Monday, August 15, 2016

How to avoid flash floods?

Another flash flood warning this Sunday night over Baltimore and Ellicott City did not materialize. But one can pretty much assure, another 500 or 1000 year rain burst will take much less time to arrive than its name suggests. What then?

A German State that experienced several unusual floods from atypical tropical rain bursts in recent years even prepared an all new type of flood maps that deal not with ponding water but with streams that form in usually dry areas.
Stormwater pond blowout at Burgess Station
(Photo ArchPlan Inc.)

While those maps may become useful for planners and insurances in areas that are new to such floods those maps can't protect assets that are already on the ground and new maps aren't of much help in places that are aware of the risk already, such as Ellicott City.

While the type of flooding changed even in a flood-experienced place like Ellicott City (water coming into buildings from down the hill and not backing up from below), what is needed is a consensus on how the existing assets can be protected or how the to be expected torrential downpours of the future can be tamed.

The SUN's expose on the flood in yesterday's paper shows through the various statements an astounding level of misconceptions, misinformation and a profound lack of a consensus on how to move forward.

Several statements in the article stand out:
  • Even if everything were still wooded, 6" of rain in two hours would have caused flooding
  • Howard County officials insist that development by itself cannot be blamed for a dramatic act of nature
  • County officials say older developments built decades ago have more significant problems with runoff. In fact, some newer redevelopments improve antiquated stormwater management systems because any project built after 1985 is subject to runoff restrictions.
    Kids playing in the drained pond at Burgess Station
    The figures provide a sense of the depth and size of the
    retention pond that cascaded downhill
    (Photo ArchPlan Inc.)
  • Limiting development in Ellicott City isn't necessary. Redeveloping older properties is one of the solutions to the runoff problem. (Building industry lobbyist Maloney)
  • 28%of the Hudson and Tiber tributary watersheds were impervious in 2006 and are estimated to be 32% now
  • Before 2011, it had been nearly 60 years since runoff from the Tiber stream produced a flash flood. But in the past five years, destructive flash floods have hit twice: Tropical Storm Lee in 2011 and last month's sudden thunderstorm.
  • There is nothing we can do to stop 6" of rain but we can have impacts on other storms (Kittelman)
  • County officials considered expensive remedies, such as multimillion-dollar stormwater storage drains at the top of the watershed, but determined that the cost wasn't worth the benefit "It was a lot of money, and it was only going to improve things by something like 5 percent," (Ulman)
  • We ought to rebuild historic Ellicott City, The question is: How do we rebuild in a smarter and more effective way? (Ulman)

From MD stormwater regulations 

Before going through these various facts and arguments, it may be useful to recap some basic physics:

Stormwater run-off follows gravity, the bigger the mass (quantity of water), the bigger the force
  • Relations between quantity, velocity and destructive force are not always linear, i.e. small incremental change near the heads of flow-lines can have compounding effects further down.
  • Of approximately 526 acres of neighborhoods that were assessed by field crews of the watershed study of the Patapsco Heritage Greenway group in 2013, sixty percent (310 acres) of area had no apparent stormwater treatment. Twenty percent (~100 acres) of that  had an impervious cover, letting stormwater run off in full and uncontrolled. 

- As for the argument that the extreme rain is something one cannot prepare for:

True, even a natural wooded area would eventually become saturated and produce runoff in a 5" per hour rain, (convective rain events). But in a wooded or vegetated natural area the entire area will absorb some of the precipitation (depending on soils and saturation) and would not discharge the excess water as a point source like a stomwater pond overflow or many paved areas do. Therefore keeping as manu vegetated natural areas is helpful and the statement that one can't do anything about those events is wrong.
Potential flooding areas in MD risk assessment map for Ellicott City

- Regarding the argument that new development is good because it improves stormwater management over old development that may not have any management:

That argument works only if new development replaces old one and only for storms that fall into the category of 100 year storms. New development is supposed to retain runoff for storms that are in the 100 year category or less. However, the Maryland stormwater regulations do not pertain to anything bigger than a hundred year rain/flood and allow discharge of the water in excess of those storms even in designated flood areas. Thus in that 6" in two hours rain all new developments would have filled their ponds and then let all additional water run off. So they certainly contributed to the floods recently experienced, especially in the case of Burgess Station where the entire pond failed and released about 6' feet of water down into the Hudson tributary.

- About the apparent increased frequency of convection rain events

This isn't just anecdotal but matches the forecast from climate researchers and is explained by higher temperatures allowing higher moisture content and more frequent convective rain. Moderate weather areas such as regions in Germany noted before also report those weather events with increased frequency.

- Retreat, upstream intervention or fortification?

Doing nothing upstream could be justified by abandoning historic Ellicott City downstream, a drastic action that some letter writers, indeed, promoted in the Baltimore SUN. 
Certainly, retreat is one of three options in building resiliency. The traditionally preferred option, fortification, doesn't appear to be very practical in the case of the historic town. Whatever dams, floodgates and elevated structures fortification would require, those measures would almost all run headlong into the core of what makes the town valuable in the first place, historic preservation. 
When the street becomes a fast moving river
The other remaining option is more subtle and requires intelligently working with nature rather than against it. Doable things downstream could include improvements on the flow capacity of Tiger and Hudson and a managed way of how they would overflow that would be less destructive. However, to take the destructive top off  "the once in a millenium flood" like the one from two weeks ago, requires to reduce the volume and velocity of the water converging on the town through many and varied measures.

There are several ways how volumes can be reduced: Less new development, retrofitting old development and increasing the holding capacity of stormwater facilities designed for just 100 year events. 

The incremental differences  may be in the 5% range for each measure individually. Cumulatively, those improvements may take the wrath out of a 5" per hour rain. Yes, there would still be flooding but incremental differences could be the all deciding differentiator between a catastrophic flood or just a flood.
destruction of historic buildings after the recent flood

While old Ellicott City rebuilds, preparation for the next mega rain has to occur now. Various action plans identify already where to start.

Klaus Philipsen,  FAIA

Related articles on this blog:

Patapsco Heritage Greenway: Hudson-Tiber watershed restoration plan

1 comment:

  1. I appreciate the information on how to avoid flash flooding. I agree that one of the best ways to avoid flash flooding is to create a good storm water drainage system, this can make the difference in a storm and can really help reduce flooding. I would imagine that most flooding inside a city happens because the water has nowhere else to go and so it just builds up.