Estimating Sanitary CSO Overflow events?

Question

I am in the process of developing a CSO model for a small client with 6 sanitary overflows. The state regulatory agency is looking for us to demonstrate the treatment plant flow vs the combined sewer overflows. We have modeled the main interceptor and overflows, and have placed five catchments, as on overflow does not have a catchment. We are looking to use the SCS unit hydrograph method and run a data file with known flow depths from the operator, ie 0.30", 1.25", etc. We have used the time-depth event, but it places the rainfall over 24 hours. We feel this is underestimating the small storms (we see no runoff from the catchments until around 0.7" runoff) and we think the model is over estimating the storms at the higher end (3.3" storm produced several million gallons of overflow and surcharge almost 80% of the manholes) this does not occur in real life. Any information is there is a better way to model the rainfall and would anyone have a good xml rainfall table for the year they could share. Thanks.

Tom Walski · Answer

Dan, 
 
This sounds like a calibration issue. With the SCS method, you have a number of parameters to adjust to get the best fit between the model and the real system including the SCS curve number and unit hydrograph type. 
 
But you also need to think about how well the SCS method applies to your system. Remember it was developed for agricultural land based on soil type. If your catchments are 80% impervious, then some of the assumptions in the SCS method may not apply and the relationship between soil type and curve number are not what you'll see in a standard curve number table. I'd advise you to read up on the theory behind the SCS method and consider how it applies to your system. 
 
Of course, having good calibration data where you know the rainfall hyetograph vs. outflow hydrograph is very important. Hopefully they have some actual flow metering data and you can get rainfall data from the US Weather Service. 
 
For highly urbanized areas, sometimes the RTK hydrograph method works better. You have more calibration values to adjust but you need a good set of flow vs. rainfall data. 
 
Good luck. 
Tom

Jesse Dringoli · Answer

Hello Daniel, 
 Chapters 7.6 and 9.3 or our Wastewater Collection System Modeling and Design book provide some information on RTK. 
 You can also find some information on RTK in the SewerGEMS help, such as the topic called "Adding Hydrographs Based On the RTK Method". Furthermore if you search on our wiki you can find some articles, though most are on more specific topics in the realm of RTK. 
 Our Stormwater Conveyance Modeling and Design book has some additional information on rainfall and runoff. For example chapter 4.4, "Obtaining Rainfall Data".