This would not be at all encouraging, but we are working on a project right now in a municipality that requires you to improve ALL downstream stormwater features if your project will impact them to the point that they will not pass the 25-year flows with an Hw/D of 1 or less.  We are having to replace a downstream 18" CMP driveway pipe with (3) 24" pipes just to pass the 25-year flow.

At a professional level, I would recommend that you design the pipe as if no other pipe exists in the area.  Don't open yourself up for future liability.  I know it will seem ridiculous to put a 30" pipe between the 18" pipes, but that would be my recommendation.  As a courtesy, you may want to notify the downstream owner of what you are proposing and recommend that they may want to upgrade.

Jeff Foster, PE
CE Group, Inc.
Apex, NC

the ditch and the "undersized" pipes may be acting like a series of detention basins.  The ditch fills up during the peak runoff and then drains slowly.  This attenuates the flow so that maybe an 18 inch pipe is sufficient.  You may be able to justify the design by calculating the stage-storage-discharge relationship and routing the flow through the basin.

cvg,
What type of storm do you recommend routing thru this "detention basin"  The DOT recommends the rational method for this size watershed but the rational method doesnt lend itself well to calculating volumes and storm routing.

One way to "roughly approximate" a hydrograph using the rational method is assume it is triangular.  For example, if you assume a 6-hour storm, the triangular hydrograph is 6 hours long.  For simplicity, the peak runoff would occur at 3 hours.  The method may tend to overestimate the total runoff volume, so the resulting design may be conservative.

SCS also has a dimensionless triangular hydrograph.  Time base of this hydrograph is approximately 8/3 of the time to peak.

volume of runoff is often calculated using the following formula:

V = A x C x D/12

A = area (acres)
C = runoff coefficient
D = runoff volume (acre-feet)

I am not familiar with this formula - how do you calculate D?  

I'm sorry, I messed up.  D is the precipitation depth in inches and V is the volume in acre-feet.

I would install an 18" culvert smooth wall pipe like ADS N-12 at the max slope of the two pipes, unless there is a significant additional inflow to the culvert/ditch system or a significant velocity change after the upstream 18" culvert and before your driveway.

However, if required, you could always model the system using any hydraulic software. (Check out the FHWA website -http://www.fhwa.dot.gov/bridge/hyd.htm - in particular the HY8 software or the HDS 5 for manual culvert methods)  I would specify a bell if well beveled pipe end at the head wall or mitered end.  (If ME are used, use a ME, same goes for vertical headwalls.  If mixed, use a vertical headwall with a well beveled entrance.)

If the pipe is too small, upstream will flood and the driveway may be overtopped.  If the pipe is too large, you just waste your clients money because the ditch or the next downstream obstruction will determine the flow.  Do no worry about what if scenairo's.  All drainage systems will eventually fail because a greater than design event will occur.  Just design it to meet the the relevant standards, rules and common accepted practices for your area, but do not flood your client in case of failure or cause a public danger.





 

Clifford H Laubstein
FL PE 58662

There is actually an 18" culvert in the existing driveway now.  We will replace it when the new driveway is built.  Currently I have 3 options.  THe Dept of Transportation requires that the 25 year flood be passed.  I have 3 options

1. Replace 18" with 18" because there is an 18" below me
2. Design the pipe with the assumption it is the only pipe there - this would be a 24"
3. Design the pipe to reflect actual field conditions: The ditch would overtop the upstream 18" culvert - our culvert in the middle would experience tailwater conditions from the downstream 18".  The design would call for a 30" RCP.
THe client really wouldnt understand that at all

Anybody got a solution

There is actually an 18" culvert in the existing driveway now.  We will replace it when the new driveway is built.  THe Dept of Transportation requires that the 25 year flood be passed.  I have 3 options

1. Replace 18" with 18" because there is an 18" below me and an 18" above me
2. Design the pipe with the assumption it is the only pipe there - this would be a 24"
3. Design the pipe to reflect actual field conditions: The ditch would overtop the upstream 18" culvert - our culvert in the middle would experience tailwater conditions from the downstream 18".  The design would call for a 30" RCP.
THe client really wouldnt understand that at all

Anybody got a solution

Your calculations may be in error or just not optimal.  Which 25 year storm? (ie - recurrence interval, 1 hr thru 7 day?)  Are you making some assumption that is too conservative, like too steep a shape factor for an SCS method or neglecting the ditch storage and considering conveyance only? Are you using a strict Rational Analysis or a modified PULS Analysis (includes resevoir routing)?  Have you conpared you model results to historic flodd data or the FDOT design model/data?  Can you make the ditch wider to increase the storage and decrease the pipe size?  How are you modeling the minor loss coefficients, ie entrance losses?  Did you check to see how other engineers are coping with the same type of problem in your area?    The driveway permits are public record, did you research some other current ones?

In any case the DOT regulations must be met.  To deliberately not meet the regulations may be misconduct, even if it saves your client money. NEVER CHEAT FOR ANY CLIENT.  I really feel that your need to take a fresh outlook on your analysis if your need a 30" pipe.  

Clifford H Laubstein
FL PE 58662

Two 15" pipes? Squash or box pipe? Is cover, aesthetics, cost, or safety the primary concern in not selecting the 30" pipe?  Does the balance for expending additional hours in design vs. actual construction force a selection of one option over another?  Sounds interesting.

    

I am using the Rational Method methodology as outlined in the DOT manual.  I am negelcting ditch storgae because I was assuming it wouldnt be  enough b/c there is 10 acres draining into the culvert.  I am not familiar with modified PULS but i will look it up.  Havent compared my results to historic flood data and wouldnt know where to find it,.  Good idea on widening the ditch.  I modeled the pipe in Culvert Master.  I know i am spending a lot of time on this one but this situation is bound to happen again and again.  Thanks for all the help and I wouldnt consider cheating.

thanks to all who replied - this is what i ended up doing:

1. modeled existing downstream pipe (discharge vs. headwater)
2. used headwater as the tailwater for proposed pipe to develop rating curve for new pipe
3. developed stage-storage curve for ditch
4. routed storm through ditch and pipe.
5. showed that overtopping would occur for 25 year but not 10 year
6. Told the DOT what my results were but that we were afraid to flood downstream neighbors.
7.  They said that they would ask their local office if there were any instances of flooding - if not then they would be in concurrence with us.  if there was then they would upgrade the downstream pipe owner

thanks again

A few notes:
1) 10 acres would normally require a 24" minimum culvert for a 25 year rational analysis in central Florida.  Unless there was percolation, a very long time of concentration or significant storage on the order of 0.5 acre foot, more or less.

2) If you do not decrease the storage capacity of the ditch or increase the flow to the ditch or decrease the flow time of the ditch, your culvert/driveway will no create any flooding to your downstream neighbor, even if it overtops.  However, if your downsteam neighbors' culvert is too small, he may flood you, the upstearm neighbor, by creating a tailwater condition for you.  Therefore by undersizing your culvert, your impact your UPSTREAM neighbor, not your downstream one.  (Rule of thumb - increasing your peak runoff may flood downstream, creating a flow restriction may flood upstream)

3) The DOT has soverign immunity for any errors of judgement, you and your firm do not.  Just because they approve a deficient design, does not make it OK, nor does that protect you from liability due to negligence.

4) I did not mean to imply that you would "cheat", however, being human and probably much older, I have had the temptation on occasion.  It is not cheating to interpret data or estimate parameters favorably, as along as the interpretation is reasonable.  For example, if you can use a C factor of 0.1-0.2 for undeveloped areas, use 0.1.(If your do not need to increase the undeveloped areas C for the recurrance interval, do not.)    Do the same for your ditch manning factor, use a high retardance.  For your culvert, use a low entrance coefficient due to the proposed beveled edge, etc...    Did you consider percolation in your ditches?

5) It is good to talk to the regulators for their input.  Also it is good to work on a problem until you are comfortable with the solution. You must be 100% comfortable with all your solutions.

6) Sorry that my advise may seem contradictory, but additional information may change my perspective.
best of luck.....     

Clifford H Laubstein
FL PE 58662

Gibfrog,

Glad to get other advice - dont worry about offending anyone - especially good advice about the DOT having soverign immunity

I would like to address comment 2 above.  Although we will not technically cause an increase in flow, our downstream neighbor will experience more flow than in the past because of the loss of the upstream restriction.  THey have lot less storage on their culvert than ours does.
Storms that are currently detained in our ditch would flow downstream to the culvert without much storage and would overtop.  Correct me if I am wrong

1) The ROW is owned by the DOT of your state.
2) DOT regulations are forcing you to upgrade your culvert from an 18" to a 24" or 30" because you must pass the 25 year event through your culvert.
3) Assumption  - You are not increasing your peak discharge to the ditch through the 25 year event, nor are you significantly decreasing the ditch storage by 10% nor increasing the ditch velocity by 10% nor increasing the ditch flow by 10%.  (I use 10% because I do not believe that any hydrologic model, nor most hydrologic parameters are more accuarate than +/-10% at best)
4) Assumption -in your model, the peak flow overtoped your 18" culvert & driveway during the 25 year event, increasing the flow and overcomming your flow restriction forcing your to upgrade your pipe size.
5) Conclusion - probably no increased downstream flooding by increasing your culvert size for the 25 year event, however this is not true for a lessor undetermined event. For this lessor undefined event (between the 10 and 25 year) which did not historically overtop your driveway, they will experiance more flow and may overtop (you are correct in this case).
6) DOT owns the ROW and if you properly follow the rules and downstream floods because their culvert is deficient, oh well.  If they flood because you made a mistake and increased your site's flow to the ditch, that is another matter.  If they flood due to poor downstream maintenance or a 50 year rainfall, oh well.
 
Best of luck...

Clifford H Laubstein
FL PE 58662

You may consider using the Santa Barbaru Urban Hydrograph method.  This method is fairly simple to do.

http://www.kesco.sphosting.com