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Rational C for Residential Developments

Rational C for Residential Developments

Rational C for Residential Developments

I am reviewing plans submitted by an engineer for a section of a residential subdivision. The development is roughly 5 units per acre in an area that is generally very sandy.  The engineer used a "composite" C-value of 0.3 to determine the Q for each inlet.  In my experience, the C-value tables have 0.5 for res. lots (5 units/ac) and roughly 0.9 for the paved areas.  Based on rough calculations, I came up with a composite C=0.6.  Obviously a drastic difference in runoff as well as pipe sizing.

The engineer's argument is that he does a composite for each lot, from side P/L to side P/L and from rear P/L to the C/L of the street.  He then determines the amount of impervious area and pervious area within that lot.  Using a C-value of 0.1(for lawns in sandy areas on the C table) and 0.9 for impervious areas, he has a composite C=0.36.     

My questions are:
1.  Is the 0.3 or 0.36 consistant with what others use?
2.  Is the C-value listed in the tables for residential lots supposed to already take into account the streets and sidewalks?

RE: Rational C for Residential Developments

I've done calculations for composite C values many times and I agree with you that 0.6 is a proper value unless these are very large lots with a small percentage of impervious area.   0.36 seems way to low but I work in an area where the soil tends more to clay.   Even with sandy soil I question the use of 0.1 for pervious areas.  The C factor published in tables fro residential lot is supposed to be a composite.

RE: Rational C for Residential Developments

In my experience,
the answer to your question 1 is NO

the answer to question 2 is YES

the question you left unasked is "is the Rational Method the best method to use for this project?"

If drainage areas are large, say 100 acres or more, it probably is not.  FEMA allows Q=CIA for areas up to one square mile (640 Ac) but this seems very large to me.

Good luck

RE: Rational C for Residential Developments

I agree with the engineer's approach of breaking it into pervious and impervious and developing an area-weighted C-factor.  This approach makes it moot what the value is in the table for residential lots, since the proposed approach takes into account the density and size of the units.  As your question indicates, it's a little hard to tell what is inherent to the tabulated values in, say, HEC-22 or a typical drainage manual, without going to the original source of the C-factors.

That said, a C of 0.1 sounds low for pervious areas unless the analysis is for a storm that would not saturate the soil much and the pervious areas are maintained in adequate condition to percolate water.  Typically conveyance is sized for 10-25 yr peak intensity storms that have a higher C-value.  If pervious areas are subject to foot traffic, or a playground, those uses would tend to compact the soils and increase the C-factor.  The design goal should be the actual expected C-factor (that takes into account compaction and how the pervious area is maintained) instead of idealized values.  

RE: Rational C for Residential Developments

Thanks for your input.

The engineer is using the rational method to determine the runoff for each individual catch basin, therefore I do not have any problem with the use of the Rational method.  I would not necessarily have as much of a either problem using a composite C value for each individual lot either if a composite C was calculated for each individual catchment area.  As of now, a generic 0.3 is used for every catchment.

His argument is that this is the industry standard for this area.  I have yet to find a set of plans that has been reviewed using this type C-value from any firm but his.  Most will use a C of 0.4-0.5 for the lots and 0.7 or greater for the right of way area.

RFW, do you have a source for your answer to question #2?  I wouldn't necessarily disagree, but the responses I have gotten from people that I have asked locally has been that streets are not taken into account.  They have no source to back up their answer, though.

Thanks again.

RE: Rational C for Residential Developments

You should be able to find an answer to this question in one or more of the following:

Chin, David A. 2000. Water-Resources Engineering. Prentice-Hall.

Chow, Ven Te, David R. Maidment, and Larry W. Mays. 1988. Applied Hydrology. McGraw-Hill.

Corbitt, Robert A. 1999. Standard Handbook of Environmental Engineering. McGraw-Hill. 2ed.

Lindsley, Ray K., Joseph B. Franzini, David L. Freyberg, and George Tchobanoglous. 1992. Water-Resources Engineering. McGraw-Hill. 4ed.

McCuen, Richard H. 1998. Hydrology Analysis and Design. Prentice-Hall. 2ed.

Singh, Vijay P. 1992. Elementary Hydrology. Prentice-Hall.

Dr. Maidment is active at the University of Texas and has a home page.

Good luck

RE: Rational C for Residential Developments

This is why the Rational Method is more an art than a science...

As for how to determine a composite C, finding the impervious and pervious areas and working it out that way rather than a "rule of thumb" value is always the best way to go.

That said, selecting the individual C values of each is something of a "guess". For example, you indicated that a C of 0.90 was used for impervious areas. In the area which I work, roads and such are assigned a 0.90 while roofs are assigned a C of 0.95. Similarily, lawns in sandy soils here are assigned a C of 0.15 (These values were assigned by a local municipality for developments within that municipality.)

Of course, there is then the whole discussion over whether single or double driveways were assumed, sidewalk areas, what building size was used, what is the actual % building coverage allowed for the lots under the zoning requirements, etc.

The "fair" resolution would be to consistently require all the engineers to calculate a composite C for each catchment and clearly state their assumptions.

RE: Rational C for Residential Developments

Allow me to pose some questions (for thought) which may have already been addressed or presummed in this discussion?

Has the Engineer reviewed the permeability of the existing soils and expected rainfall intensity for the design storm event?

On the face of it, 0.35 does not seem too low.  I have used similar values in some lower density subdivisions, depending on the nature of the land development, the "form and character " of the existing land and the local statutes.

You have abviously spoken with the Engineer and it is obvious that his/her explanation did not satisfy you.  Since you are questioning the Engineer's assumptions, I would suggest you review some of the previous subdivisions (as you have done) and compare those storm assumptions to the performance of the system.  Next, I would solicit advice from other local Engineer's familiar with the area.  See what they have done, or what they would suggest.

Now don't take this out of context or upset, but from your opening comments, I am presuming that you are reviewing on behalf of the municipality or authority having jurisdiction.  I do not want to presume your experience level nor education nor training, but having done this for many years, but the nature of your query suggests that you may be taking your review a little too deep.  Maybe you have been instructed to verify the Engineer's presumptions and calculations, but I doubt it.  Unless his design of the storm system is grossly undersized or drafted with a crayola crayon, some aspects of your review may be treading close to challenging another Engineer's work, particularly if the municipality or authority  does not specify the criteria for design of stormwater systems.

KRS Services

RE: Rational C for Residential Developments

KRS,  Yes I am a municipal engineer, I am an EI five years out of college, I work with a handfull of PE's and I normally do not delve this deeply into a review(at least without reason).  I started digging a little deeper into this after a cursory review of the calculations that were submitted.  The pipe sizing calculations did not include the capacity of the pipes.  I calculated the capacity of a few pipe sizes at random using Manning's eqn.  Several turned up undersized (some having half the capacity needed).  At that point I ran the pipe system through Hydraflow Storm Sewer only to find that about 2/3 of the system was undersized to the point that several feet of surcharge was being generated in the streets.

At that point I did get a little more serious about the review.  The issue about the C-factor did also come about because of what appears may be some confusion (on my part as well as several colleagues) about what a C-factor for Residences actually includes, which led to my questions above.  I have not, however, found that other engineers in the area use C vlaues as low as this.

RE: Rational C for Residential Developments

C coefficients for the City of Phoenix are established by the City Drainage Standards.  For residential areas with single family homes they range from 0.45 to 0.85 and are based on the actual zoning.  The values are generally higher for 100-year runoff than for 2- or 10-year.  

Separate coefficients are required for streets, residential lots and landscaped areas.  Streets and parking lots are 0.95.  Lawns, parks, cemetaries etc. are 0.25 - 0.30.  Undeveloped land is 0.35 - 0.45.  Agricultural areas are 0.15 - 0.20.  Nothing is lower than 0.15.  

In addition, the engineer can propose alternative weighted coefficients where appropriate justification and documentation can be provided.

RE: Rational C for Residential Developments


Thank you for your prompt reply, I understand better the reasoning behind your question.  Believe me, I have had to deal with issues such as the one you are describing.  In all cases, the Engineer in question is either working directly for a developer, whose interests and motivations are divergently different than the municipality's, or are a partner/co-owner of the development.

You should be commended for your didlgence as some reviewers may have stopped at the question of the C value, rather than taking the time as you did to model the design to ascertain concurrance with the municipal standards.  As the individual whom would sign off the plans for the municipality, I have challenged a more than a few   designers and Engineers.

More to the point and your query, since you have already checked that other Engineer's in the area do not use the low C value, then I think it is safe to presume that his/her assumptions and values are too low.  I also assume that you have discussed this matter with the other PE's in your office.  In my experience, there are three ways in which I would proceed.

1)  Inform the Engineer that his design appears to be undersized when verified against the model.  Allow him/her the opportunity to revise and resubmit based upon your criteria.

2)  Provide your calculations and concerns to the Engineer and order the design to be upgraded as a condition of approval.  You can provide him with the municipalities standards (if any) and also provide or instruct that the design meet with good practices.  Your local Engineering Association may be instrumental in providing resources or reference material from which the "good design practices" can fall into.

3)  Inform the Engineer (and your local PE Association) that you intend to have the design reviewed by an independent third party.  It's likely that the municipality may have to bear this cost, and perhaps recover later as part of the development.  The reason I suggest this extreme suggestion is due to the fact that I am presuming that there may be an impasse in discussions between your office and the Engineer in question and either emotions may be getting the better of the parties or the Developer/Engineer are begining to mount pressures on senior administration or some elected officials.  An independant review with provide an unbiased technical review and report the facts accordingly.  If the design is undersized, it will be reported as so.  This will assit your department in defending your actions against unwarranted allegations by a COuncillor or Developer.

Good luck to you and please let me know how it all shakes out in the end.

Gene (780)573-1884  

KRS Services

RE: Rational C for Residential Developments

I would also commend your questioning of the consultant's design.  When a problem occurs down the road, the consultant will not be the one to receive the angry phone calls from residents and politicians.  Furthermore, the cost in installing a larger diameter pipe during original construction is significantly lower than having the taxpayers burdened with upgrading the system when flooding occurs.  

  Your role as the reviewer is to question the work of a private developer.  Why have the municipality involved at all?  Why have standards of construction or design if we are not allowed to question the work of others?  Without protective measures engineers will be pressured into saving the owner money any way possible, including undersizing a storm sewer.  

  What is most complexing to me is the level of reverse questioning from the consultant to you the municipal reviewer.  I don't want to sound as though it is our way or the highway, but you should have the final decision in this matter.  By the time you have exhausted your research of the topic, the consultant could have updated the design.   

RE: Rational C for Residential Developments

Send it back"REJECTEDFOR THE FOLLOWING". Then list the problems with his .36C and pipe sizing. You shouldn't have to spend the next year explaining. He is a PE right? He designs, you review. He has to make you happy.

RE: Rational C for Residential Developments

ngneer, dicksewerrat,
I feel the need to respond to your questions concerning my need to justify and explain my position.  I am relatively new to a municipal government of a city of 120K residents that: 1. Has had no City Engineer for the better part of ten years, 2.  Has never had any form of drainage standards, 3.  Has apparently only required the review of residential subdivisions, not commercial or industrial sites, and 4.  Has let the local engineering community dictate how the engineering dept., development standards, etc. are to work.

That said, it is a big shock to the local engineers that: a.  someone is actually looking at their plans, b.  someone actually has put some thought into reviewing their plans, and c.  someone is actually looking at their plans.  People have been reviewing their plans, but primarily for construction issues, not design issues.  It is not uncommon for local engineers to do things such as round down on a calculated pipe size instead of rounding up (I have been told this by at least two engineers that do it, not hearsay). But nobody has ever been required to submit actual pipe sizing calcs for review so that this could have been caught.  These pipes usually end up in somebody's back yard and flood every time it rains.

The status quo for years has been such that if a developer doesn't like your review comments are, their first step is not back-up information to the review, but a call to the City Manager or a Council person.

I will stop there.  I could rant all day.  

RE: Rational C for Residential Developments

that is precisely the reason that we wrote drainage standards and policies for the City of Phoenix.  The 5th largest city in the nation, yet they had no complete set of written drainage standards.  It has taken 7 years for us to complete the standards and now the city code still needs to be revised to make the standards effective as law for the developers.  See the following link for our drainage standards: ftp://www.ci.phoenix.az.us/pub/payf/swpolicy.pdf

RE: Rational C for Residential Developments

Just to show you how backwards we are, in 1998 the stormwater utility paid to have a stormwater manual prepared.  I have a copy of it on my shelf.  The manual and revised ordinance were presented to council for a vote and was denied because 1 developer and 1 engineer (obviously with some pull) stood up to speak out against a proposed on-site retention requirement.  By all accounts, those presenting the manual thought that all concerned were on board with the improvements.

RE: Rational C for Residential Developments

Clearly, as the reviewer you have the authority and responsibility to look out for the public interest by rejecting plans that fail to do so.

But, if there is no drainage manual for the engineer, and his previous projects have been approved with the same C-value as is being used in the current proposal, then the situation stinks.

It's my opinion that engineers should (and most do) use sound judgements and assumptions in preparing plans and analysis - it's inherent when you put your stamp on something, and ultimately the responsibility of the engineer of record (not the reviewer) to ensure that the design meets the standards for engineering practice in your community.

So before you tersely "REJECT" the plans as suggested by dicksewerrat, you should consider what guidance was offered by your agency, the fact that the engineer is ultimately responsible for any type of design shortcomings(hence the engineer's seal!), and your role as reviewer is to protect the public interest (but you don't have to stamp the plans), and one of the roles of your department should be to identify requirements for the design community.  If your department is now increasing the scrutiny of submittals and calculations then you need to make sure that engineers/designers have enough information to understand what you want and expect in their submittals & calculations, otherwise time and money will be unnecessarily wasted by everyone involved.

I highly suggest you adopt some sort of drainage manual as soon as possible, maybe there's a state or county manual you can use, or one from a neighboring jurisdiction.  While some developers would fight it, everyone would benefit from the added clarity. Or have pre-app meetings and give the designers a copy of a drainage study that you accept to use as a model.

RE: Rational C for Residential Developments

We spent about 3 years coordinating our manual with the development advisory board, the home builders association and with many of the developers engineers.  It went through the council easily because after many re-writes, we finally got something everybody could agree to.  However, without the city code, we still can't enforce it.  Hopefully by the end of this year, the City will have the code re-written to require use of the manual.

RE: Rational C for Residential Developments

As I stated earlier, this disagreement did not start just because I chose to be picky about this particular project.  The engineer brought the scrutiny onto himself by submitting design calculations that were incomplete, but more importantly, clearly showed me that the pipes that he put his stamp on could not handle the amount of flow that he calculated using his lower C-value=0.30.  To that end, he did not "esure" that his own system even worked, much less that it met City standards.  When asked about it, he told me that he wasn't the person that did the calculations.  His argument was that one of his office worker had done the calculations.  HIS stamp was on the calcs.  So hopefully you can see why the argument that the engineer took the time to ensure his work is right doesn't hold much water with me at this point.

As for his liability in the matter because it is his seal on the plans.  We are not talking about differences in flow or pipe sizes that mean the difference between flow that is safe and flow that is deadly.  The primary issue at hand is that the engineer is attempting to save an increase in one pipe size for the majority of the subdivision.  Where we have considerable problems in this area are the pipes that are run through people's back yards or side yards that back up and flood their lots during every heavy rain.  The engineer of record is not the one being called when this happens.  The City engineering dept. is having to deal with these issues.  We have several engineer's, this one included, that have admitted rounding their pipe sizes down instead of up.  Guess what,  when an analysis of a majority of these back yard issues is done, we find that all that is needed is an increase of one pipe size.
As I stated earlier, with respect to drainage standards, I have only been here a relatively short period of time.  I am in the process of mailing out a copy of minimum design standards that were generated with the input of the local HBA and engineer's.  A manual is to follow and supplement.  Unfortunately, most people in upper management and elected officials are more than willing to wait for the Phase II stormwater regs to be implemented.
I apologize for the long rants.  Therer are not many people around here that can understand the issues that this City has with it's development standards.  By the way, the SOP in the past for design review has apparently always been to bully your way into what you want.  This situation is no different.

RE: Rational C for Residential Developments

Ask the Engineer for the name of his malpractice insurance company and the policy number. Keep it on file for problems in the future. I guess I'd call the insurance company to verify. Just tell them you are dueing due diligence for the City. If a few yards flood after a 1 year storm event and the citizens come to the City for help, you hand out copies of the insurance info.Maybe this should be in the Design manual.

RE: Rational C for Residential Developments

Sorry about the invective attitude, but I have had to fix a bunch of designs with city money. and it irritates me. they save less than .5% of a project on the storm water infrasturcture and then pay out 5% for landscaping that is washed away within a year or two.

RE: Rational C for Residential Developments

dicksewerat is right on with his comments.  consultants many times fail to experience the actual results of their design work.  although consultants may be more on the cutting edge, engineering takes a new twist when your design negatively impacts the citizens that rely on our work.  I never expected the disputes /complaints that I now find myself in each day as a municipal engineer.  I certainly do not remember a course is public relations during my undergrad bsce, yet a large portion of my work is just that.  In reality, it's almost impossible for the municipality to track down the original culprit of a faulty design when a problem does arise.  What this argument is about is that ths reviewer is correct to be proactive versus reactive.  If they are uncomfortable with the current design, request the proper pipe size.  After all, I certainly would not inform Joe and Mary Resident that the problem was that the runoff coefficient was underestimated thereby reducing maximum flow values and insufficient pipe diameters.  The next words out of mouth better be the township will fix the situation asap.  This was defintely an enjoyable discussion topic.   

RE: Rational C for Residential Developments


In reference to your previous comment regarding how backward you think the community is, I have to say that in my 20 or so years, it it not backward, but typical.  Very simply put, I completely understand your situation, in fact, my previous comments support your position.  

I will be brief, but allow me to share my experienced viewpoint.  The is a vast difference between what the Council percieves as the necessary standards for the community and what the administrative staff (which are usually professional engineers and such) require.  In many instances, politics will dictate the course of action.  You must however realize that no matter what, by statute and ordinance, the Council is the only body which establishes policy.  That being said, it is very likely that at the very senior level of your municipality, or even at the Council level, there are other prevailing issues that resulted in the voting down of the standards.  I'm not saying this is the case in your organization, but quite likely the Council may not trust the Engineering staff for some reason, there may be conflicts with the CAO, or municipal manager and Council, Council itself may be divided and this may have been an instance of block voting or the engineer and/or developer may have been sucessful in lobbying the Council vote accordingly be, and I quote the famous developer's line :

"...the proposed standards...in our opinion...are not consistant with the way we have always done things...they are Big City rules...which ultimately increases our costs needlessly...which will then be passed onto the purchaser...which makes the properties more difficult to sell...and increases inflation as well...thereby increasing your (municipal) operations and maintenance as well."

Sound familiar?  The best that I can suggest is to do your best, document your concerns in a memo to file, bring to that attention of your supperiors your findings, and above all...don't take the decisions of others (Council and Sr. Managers) personnally, they are politicians, and that's what they do!  It' not only not worth the stress, but if it's really too much to take, perhaps another organization may be better suited to your talents and sincere concerns.

KRS Services

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