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New to urban drainage design, struggling with general design procedure

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TransCivil

Civil/Environmental
Jun 8, 2006
3
I am new to drainage design and currently putting together a design for an urban road with curb & gutter, inlets, manholes, etc. and the storage is going to be infiltration trenches. I am able to get through the details of designing the storm drain pipe system, but I am having a hard time coming up with the volume for storage. The storm drain is designed for a 10-year frequency and the storage will be 100-year. When I read HEC-22 and other manuals, the storm drain system and the storage facilities are talked about completely separately (it never says "When you get a discharge calculated from your storm drain, do the following to determine the storage volume.") What is the proper connection between the two in hydrologic terms? Do I use the rational method to get all the inlets spaced, pipes sized, etc. and then develop a completely independent hydrograph for the same area just to calculate the total storage, ignoring the peak discharge from the storm drain system?

A friend that is a more experienced drainage engineer suggested that I take the peak discharge from the last pipe in the storm drain and the overall time of concentration produced by the software and apply those parameters to a triangular unit hydrograph. Time to peak = time of concentration, storm duration = 2.67*Tc, and calculate the area under the triangular hydrograph. Sounds pretty straightforward and makes sense, but how come none of the manuals outline a procedure like this?
 
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First decide whether or not to use the Rational Method or any of its variations. If your drainage area is large, say 20 or more acres, this is probably Not the method of choice. Your friends suggestion, although widely misused, is probably bogus.

To design a storage volume you need a hydrograph. The Rational Method does not produce a hydrograph. You may use the NRCS Unit hydrograph method, the Santa Barbara Urban Hydrograph Method or an actual hydrograph from an historic storm. There is free and low cost software to do the calculations for you or you may develop Excel spreadsheets to do them. WIN TR-55 is one such free program. There are other methods, such as regional regression equations, which might be appropriate.

You also need some criteria to determine the allowable outflow from the detention storage. Such criteria should be available from the agency responsible for reviewing your design.

Finally, consider the consequences of failure of your design. Don't assume that because you have met some agencies criteria you have a good design. Be more conservative if the consequences of failure would result in extensive flooding with possible loss of property or even life.

good luck
 
if you are designing a "retention" basin, you do not need a hydrograph. Determine 100-year precip, drainage area and loss coefficient and calculate total runoff in acre-feet. Provide storage for all of it plus safety factor / freeboard. however, your post is not clear as you do not indicate if this is "retention" or "detention" basin. My experience is that most agencies require retention basins - not detention basins. Detention basins are usually only allowed for larger regional developments to reduce the runoff to pre-development levels. They are not designed to capture the entire storm runoff volume, only part of it. If you are designing a detention facility, then you will need to perform rainfall-runoff analysis and estimate the storm hydrograph for the 100-year storm.
 
Why the "100 year" storm ? It has nothing to do with 100 years. It is NOT the largest possible storm. It may, or may not, provide the level of protection needed based on possible effects downstream. The probability of it being equaled or exceeded in 50 years is abour 24% ( 1 in four chance) and in 100 years the probability is 63% ( more likely than not.)

Many local agencies require detention for sites as small as 0.50 acres and even less. Hardly "regional detention, is it ?

 
The 100 year storm would be better described as the 1% storm, as there is a 1% chance of getting a storm that severe each year. Through the magic of compounding, you come up with the percent chances RWF7437 describes.

Also, I've heard that in many areas, what used to be a 5-year storm (20% chance/year) is now a 4 year storm, as rainfall events have been getting more severe on average. The report I saw didn't say anything about 100-year storms, though.

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TransCivil said:
Do I use the rational method to get all the inlets spaced, pipes sized, etc. and then develop a completely independent hydrograph for the same area just to calculate the total storage, ignoring the peak discharge from the storm drain system?
Yes. This is because the storm event that will give rise to the highest peak flow will be short and intense, but it will not also give rise to the highest volume. Your concern with designing the pipes is peak flow, while your concern with designing the retention/infiltration trenches will be volume. The storm that will give rise to the highest volume will be of much longer duration, and therefore less intense.

Because you're designing a road, there is a regulatory agency that has a standard method of performing the drainage calculations. We can argue at length what's right and what's wrong, but it's not going to help you get your plans approved.

It sounds like your agency has decided to design storm drains for 10-year (or 10%) storms and drainage for 100-year (or 1%) storms. That's half the battle, but the other half you need to decide upon is the storm duration. Usually the Rational Method is acceptable for storm drains in areas under 20 acres (which is probably the condition you have given you're designing a road; rare is the road that has storm drains only every 10,000+ feet). It is typical to pick a duration equal to the time of concentration, and for practical purposes to use a minimum duration of 5 minutes. You would therefore look for a 10-year, 5-minute storm duration on an IDF chart for your area. For the detention/retention/infiltration problem, you're best off using software that's capable of analyzing the system for a variety of storm durations. Typically for detention basins the 100-year, 24-hour storm is a "worst case" but for smaller areas, a shorter storm duration could possibly be more appropriate. It depends how quickly your water is going to infiltrate out of the trenches.

Your regulatory agency may not be used to dealing with infiltration trenches as a method of retention/detention, so their drainage manual may not address this. Infiltration capacity diminishes with time (as the ground becomes more saturated), so you may have to calculate by hand your outflow volume and change in storage after developing an inflow hydrograph (or series thereof) using standard methods.
 
Thanks for the help so far, it helps a lot.

Here's a little more detail and I'll try to keep it brief. This design is for an Indian Reservation and they are just beginning to administer the design and construction of their roadways themselves, so standards and policies are non-existent. We are basically taking on state DOT standards since they are federally approved and when necessary, the backup is other federal standards or guidelines like HEC-22, etc. The DOT drainage manual here is very incomplete and not very useful. They list many references and many calculation methods and there always a disclaimer that says "The following is not DOT policy or standard". Well then, what is the standard? That's a question I hear frequently around here.

Most local agencies will use a 10-year storm for storm sewer design and 100-year storm for storage. The duration I am not sure about. The infiltration trenches on this project are retention and I do not necessarily need to account for the infiltration that occurs during the storm. This could be a safety factor of sorts. Since this would not require a hydrograph, is there a quick and dirty way to calculate the 100-year storage volume by using only a depth and applying it to the entire area somehow with a runoff coefficient?

I need to take a look at the software available for developing hydrographs. Is the time of concentration a required input for the software? If so, how do you determine it when the runoff is being transmitted mostly through a storm sewer and not overland flow?

THanks again
 
Most hydrographs are determined by first determining the volume of the storm and then distibuting it according to a Type I/Type II/etc. distribution. Storm volumes are usually given in terms of depth (e.g. 12" in 24 hrs). To get a true "volume" you multiply the depth by the area.

Taking this approach (instead of letting software develop a hydrograph for you e.g. using SCS unit hydrograph method) you would have to come up with initial abstraction on your own. For a road dumping into a storm sewer, you wouldn't have much by way of depression storage, infiltration, evaporation, interception, etc. You can't use a runoff coefficient (Rational method) to do this, but you can use the SCS unit hydrograph method that uses a Curve Number to calculate initial abstraction.

You do need time of concentration for determining a hydrograph; you don't need it if all you're interested in is the storm volume. You determine the time of concentration through the storm sewer by calculating the velocity in the storm sewer. You will also have to add in the overland flow component. Look at TR-55 for breaking down the sheet/shallow concentrated/concentrated flow components of time of concentration (or use Hydraflow Hydrographs that has a form prompting you for all the inputs and calculates it for you).

Because you're doing retention, you're going to have to account for the entire storm volume, which poses the question of storm duration. You'll have a higher volume for a 24 hr storm, and higher still for a 48 hour storm assuming no outflow.

Many agencies arbitrarily selected/accept a 24-hour, 100-year design storm for detention/retention calculations. All standards are arbitrary to some extent, and caveats like you're finding are there to leave the liability with the design engineer, which is fair enough. If you're not experienced enough to be confident taking liability, you shouldn't be doing the design.
 
note that around here, 100-year, 2-hour storm is the requirement for retention and that calculation of a hydrograph is not required. Must provide storage for the entire storm plus freeboard and then dispose the water using drywells or bleed off pipe within 36 hours.
 
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