Off-site Lake for detention

[EITZM]

I got a 6 acre parcel that will be developed into 6 lots and that will discharge directly into a floodplain with no structures directly downstream between the discharge point and the floodplain , however a 2.5 acre lake is contained within that floodplain and is directly downstream from the site. I know requirements strictly depend on each municipality, however what if I am able to provide a downstream analysis to determine that the peak flows and the peak flow timing are not contributing to any downstream flooding to justify no need for detention. What studies /analysis should be done if any to the lake directly downstream. Any guidance will be greatly appreciated.

 

[Ron]

What is your Plan B if the lake elevation rises to the point that it cannot accept runoff or if it backs up onto your site?

 

[sam74]

It has been a while since I have done this but the municipality where I once worked had approved the methodology several years ago, things may have changed.

Determine the watershed limits for the lake and then determine the time of concentration for the entire watershed hydrograph. You can figure out peak flows by NRCS method with general estimation of the land uses in the area backed up with field visit or recent aerial photos (google earth).

Then do a drainage analysis for your subdivision and determine the time of concentration for it in pre and post development and make the NRCS method peak flow calculations. You should be able to show that by decreasing the time of concentration by developing the site you would actually lessen the peak runoff generated by the overall lakes watershed.

We had used this methodology for sites further away from the lakes in question that say drained to a blue line stream that drained to the lake because the municipality already had statutes that you could discharge with out detention (other than first flush detention requirements) directly into lakes if your site was bordering them.

 

[cvg]

before you waste any time evaluating this, you should contact the other property to get the go ahead to discharge your site water to their lake. assuming the give the go ahead, then Ron hit the key issue. My guess is there may not be enough freeboard in the lake to do it.

 

[EITZM]

Ron and Sam74, I appreciate the input, but these are my thoughts, Sam74, What you stated is what my initial approach was but since the runoff volume and timing increases form my site will increase the combined(WS and site) flow and impact the downstream, shouldn't the on-site detention pond by oversized to accommodate the timing and the increased runoff volume? Let me know your thoughts.
Also other than the overall watershed study what studies /analysis should be done if any to the lake directly downstream to ensure the increase in volume will be accounted for?

 

[sam74]

My situation at the time was discharging into TVA's lake system which had dams to control flooding situations and TVA owned the land up to a certain storm event elevation. So they were considerably larger than 2.5 acres and landowner issues didn't really exist.

I think it would still be interesting to see what the overall watershed area of the lake is in comparison to your six acre site. Also what information is available to you with out too much work. Does the lake have an outlet structure of some sort you could model? Do you have access to contours of the lake you could use to model? A lake is just a big existing detention pond. You would just need to model it to determine if you would be actually be increasing the peak runoff or maximum water surface elevation. You may not be effecting those worst case scenarios and in my opinion you may not even require a detention pond depending upon the results of a study. Although your reviewers may not consider a logic/rational argument and require detention regardless.

Your six acres may be a drop in the bucket compared to the overall watershed area and might be almost non-measurable in terms of a water surface elevation increase but you won't know until you spend a little time studying it. Assuming you are in the USA, if you have some sort of CAD to import PDFs you can get USGS quad maps for free and bring them into CAD at scale to get some of your data.

In regards to volume, at that time we never considered or were required to consider volumes increases. If we didn't increase the peak discharge we were within municipality regulations. And they were fairly stringent regulations I thought but I'm sure more stringent regulations exist.

 

[TerryScan]

I am a bit curious:
-If the downstream lake is *in the floodplain*, is there ANY storage available? (is the lake already inundated during a flood event?)
-Without more information, I don't see how anyone can comment on the need for additional on-site detention. As mentioned, timing may be a key element. If your site is near the bottom of a larger watershed, one may want to release flow with as quickly as possible (no detention at all) so that the site peak passes before the main watershed peak. If you are at the top of a watershed, you may want to maximize detention. In between? That's where a study tells more of a story.
-One 6 acre project may not be a big deal compared to a larger watershed, but small impacts can add up. If one ignores 500- 6 acre sites, one is dealing with 3000 acres of disregarded area. (again, I have no idea of all the related details for the subject site here).

 

[beej67]

Does the lake fully contain and control the 100 year storm according to the FIRM, or is it under water during the base flood?

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

http://www.campbellcivil.com

 

[EITZM]

Project did not go through. However was informed that a Dam breach analysis would be required. Detention could be provided in lake if Owner agreed.(which was not an issue)
The site is about 6 acres within the approximately 280 acres watershed draining to the lake. beej67, according to FIRM, lake is under water during the base flood. Thanks for the great feedback everybody,