New to Storm Water - some basic questions

[geosavvy]

I've worked for a geotechnical firm for 5 years and now im suddently working for a municipality and getting caught up to speed on storm water design.

I'm still trying ot get clear in my head the basic methodologies involved with designing a storm water conveyance and detention system.

We have some basic criteria that specify using the rational method for sites under 10 acres with a Tc less than 15 minutes, and using TR-55 for bigger sites.

My confusion is understanding what is meant by 'The Rational Method'. All I have seen, indicates that the Rational Method involves applying the equation Q=CiA to calculate peak flow rates... but what do you do with that peak flow rate? the Rational Method doesnt appear to encompass a procedure for creating a hydrograph after you have calculated a peak flow rate.

Also, I guess im not sure what procedure TR-55 uses to calculate peak flowrates. I have read that it uses either the 'Tabular Method' or the 'Graphical Peak discharge Method' to calcualte a peak discharge. I have used Win TR-55 but I did not see an option to choose which method it uses.

Win TR-55 will spit out hydrographs too. What procedure does it use to generate a hydrograph? Aren't there several ways to create hydrographs (such as the 'unit hydrograph' method?).

It appears that the Rational Method, Graphical Peak Discharge Method, and the Tabular Method all require the time-of-concentration to be calculated.

After one of these three methods are used to estimate a peak flow rate, another procedure is applied to come up with a hydrograph.

I know there are other program out there that people use and im not sure which procedure those programs use to calculate peak flow rates and to create hydrographs. Some I have heard mention of are Pond Pack, Hydraflow, Hydrocad, and SWMM.

I'm assuming most of these programs use procedures other than the ones ive listed to calculate peak flows and and to create hydrographs?

I guess ive asked enough questions. I appreciate any feedback.

 

[francesca]

I have used Hydroflow. It uses the TR-55 method (and three others) to calculate time of concentration. It can calculate peak flow using either the Rational Method or the SCS Curve Number method. I never used it for the Rational Method (just used a spreadsheet) so I'm not sure if it's the "Modified Rational Method" (which does produce a hydrograph/volume) or not.

Search the forums for "Modified Rational Method" or browse through the last few pages and you'll find a wealth of information on it.

 

[cvg]

Rational - Q=CIA This doesn't give you a hydrograph, if a hydrograph is needed, you will need to do further analysis to generate it. But the method is useful for small areas if only the peak flow is need. May not be the most accurate method, but generally provides a quick, conservative estimate of the runoff.

Rainfall - Runoff modeling - requires rainfall distribution, unit hydrograph and estimate of loss rates. Many methods for doing this, including SCS (NRCS) methodology. Generally assumed to provide a more robust analysis and will generate a hydrograph. The procedure is required for any analysis which requires routing the runoff through a basin, dam, bridge etc. I generally use HEC1 to perform the analysis.

Regression Equations - a method for quickly estimating the runoff with order of magnitude accuracy. Often used to check if the results obtained by rainfall - runoff modeling are reasonable.

 

[bltseattle]

You should consider looking over some general references:
HEC-22 Manual by FHWA

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=22&id=47

National Engineering Handbook part 630: Hydrology ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/

I have also found "Stormwater Collection Systems Design Handbook" by L. Mays to be a useful general reference.

The (Ir)Rational method is used to compute peak flow rates to size pipes. It does not produce a hydrograph. Time does play into the method because the time of concentration for the basin will determine what rainfall intensity you use. Also note that rational runoff coefficients will vary depending on the size of the storm (and antecedent moisture, though that is usually not factored into analyses that use the (ir)Rational method).

Some jurisdictions use an approach that assumes an ascending/descending runoff rate rate that reaches the rational-peak rate at a certain time - this is a modified rational approach. For example, assuming that the flow increases linearly from zero to the peak rate over the time-of-concentration and descends back to zero over 2x the time-of-conc. This is arguably an overly simplified hydrograph method (little connection to real runoff processes) though you can use it to come up with a runoff volume, and if it works for a certain jurisdiction's needs, so be it.

TR-55 method is documented in the TR-55 manual by NRCS. The method provides a way to compute runoff volumes and hydrographs. This reference includes time-of-conc computation methods (graphs & equations). See

http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html

Most of the softwares you named will provide several methods, including rational and TR-55. They are used rather than the Feds' freeware because they provide greatly enhanced functions for entering/accessing data, combining/routing hydrographs, level pool routing to size detention facilities, and provide numerous options for presenting the results. There have been many threads about the merits of various software if you do a search on this forum.

Good luck

 

[env21tech]

By all means use the Rational Method and IDF curves (Intensity = average intensity during the duration). Avoid software that requires a relation between Tc(what is happening on the ground) and average rainfall intensity (what is happening in the sky).The raincloud will act the same whether it is raining on a parking lot or a heavily wooded forest.
The 5-minute IDF duration provides more than enough raincloud size to deluge even a 50-acre urban area.
Calculating rainfall volume is easy with the IDF curves -- just multiply duration x average rainfall intensity.

 

[psmart]

env21tech: The problem with using only a 5-minute event is that some longer duration (which will also have a larger volume) may create a higher water level in the detention pond. Using a fixed duration *might* be OK for predicting peak flows, but ponds are sensitive to what happens over a longer time span (depending on the outlet configuration), hence the need to generate a full runoff hydrograph.

If you have questions about this, I suggest you start a new thread on hydrograph procedures (such as the SCS/NRCS method), or see one of the many previous threads on this topic.

 

[cvg]

psmart, you are absolutely correct and in fact the requirement in this area is generally full retention of on-site flows for the 100-year, 2-hour storm

 

[env21tech]

psmart --the five minute duration would be used for sizing storm sewers and I agree that a longer duration (e.g. 1-2 hrs) is needed to estimate detention.
I ususally assume the rainfall hydrograph = the inflow hydrograph because the site will have been saturated by the leading edge of the storm and most of the rainwater will find a way to get to the main detention basin within 10-15 minutes.
The site may also include mini-detention basins such as flooded rear yards, etc.