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Channel System                                                          


Aerial 2007, Google                         

This segment of channel is part of a much larger highly urbanized watershed.  Runoff from more than 15 square miles drains through this reach.  Not too many years ago, a large corrugated metal plate arch formed an underground tunnel between nodes C and P.  The pipe had corroded severely over the years and eventually collapsed between nodes C and F during a major storm.  It had to be replaced under emergency conditions with a sheet pile system.  Later, as part of a river restoration project, the tunnel was completely removed and replaced with the open channel system shown above.  A combination of spillways and grade breaks, sheet piles, gabion and riprap lining, and earthen sections with wetland plantings are used in this high energy section of the watershed.      

 

Selected Photographs

              

                   From E to F                    From G to H                   From H to G                   From L to K

 

              

                    From M to N                    From O to P                   From P to O                   From P to Q

 

 

ICPR Modeling Considerations

 

The yellow circles in the topmost photograph indicate node locations along the channel.  Runoff hydrographs are assigned to these nodes and ICPR then routes the hydrographs through the drainage system, calculating stages and flows throughout the storm event.  Comments regarding specific modeling issues related to this system, including proposed link types, are provided below.

 

From Node To Node

Comment

A B

A "channel" link should be used to connect these two nodes together.  It is the approach to the spillway between nodes B and C.  


B C

This spillway is actually the original entrance to the tunnel system.  A weir controls water levels in the upstream channel and then a spillway drops approximately 4-5-feet onto a concrete apron.  The original headwall to former tunnel system was left in place immediately downstream of the spillway because it supports a water main located on top of it.  Therefore, after water drops over the spillway, it must pass through an arch opening (approx. 2 feet in length) before moving into the sheet piled open channel system.

A "drop structure" link should be used to connect node B to node C.  A drop structure in ICPR is a weir in series with a pipe.  In this case, the pipe would have an arch geometry with a pipe length of only 2 feet.  This would restrict the cross sectional area beyond the spillway to the original tunnel opening size.  In essence, the opening acts similar to a very large orifice.


C D

Vertical sheet piles are used along this segment of channel forming a rectangular cross section.  There is cross bracing at the top to stabilize the sheet piling.

A "channel" link should be used to connect node C to D.  Either a trapezoidal cross section with zero side slopes could be used or an irregular cross section.  Manning's n could be adjusted to account for the cross bracing.


D E

This roadway crossing could be modeled as either a "bridge" link or a "pipe" (culvert) link.


E F

A "channel" link would be used to connect these two nodes, similar to link C-D.


F G

This roadway crossing could be modeled as either a "bridge" link or a "pipe" (culvert) link.


G H

As water exits from under the roadway, the channel rapidly flares out to a wide section and then tapers down again as it approaches node G.

A "channel" link should be used here with irregular cross sections defined at each end of the link.  ICPR allows non-prismatic channel links and consequently can accurately account for expansion and contraction losses. (Note: Many programs similar to ICPR (e.g., SWMM) do not allow non-prismatic channel links between two nodes.) 


H I

There is a grade break (approx. 2') at node H formed by gabions in the bottom of the channel.  Although another node (say, H') could be placed immediately downstream of node H and the two connected together with a weir, it's not necessary because of the way geometry and inverts are specified for channel links in ICPR.

Instead,  node H is connected to node I with a channel link.  The upstream invert elevation of this channel link would be approx. 2' below the downstream invert of link G-H and the drop would automatically be included in the calculations.


I J

A "channel" link would be used to connect these two nodes together.  This reach of the channel system is flatter than other parts and is earthen-lined with selected wetland plantings.


J K

There is a narrow pedestrian bridge crossing the channel between nodes J and k that could influence the hydraulics for major storm events.  Additionally, there is a 2-foot (+/-) grade break directly below the bridge.

Node J could be connected to node K with either a "weir" link or a "bridge" link.  Either would work in this case although the bridge might be a better choice for major storm events.


K L

A "channel" link would be used to connect nodes K and L together.  This is a high energy area and the bottom of the channel and a portion of the side slopes are lined with gabions and riprap.


L M

A "channel" link would be used to connect these two nodes together.  This reach of the channel system is flatter than other parts and is earthen-lined with selected wetland plantings.


M N

There is another grade break (approx. 2') at node M formed by gabions and riprap in the bottom of the channel.  This is similar to the situation at node H and is handled the same way.

Node M is connected to node N with a channel link.  The upstream invert elevation of this channel link would be approx. 2' below the downstream invert of link L-M and the drop would automatically be included in the calculations.


N O

This is a roadway crossing with another grade break below the bridge similar to link J-K.  It can be modeled the same way, with either a bridge link or a weir link.  The bridge link is probably better suited for major storm events.


O P

A gabion lined rectangular channel section is located beyond the roadway crossing N-O.  A "channel" link should be used here and an exit loss should be applied because the channel expands abruptly at node P.


P Q

This channel segment gradually tapers from a very wide cross section to a narrower one at node Q.  A "channel" link should be used here.  Contraction losses due to the narrowing cross section are included in the computations.