Tuesday, August 27, 2013

Building Smart on the Riverfront

Plans are underway to develop Norristown's riverfront. I'm not saying they shouldn't. Good development along the river could be a big boon for our economy. But rivers are tricky things.

I took a geography course in college (strictly for a necessary general elective--never thought I'd use what I learned.) One of the topics covered was the dynamics of rivers and what causes floods in certain places. Simply put, as water flows, it takes the path of least resistance, eroding the softer parts of a riverbank. Eventually this is what causes bends in an old river like the Schuylkill. The outer part of each bend is called the cut bank, where the land's being slowly washed away. The inner part is the slip off bank, where the river deposits the sediment from the cut banks upstream.

If there's a big storm, or a weeks' worth of heavy rain, the process speeds up--more land erodes from the cut banks. It can undermine bridges and buildings, or if he cut bank is low, the whole area floods. You could build a flood wall, of course, but the water has to go somewhere, so the slip off bank on the other side of the river will take the brunt and wash away. Sort of like pool balls--bounce them off one side of the table, they'll hit the other side.

On the photo of the Norristown/Bridgeport riverfront above, I've marked the cut and slip off banks. On the upper bank, you can see 2 vacant lots, one to the southeast of the "A" and one right above the lower slip off bank. Both are under consideration for immediate development.

Building on the lower lot might not cause serious problems. That's where the river deposits sediment anyway. The upriver lot is on a cut bank though, so if the development is too close to the river, or uses too many hard surfaces, or a flood wall, the water will cut into the slip off bank on the Bridgeport side. Now, that's where there was a big fire several years back, so at present, there's nothing much on that bank to be damaged by a flood. But, if we have a REALLY big flood, the cut and slip off banks can actually reverse themselves, which would flood that next new development downstream.

The more surfaces that can't erode along the river, the faster the water flows. In a flood, the force created can be as destructive as a power-washer. The next large bend downstream in the river contains a water treatment plant, the supports for the Turnpike bridge, and high-tension electrical wires. If a violent flood hits there, a lot of vital infrastructure is history.

Here's a photo of the riverfront development in Conshohocken. Note how much narrower the river is there now that the banks are developed. I worked in one of the Tower Bridge buildings, on the NW side of Fayette, when Hurricane Floyd came through. Gulf Creek (upper left) is already a fast flowing stream, naturally channeled and fed by runoff from the high hills there. All the extra water that day couldn't get out into the river fast enough because of all the development--there were few surfaces that would "give" and slow down the water. The bridge over Route 23, much of Ballogomingo Road and parts of Route 320 were demolished. It took 3 months and millions of unbudgeted taxpayers dollars to repair.  Lately, with the newest development, the businesses and railroad along the river below Fayette Street flood with every large storm.

So sure, develop along the river in Norristown, but keep the structures and paving back far enough that they won't cause future catastrophies. Leave some open space, with natural grasslands, or a wide forested area--something to act as a buffer when the river rises. A non-paved nature trail off of the existing bike trail would be a great use of this buffer zone, allowing residents to enjoy the riverfront, and wouldn't cost a fortune to reconstruct after each flood.


  1. You also need those buffers of vegetation to absorb run-off from concrete, etc, that would go right into the river if it couldn't be at least partially absorbed by the vegetation.

  2. I just hope the development planning includes people who are willing to think about the situation and problems it might entail as thoroughly as you have, Elena! The high rises on the island near Manayunk have been evacuated several times during high waters, and I don't know what other kinds of problems they may have caused. Norristown could learn from others' mistakes.