Fundamental stormwater mgmt question. 3

[wesoasis]

I am working on a stormwater management plan for a development that covers approximately 700 acres and intends to increase the overall imperviousness by approximately 20%. There is about 1500 ft releif from one site boundary to the other and four existing drainages (creeks) which currently convey stormwater runoff and snowmelt. Without using pumps and excessive infrastructure, it is tough to keep runoff from entering creeks, as obviously this is what it has been doing naturally; so, what alternatives are available to attenuate flows that have already entered a creek to avoid erosion/water quality issues downstream?

 

[msquared48]

Develop your own stormwater drainage courses parallel to each side of the creeks to catch most of the runoff before it enters the creeks.

Mike McCann
MMC Engineering

 

[cvg]

are you sure that your increase of 20% in site runoff will really have a major impact on the peak flow in the creeks? Will that increase in peak flow really result in significant increases in erosion? You should answer this question first by doing the hydrology and you may be surprised in the results. If the creeks have significant upstream watershed, your impact may not have a large effect.

That said, with our tough environmental permitting laws (I assume you are in the US), you would be ill advised to try and modify the creek to attenuate flows. The 404 permits will be a long time coming if you do. Better to retain and attenuate flows onsite to the maximum extent possible.

If you do want to pursue the stream modifications, grade control structures can slow or stop bed degradation. Off-line detention basins could be used to scalp peaks off your post construction hydrographs. Bank lining can be used to prevent scour and lateral migration of the channel.

 

[wesoasis]

It appears that for at least one of the drainages, which doesn't have significant upstream watershed, that there is a significant increase in the total volume of runoff as well as peak runoff. I would like to propose building an in-line detention basin in a non-fish bearing creek in Canada. Anyone have any clues about permitting for this?

 

[cvg]

You are talking about constructing a small dam. If there is truly very little upstream impact, than perhaps you should build the dam on your site and discharge it to the creek. You can then avoid any impact to the river. You can also do a hydraulic and sediment transport study of the river and may be able to better quantify the impacts. Perhaps once you do that, you will find out that the impacts are only to a short reach and perhaps one or more grade controls could be constructed that would be environmentally friendly.

I'm in the US, so can't comment on Canadian permitting issues - although I believe the environmental laws in Canada are very stringent. If it is a perennial stream, you may have to prove that there are no fish. That goes for minnows and fry. We have an endangered minnow species here which limits construction in many streams. Plus don't forget about other species such as frogs and beavers.