Pond Boss Forums INTRO Questions & Observations Spillway sizing questions

I have a 8.5 acre lake. The current drainage is a standard 24" overflow system. There is no emergency spillway. The lake is 15 - 20 years old with no visible signs of previous flooding, so I am assuming the current 24" pipe has adequte capacity. I want to:

1. Replace the old stand-pipe with syphon system. From a cost perspective I would rather use 12" or 18" pipe vs. 24".

2. Add an emergency spillway.

Question: Is there a way to determine the minimum size of the new pipe and emergency spillway pipe, given that the old system 24" worked well?

Dan,

I can't answer your specific question on pipes but can offer another one...have you considered natural spillways? They are cheap, virtually 100% reliable, and require little or no maintenance. Best of all they never leak.

Meadowlark,

You bring up a good point.

Here is my issue - the road that my spillway will travel over is the only access to the backside of my property. If the spillway is inaccesable, then the backside (about 15% of the total acerage) will be inaccessable.

Its hard to know how often that would happen considering that I want to decrease the size of the drain from 24" to 12" or 18".

darnold:

There is a way to calculate the excess runoff for a 100-year or other rain event. The input data would be the size of the drainage basin, vegetation type, effective slope, soil type and storage capacity of the pond and other waterbodies in the watershed. The calculation is not bad once you have all the data. Once you determine the pond overflow minus the stand pipe capacity you can size the spillway (if needed).

I'm sure the NRCS can do the calculation for you or provide the variables for the calculation. Here is a link to part of the training manual for Michigan, which will give you all you need except the soil and vegetation constants for your area. You basically assume the pond is a stormwater detention basin and go through the runoff calculation.

Homework Problem

Steve and Meadow, thanks for the replies.

At the speed my NRCS folks move, I may never collect all of the watershed information. I spoke with them a few weeks ago and they said they were too busy and check back in late May.

Steve, that is an excellent article and great deal of knowledge in there. Thanks.

Meadow, thanks for your suggestion and saving me about $1 zillion dollars by not putting in expensive piping on my spillway, not to mention the future maintenance issues.

I met my contractor yesterday and walked thru my options and costs. We decided to replace the old 24" system with a 12" drainage pipe and add a natural emergency spillway area about 1' above the water line (no pipes in the spillway = big $ savings). We estimated that the emergency spillway will rarely be in use and therefore I don’t have an access problem. Also a natural spillway will give me virtually unlimited overflow capability (which allows a smaller drain pipe, save $). If I do end up with and access problem, my contractor suggested putting in an inexpensive type of wooden bridge later on...thanks again. :p

Dan,

You won't ever regret that natural spillway...especially when we get another of those 20 inch 100 year rains every year in East Texas. Its the way to go.

Yes, I agree. What do you plant for ground cover in your spillway?

Dan,

I just let natural vegetation, grasses take over. I haven't had any erosion problems there.

Dan:

Here's the link on vegetative stabilization. More good information.

Vegetative stabilization