Docstoc

Runoff Hydrographs

Document Sample
Runoff Hydrographs Powered By Docstoc
					RUNOFF HYDROGRAPHS
The Unit Hydrograph Approach
     STORM WATER HYDROGRAPHS

   Graphically represent runoff rates vs. time
     Peak runoff rates
     Volume of runoff

   Measured hydrographs are best
       But not often available
   Methods are available to develop a “synthetic” hydrograph
       Use a unit hydrograph (UHG)
 DEVELOPMENT OF RO HYDROGRAPHS
 Most often interested in hydrographs at the watershed
  outlet (and possibly some selected points in the
  watershed).
 We modify the rainfall hyetograph to reflect watershed
  characteristics.
 The volume under the effective rainfall hyetograph is
  equal to the volume of surface runoff.
HYDROGRAPH COMPONENTS

 qp  is the maximum
  flow rate on the
  hydrograph
 tp (time to peak) is
  the time from the
  start of they
  hydrograph to qp.
 tb (base time) is the
  total time duration of
  the hydrograph.
HYDROGRAPH COMPONENTS

 tc (time of concentration) time it takes water to flow
  from the hydraulically most remote point in a
  watershed to the watershed outlet
 tL (lag time) is the average of the flow times from all
  locations in the watershed and can be estimated as
  the length of time from the center of mass of the first
  effective rainfall block, to the peak of the runoff
  hydrograph.
 If each block of effective rainfall has a duration of D

                                 D
                      t p  tL 
                                 2
UNIT HYDROGRAPH
   Hydrograph of runoff resulting from a unit of
    rainfall excess occurring at a uniform rate,
    uniformly distributed over a watershed in a
    specified duration of time.
        UNIT HYDROGRAPHS

   Assumptions:
       Rainfall intensity is not considered
       Linear relationship between stormwater runoff and rainfall
       UHG is independent of antecedent conditions
       Uniform rainfall distribution
DURATION OF THE UNIT HYDROGRAPH
   Each unit hydrograph has a duration that is the
    same as the duration of the rainfall excess that
    produced it.
     Conceptually can have an infinite # of hydrographs
      corresponding to different durations.
     Practically, a unit hydrograph is applied to rainfall
      excesses of duration as much as 25% different than
      the duration of the unit hydrograph.
HOW IS THE UNIT HYDROGRAPH USED?
   For a unit hydrograph of
    duration, D, the volume
    underneath the
    hydrograph is always 1,
    produced by 1 unit of
    excess rainfall.
   A hydrograph for a block of
    rainfall excess of any
    depth is obtained by
    multiplying the ordinates
    of the unit hydrograph by
    the depth of the rainfall
    excess block.
    The result is the ordinates
    of the runoff hydrograph.
HOW IS THE UNIT HYDROGRAPH USED?

   Rainfall excess is divided into
    blocks, each of a uniform
                                            j m
    duration, D.
   A component hydrograph for
                                      q j   ri u ji 1
    each block of rainfall is               i 1
    calculated.
   The starting time for each
    hydrograph coincides to the         ri  0 i  m
    starting time of the
    appropriate block of rainfall
    excess.
                                        ui  0 i  n
   All the component
    hydrographs are added
    vertically to obtain the total
    runoff hydrograph for the
    storm.
CHECKING THE VOLUME UNDER A
UNIT HYDROGRAPH.
                 Where:
                   t is the duration time
                    increment of the

   t  q i         hydrograph (min)

V                 Sqi is the sum of the
                    ordinates of the runoff
                    hydrograph (cfs)
   60.5A           A is the area (acres)
DERIVING UNIT HYDROGRAPHS
 Unit hydrographs can be derived from
  records of observed rainfall and
  streamflow
BUT
 For small watersheds, synthetic unit
  hydrographs are generally used.
 Synthetic unit hydrographs provide
  ordinates of the unit hydrograph as a
  function of tp, qp and a mathematical or
  empirical shape description.
ESTIMATING THE TIME PARAMETERS
 Time of concentration (tc)
 For some areas, we can sum the time for various
  flow segments as the water flows toward the
  watershed outlet.
 Segments
     Overland flow
     Shallow channel flow
     Flow in open channels.                 nLi
                                    tc  
                                         i 1 v
                                                i
  LAG TIME, TL

SCS Equation to calculate
 time lag
    L = hydraulic length of
       watershed (feet)
    S = curve number parameter           L (S  1)
                                          0 .8     0.7
       (inches)
    Y = average land slope of the
                                    tL          0.5
       watershed (%)
                                          1900 Y
    tl = time lag (hours)
TIME TO PEAK AND DURATION

 Duration of rainfall
  excess should be 1/5 to             D
  1/3 tp.                   tp  tL 
 Base time, tb                       2
       tb=2.67 tp.
       Some use tb = 5tp
       Some use tb = ∞
ESTIMATION OF PEAK FLOW PARAMETERS
 General     form                      KA
                                   qp 
                                         tp
 For a triangular unit
  hydrograph with                       484 A
  tb=2.67 tp                       qp 
                                          tp
 Where:
   tp is the time to peak (hrs)
   qp is peak flow (cfs)
   A is watershed area
    (square miles)
SHAPES OF UNIT HYDROGRAPHS
 SCS   uses a dimensionless unit hydrograph
  (Figure 3.34).
 Also have a triangular unit hydrograph
  derived to have the same tp and qp as the
  dimensionless hydrograph.
 If either of these are used the qp and tp are
  related through      484 A
                qp 
                       tp
TRIANGULAR HYDROGRAPHS
 Good approximation for peak and duration.
 Find the qp and tp of the unit hydrograph.

 Multiply qp by the depth of the rainfall excess
  block.
 Use the tb approximation to find the duration of
  the hydrograph.
    EXAMPLE 5.10 IN TEXT (MODIFIED)
Solution:
    HSG = D / Commercial  T. 5.1  CN = 95
       S = 0.53 in.

    Assume AMC = II  Q = 1.96 in. of runoff
    Find points to develop the unit hydrograph
       tl = 0.75 hr (45 min)

       tp = 1.25 hr (75 min)

       tb = 3.33 hr (200 min)

       qp = 302 cfs / 1 in. of runoff

    Plot unit hydrograph
    Check area under the triangle  1 in.
          600


          400
Q (cfs)




                 qp = 302.5 cfs
          300
          200



                50 tp = 75 100     150   tb = 200
                          T(min)
Volume under triangle = (302.5 cfs x 4,500 sec) / 2 +
[(302.5 x (12,000 – 4,500 sec)] / 2 = 1,812,000 ft3



  Surface runoff depth = 1,812,000 ft3 / 21,780,000 ft2 =
  0.08 ft = 1.0 in. ok
   EXAMPLE 5.10 IN TEXT
Solution:
  qp 2.5” rain = 302.5 cfs x 1.96 in. of SRO
  qp 2.5” rain = 592.9 cfs
  Plot storm hydrograph
  Check area under the triangle  1.96 in.
 Q (cfs)
           qp = 592.9 cfs
600
                                Surface runoff depth
                                = 1.96 in. ok
400
                               Volume under triangle
300                            = 3,557,400 ft3

200




                                                T(min)
              50 tp = 75 100   150   tb = 200
AGSM 335
   Homework #6
SCS DIMENSIONLESS UNIT HYDROGRAPH
   Haan equation                                               1.92
                                             qptp 
                                     K  6.5
                                             V   
                                 K
                                                  
q( t )  t 1       t        
       e
                        tp
                                     Where:

q(p)  t p
                            
                             
                                      •qp is peak flow (in/hr)
                                             •To convert from cfs to in/hr
                                             divide by A (acres) and
                                             1.008.
                                      •tp is time to peak (hrs)
                                      •V is volume under the
                                      hydrograph (V=1 for unit
                                      hydrograph)
                                      •For a unit hydrograph K should
                                      be close to 3.77
EXAMPLE 5.11 IN TEXT USING SCS UHG
 Rainfall excess for each time block of
  duration 15 minutes 0.17, 1.10, 0.84
 tp =52.5 minutes qp = 432 cfs

 Convert qp to in/hr.

 Calculate K

                    1.92
            q pt p             0.86  .875 
                                              1.92

            V 
    K  6.5               6.5                    3.76
                                    1      
COORDINATES OF THE UHG
   For each time block of duration D, calculate q(t)
    using:                        3.76
                     t 1 .875 
                             t
            q(t )        e      432
                     .875      
 These are the ordinates of the UHG.
 For each excess rainfall block multiply the depth
  of the excess rainfall by all the ordinates of the
  unit hydrograph.
 Each hydrograph should begin at the time the
  rainfall excess block begins. i.e. If the rainfall
  excess block occurred at 30 minutes, the
  hydrograph should start at 30 minutes.
CREATING THE TOTAL RUNOFF
HYDROGRAPH
 Add the contents of each row (not including the
  UH ordinates) to get the total runoff at each time
  t.
 Plot t vs. Q.
UNIT AND RUNOFF HYDROGRAPH
CALCULATIONS
  Example of a Unit Hydrograph


  qp                               432cfs
  K                                3.76
  tp                              0.875hr
                                                             Excess Runoff (in)
          t               q(t)              0.17           0.84              0.71            0.24            Runoff (cfs)
               0.00                0.00             0.00                                                                      0.00
               0.25               57.04             9.70            0.00                                                      9.70
               0.50              263.93            44.87           47.91              0.00                                   92.78
               0.75              414.04            70.39          221.70             40.50            0.00                  332.59
               1.00              417.11            70.91          347.79            187.39           13.69                  619.79
               1.25              329.67            56.04          350.38            293.97           63.34                  763.73
               1.50              223.48            37.99          276.92            296.15           99.37                  710.44
               1.75              136.27            23.17          187.73            234.07          100.11                  545.07
               2.00               76.90            13.07          114.47            158.67           79.12                  365.34
               2.25               40.90             6.95           64.59             96.75           53.64                  221.93
               2.50               20.76             3.53           34.35             54.60           32.71                  125.18
               2.75               10.14             1.72           17.44             29.04           18.45                   66.65
               3.00                4.81             0.82            8.52             14.74            9.82                   33.89
               3.25                2.22             0.38            4.04              7.20            4.98                   16.60
               3.50                1.00             0.17            1.86              3.41            2.43                    7.88
               3.75                0.44             0.08            0.84              1.57            1.15                    3.64
               4.00                0.19             0.03            0.37              0.71            0.53                    1.65
               4.25                0.08             0.01            0.16              0.31            0.24                    0.73
               4.50                0.04             0.01            0.07              0.14            0.11                    0.32
               4.75                0.01             0.00            0.03              0.06            0.05                    0.14
               5.00                0.01             0.00            0.01              0.02            0.02                    0.06
               1.00                                                 0.01              0.01            0.01                    0.02
               1.25                                                                   0.00            0.00                    0.01
                                                                                                      0.00                    0.00
                                                                                                                              0.00
PLOTS OF UNIT AND RUNOFF
HYDROGRAPHS

                            Unit and Total Runoff Hydrographs
                 900.00

                 800.00

                 700.00

                 600.00
  Runoff (cfs)




                 500.00

                 400.00                                                          Unit Hydrograph
                                                                                 Total Runoff Hydrograph
                 300.00

                 200.00

                 100.00

                   0.00
                          0.00   1.00   2.00      3.00      4.00   5.00   6.00
                                               Time (hrs)
BAEN 460
   Homework #6

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:9
posted:10/9/2012
language:English
pages:31