Wednesday, August 3, 2016

What nearly wiped out Ellicott City and what should be done about it

Ellicott City at Patapsco after Tropical Storm Agnes 1972 (SUN Paper)
I always was in awe when I walked along the banks of the Patapsco River and the remnants of the little mill town Daniels came into view that the river wiped out in the wake of tropical storm Agnes (luckily it had already been condemned and was vacated).
Agnes turned River Road into a trail and was never opened to traffic anymore, here and there lurked a rusting auto wreck in the brush.

Before the Grist Mill trail was constructed a few years back, one could also spot an axle from a rail-car that had once toppled off the rails high above the river; the raging river had washed out the track-bed. The Patapsco rose high enough to reach Caplan's department store in Ellicott City and destroy the bridge into town. Such was the power of the river.

The streams feeding into the Patapsco (historic map)
This time was different: the danger for Ellicott City came not from the river below but from the hills above. The AME church on Main Street located west and above the old fire house is way above and far away from any water that ever came from the Patapsco. Still, the church lost its entire embankment on Saturday night.

Each of the tributaries that feed into the little Tiber stream that runs parallel to Main Street had already become a raging torrent before reaching the Tiber. That totally overwhelmed its riverbed which is partly covered up by buildings. So the Tiber began running down Main Street instead, turning the picturesque historic street into a mountain river which carried mud, logs and cars into storefronts and finally inevitably into the raging Patapsco.

What caused this and what should be done to avoid it in the future?
AME Church on upper Main Street
(Google Streetview)

From what is known to date, the causes were
  • a stalling weather-front that carried copious amounts of moisture and let out more water in one hour than any previous rain event in the area. That may or may not have to do with climate change; at any rate, it is suspicious that two storms that, according to past statistics, would be 100-year storms happened within a few years. 
  • The other contributing factor is development. Denuding, shelving and paving over the steep slopes around Ellicott City is an ongoing development pattern fueled by the incredible growth that the fastest growing jurisdiction in Maryland is experiencing. 
  • Even the most progressive stormwater management techniques don't deal with 5" of rain in one hour and will overflow, doing nothing to protect the valleys below. Although a forested slope would release some of an extreme downpour as well, it wouldn't be nearly as much.
  • Stormwater pond at Burgess Station: Blown out (Photo: Philipsen)
  • Lastly, there is circumstantial evidence that at least one totally failing stormwater retention pond released all its content in a sudden burst into the creek about 30' below. That may have contributed to the wash-out of the AME church and/or created some type of flood wave with a surge of energy all the way down Main Street
Marsha McLaughlin, the former Howard County Planning Director suggested that Ellicott City shouldn't be rebuilt as it was. 

Rebuilding was precisely, what the County Exec Kittelmann had vowed in the first hours after the destruction. 

Not rebuilding right into the danger zone is normally wise. However, in this case, this would mean abandoning all of historic Ellicott City. This flood endangered almost the entire town.
The matter had been on the radar for some time. Early 2015 County Exec Kittelman convened a Historic Ellicott City Flood Work Group which published a report in November of last year. Its final words show that while accurate, the efforts need to be ramped up far beyond what was included as short and mid-term action.
Climate change could make future flooding more frequent and larger in scope. Development in the watershed has contributed to the flooding danger and this needs to be considered if proposals for new development occur. We hope this report will help prevent loss of life and damage to property in the place where we live, work and recreate, Historic Ellicott City. (Work Group Report)
location of the pond at Burgess Station (Graphic Elliot Plack)
Likely the best solution for protecting old Ellicott City from future floods resides above the town, near the top of the valleys leading into the town center, in many decentralized places that either prevent or catch run-off long before it gets destructive. A sustainable effective solution is less likely to be one or two heroic structures in the low areas such as under the town's parking lots D or F as suggested in the report. 

Contours and pond location at Burgess Station (Graphic Elliot Plack)

Facilities need to be constructed that are large enough to hold those massive rain bursts long enough to avoid destruction and overflows. Since so much of the area is developed, those structures would probably have to be underground. These measures are not rocket science and have been used and refined around the world, either as surface measures such as dams, retention ponds or water parks (Rotterdam) or as underground measures in cavernous underground storage tanks or large diameter pipes.
There appears to be little that can be done along Main Street itself except for protection against water backing up from the Patapsco. 

Klaus Philipsen, FAIA

AME Church after the storm: Embankment washed away (Photo Philipsen)
Stabilization measures taken on Sunday to rebuild the embankment with rock (Photo: Philipsen)

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