- Step 1: Determine Inflow Hydrograph based on Stage-Discharge and Stage-Storage Graph
- Step 2: Determine Routing Time Period (t)
- Step 3: Develop ‘storage indicator numbers’ table
- Step 4: Plot Storage Indicator Number Graph
- Step 5:Plot Supplementary Curve of Storage Graph
- Step 6: Develop Routing Table
- Step 7 : Plot Outflow Hydrograph
- MSMA 2nd Edition; Chapter 2; 2.5 Hydrological Routing
- MSMA 2nd Edition; Chapter 7 – Detention Pond
How to perform routing through a reservoir or storage facility using Equation 2.17
Step 1: Determine Inflow Hydrograph based on Stage-Discharge and Stage-Storage Graph #
i. Inflow hydrograph
- Rational Method refer Chapter 2; 2.3.2 Rational Hydrograph Method)
- Time Area Method refer Chapter 2; 2.3.3 Time Area Hydrograph Method)
- Refer Chapter 7; 7.5.2 Stage Storage Curves Development
iii. Stage-storage curve
- Refer Chapter 7; 7.5.3 Stage Discharge Curves
For the proposed facility (pond)
Step 2: Determine Routing Time Period (t) #
Select a routing time period (t); and provide a minimum of five (5) points on the rising limb of the inflow hydrograph
Step 3: Develop ‘storage indicator numbers’ table #
Develop ‘storage indicator numbers’ table from ‘stage-discharge curve’ & ‘stage-storage curve’
Typical storage indicator numbers table contains the following column headings;
- Discharge (O) and stage (S) – obtained from stage-discharge & stage-storage curves
- Subscript 2 is arbitrarily assigned in the table
- Time interval t must be the same interval used in the tabulated inflow hydrograph
Step 4: Plot Storage Indicator Number Graph #
Storage indicator number curves are plotted by:
the outflow (column 2) vertically against the storage indicator numbers in column 6.
Step 5:Plot Supplementary Curve of Storage Graph #
Supplementary curve of storage: Column 3 vs Column 6 (S2 /t + O2 /2)
*refer figure 2.12 above.
Purpose: useful for identifying storage for any given value of S2 /t + O2 /2.
Step 6: Develop Routing Table #
Now routing can be performed by developing a routing table for the solution of Equation 2.17 as follows:
Column 1 & 2
To calculate Inflow (Column 2); user can used equation 2.3; Q = CiA / 360
Column 4,5,6 & 7 (at time 0min)
Referring to Equation 2.17;
- From equation 2.17; the left side of equation is determined algebraically as columns (3) + (4) – (5). And this value equals the right side of the equation and is placed in column (6)
- Discharge (O) and storage (S) are obtained from the stage-discharge and stage-storage curves respectively
- Time interval (t) must be the same as the time interval used in the tabulated inflow hydrograph
Enter the storage indicator curve with S2 / t + O2 / 2 (column 6) to obtain O2 (column 7)
Used Stage-Discharge Relation and Storage Indicator Numbers table to calculate O2
take t = 9min ; S2 / t + O2 / 2 = 5.88 m3/s
Hence the O2 at t= 9min is 0.12 m3/s
(refer image below for reference)
Next is how to proceed the pond routing?
Column 6 (S2 / t + O2 / 2) and Column 7 (O2 ) are transported to the next line and become the Column 4 (S1 / t + O1 / 2) and Column 5 (O1 ) respectively;
- Peak storage depth and discharge (O2 in Column (7) will occur when Column (6) reaches maximum.
- The storage indicator numbers table developed in Step 3 is entered with the maximum value of S2 / t + O2 / 2 to obtain the maximum amount of storage required. The table also can be used to determine the corresponding elevation of the depth of stored water.
- Designer needs to make sure that the peak flow in Column (7) does not exceed the allowable discharge as prescribed by the stormwater management criteria.
Step 7 : Plot Outflow Hydrograph #
Plot outflow hydrograph; O2 Column (7) vs time Column (1)