Catch basin design
Catch basin design
(OP)
To all;
I am seeking some help. I have designed out a small subdivision and placed my catch basins a part based on a past design requirement that I learned. Well, the local authority wants the CBs every 400 ft which seems a little far for my design application. I have been using the Federal standard but this guy seems to be nagging about this one little thing. I get the slopes alright but would like to know what everyone else uses.
Thanks in advance.
I am seeking some help. I have designed out a small subdivision and placed my catch basins a part based on a past design requirement that I learned. Well, the local authority wants the CBs every 400 ft which seems a little far for my design application. I have been using the Federal standard but this guy seems to be nagging about this one little thing. I get the slopes alright but would like to know what everyone else uses.
Thanks in advance.





RE: Catch basin design
Mike McCann
McCann Engineering
RE: Catch basin design
Typically the spacing is tied to the allowable spread of water in the street. Agencies usually specify the maximum allowable "spread" and the spacing of catch basins is up to you to determine based upon the amount of rainfall, cross slope of road and longitudinal slope of roadway which is used to calculate the spread. That said, for subdivision local streets, spread is often allowed to inundate the entire street and catch basins are required to capture the flow before entering the nearest arterial road. Arterials are often designed so that only 1 lane is flooded during the design storm. So spread would be about 12 - 16 feet typically.
RE: Catch basin design
RO
RE: Catch basin design
Jurisdictions usually state that length of flow in the gutter must not exceed "___" feet. In this case are you saying 400 feet? I have seen some requirements as high as 600'. If you have low points in the street, you could conceivably have 800 feet between CB's (400 feet on each side of the CB to the high points). This is usually the situation in flat land areas.
I once had a client who judged the storm drain design based on the number of CB's in your system. He always felt engineers were excessive (conservative) in their design, even though the jurisdiction dictates the criteria.
Remember CB's cost money and if you can show the design meets the jurisdiction's design standards and you know the system handles the design storm, you are endeavoring to produce an efficient design for your client. Obviously, public safety is the primary concern, but the jurisdictions' design criteria is based on achiving the public safety. Having too many CB's doesn't do service to your client.
RE: Catch basin design
RE: Catch basin design
RE: Catch basin design
Hey thanks for the responses. This is the kind of thought process I like going through to check my reasoning. PELS you are right and I did calculate possible overland releases and conducted a very intense evaluation for each road. My calculations show that the spread on eather side of the road will meet my 250' design based on my cross slope of road and longitudinal slope. With a 400' length, there is a possibility of three places where it overloads my combination inlets and depending on the slope at each station, I could end up with some water in front of someone's driveway. I recalculated that with a spreadsheet I use to verify inlet designs and based it on a 25 year event. My spread is within a 10 foot design, since this subdivision is only a two lane road. I have maintained my slopes at 2.0 to 3.5% but there are the three points I mentioned above where the slope is 1.45 to 1.5% and that is my arguing/worry point. I feel they need to be closer together.
I spoke with some of my colleagues yesterday and we all feel that I keep my design the way it is. It might be conservative but it is better than risking a law suit for our firm. If I have to, I will take it over the permitting group to argue my points because they are making a decision from someone that is not the engineer and is going off of an ordinance that just does not make sense. Thanks for the help. Any other advice would help if you have any.
RE: Catch basin design
RE: Catch basin design