Hydrograph Method

[c2h2u2]

Hi Dr. Quek,

I would like to ask you some questions as stated below:

1. How to determine the Rainfall Ratios as stated in the Appendix 13.B Design Temporal Pattern (MASMA) if the duration is not same as stated in the Table 13.B2?? According to the Masma, it stated that use the nearest standard duration, meaning to say, if the duration i get from my calculation is 75 mins, so should i take 60 mins as my reference?? How if i get duration of 45 mins?? should choose 30 mins or 60 mins?? or choose the critical one??

2. How to determine the losses from Table 14.4?? If the site is of pervious areas with loam soil (Lawas, Sarawak). From the Table 14.4, for pervious areas, there are 3 different ways to determine the appropriate losses to be used. Among the 3 methods, what conditions to be considered in order for us to use either the Initial loss-proportional loss, Initial loss-loss rate or Horton Model?? At first, i was using Horton Model but i got some negative values for the ER.

3. How much different is considered as reasonable if I compared the results that I get from Rational Method with Hydrograph Method??

I thanked you in advance for solving my questions.

Best regards,
c2h2u2

[drquek]

1. How to determine the Rainfall Ratios as stated in the Appendix 13.B Design Temporal Pattern (MASMA) if the duration is not same as stated in the Table 13.B2?? According to the Masma, it stated that use the nearest standard duration, meaning to say, if the duration i get from my calculation is 75 mins, so should i take 60 mins as my reference?? How if i get duration of 45 mins?? should choose 30 mins or 60 mins?? or choose the critical one??

The answer is: as a general rule you should use interpolation. But if the duration (say 75 min) is quite close to the standard duration listed in MSMA (60 min), then you may use the nearest duration (75 min). But notice in this case the next higher standard duration from 60 min is 120 min, and 75 min is comparatively close to 60 min. Hence we adopted 60 min.

But if you have a duration of 45 min, I recommend you do an interpolation between 30 and 60 min.

Do you know how to do this?



2. How to determine the losses from Table 14.4?? If the site is of pervious areas with loam soil (Lawas, Sarawak). From the Table 14.4, for pervious areas, there are 3 different ways to determine the appropriate losses to be used. Among the 3 methods, what conditions to be considered in order for us to use either the Initial loss-proportional loss, Initial loss-loss rate or Horton Model?? At first, i was using Horton Model but i got some negative values for the ER.

Have you tried the Initial Loss-Proportional Loss model. I have tried this and it gives good result.



3. How much different is considered as reasonable if I compared the results that I get from Rational Method with Hydrograph Method??

From my experience, the Hydrograph Method (if you are referring to the Runoff Routing Method) it generally gives better estimate than the Rational Method which often gives less accurate answer. This is because of the calibration process used in the Runoff Routing Method. My experience with anther Hydrograph Method (Time-Area Method) is its accuracy depends on the skill of the modeller.