user's manual for corhyd: an internal diffuser hydraulics model - IfH

should therefore result in feeder velocities v f,nf > 0.5 m/s (DIN EN 1671, ATV-DVWK-A 110
(2001) and ATV-DVWK-A 116 (2005)). This corresponds to a maximum feeder pipe
diameter of D d,max = (Q nf 8/π) 0.5 ≈ 1,6 Q nf 0.5 . The feeder velocity for the far future design
flowrate Q ff and the same diameter results then in v f,ff = v f,nf Q ff /Q nf . Generally flowrates do
not more than double or triple during the lifetime of an outfall, so far field feeder velocities
are from 1 to 2 m/s, what is clearly acceptable in terms of operational viewpoints considering
the related energy losses.
As the feeder also the diffuser pipe is constraint by a maximum scouring diameter.
Theoretically this would result in a diffuser with different pipe segments as much as risers,
and, under the assumption of a homogeneous discharge distribution, each with a maximal
diameter of D d,max,i = (8Q nf i/(Nπ)) 0.5 , where N denotes the total number of risers and i the
observed pipe section before the i-th riser starting counting offshore. Although best for
sediment removal this solution, also called tapering generally is too expensive to install (about
20 % more expansive than single diameter diffuser) and maintain (i.e. cleaning). Besides the
continuous tapering after one or more branches the only alternative is decreasing the diffuser
diameter as a whole. Thus a simple configuration is achieved, although increased friction
losses and separation losses in the diffuser pipe will increase the total head. Changes of the
diffuser diameter cause only moderate changes in the discharge profile. If tapering is applied
the changes in the discharge profile are even smaller than in the case of changing the diameter
generally.
By applying different diameters for CorHyd calculations it has to be considered, that pipes are
not available in all sizes and only diameters are applied which are given as internal diameters
in catalogues of pipe producers.
4.4 Port / Riser configurations
Instead of typing in ports or risers one by one the concept of port/riser groups was used (Delft
Hydraulics, 1995) for easy and fast data input. The user should try to use as less groups as
possible but as much as necessary to achieve optimized design.
The total number of different groups is N g . For each port/riser group the number of used risers
N gp and the location on the diffuser pipe section has to be given. E.g. group number one
consist of N gp = 15 risers each of them with the same specific port/riser configuration and is
mounted along the pipe section number one. Details of the parameter definitions are
visualized in Fig. 15. The next input denotes the spacing L g between each group and the
spacing S between each riser in one group (often both are the same). It follows the input of
the port elevation L r above the diffuser centerline (necessary for calculating the external
pressure at the outlets). If no risers are applied the value should be zero. It follows the input
for the port and riser diameters D r , D p and the roughness k s,r . If no risers are applied riser
diameter and roughness should be zero. If more than one port is located at one position or at
one riser the number of ports N p has to be given. If ports consist of little attached pipes their
length L p and related roughness k s,p should be given.
A 50 mm minimum port size for secondary- or tertiary-level treated effluent and storm water
inflow to the sewage system was suggested by Wilkinson and Wareham (1996) for avoiding
the risk of blockage. Furthermore a minimum port size of 70 to 100 mm for primary treatment
plants (just screening and settling tank). The maximum port diameter should generally be
smaller than the diffuser pipeline diameter D d at upstream position to achieve higher
discharge velocities and avoid saltwater intrusion during low flows. The riser diameter D r
Institut für Hydromechanik, Universität Karlsruhe 39