Hydrographs

A hydrograph is a representation of flow for a particular stream and is used to evaluate the watershed runoff volumes and the peak flow rates from a single storm or set of storms. This information is then used in the design of flood control facilities.
Watershed runoff is a function of several things, including
  • rainfall intensity
  • storm duration
  • type of vegetation in the watershed area
  • size and shape of the watershed
    and many other factors
    • The synthetic hydrograph approach was initially developed in 1932 by L.K. Sherman and has been fine tuned by several others since then. The unit hydrograph serves to represent the run-off over a certain timespan from one inch of rainfall and is contructed as follows:

    Provided information: 

     Time   Tot. Runoff
 Unit	  (cfs)

   0	   40
   1 	   50
   2	   250
   3	   380
   4	   400	
   5	   280
   6	   220
   7	   170
   8	   100
   9	   80
   10	   50

    and an effective precipitation of 1.5 inches.
    Effective rainfall is the total rainfall less infiltration, evaporation, transpiration, absortion and detention.

    Graphed, the above looks like...

    The next step is to subtract the base flow from the total runoff. Base flow is defined as the normal flow of the stream without the influence of storm-runoff. To determine the actual flow from runoff it is necessaary to subtract the base flow from the total runoff. To find base flow, draw a line from the minimum beginning value of the hydrograph curve to the minimum ending value. The base flow for each time unit can then be read off the graph. In this example, the base flow is constant at 50 cfs. 

     Time    Total    Base-   Direct
 Unit	 Runoff   flow     Flow
	 (cfs)    (cfs)    (cfs)

   0	   40	   40       0
   1 	   50      50       0
   2	   250     50       200
   3	   380     50       330 
   4	   400     50	    350 
   5	   280     50       230
   6	   220     50       170
   7	   170     50       120
   8	   100     50       50
   9	   80      50       30
   10	   50      50       0
    And this graph looks like...

    The dotted line is the base flow and the dashed line is the total direct flow from the storm.

    The final step is to divide the direct flow by 1.5, the effective precipitation, to find the flow if just one inch of rain fell. 

    
 Time    Total    Base-   Direct  Unit Hydrograph
 Unit	 Runoff   flow     Flow    ordinate 
	 (cfs)    (cfs)    (cfs)   (cfs) 

   0	   40	   40       0       0
   1 	   50      50       0       0
   2	   250     50       200     133.33
   3	   380     50       330     220
   4	   400     50	    350     233.33 
   5	   280     50       230     153.33
   6	   220     50       170     113.33
   7	   170     50       120     80
   8	   100     50       50      33.33
   9	   80      50       30      20
   10	   50      50       0       0
    The graph for the unit hydrograph looks like...

    References
    Computer Methods in Urban Hydrology. Theodore V. Hromadka. Lighthouse Publications. 1983.

    Introduction to Hydrology. Warren Viessman, Jr. et alia. Harper Collins.1989.

    Return to hydrology index.