Partition into different cover types and slopes along path l Apply velocity model on each part add times for entire path
Upland Method Velocity Chart
Kerby-Kirpich Method for Estimating Tc l Appropriate l Compute for many conditions up to two times: overland flow time and channel flow time
Kerby-Kirpich Method for Estimating Tc l Overland l Channel flow time l Combine the two to estimate time of concentration (if there is no channel component, then omit) l Overland length < 1200 feet
Kerby-Kirpich Method for Estimating Tc Kerby Retardance Coefficient Generalized Terrain Condition Dimensionless Retardance Coefficient (N) Pavement . 02 Smooth, bare, packed soil . 10 Poor grass, cultivated row crops, or moderately rough packed surfaces . 20 Pasture, average grass . 40 Deciduous forest . 60 Dense grass, coniferous forest, or deciduous forest with deep litter . 80
NRCS Method for estimating Tc l Comprised of up to three components l The sheet and shallow concentrated are of importance in urban systems. Module 3
NRCS Method for estimating Tc DDF Atlas; NOAA 14
NRCS Method for estimating Tc
NRCS Method for estimating Tc
NRCS Method for estimating Tc
Example l Figure to right is a Google-Earth image of a 550 acre watershed in Virginia l The area is agricultural, Virginia SH 58 runs along the south part of the watershed. The other visible roads are county roads or private (farm) roads.
Example l Figure to right is a topographic map of the same area, but without the watershed boundary depicted. l The outlet is the red square on the map. The yellow square is a reference area of 1000 X 1000 meters.
Example l Watershed delineated l Main channel delineated l Overland paths indicated
Example l Acrobat. Pro measure reference area and watershed area l Acrobat. Pro measure main channel length l Acrobat. Pro measure overland length
Example l Use Kerby-Kirpich to estimate Tc l Report as Tc =67 minutes
33 Introduction to HEC-HMS l History l l Evolved from HEC-1 as part of“new-generation” software circa 1990 Integrated user interface to speed up data input and enhance output interpretation l HMS l l is a complex and sophisticated tool Intended to be used by a knowledgeable and skilled operator Knowledge and skill increase with use
34 HEC-HMS l Data l l Graphical User Interface (GUI) Multiple input files Multiple output files Time-series in HEC-DSS l All l l management files arranged in a “Project” Paths to individual files Can e-mail entire project folders and have them run elsewhere
35 HEC-HMS l Conceptualizes precipitation, watershed interaction, and runoff into major elements l l l Meterological model l Raingage specifications and assignment to different sub-basins Time-series models l Supply input hyetographs l Supply observed hydrographs Simulation control l Supply instructions of what, when, how to simulate
36 HEC-HMS l Conceptualization: l l l Basin and sub-basin description l Supply how the system components are interconnected Loss model l Supply how rainfall is converted into excess rainfall Transformation model l Supply how the excess rainfall is redistributed in time and moved to the outlet
37 Applications l HEC-HMS is a Hydrologic Model l Peak Flows l Hydrograph Routing l Stream reaches l Reservoirs and detention basins l Hydrograph lagging and attenuation l Sub-basin modeling (if appropriate)
38 HEC-HMS l Precipitation l Abstractions l Fraction of precipitation that does not contribute to runoff (and ultimately discharge) l Routing l l l Watershed routing Stream (Channel) routing Reservoir (Storage) routing
HEC-HMS Example l Minimal model l SCS Type Storm l 640 acre watershed – no process models; completely converts rainfall into runoff l Illustrate how to mimic rational method by adjusting drainage area l Repeat with a 24 -hour Texas Hyetograph same location
Next Time l l Loss Processes l Evapotranspiration l Infiltration SCS Curve Number
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