Question for those who know something about river and stream flow and flooding...

[Voyager]

I have a question for you guys. In this post are two photos of the Monocacy Creek in Bethlehem, PA. The first one is depictiing the normal conditions and normal stream flow in the location. The second one was taken during the Hurricane Irene flooding in 2011. What I want to know is if removing the (man made) waterfall would have prevented (or minimized) the flooding that occured in the location.

 

 

[vogelhund]

If you were to remove the railroad grade, that would probably help.

[Voyager]

Thanks. My thinking is that first off, the channel is too narrow beyond the waterfall. Other than that, I believe that if the waterfall, and in actuality, it was at one time some sort of dam, but years of sediment have built the creekbed up level to the top of the concrete, were to cause a flooding issue, it would be upstream and not downstream.

 

Correct me if I'm wrong.

[ice1972]

I have a question for you guys. In this post are two photos of the Monocacy Creek in Bethlehem, PA. The first one is depictiing the normal conditions and normal stream flow in the location. The second one was taken during the Hurricane Irene flooding in 2011. What I want to know is if removing the (man made) waterfall would have prevented (or minimized) the flooding that occured in the location.

 

1185682_578987475490916_1300713105_n.jpg

 

9940242-standard.jpg

 

Short answer:  Probably not.

 

Based on your pictures I notice that the bad flooding seems to be associated with the region just downstream of the dam.  The actual rise in water surface elevation upstream is not so bad compared with the rise downstream (just look at the stone wall).  Obviously the same flow rate is in play across the dam so the question is what is causing the flooding downstream and not upstream (granted lacking more detail I'm assuming minimal flooding upstream).  The answer is that some downstream control is in play that doesn't translate upstream at the dam.  I looked on Google Maps to see what was downstream.  The first downstream hydraulic structure in play is the Ohio Road bridge - a double arch structure that looks like under normal flows may already be too high to allow higher flows through.

 

Question 1:  Do you know if there was significant flooding on the downstream side of this bridge?  If not then the bridge is the culprit.  If there was then we have to look further downstream.

 

Further downstream you have the Main Street bridge crossing and the West Lehigh Street bridge crossing.  Did any of these crossings suffer problems during Irene?  Any one of these crossings could cause problems if the available room for water to flow through is restricted.

 

Question 2:  Was the Lehigh River way up?

 

Since the Monocacy Creek discharges into the Lehigh River then the stage in the creek will rise as the stage in the river rises.

 

After looking on Google Maps it seems there is some control structures right down by the Lehigh River where flow is diverted into the Lehigh Canal - but I can't be sure of that.  That's an interesting system of hydraulic control structures whose behavior is complex as all the flows come downstream on the Lehigh River and its tributaries (i.e. The Monocacy Creek).

 

Of course Irene was an exceptional situation and its possible that the flooding experienced in those pictures may not have ever happened before.  You just can't design stuff for extreme events - its hard to anyway given the cost.

 

Anyway, hope that helps.

[ice1972]

I should clarify:  Influences that can translate upstream are more likely if the channel slope is not steep.  If there is a significant drop from the location of your pictures out to the Lehigh River or to those other crossings I mentioned then its more likely that channel geometry (width and depth) and roughness play a larger role.  These things are complex and require significant study to determine actual causes of flooding and to determine what improvements can be made at a reasonable cost that have the biggest bang for the buck.

[Voyager]

Wow, there is so much to answer in this post that it's going to take time, but a big thank you for your well thought out answers and research, Ice!

 

In your first paragraph, your think is pretty much in tune with mine. I was thinking that flow rate is flow rate, whether the waterfall (dam) is in place or not.To answer your question about upstream flooding, it's been miniumal, except during rare events like Irene. Union Blvd, the most major road with a low bridge crossing, has rarely been affected by floodwaters. Mostly, a low lying strech of Conestoga Street, which is to the right of the railroad tracks in the photos, is the first to go under water, and that is only about a 200 foot section of the road. The Ohio Road bridge may have an effeect, but I think it's minimal.

 

If you look in the first photo with the normal stream flow, there is an open stairway in the wall just downstream of the falls. I have personally seen water flowing through that opening even when the stone wall is not breached. Also, beyond the Ohio Road bridge and the historic structures on the left side, is a low spot in the creek bank that is about 1-3 feet above the normal water level. In this area is a "trough" that was used by the historical Moravian community for "industrial" purposes, the remnants of which can be seen in this photo. http://www.panoramio.com/photo/45745780

 

It's my belief that any high water, once it tops the low bank, is going to back up into the industrial area. These two features, the opening in the stone wall, and the low bank farther downstream, are the big players in the flooding in the area. As for the Main and Lehigh Street bridges, they have never been under water, but some of the lower areas near the Hill to Hill Bridge viaduct have been during exceptional events such as Irene. As for the Lehigh Canal area, the canal actually crosses the creek via a water bridge, so the creek empties into the Lehigh River and not the canal.

 

My reason for this thread and question was I had gotten into a little "argument" with another person on a Facebook page. The city is going to remove the dam because they claim it causes the flooding in the historic industrial area. Clearly, from my own personal observations, having lived in the area for 28 years before moving elsewhere, I feel that it's not going to solve the problem, but this person (as well as the city) does.

[ice1972]

It may very well be the case that more localized high intensity storms like a summer thunderstorm could be causing most of the flooding issues....in this case removal of the dam could be a viable solution.....I suspect that an Irene level event would flood that area dam or no dam......

[GramaxRefugee]

Voyager, try posting the same photos here:

http://www.eng-tips.com/threadminder.cfm?pid=162

I think the readership will enjoy a look at that flood, and you'll get some answers.

 

 

 

For my part, I think that little dam does increase flooding somewhat. (A bit).  It's hard to tell the height of dam from the photos, but it is still an obstacle to flow. No, it should not cause a downstream increase in the typical channel slope found in this area.

[ApacheTrout]

Ice1972 has it right in that the dam is not causing the problem.  The tell is the stone wall adjacent to the dam on the left side of the channel.

 

The flooding is caused by the constriction in the channel downstream of the dam.  The channel is too narrow to allow the flood to pass through, which causes the water elevation to increase to the point where the stone wall that forms the left bank of the channel.  The historic district is serving as the floodplain for the Irene discharge. 

[ice1972]

Ice1972 has it right in that the dam is not causing the problem. The tell is the stone wall adjacent to the dam on the left side of the channel.

The flooding is caused by the constriction in the channel downstream of the dam. The channel is too narrow to allow the flood to pass through, which causes the water elevation to increase to the point where the stone wall that forms the left bank of the channel. The historic district is serving as the floodplain for the Irene discharge.

Stream flow is a very interesting phenomenon in that no two storms will cause the exact same flow/stage characteristics and in fact more localized "flashy" events where it pours over a few square miles only for only 20 min can cause worse flooding than an Irene level event....not saying that happened here without more info...

[SVT450R]

I agree the dam removal would not help much.  After looking at the images again i think i found the main culprit if you look down the narrow channel which is also a contributor you can see a stone/concrete bridge in the background i found a better picture of it and to me the openings look to be not large enough.  They seem to be way undersize compared to the amount of flow capable of coming down the river which probably adds to the back up.

http://www.panoramio.com/photo_explorer#user=971344&with_photo_id=19165585&order=date_desc