Small Scale Storm Water Detention Effectiveness 8

[655321]

Could I get some of your thoughts on how effective you believe storm water detention basins are in small developing urban watersheds with the goal of creating "no net increase" in run-off rate from individual sites in the 2, 10, 100 Year event?

I just don't believe in most cases these basins really do anything. How can you can realistically design these “structural” methods for such large storms on a small scale; some sites our jurisdiction has required it on have been as small as 3 acres. And, if they did work exactly like you planed, all your doing is spreading out the same volume over a longer time. In some circumstances, if the peak was moved to the right time, you could actually be making the overall system worse.

I’m sympathetic to environmental causes, and I think I can still be objective and open minded on this topic. It seems to me our energy might be better spent elsewhere besides these basins, such regional facilities for large storm flood control and micro-measures for water quality improvement.

I'd really appreciate your thoughts and ideas on this.
Thanks in advance.

 

[WILDWINGS27]

I do agree that sometimes we are better off to require a cash donation to improve or develop regional detention basins instead of having numerous small basins with unmaintainable restrictor sizes.

 

[francesca]

I think the argument that they do very little and could do harm is a belly ache from people who long for the glory days of not having to do anything. Small detention may not be a substitute for regional detention and/or infiltration, but at the very least they protect the small local drainage channels and reduce the size of necessary regional detention facilities.

I wonder how long it will be before regional drainage models are required for developments, like regional traffic models that look at impacts beyond the immediate limits of the site.

 

[psmart]

As stated in the OP, the common requirement of maintaining the pre-existing peak flow is usually accompanied by a significant increase in volume. This causes an increase in peak flow further downstream, as the broader (developed) peaks from multiple sites are added together.

So we certainly need to do more to look beyond the immediate site impacts - but this doesn't mean we should give up on on-site management. Even if regional facilities were more cost-effective for peak management, they would still be pushing the problem of volume increase further downstream. The only way to handle this is on-site infiltration. And the ultimate goal would be no net change in the discharge hydrograph.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Maury]

My opinion: If the choice is between regional detention and small on-site detention, than regional is clearly a better option. If the choice is between smaal on-site detention and no detention at all, then the on-site is better. Spreading out the runoff over a longer time is worthwhile and it rarely does more harm than good.

 

[cvg]

I would agree that providing localized retention does more good than harm. It provides some measure of sediment control and it protects from the vast majority of smaller, more frequent storms. It may or may not result in increase in peak flows at some point downstream. Of course, even regional basins if not analyzed under every scenario could cause an increase in peak flow downstream if all the peaks from all the regional basins happen to coincide. In my opinion, the frequency of this set of events occuring would far exceed the base 100-year flood frequency.

 

[RWF7437]

Another 2 cents worth:

Regional detention, planned, operated and maintained by a public agency but paid for by the beneficiaries ( that's all of us )is the fairest way to go for the reasons pointed out by others.

To draw a parallel, Cities do not require on-site wastewater disposal and usually, in fact, prohibit it. This is done not only for public health reasons but also for economic reasons. It simply doesn't make economic sense to build multiple small mud puddles whose performance cannot be controlled or even predicted. These puddles not only do not work, are costly, and result in endless review obstacles, they create liability and maintenance problems for the private property owner who has no choice but to build them.

Been arguing this for thirty years. I'm tired now. I think I'll go do something useful. If you want my full rant visit

http://www.PDHOnline.com

and view "Eclectic Hydrology".

 

[tsgrue]

.

This response is somewhat oversimplified, but...

I cannot state strongly enough that, almost without exception (probably 99.99%+ of the situations), the closer you manage stormwater to the location at which precipitation reaches a surface, the better stormwater management you will achieve - locally and regionally. This can be accomplished in an extremely low-cost and sustainable manner, actually reducing the cost of many (or most) real estate development projects relative designs which focus on conveyance.

Such is fundamental to mimicking the predevelopment watershed response from a site. Also, retention - as opposed to detention - is needed to mimic the predevelopment watershed response and should be the primary focus. Detention should be used only when retention is not feasible. Measures that includes retention as feasible are also lower cost than systems that don't include retention. Detention basins that *sustain* an elevated postdevelopment runoff flow rate (relative to predevelopment) can be worse than no detention at all relative to stream erosion and downstream flooding depending upon runoff flow timing.

Full stormwater management would address the following:

1. Quantity: Postdevelopment total quantity (mass, volume) of stormwater infiltrating the soil surface at the site, evapotranspirating from the site, and flowing from the site by way of surface runoff (eg, open channel flow, sheet flow, pipe flow) should mimic the predevelopment total quantity (mass, volume) of stormwater infiltrating the soil surface at the site, evapotranspirating from the site, and flowing from the site by way of surface runoff (eg, open channel flow, sheet flow, pipe flow).

2. Character: Postdevelopment character (instantaneous flow rates, site hydrograph shape) of stormwater runoff from the site should mimic the predevelopment character (instantaneous flow rates, site hydrograph shape) of stormwater runoff from the site.

3. Constituents: Postdevelopment instantaneous, event, and total mass of constituents transported from the site by surface flow (stormwater runoff) and subsurface flow (soil water) should mimic the predevelopment instantaneous, event, and total mass of constituents transported from the site by surface flow (stormwater runoff) and subsurface flow (soil water).

I suggest the following documents and W3 sites as a start in investigating this matter.

Design: Low Impact Development Manual (

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_210_10.pdf

Interim Code of Practice for SUDS

http://www.ciria.org/suds/icop.htm

Low Impact Development Technical Manual
(

http://www.psat.wa.gov/Publications/LID_tech_manual05/lid_index.htm)

Sustainable Urban Drainage Systems Network

http://sudsnet.abertay.ac.uk

The Practice of Low Impact Development

http://www.huduser.org/Publications/PDF/practLowImpctDevel.pdf

Water Sensitive Urban Design

http://www.wsud.org

Water Sensitive Urban Design Guidelines for the City of Knox

http://www.knox.vic.gov.au/Files/KnoxCityCouncilWSUDGuidelines.pdf

Low Impact Development (LID), Water Sensitive Urban Design (WSUD), Green Coving (GC), Minimal Impact Development (MID), Sustainable Urban Design Systems (SUDS), and various other terms are generally slight variations on the same theme: "buzz" words/phrases for marketing a stormwater management approach intended to be sustainable.

.


tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[francesca]

If you retain for a no net increase in volume discharge and therefore succeed in infiltrating an equal volume as in the pre-developed case, but have substantially increased your impervious surface, are you not making your ground more saturated in the pre-developed case because you have reduced evaporation and transpiration?

In a karst environment, retention is easy through the use of Class V injection wells, but what are the long-term consequences of possibly elevating the ground water as more water is injected lower down the soil horizon?

Also, how do you handle retention so that you are not retaining all of a small, frequent storm, but are retaining enough in a large, infrequent storm? Remember that there is also an onus not to decrease runoff in the post-developed condition. I suppose you could have a small detention basin with a weir that discharges into a large retention basin, and that retention basin could be used as a sports field or playground.

 

[655321]

Thanks for all your replies. RWF7437, I couldn't agree with you more, and I think your analogy to wastewater treatment is perfect.

What do you think of a policy that would address small storms on a micro lever, and the large events regionally? And how big do you think an area needs to before it qualifies as regional?

Also, wouldn't an outfall structure designed to detain a 2 year (or bigger) event allow the vast majority of storms to pass uninhibited through the basin? After all, a 2-year storm is still a big rain.

 

[cvg]

I agree with both tsgrue and rwf7437 - Regional detention and local both have their place.

Regional basins can be very effective for reducing large, off-site flows. They can be built cheaply if done before development pressure begins. However, they are large, expensive projects and usually require bond issue, flood control district tax base or community facilities district or other such government organization in order to fund the construction. Often, at the slow pace that government moves (and with lack of planning or political leadership), the decision to build the regional basin, channel or storm drain comes well after the development has been completed. This makes it more difficult to acquire the property and to implement a regional system. The planning and funding have to be done well before development occurs. This takes a very proactive government agency to be successful.

Local drainage facilities are smaller and very easily constructed and are generally required by local ordinance as a requirement to obtain a grading and drainage permit. Channels and drains are often unnecessary and flow can be on the surface to the basin. They are thus constructed incrementally with development without any involvement of the government or requirement for special taxing district. Politicians often find it easier to leave the planning and local drainage facility design and construction up to the developers rather than try to raise taxes to protect against a (100-year) flood that most of their constituents feel they will never see in their lifetime...

 

[RWF7437]

653321,

I'll attempt a brief answer to your questions. Bear in mind that some of this is only my opinion.

"What do you think of a policy that would address small storms on a micro lever, and the large events regionally?"

I think that might make some sense. Especially if the local, small devices, whatever they are, are designed to reduce "pollutants". In other words, if they are intended to improve water quality, rather than being sized based on flow or volume in order to control water quantity, as is now the general practice on the US. On this question I tend to agree with tsgrue that treatment at the source makes sense.

"And how big do you think an area needs to before it qualifies as regional?"

The answer to this question should be determined by economics. Thus the answer may be different in different areas of the world. Neil Armstrong has said that all engineering is a search for efficiency. This includes efficient use of resources and resources include money. If you don't care what it costs or who pays you don't need an engineer. In a stormwater system we're trying to balance the costs of conveyance, treatment, damages due to flooding, inconvenience, public safety, environmental effects and intangibles.

"Also, wouldn't an outfall structure designed to detain a 2 year (or bigger) event allow the vast majority of storms to pass uninhibited through the basin? After all, a 2-year storm is still a big rain."

Yes.

Thanks for raising these questions.