Which method to use with drainage area greater than the limit for Rational Method? UCS method?

[DrainageEnthusiast]

I've been practicing road design for quite some time now and along with it is the road drainage design for urban areas. But majority of my work in drainage requires me to use only rational method because most of the projects are residential subdivisions with small commercial areas and parks (around 20 acre min to 200 acre max project area). I'm only using Rational Method to calculate peak flows for each sub-areas and Bentley Flowmaster to do the hydraulics of pipes/culverts sizing, catchbasin sizing. Next is the calculation of pipe slopes and inverts and preparing of profiles. Afterwards, I use WSPG to check to storm sewer system's HGL as I tie-in in the propose line to existing line. If greater than 200 acre or 500 acre catchment area, our hydrologist will calculate the Qs as I size the pipes based on the values he obtained. As simple as that. Everyday, every project. (sorry for long intro)

And then I transferred somewhere in the middle east to work. The district where I'm working right now has a total drainage area of around 13 sq. km. It is composed of existing residential, commercial and small parks. All existing drainage system, gully-type catchbasins to be abandoned. New lines/networks, gully-type catchbasins to be proposed. Big budget.Major overhaul of old storm drainage system.
Definitely I cannot use rational method here. Can anyone here suggest what method or approach I should use with a drainage area as big as this to size the pipes/culverts? After reviewing the survey shots of the existing condition of the roads (some major and small roads)I was able to create a preliminary layout of the proposed storm network. And divide the catchment areas into sub areas.
I was given a criteria of 70m to 120m manhole spacing for pipes 400mm to 1200mm in dia. 120m to 150m manhole spacing for culverts should I need a conduit higher than a 1200mm dia pipe.
Our principal hydrologist delineated all the drainage boundaries and flowpaths using HEC-HMS program for the hydrology analysis.
Each storm line ranges from 500m to 3km in length.
Can I use the SCS method to calculate Qs per each sub-areas to size the pipes/culverts from upstream to downstream of a mainline system with a total length of 3km?I will only create a spreadsheet for the method I'm going to use if no applicable software available to help me solve this problem.
The municipality is not strict with regards to the method to be used, however, I wanted to use the correct approach..which basically...I don't know right now. I have to submit a design approach report afterwards..to justify everything.
Thanks in advance for feedback.

 

[gbam]

There are many hydrologic methods that can be used in this situation. SCS is valid as well as Clark unit Hydrograph and the Green and Ampt infiltration. It will all depend on the climate and fit for methodology. Let me ask this; will there be seperate offsite drainage hydrology and culverts or will everything be captured and conveyed in a single storm drain system? If the system is broken into several smaller systems with out-falls at cross culverts then the rational method may be valid provided your total area does not exceed the maximum allowable area (160-ac). If you are evaluating the downstream impacts resulting from the new storm drain system you may want to use the SCS or other approach to evaluate the "Big Picture" hydrology.

There are a number of inexpensive models out there that may simplify your analysis and need to code a huge spreadsheet. We use either StormCad from Haestad or InRoads Storm and Sanitary. These are a bit more expensive. I hear that HydroCAD is a rather inexpensive modeling tool that will account for storage in the system.

 

[beej67]

Middle East meaning Iraq / Afghanistan / etc?

You can use SCS method, if you have all the relevant variables. The problem with using SCS method in foreign countries is you may not have the Hydrologic Soil Group delineated, and your rainfall distributions aren't likely to be anything standard either. If you don't have either of those, you could perhaps make some assumptions and use USA-based storm distributions and equivalent soil types, but honestly by that time the error introduced is probably as much as would be in a rational method analysis anyway.

If it was me, and all I was doing was sizing pipes and not worrying about any sort of storage analysis, I'd probably just use Rational plus a factor of safety and move on with my life.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

For a SCS method, you're not necessarily limited to the standard rainfall distributions (e.g. Type I, IA, I, III). You can also generate a site-specific synthetic rainfall distribution based on local IDF data.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DrainageEnthusiast]

@gbam:
The project area has narrow valleys (they call it wadi here, very dry and I think they have resistant to erosion, but I haven’t read the soils report yet) along the southeast and the northern part. Per hydrology study, the flowpath direction is from southwest to northeast. Yes there is a separate offsite catchment (a small portion coming from southwest of the project area)that will drain into the project onsite. Our principal hydrologist is taking care of all hydrologic parameters in order for me to design the offsite by providing a channel to catch the offsite and drain it to the southeast valley. Now my main concern is the onsite, in which my primary objective is to catch them all at least (without letting any stormwater pass the northeast boundary). This is an urban area in which we’re proposing networks of circular pipes and box culverts. After delineating the lines and boundaries, I ended up with two long main systems with branches and 2 outlets (one draining to the north and the other one to the southeast). When I checked, both system will carry a drainage area higher than 80 hectares( 200 acres), around 600 hectares(1483 acres) each to be exact. I remember before I did some projects in County of Riverside and City of Hemet, they allowed me to use rational method up until 500acres, with the CivilD and WSPG (both are DOS softwares).

@beej67:
I’m currently somewhere in Saudi Arabia. It has not crossed my mind to work in Iraq and Afghanistan, though they are quite interesting country to work on now that you mentioned it.

@beej67 and psmart:
There is actually a local IDF data. They have a latest data from year 2001 to 2008. Tried to compare it with their local IDF from 1960s to 2000, there’s a small rise in rainfall depth. I have all the relevant variables (such as land use, soil types, AMC) as well.
Now I have another question. Does anyone here understand the SCS method in HEC-22? I don’t have any softwares to use right now, so I’m planning to code in a spreadsheet the HEC-22 design approach in terms of SCS Peak Flow method. Honestly I’m not familiar in calculating or sizing the conduits manually. I have an idea how the usual practice works but I usually rely on available acceptable software for my design. Just try to understand how it works and what parameters or data needed for input. Analyze the result and that’s it. I know this is long, but please if anyone can answer my curiosity, pls. do so.
1. Given that the total catchment of my main line would exceed the rational method limit, how will my design approach be for the branches if a branch (connecting to a main line) has a contributing drainage area of 20 hectares (49.4 acres) only? Will I calculate it by rational or SCS (since I’ll be using SCS with the main line) to size the pipes connecting to the main? Isn’t I should only use one method for all? Or should I consider the area contributing to the flow to change the method I’m using for that particular line?
2. I understand the sheet flow and shallow concentrated flow procedure to get Tc when you size the 1st section of a pipe line (from 1st to 2nd manhole) with the initial drainage area upstream of a system; and plus pipe flow travel time procedure on calculating the Tc using cumulative area when you size the 2nd section of the pipeline. On the third section of the pipeline, I will use the Total Tc from 2nd section to add it to the pipe flow Tc for the 3rd section using cumulative area again. Am I right or wrong? If not, please tell me the correct and proper way.
3. HEC-22 states that shallow concentrated would occur “after short distances of at most 130m (400ft)”. I’m a bit confuse with the assumption of length of travel for the shallow concentrated flow procedure since it’s all about the travel time from roof to drain and driveways, to gutter and inlet. What is the most logical length of travel for me to calculate Tc? Based on the procedure, it would appear that I only have one value of Tc (for shallow concentrated flow) and one value of Tc (for sheet flow) throughout the duration of my calculation for one line. The only one changing is the pipe flow time as I go downstream.
4. How would I consider branch line drainage areas when I’m calculating the Tc for main line? Should I consider that as an “add area” only or add the Tc (of branch line) to the Tc of the main? For example, I will now calculate the Tc for section 6 (6th and 7th manhole) of the main. However, there is a branch line connecting to the 6th manhole with a contributing area of 20 hectares. Can anyone tell me how to properly calculate the Tc if there are junctions like these? Especially if there are more than one junction in a main line.
5. The proper procedure in sizing a box pipe is that it should be analyze at full flow condition which makes it like under pressure condition but apparently is not (due to the manholes and junction energy losses). I’m wondering why some who analyze it at full flow (closed) still uses the formula wetted perimeter=b + 2y for open channel, considering it should be the whole perimeter of a rectangle which is 2b+2y? Can anyone elaborate on this one?
6. Which one is a better approach, a full flow conduit or design it with free board, like maybe around 90 to 94% full? It will be a bit conservative if I do design it at full flow and if I maximize the criteria for velocity, I maybe able to use a smaller cross-section than a full flow condition would give.

I would like to ask some more, but I have to know your opinions on my questions first.

 

[beej67]

If you've got all the input variables, and all you're worried about is a peak flow, then generating one from the NRCS method is relatively simple. Download TR-55 - Peter's got it hosted here:

http://www.hydrocad.net/pdf/TR-55 Manual.pdf

Chapter 2 gives you your runoff, Chapter 4 gives you your unit peak discharge. Watch units, NRCS is in Imperial.

To your questions..

1) I don't think 50 acres is too much to use rational method if all you're using it for is to size pipes in a storm drain network. But if you're married to NRCS, I'd use NRCS for the whole thing.

2) That's normally how you do it when doing a Rational Method analysis, because with Rational Method your design storm changes pipe by pipe. With NRCS method, it seems to me you should do the same thing as you step your way through the system, but instead of pulling a new storm intensity from an IDF curve based on Tc at each juncture, you pull a new unit peak discharge based on the chapter 4 charts in TR-55. That's presuming your storms follow the same distributions as one of the standard storm types in TR-55. If you don't, then maybe just default to adding the flows up pipe by pipe and simplify your life some.

3) This goes back to question 2. If you're doing NRCS peak flow and adding the flows up pipe by pipe, then each watershed should have it's own Tc that determines the flow added in the network at that node. If you're re-figuring your NRCS peak flow at each juncture in the network, then you'll only ever have one stretch of sheet flow and shallow concentrated flow, then a bunch of additional travel times through pipes as you step your way through the system. The only exception, is if you add a branch or catchment that has a higher Tc than your current total system time, in which case you default to that longer time for the remainder of your system calculation.

4) ..this goes back to 3 and 2. In a Rational Method analysis where you're re-figuring Tc junction by junction, you take the single longest Tc to that junction as your Tc. If you're just adding peak flows from contributing pipes, this whole headache goes away. As I say above, you can't do that in Rational, but you may be forced to do that in NRCS.

5) If you include the friction along the top of the box, the capacity goes down to the same capacity as some level below the top of the box, which creates a diminishing return for modeling the box, and doesn't give your true capacity. You can think of it like this - the true capacity of the box is when there's a tiny bit of air between the water and the top of the box. How tiny? Maybe enough to neglect. This is also why circular culverts have their maximum free flow capacity around 92% full by the way - any fuller and the extra cross section of flow is overcompensated by the additional friction of the pipe's crown.

6) Either way is fine, as long as you know what you're doing and include a proper factor of safety based on your engineering judgment. You can include factor of safety in freeboard, or you can design it full and choose a larger design storm. In the states, regs vary on how they want you to do it.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[DrainageEnthusiast]

@ beej67:

Exactly! Thank you for clarification. I appreciate your help and the info.

Two more thing:
1. Do you think HEC-22's sheet flow of travel length 130m(400 ft)is a bit much? I think WinTR-55 lessen this to around 30m (100 ft)only recently when they upgrade their free software. I haven't used any downloadable softwares yet, except the HY-8 (it's pretty awesome btw) for culvert hydraulics. I can't seem to agree to with the limits, though I usually use 30m (100ft) and 30m(100ft) (minimum) for sheet flow and shallow concentrated and 130m (400ft) and 50m (160ft) for maximum limit. Can you recommend a logical length for my initial drainage sub-area upstream right before the first inlet/manhole of the system?

2. What method would you recommend in calculating Tc for sheet flow and shallow concentrated flow for urban watershed or developed condition? I do prefer Kinematic Wave coz it uses the IDF as it relates the rainfall intensity rather than a certain factor that Kirpich or Kerby equation does.