Estuary Processes and Managment - River Dynamics and Integrated River Management - page 520

Fig. 8.63 Seawater intrusion into tunneled sewage outfalls; sea water intrudes into the tunnel through the seaward ports
number exceeds unity (Wilkinson, 1984; Yau, 1997). Outfalls are generally designed to ensure that this
criterion is satisfied by a reasonable safety margin. However, at start-up, the tunnel is flooded with sea
water and at low flow period (e.g., during breakdown) sea water may intrude into the outfall. Once this
occurs, a discharge many times that required to prevent intrusion at the diffuser ports is needed to purge
the sea water from the tunnel. In recent years, one-way non-return “Duckbill” valves have increasingly been
used on riser heads as an effective means to prevent sea water intrusion into outfalls.
Hydraulics of duckbill valves —In this section more details about the duckbill valve (DBV) are presented
to better demonstrate the optimal design of diffusers. Duckbill valves can be easily installed on wastewater
effluent diffusers as well as stormwater outfalls and industrial flow systems (Fig. 8.64): ķ a DBV is
manufactured of neoprene flexible elastomer material reinforced with synthetic fabric, much like a car
tire; ĸ it resembles a short piece of rubber hose that has been flattened at one end (the “bill”); Ĺ at the
other end of the valve it is typically clamped onto an existing nozzle port (the “cuff”); ĺ the transition
between the bill and the cuff is termed the “saddle”. Duckbill valves have been increasingly applied to
submarine wastewater outfalls in many countries (Lee et al., 1998; Roberts et al., 1989). A case in point
is the Urmston Road Outfall in Hong Kong—an outfall system consisting of dual 2.6 km long 1.8 m diameter
steel and reinforced concrete submarine pipelines that discharge sewage into water 22 m deep; the last 600 m
of the outfall contains 30 diffusers, each with four DBV discharge ports (Smith-Evans and Dawes, 1996).
Hydraulic characteristics —Conventional diffusers are often fitted with round nozzles or pipes with a
fixed diameter; this is the easiest terminus to build and maintain. However, as the jet velocity in a constant
diameter port varies linearly with discharge, intrusions of salt-water and sediment can occur at low discharge
flows (Larsen, 1995). In contrast, experiments and theory show that the jet velocity through a duckbill
valve varies nonlinearly with the discharge (Lee et al., 1997). A case with and without duckbill valves is
shown herein to illustrate the effect of the duckbill valves on diffuser hydraulics (Lee et al., 1998). The
most important hydraulic characteristic of this flow device is the relationship between pressure and flow,
the so called “head-discharge” relationship. Figure 8.65(a) shows the computed variation of pressure with
flow; it is seen that the relationship is very linear for all the thicknesses tested. On the other hand,
Fig. 8.65(b) shows that the valve opening area (at the bill) varies nonlinearly with the discharge (Lee et
al., 2004). Figure 8.66(a) shows the comparison of the best fit of the pressure-flow numerical results with
the experimental data of a 305 mm valve. It is seen that the experimental data support the linear P - Q
relation except at large flows, when the pressure measurement is probably affected by the size of the
experimental setup; the high flow data also deviate somewhat from energy conservation checks. On the
other hand, the comparison of the best fit numerical result with measured jet velocity (Fig. 8.66(b)) shows
excellent agreement (the jet velocity for the highest flow was outside measurement range).