Roadside Ditch
Roadside Ditch
(OP)
Hello, i'm doing a review of runoff calculations & noticed that the applicant hasn't set up the model as they described the actual conditions, and i've been asked how the shoulda/coulda done it. there are a series of subcats that flow to a continuous roadside ditch that has culverts and they've delineated the subcats from the culverts, so that each subcat flows to a culvert.
as the ditch is (presumably) continuous, and they have not described different, whatever can't flow through each culvert should cascade downstream through the ditch - this is how their Drainage Diagram is set up for routing, as dashed lines connect the nodes, but their output shows zero flow from the Secondary Outflow for all storms and each culvert, and impossibly high flows through the roadside culverts (54 cfs for an 18-inch CMP?! c'mon...)
please note that they did not set up the culverts as catch basins, and their output reports don't include details on how they set up any sort of storage, which they must have, otherwise HCAD won't let you save a pond node, right?
Anyway, at each design point (culvert), I think they should:
1) define the culvert as the primary outlet;
2) define a trapezoidal weir as secondary outlet at the same or slightly
higher invert as the culvert (the weir having the same geometry as the ditch); and
3) provide for overflow along the road through a broad-crested weir as tertiary outlet routed to same place as the culvert (it's a small ditch).
they also should use reaches in-between the culverts to represent the roadside ditch.
sound reasonable? not having done this particular physical setup before, i'd appreciate input. thanks in advance!
as the ditch is (presumably) continuous, and they have not described different, whatever can't flow through each culvert should cascade downstream through the ditch - this is how their Drainage Diagram is set up for routing, as dashed lines connect the nodes, but their output shows zero flow from the Secondary Outflow for all storms and each culvert, and impossibly high flows through the roadside culverts (54 cfs for an 18-inch CMP?! c'mon...)
please note that they did not set up the culverts as catch basins, and their output reports don't include details on how they set up any sort of storage, which they must have, otherwise HCAD won't let you save a pond node, right?
Anyway, at each design point (culvert), I think they should:
1) define the culvert as the primary outlet;
2) define a trapezoidal weir as secondary outlet at the same or slightly
higher invert as the culvert (the weir having the same geometry as the ditch); and
3) provide for overflow along the road through a broad-crested weir as tertiary outlet routed to same place as the culvert (it's a small ditch).
they also should use reaches in-between the culverts to represent the roadside ditch.
sound reasonable? not having done this particular physical setup before, i'd appreciate input. thanks in advance!





RE: Roadside Ditch
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Roadside Ditch
I’ve never seen a model that shows a secondary outlet connected to another node if no secondary outlet is defined… the option to connect the secondary outlet to another node doesn’t become available until a secondary outlet is defined. I was able to duplicate their setup in HydroCAD by defining a secondary outlet, routing it to another node, and then deleting the secondary outlet definition – the secondary outlet remained routed to the node that I chose despite having been un-defined in the program.
So i think they deleted the secondary outlet definition after routing it, or set the multiplier to zero like you said.
regardless, did i describe modeling the ditch/culverts correctly? i'd use CB's instead of ponds...
RE: Roadside Ditch
As for the ditches and culverts, I'm not really clear if the culverts are in the main ditch line or on the contributing branches.
Too much speculation here - I really need to see the HydroCAD project file in order to have an informed comment.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Roadside Ditch
my basic question isn't specific to their project - i've often thought about how to model the continuous ditch/intermittent culvert situation in HCAD, and know how i'd approach it, just haven't had a project to do so yet.
RE: Roadside Ditch
If they have that much flow going through an 18 inch culvert, they've probably got the culverts set at wrong (very low) elevations in their model, compared to the elevation of the ditch. And if that's the case, the water's all falling down an imaginary hole and going through the culverts, instead of some of it going further down the ditch.
You should be able to tell this from the output file, I would think, if they provided a complete printout of the routing calcs.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Roadside Ditch
Beej, the problem is that, even though they appear to recognize that water would continue down the ditch, they don't define ANY outlets at the ditch culverts. as a result, they have, for example, 88 CFS being somehow taken care of through an 18" culvert, with no overtopping of the road and no flow continuing down the ditch to the next node THAT THE SECONDARY OUTFLOW IS GRAPHICALLY CONNECTED TO! (sorry for yelling ;)
i have to explain the situation to the atty's & want to be able to explain how it arguably should be done otherwise - i'm just trying to troubleshoot my technique. we may even get the chance (i.e. be paid to :) to reconstruct the model & play around with it to see what effect it has.
RE: Roadside Ditch
You can always contact HydroCAD support with any questions.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Roadside Ditch
* i still don't see how to do that without defining one, routing it, then removing it when editing the node...
RE: Roadside Ditch
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Roadside Ditch
road & ditch are 5 to 10% grade. distances between culverts range from ~300 to 700 ft. i don't believe there are check dams or submerged inlets to encourage water to the culverts.
i think i understand why they did it so simply - their design points end there (along the road), and because they've shown that peak flows will be attenuated to each point, they (obviously) don't feel the need to address the predicted flooding
i'm wondering how the hydrographs add up when properly routed, though... it's my contention that they can't do a valid pre- / post- comparison until the routing is set up correctly to model the time-varying response of the downstream nodes that should be receiving flows that the upstream culverts can't get rid of.
RE: Roadside Ditch
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Roadside Ditch
RE: Roadside Ditch
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Roadside Ditch