SIDEWEIR FOR COMBINED SEWERS

 

Giuseppe DEL GIUDICE ¹ and Willi H. HAGER ²

 

Dip. di Ingegneria Idraulica, Università di Napoli, I-80125 Napoli

Tel.: +39 081 768 3449, Fax: +39 081 593 8936, e-mail: delgiudi@gds.unina.it

² VAW, ETH-Zentrum, CH-8092 Zurich, Switzerland

Tel.: +411 632 4149, Fax: +411 632 1192, e-mail: hager@vaw.baum.ethz.ch

 

 

ABSTRACT

Sewer sideweirs with a converging plan into a throttling pipe are experimentally analyzed. The main findings relate to the overall optimum design in terms of hydraulic performance and discharge capacity. Those sideweirs should be applied for subcritical approach flow.

 

Keywords: Hydraulics, lateral, sewer, sideweir, storm drainage.

 

INTRODUCTION

Sideweirs in combined sewers are used to separate sewage from stormwater. Such sideweirs are essentially different from canal sideweirs because they involve a converging U-shaped geometry with a two-sided lateral outlet, and a throttling pipe at its downstream end to limit the discharge to the treatment station. The crest of the sideweir can be low or high relative to the approach diameter. Low-crested sewer sideweirs have a large discharge capacity but their hydraulic performance is poor. Typically, even a subcritical approach flow gets supercritical along the weir and the obstruction of the throttling pipe forces a hydraulic jump close to the downstream sideweir end. Therefore, the high-crested sewer sideweir is a design standard.

Sewer sideweirs have received little attention so far, mainly due to complexities both in flow conditions and structural design. The current guidelines have been modified from earlier recommendations in so far that separation of both water and solid matter was realized to be impossible (Oackley 1967). Currently, a stormwater basin is normally added to the outflow branch into the receiving water in order to separate sediment and floating matter.

The hydraulic design of a sewer sideweir involves at least three discharges:

(1) No sedimentation for minimum discharge,

(2) No lateral discharge up to the design discharge of the treatment station, and

(3) Limit of treatment discharge for maximum approach flow.

 

 

Fig.1 Definition of sewer sideweir.

 

This research study was conducted to explore the current sideweir design. Because of the complex flow patterns, experiments were made to answer engineering questions regarding performance and capacity of sewer sideweirs. These two items influence the performance of a combined sewer system and the present results can be considered as a design basis. The experiments and the experimental program are described by Del Giudice and Hager (1998). The model sideweir had a D_o=240mm approach pipe and a D_u=100mm throttling pipe. The approach flow could be set with variable approach flow depths h_o and approach Froude numbers F_o<1.5, i.e. with a subcritical or transitional approach flow. The sideweir was 1000mm long and had two-sided sharp crests of height 40, 60, and 80% of D_o. The relative length of the throttling pipe was 20, 40 and 60 D_u and the outflow was always free.

Free surface profiles along the sideweir and the approach channel as well as velocities along the axis and the crest section were measured. The following is a summary of the main results.

 

FREE SURFACE PROFILE

For a converging sideweir, the free surface profile h(x) may increase or decrease along the crest, with x as the streamwise coordinate measured from the sideweir beginning. Because the energy loss along a sideweir is small and a bottom slope of 0.3% was allowed for its compensation, it will be neglected in the following. Further, to avoid complications with the cross-sectional geometry of the converging U-shaped channel, a rectangular substitute section is adopted. The Bernoulli equation applied between the upstream (subscript o) and a sideweir section requires

 

. (1)

 

With the approach Froude number F_o=Q_o/(gDh)^1/2, this also reads

 

, (2)

 

where y=h/h_o, and . Solving up to order of y gives with

 

. (3)

 

Observations indicate agreement with Eq.(3) except close to the singularity f=1.

According to Eq.(3) the flow depth along a sideweir is constant for either , or for . This flow condition is also referred to as pseudo-uniform (Hager 1995). A general condition for pseudo-uniform flow to occur is D/D_o=Q/Q_o, i.e. the sideweir with D(x) has to decrease exactly as does the local discharge Q(x). Introducing the pseudo-uniform parameter =[(Q/Q_o)/(D/D_o)], Eq.(3) can be rewritten as

 

. (4)

 

Accordingly, the basic hydraulic features of the converging sideweir are:

(1)   For F_o=0 the free surface is horizontal, and the deviations increase quadratically with F_o,

(2)   Depending on the value of P the flow depth may increase or decrease along the sideweir, provided transitional flow is excluded,

(3)   The effect of local Froude number is approximately accounted for by f=PF_o, and the flow depth either increases for P<1 and f<1 (subcritical flow), or P>1 and f>1 (supercritical flow),

(4)   Because all hydraulic losses have been neglected, Eq.(4) is valid only for the near field, i.e. for relatively short sewer sideweirs without hydraulic jumps and impact flow (see below).

Eq.(4) retains the features of converging sideweir flow and may explain the flow complexities.

 

EXPERIMENTAL RESULTS

 

End depth ratio

The ratio between the axial flow depth at the downstream (subscript u) and upstream sideweir ends is referred to as the end depth ratio Y_u=h_u/h_o. This ratio defines both the sideweir performance and the throttling discharge. Based on the momentum equation, and with =(4/)(/)(h_o/D_u) where _u=Q_u/Q_o and =D_u/D_o Del Giudice and Hager (1998) found

 

. (5)

 

Accordingly, Y_u is independent of the sideweir length. For the design discharge, the discharge ratio is small and. Experiments indicate agreement with Eq.(5) up to F_o=0.8.

 

Discharge distribution

The local discharge Q(x) along a sideweir depends on many parameters, including the approach flow conditions, the lateral overflow features and the sideweir geometry. To introduce a simple approach the normalized discharge distribution q=(Q-Q_u)/(Q_o-Q_u) is considered. Previous results by Gisonni and Hager (1997) pointed to the dominant effect of approach Froude number F_o. With X=x/L where L is the sideweir length the previously found result may be generalized as

 

q = 1 - X^1+F_o . (6)

 

Evidently, normalization requires that q(X=0)=1 and q(X=1)=0. Further, the asymptotic case F_o=0 provides a uniform discharge distribution q=1-X, and the uniformity decreases as the Froude number increases. Therefore, a sideweir with a large approach Froude number discharges almost all lateral flow at its end, and the upstream portion has practically no overflow discharge. Eq.(6) was experimentally verified for relative weir heights W=w/D_o=0.40, 0.60 and 0.80.

 

Discharge coefficient

As the discharge distribution Q(x), the lateral discharge Q=Q_o-Q_u varies with many basic parameters, and a simple outflow equation is (Gisonni and Hager 1997)

 

(7)

 

Here, C_da is the average discharge coefficient of a double-sided sideweir with sharp crests, L is the sideweir length and w is the weir height measured from the invert. Because both the free surface profile and the discharge vary essentially with F_o, the discharge coefficient for intermediate sideweir length (3<L/D_o<6) may be expressed as

 

C_da = 1 + F, (8)

 

where =(1/2)( L/D_o). Eq.(8) was tested for (L/D_o)^1/2F_o<2.5.

 

Discharge ratio

According to ATV (1993) regulations, the ratio of throttling discharge Q_p to the design discharge for zero overflow Q_pD should be less than 20%. This criterion limits the treatment discharge and has to be verified for the maximum approach discharge. From a detailed analysis conducted by Del Giudice and Hager (1998) it can be demonstrated that the discharge limit is satisfied either if F_o is sufficiently small (F_o<0.50), or if the relative weir height is sufficiently large, W>2/3. Obviously, sideweirs with either a large approach Froude number or a small weir height have a poor hydraulic performance and ATV regulations cannot be satisfied. As a design recommendation, we favour sideweirs of intermediate length (3<L/D_o<6) with a relative weir height W≥2/3 and a subcritical approach flow (F_o<0.75). Such a sewer sideweir corresponds to an optimum design in terms of hydraulic performance and discharge capacity.

 

Throttling discharge

Throttling pipes behave essentially like a culvert. In general, four basic flow types may occur: (1) Critical flow, (2) Uniform flow, (3) Gated flow, and (4) Pressurized flow. For sideweirs with a high crest (W>2/3) and for typical diameter ratios D_u/D_o<1/2, only flow types (3) and (4) are relevant, because h_u/D_u>1.2 for design flow.

The present results were obtained for gated flow with a bottom slope S_o larger than the critical slope. With F_D=Q/(gD)^1/2 as the throttling pipe Froude number and the downstream depth ratio y_p=h_u/D_u, the discharge equation for the throttle is

 

F_D = 1.11C_d(y_p-C_d)^1/2. (9)

 

 

Fig.2 (a) Experimental setup with critical treatment discharge, (b) F_o=0.38, (c) F_o=0.83.

 

The discharge coefficient of the throttle intake downstream of a contracted sewer sideweir is C_d=0.70. Eq.(9) agreed with model observations.

 

PHOTOGRAPHS

Sideweirs with a relative weir height w=0.60D_o were considered and the following is intended to give an outline of typical experiments. Fig.2a) shows the experimental setup, with the critical treatment discharge in the approach pipe and just no overflow.

Fig.2b) relates to an approach Froude number F_o=0.38, with an almost horizontal surface profile and a small swell at the downstream end. Fig.2c) shows that for even F_o=0.83, the approach and the sideweir flows remain practically horizontal but the swell has now a significant height.

Fig.3 shows the same situations in the streamwise direction as Fig.2. For both critical discharge and F_o=0.38, the flow is extremely smooth, except for the end of the sideweir, whereas the swell is highly turbulent for F_o=0.83.

These photographs may give an impression of the highly complex flow pattern that is typical for sideweirs. The present project was conducted mainly to come up with a rational approach for a hydraulic design.

 

 

Fig.3 Streamwise views for hydraulic conditions of Fig.2.

 

CONCLUSIONS

Sewer sideweirs are essential for water quality considerations in a combined sewer system. The hydraulics of this structure are explored based on 1D-computations and extensive hydraulic modelling. Based on a perturbation approach, the various types of free surface profiles are given, with particular reference to pseudo-uniform flow. Then, the end depth ratio is obtained using the momentum approach. The discharge distribution and the lateral outflow are demonstrated to depend essentially on the approach Froude number. Finally, a sewer sideweir has an optimum performance for relatively small F_o and a relatively large weir height. Photographs illustrate the complex flow features.

 

REFERENCES

Abwassertechnische Vereinigung (1993). Richtlinien für die hydraulische Dimensionierung und den Leistungsnachweis von Regenwasser-Entlastungsanlagen in Abwasserkanälen und -leitungen. Arbeitsblatt A111. ATV: Hennef, Germany (in German).

Del Giudice, G., Hager, W.H. (1998). Sewer sideweir with throttling pipe. Journal of Irrigation and Drainage Engineering, submitted.

Gisonni, C., Hager, W.H. (1997). Short sewer sideweir. Journal of Irrigation and Drainage Engineering 123(5): 354-363.

Hager, W.H. (1995). Abwasserhydraulik. Springer: Berlin, New York (in German).

Oackley, H.R. (1967). Practical design of storm sewage overflows. Storm sewage overflows 10: 115-122. Institution of Civil Engineers: London.