Disclaimer

 

Use at your own risk.  Not officially endorsed.

 

This is a very simple—but time saving—mass-balance model intended for “back-of-an-envelope” type of calculation. For elaborate routing modeling, there is SWMM5 downloadable from EPA website for free.

 

Basically, it is a reservoir routing technique.

Verify the results as errors and bugs are possible.

Email errors or suggestions to info@hydrocalc dot com

 04/02/08

 

See examples

 

Introduction

First, a detention unit of infinite volume is assumed.  It can lose water at a constant rate.  Let us call it loss rate. The loss rate is a lumped value—due to infiltration, pumping and constant discharge from the outlet.  Flow from a catchment area is directed to the detention unit. The catchment area has only two properties: acreage and C-factor.  Hence, it has no routing effect.  A San Francisco design storm (not official) hyetograph of any duration or SCS-24 hour hyetograph—which is idealized rainfall intensities at different time steps—is used to load the catchment.

 

At each time step, the water collected by the catchment enters the detention unit. The water can exit at the loss rate defined earlier. However, for each time step, the lost volume cannot be greater than the sum of what came in and what was there earlier.

 

Goal:

• Find maximum volume ever reached

 

Known:

• Input hyetograph
• Maximum Loss rates
• Infiltration
• Pumped out
• Outlet discharge

 

Assumptions:

• Configuration of pipes, inlets, outlets ignored
• Head not taken into consideration
• Routing effect neglected for any other path except for the detention unit
• Loss after the rainfall duration is not shown in the table

 

Sources of Error

• Numerical engine inherent in javascript
• Rounding error
• Incremental time step used is 5 minutes
• Others

 

Tips

• To plot chart, use MS-Excel spreadsheet:
• To Paste the table data into MS-Excel spreadsheet, use Edit->Paste Special->Select Unicode or Text

 

Explanation of each column

Step	Minutes	Intensity	Inflow	Volume In	Potential Volume Out	Balance	Storage
n/a	Minutes	Inches/hour	cfs	cu. ft	Cu. Ft	Cu. ft	Cu. Ft
	Time from beginning of storm	Intensity obtained from design hyetograph for particular return period and duration	Inflow rate =intensity/12/60/60*C*catchment area*43560	Volume entering for each time step=inflow rate*time step*60	Max volume exiting =Lumped loss rate*time step*60	Volume in – volume out (no physical meaning)	Sum of the balance up to this time step; only positive values; if negative, assume zero—because that means all inflow goes out
Last Row	-	Total Depth (inches)	-	Sum of Volume In	-	-	Max Volume (cu. Ft)