Tuesday, September 13, 2016

Tough choices for Ellicott City

  • To catch the water volume that makes the difference between a 10 year and a 100 year flood event in the Hudson Branch watershed alone would require water storage akin to 33 football fields 6' deep.
  • If the entire watershed were wooded the water arriving at Main Street would be cut almost in half. 
  • If all 23 stormwater ponds in the watershed were not there it would make only an 18% difference for the 10 year storm and 10% in a 100 year storm. 
  • Fixing two culverts under Main Street and Frederick road at Ellicott Mills and Rogers Avenue could reduce local flooding in those local areas. (improved conveyance)
  • An additional 48" storm drain under Main Street all the way to the Patapsco would make only minimal difference, though.

Councilman Jon Weinstein explains his bill
This along with terms like gauge-calibration, Standard TR-20, back-watering, one and two-dimensional modelling, conveyance, scouring and velocity was swirling around in the council's meeting room on 3430 Courthouse Drive, the seat of the Howard County Government in Ellicott City. The government complex itself is an example of excessive pavement in the watershed that contributed to the flood.

In the room: The Ellicott City Flood Workgroup, 14 mostly white men hand-picked by Executive Kent Kittleman through executive order after the flooding that occurred as a result of Tropical Storm Lee in 2013. The group would sunset in November 2016 were it not for the wake-up call of the much worse flood that occurred this July.

As a result of that earlier storm (another "100 year event") the County had commissioned a comprehensive hydraulic study from the engineering firm McCormick Taylor. At yesterday's meeting the results of that study were explained in detail by the expert engineers and the results were not encouraging. There is neither a silver bullet of one heroic project that would solve all problems, nor is the continuation of business as usual an option.

To prevent business as usual Councilman Jon Weinstein explained his bill 65-2016 which would impose a nine month moratorium on all development in the watershed of the Tiber and Hudson. He reported that his first problem was a clear definition of the watershed from various conflicting maps . He was very relieved to find that group members had prepared a, what they considered accurate map of the watershed, which graced a wall of the meeting room. The moratorium would stop all development review except for those already in the pipeline including public works projects except if they are part of the recovery efforts. "Calamity repairs" or personal property improvements are also exempted.
Modelling the Tropical Storm Lee was part of the
McCormick Taylor study. This map shows the
flooding modeled for Lee.

Under chairwoman Debbie Slack Katz and guidance from Deputy Director of Planning Raj Kudchadkar the group debated its future (it wants to stay together after November), its relation with the parallel Citizen Advisory Council (can there be a merger?), the council bill (support) and what kind of storms the group wants to prepare for. ("Are we planning for another 1000 year storm?").

How much is a 100 year storm? In Maryland's piedmont around 7" of rain in 24 hours. 
The storm that hit Ellicott City recently brought just under 6" in only two hours. The tropical storm Lee in 2013 dropped 4.89" in two hours. The ratings of storms are based on rain in 24 hours.

The effects of rain used to be measured simply by flooding (i.e. backwater from the Patapsco) and mapped in FEMA flood maps. Lee and the recent storm showed that this isn't enough any longer.

Now the talk is about flash-floods, little branch streams turning into raging rivers that go over the banks and find other ways to rush downhill. The most obvious route: Main Street Ellicott City. The damage now occurs far away from the Patapsco which is shown as the biggest source of risk on traditional flood maps and the damage is not that things get wet and muddy: Whole walls and foundations got washed away by the strong currents, trees, cars and debris clogged culverts and channels.

Engineer Chris Brooks from McCormick Taylor admitted that the regulations are trying to catch up with the changing realities. (The to be assumed 100 year storm 24 hr rainfalls were recently slightly increased). His assignment after Lee was to identify what can be done that is "feasible and practical". Based on the damage of Lee his study was limited to the area from Roger's Avenue to parking lot "D" and addressed only the Hudson watershed of 1.55 square miles.

A new study commissioned by the County now has more urgency: In six months the full watershed all the way to the Patapsco is to be studied, including the Tiber watershed. "Everything is on the table", Brooks said, "we can do anything as long as it doesn't conflict with the laws of physics", it all depends on how much the community is willing to do".

He noted, that a 12' wide and 8' high storm culvert under main street would mathematically be enough added "conveyance" but it would require blasting rock, relocation of many existing utilities, conflicts with archaeological artifacts ("this is Maryland's oldest road") and would possibly back up when the Patapsco would carry high water as well in a more extensive slow large area storm. He also cautioned that the Department of the Environment frowns on inline ponds (dams), that a huge pond upstream at Walmart or at the 29/40 interchange was technically possible but could pose a hazard of another kind with dams more 20' high and above the town and may be too high up in the watershed to catch enough water.

Was he "optimistic that the problem could be solved at all", one member of the work-group asked in desperation. Brooks responded, that this was the most complicated  and difficult case he has encountered in his career (He is relatively young).

The Flood Group will have its hands full beyond November.
Velocities modeled by the two-dimensional flood
model used for a part of the Hudson watershed

It should remember that 50% of the total water volume comes from development, that all current ponds are designed only for 24-hr 100-year storms and release all excess water beyond that as if they didn't exist, that probably in excess of half of the existing development has no stormwater management at all.

Solutions that involve upgrades to existing facilities, additional mandates for development without ponds, a prohibition of additional larger development and fortification of buildings in the valley seem to be in order.

The real culprit in the last flood was the force of the water as a result of velocity and volume. If the force is broken, fortification against relatively still water occasionally rising into buildings should be possible. Regs and the engineering toolkit are very focused on volume. The dynamics and energy of water velocity need to be added to the consideration. No longer is it just flood-maps that matter.

The difference in damage between storm Lee with its 4.89" in two hours and the recent one with its near 6" in two hours proves that an incremental difference of 20% can make a critical difference in outcome after all.  20% is just about the cumulative impact that Brooks attributed to a variety of practical improvements in the watershed.
In hydraulics few things are linear.

Klaus Philipsen, FAIA

From the Maryland Stormwater Manual about Extreme Flood Volume (Qf):
 The intent of the extreme flood criteria is to (a) prevent flood damage from large storm events, (b) maintain the boundaries of the pre development 100-year Federal Emergency Management Agency (FEMA) and/or locally designated floodplain, and (c) protect the physical integrity of BMP control structures. This is typically done in two ways: 100-Year Control: requires storage to attenuate the post development 100-year, 24 hour peak discharge (Qf) to pre development rates. The Qf is the most stringent and expensive level of flood control and is generally not needed if the downstream development is located out of the 100-year floodplain. In many cases, the conveyance system leading to a stormwater structure is designed based on the discharge rate for the ten-year storm (Qp10). In these situations, the conveyance systems may be the limiting hydrologic control. Reserve Ultimate 100-Year Floodplain: 100-year storm control may be required by an appropriate review authority if: • buildings or development are located within the ultimate 100-year floodplain, or • the reviewing authority does not completely control the 100-year floodplain. Hydraulic/hydrologic investigations may be required to demonstrate that downstream roads, bridges and public utilities are adequately protected from the Qf storm. These investigations typically extend to the first downstream tributary of equal or greater drainage area or to any downstream dam, highway, or natural point of restricted stream flow