HEC-HMS Project - Storm Drain Report

[a_25y]

Greetings,

Context:
I'll do my best to explain my situation the best I can.

I have been given the task to use HEC-HMS to do a storm drain report (find storm discharge from a 10 year, 25 year, 100 year storm, size detention ponds, size pipe capacities, etc.) for a subdivision development project that is approximately 200 acres in Hurricane, UT.

Usually, I use Rational method to accomplish this task, however, Hurricane City requires the use of HEC-HMS for any project bigger than 10 acres.

I am very new to this program and have relied on the downloadable manual, youtube tutorials, and other materials to learn more about HEC-HMS. Therefore, I wanted to share some simple questions to see if I am able to adequately use HEC-HMS and its components.

Objectives:
1 - Size Pipe Capacities.
2 - Size Detention Pond.

Givens:
[li]200 acre approx. drainage area [/li]
[li]I Divided the Basin into 18 subbasin components, each with its respective drainage area value[/li]
[li]A "Sink" Component is located at the end of all the interconnected subbasins and renamed as "Outlet" [/li]
[li]Base flow method = NONE[/li]
[li]Loss Method = SCS Curve Number[/li]
[li]Transform Method = SCS Unit Hydrograph[/li]
[li]Design Storm = 3 hour 10 year storm & 100 year storm, defined as Gage 1 and Gage 2[/li]
[li]Soil Type = B[/li]
[li]Impervious Area = 20%[/li]
[li]Curve Number calculated from WinTr-55 = 68[/li]

Questions:
[li]I am not sure how to have the subbasins connected. For example, if I had subbasins A, B, and Outlet. And I wanted A drain to B and B to Outlet, do I use a "Reach" component? [/li]
[li]If I do use a "Reach" component, do I set the routing method to "Muskingum-Cunge Method"? I read this allows me to use this as a means to simulate primary storm drain conveyances. So I inserted the length, slope, Manning's n, shape, and diameter.

However, I get two warnings when I run this model(examples shown below):

NOTE 41054: Routing parameters for reach "Reach-1": Delta t (sec) 32.3 Delta x (ft) 1,160​

WARNING 41070: Flow depth exceeded 0.771D criteria 344 time steps at reach "Reach-1".​

&

WARNING 41074: Length for reach "Reach-2" is very short and may cause numeric instabilities.​

[/li]
[li]I tried the manual and google on how to fix these "Warnings" but I always up in a dead end. [/li]

If you know how to correct these warnings, that would help a lot! If there is a better way of doing this, I would greatly appreciate your comments. If you have any questions, I'd be happy to clarify as much as possible. (I can upload my file if needed)

If an alternative solution is just having the "Reach" components simply connect to each without a routing method, would that help this model out?

Thank you

 

[Ekenny_G]

It appears you have sub-basins or sub-areas within a large drainage area. And you want to route the drainage area runoff thru an outlet structure to determine water surface elevations?
I normally use winTR-20 to accomplish this. That would be my first attempt.

 

[a_25y]

Greetings Ekenedilichukwu,
Yes. I have a residential development area. Within that area I have set sub-basins and calculated its area so I can size my SD pipe network. If I know my intensities, areas, and coefficients, I can find my discharge for that sub-basin and size an adequate pipe for that region. Ultimately, all the pipes lead towards the detention pond. As the pipes approach the pond, the pipes will need to increase in size to have sufficient capacity for the increasing volume of water.

My goal is to calculate the total amount of storm water discharge to size my detention pond with HEC-HMS. I assumed that I had set my outlet component to calculate the total amount of discharge expected. I want to use HEC-HMS to find the discharge into the pond and then calculate the volume necessary for the pond to detain all the sub-basins.