Rational C for Residential Developments 5

[packgrad98]

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?
3.

 

[dicksewerrat]

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.

 

[ngneer]

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.

 

[KRSServices]

Packgrad98,

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

http://www.krs-services.com