WaterSubject

Based on previous observations of the 45° junction manhole for supercritical flow in the main and lateral branches, the hydraulics of the more common 90° junction manhole were explored. Using a selected manhole geometry involving: (1) a short straight piece in the lateral branch to inhibit full development of the bend wave, and (2) the addition of the junction extension as used in previous designs for the bend manhole, the present study gives results that are in basic agreement with those collected in the 45° junction manhole. This surprising result thus allows for a design basis independent of the junction angle.

The present paper defines three waves that may occur in a junction manhole, i.e. bend wave, junction wave and the swell at the manhole outlet into the downstream pipe. In addition, the position of the determining junction wave was established. Important for the junction design is the discharge capacity for which supercritical flow can be maintained across the manhole. It was found that the lateral branch flow depth and the pipe diameter have an important effect on this capacity, for both branches or only one of the branches in operation.

Combined sewers constitute a collecting system characterised by numerous combining junctions. For relatively small bottom slopes of the order of 1%, uniform sewer flow is supercritical. At junctions or at any other manhole, the uniform flow will thus be significantly perturbed by the generation of shock waves. A shock basically originates from arbitrary perturbations such as contractions, expansions, bends, junctions or even changes of roughness and bottom slope. The shock wave may be described as a local surface wave generation across which the velocity remains almost constant. Accordingly, the main engineering features of these waves are their wave height and location.

Waves generated by shocks have an adverse effect in combining sewers because overtopping in a manhole may result in a breakdown of supercritical flow, associated with a normally abrupt transition from supercritical to pressurised two-phase air–water flows. This phenomenon is also referred to as choking, and it may occur at three distinctly different locations in a junction manhole: (1) at the inlet of the upstream branch, (2) at the inlet of the lateral branch, and (3) at the outlet of the manhole into the downstream branch. Combinations of the three may even result during advanced choking conditions. Once choking has occurred, the flow undergoes a significant and normally rapid change. Usually, the junction flow then resembles a hydraulic jump, with a flow depth much larger than would be needed for supercritical flow. For choking at the manhole outlet, the water level in the manhole rises similarly as in a surge tank, and pulsations may become so large that the pressure on the manhole cover may lift it off and discharge sewage on public roads. This so-called geysering has to be prevented under all circumstances.

In a recent paper by, the junction manhole for the 45° lateral branch was investigated, together with a review of past research. It was found that practically no work is currently available that accounts for open channel flow in closed conduit systems. The effect of pipe choking has thus so far not been accounted for. The present paper aims to introduce observations relating to the more common 90° junction manhole. The results of this paper provide design guidance for all usual junction manholes with a lateral branch angle between 30° and 90°. As already noted in earlier work with bend manholes, the 90° junction manhole does not correspond to the determining design case, as one would initially think. The results of the present paper indicate that all junction manholes can be designed with the same engineering approach, provided the junction extension as previously introduced for the bend manhole is added. This paper thus represents a general hydraulic design guide for junction manholes.

When starting this project it was not at all clear that the 45° and the 90° junction manholes would behave similarly under supercritical approach flow conditions. The relevant design equations of the present paper thus essentially agree with earlier observations. This may be disappointing on the one hand because nothing new seems to have been obtained. It is highly satisfactory on the other hand that the selected design of the 90° junction manhole with the small intermediate straight piece reduces the effect of the bend wave and leads to a generalised design independent of junction angle.

The height of the bend wave in the lateral branch is much smaller than in a normal bend, such that only the junction wave J has to be considered, except for small Froude numbers F_L<2.4. The freeboard required is thus imposed. More important is the discharge capacity Q_C=0.6g^1/2F_oh_o^1.2h_L^?0.2D^3/2 as expressed with. For both the 45° and the 90° junction manholes it increases linearly with F_o, almost linearly with h_o, and with the power (3/2) of the pipe diameter D.

As stated by, the capacity of junction manholes is intermediate between (1) bend manholes with a smaller capacity and (2) through manholes with a larger discharge capacity. Given that filling ratios are often similar, such that y_oy_L, reduces to F_C=0.6F_oy_o. From experiments a maximum value F_C=1.4 may be observed. Therefore, the main branch flow reduces the maximum wave height and increases the discharge capacity of a junction manhole as compared with a bend manhole. For a total discharge Q the lateral discharge perturbs flow in a junction manhole. If most of the discharge originates from the main branch, both wave heights are relatively small and the discharge capacity large.

Supercritical flow including the limits for minimum and maximum flow conditions were investigated. Supercritical flow in either of the two branches, or in both branches is possible provided: (1) The junction has an adequate geometry, and (2) the discharge falls between a lower and an upper limit. For discharges smaller than the minimum subcritical flow occurs; for discharges larger than the maximum, the supercritical flow structure also breaks down and dangerous two-phase pressurised flow is generated that may result in adverse flow features.

The present work answers questions relating to the type of flow, the maximum wave heights in the lateral and in the straight-through branches and the maximum swell height. All these parameters vary exclusively with the Froude number in a circular pipe and the filling ratios of the lateral and main branches. The junction structure may be designed with the observations relating to the discharge capacity. It was demonstrated that both the 45° and the 90° junction are governed by almost identical relations, provided the junction geometry as adopted in the present project is selected. This finding renders results independent of the junction angle, and simplifies junction design.