ERT 246 Hydrology & Water Resources Engineering Siti Kamariah Md Sa’at School of Bioprocess Engineering, UniMAP

Unit Hydrograph Cont.

More Example 4: Two storm each of 6-hr duration and having rainfall excess values of 3.0cm and 2.0 cm respectively occur successively. The 2-cm ER rain follows the 3-cm rain. The 6-hr UH for the catchment is the same as given in previous example. Calculate the resulting DRH. Page 3

Unit Hydrograph of Different Durations Under condition where lack of adequate data in developement of unit hydrograph D-hour unit hydrograph is used to develop unit hydrographs of differing durations nD Two method available: Method of superposition The S-Curve Page 7

Method of superposition If a D-h unit hydrograph is available, and its desired to develop unit hydrograph of nD, its is easily accomplished by superposing n unit hydrographs with each graph separated from the previous on by D-h. Page 8

D = 2-Hr Unit Hydrograph Adjusted Net Rainfall one inch over basin Qp Page 9

Change UH Duration Consider 1 hr UH Add and Lag two UH by one hour Sum and divide by 2 Results in 2 hr UH Page 10

Example 5 Given the ordinates of a 4-hr unit hydrograph as below derive the ordinates of a 12-hr unit hydrograph for the same catchment Time (hr) 4 8 12 16 20 24 28 32 36 40 44 Ordinates of 4-hr UH 80 130 150 90 52 27 15 5 Page 11

Equilibrium Hydrograph Uniform Rain Inflow = Outflow Equiv to summing n hydrographs in T Produces S curve One Hydrograph Page 14

S-Curve Also known as S-hydrograph Hydrograph produced by continous effective rainfall at a constant rate for infinite period. Curve obtained by summation of an infinite series of D-h UH spaced D-h apart. Page 15

S-Curve Construction Obtained by adding a string of D-h UH each lagged by D-hours from one another. If Tb= base period of the UH, addition of only Tb/D UH are suffcient to obtained S-curve U(t) = S(t)-S(t-D) or S(t) = U(t)+S(t-D) Where S(t-D)= S-curve addition at time t S(t) = Ordinate of D-h UH at time t Page 17

S-Curves for UH Page 18

S-Curves S-curve Convert 2 hr Unit hydrograph to 3-hr Lag each 2-hr UH by Duration D Add to produce S-curve Page 19

Example 6 Solve previous example (Example 5) with S-curve method: Page 20

Hydrograph Convolution Add and Lag Method 1 2 3 1 2 STORM HYDRO 3 Add up the ordinates of all three to produce storm hydrograph Page 22

Multi-Period Storm Pg - F = Net Rainfall NET RF Multi-Period Storm Pg - F = Net Rainfall RF units = UH duration (i.e. 1 hour) Adjust UH by Net RF Add and Lag UH Produce Storm    Hydrograph UH STORM Page 23

Development Effects on UH Natural Partial Fully Developed Page 24