More crossection types can be implemented on demand

[smultsch]

Hello,

would be great to have a "TrapezialReachType" for simulating stream flow in addition to existing types as listed in the tutorial:

"If you create a reach for the project, you must give the crossectional geometry of the river. The different basic geometries are:
TriangularReach - T, a river with a triangular crosssection
SWATReachType - S, a river with a trapezoidal crossection, with a flat flood plain, as in the model SWAT
RectangularReach - R, a rectangular crosssection
PipeReach - P, a circular pipe
More crossection types can be implemented on demand."

Please find attached some examples of stream types which I am trying to simulate. The wet surface of the stream is a sensitive parameter I am interested in. The existing channel geometries are not sufficient to represent all of them and result in unrealistic estimates of the wet surface.

Cheers,
Sebastian

[philippkraft]

While the reach geometry has of course some influence on the transport equation, I would deem the parameter uncertainty of the Manning n as the greater problem. From that perspective I would use for the picture on top SWATReachType (which is a trapezoid crosssection + bank overflow mechanism), the 2nd a TriangularReach, assuming bank overflow never happens (else use SWATReachType again) and the 3rd a RectangularReach.

However, I have thought a long time ago about a generic reach type using a vector of coordinates for the functions P(d) and A(d). The ideas are conserved in the end of ReachType.h:488-503. But I stopped there, because this was not important for me.

I do not think that we gain anything by yet another special reach type - lets rather go for a generalized formulation. I do not have the time to do the math, but I can translate a Python prototype into cmf's C++ code.

[philippkraft]

If someone provides the Python prototype, please use the following (Python 3.5+) template:

class CrossSectionReach:
    
    def __init__(self, n_manning: float, x: Sequence[float], depth: Sequence[float]):
        self.x = x
        self.depth = depth
        self.n_manning = n_manning
        
    def set_nManning(self, val: float) -> None:
        self.nManning = val
    
    def get_nManning(self) -> float:
        return self.nManning
    
    def typecode(self) -> str:
        return 'C'
    
    def get_channel_width(self, depth: float) -> float:
        """
        Calculates the flow width from a given actual depth [m] using the actual geometry
        :param depth: Actual depth in m
        :return: Actual channel width in m 
        """
        ...

    def get_wetted_perimeter(self, depth: float) -> float:
        """
        Calculates the wetted perimeter from a given actual depth [m] using the actual geometry
        :return: 
        """
        ...
    
    def get_depth(self, area: float) -> float:
        """
        Calculates the actual depth of the reach from the wetted area using the geometry
        :param area: The actual wetted area in m², can be obtained by V/l, 
                     where V is the stored volume and l is the reach length
        :return: the actual depth in m
        """
        ...        
    
    def get_flux_cross_section(self, depth: float) -> float:
        """
        Calculates the wetted area from a given depth using the IChannel geometry.
        :param depth: depth of the reach m
        :return: Wetted area of a river cross section in m²
        """
        ...

[smultsch]

Ok, thanks. A generic reach type would be very interesting, but is not necessary for my current work. So, in order to have trapezoid cross-section I can just use the SWATReachType and disable the flood plain (e.g. by setting FloodPlainSlope=90%)?

[philippkraft]

Just make a huuuuge channel depth, eg. 20m

[smultsch]

Thanks, this solution seems to be working. But it is a little bit tricky to figure out what parameters of the SWATReachType must be set by the user and which one are calculated by CMF. For example, in case of a trapezial shape the user must initialize the reach with "FlowWidht,BankWidth,ChannelDepth" and additionally set the BottomWidth by accessing the related property. Maybe this could be clarified in future.

reachshape = cmf.SWATReachType(FlowWidht,BankWidth,ChannelDepth)
reachshape.BottomWidth = 0.3

test_reachshapes.zip