A COMBINED STORM OVERFLOW STRUCTURE: DESIGN PROCEDURE

 

GIUSEPPE OLIVETO, MAURO FIORENTINO, and GIANLUCA MININNI

 

DIFA, University of Basilicata

I-85100 Potenza, Italy

Phone: +39 971 474676; Fax: +39 971 56537; e-mail: fiorentino@unibas.it

 

 

Abstract

In combined sewer systems suitable locations are selected where only a fraction of the maximum discharge is allowed to continue towards the sewage treatment station. To this purpose, overflow structures are used. In this paper a storm overflow structure constituted by a sideweir and a bottom opening is proposed. It is particularly suitable for subcritical approach flows. A hydraulic design procedure is described and, based on systematic observations and by using governing flow equations, the main hydraulic features are specified. The results reveal that the proposed overflow structure has advantageous characteristics with reference to efficiency, reliability, and maintenance.

 

Keywords: Sewers, storm overflow structure, sideweir, bottom opening, hydraulic performance.

 

Introduction

Storm overflow structures are key devices in combined sewers. For discharges up to the design discharge of the sewage treatment station no overflow should occur. Whereas, the discharge exceeding the so-called critical treatment discharge should be conveyed towards a storage basin or a receiving water body.

Over the last decades two standard designs have been favoured: sideweir and bottom opening. Side weirs are exclusively adopted for subcritical approach flows; bottom openings are particularly suitable for supercritical approach flows.

With reference to sideweirs, those of high weir height are needed for an overflow structure alone, whereas the combination with an overflow chamber can involve also sideweirs of medium or low weir height. An acceptable efficiency of a sideweir of high weir height can be obtained by locating a Venturi flume and/or a sluice-gate slightly downstream the end of the weir (Biggiero, 1969); that, however, can involve small velocities close to the weir and, thus, deposition can occur. Vice versa sideweirs of medium or low weir height require little maintenance, but their efficiency is poor (Biggiero et al., 1994).

Based on the foregoing considerations, in this paper a storm overflow structure constituted by a sideweir and a bottom opening is proposed in order to improve the efficiency of sideweirs of low weir height.

 

The hydraulic scheme

The proposed overflow structure is constituted by a prismatic sideweir of low weir height and a bottom outlet located slightly downstream the end of the weir. A similar device was also mentioned in Biggiero (1969). The approach flow is subcritical or close to critical flow. For discharges up to the critical treatment discharge no lateral outflow occurs; thus, all discharge passes through the outlet and continues towards the sewage treatment station. For the maximum discharge, usually much larger than the critical treatment discharge, the sideweir determines the transition from sub- to supercritical flow slightly upstream the inlet section, so that the flow along both the weir and the bottom outlet is supercritical. The discharge that passes through the outlet continues towards the sewage treatment station, the remainder and the lateral outflow are conveyed towards a storage basin or a receiving water body. More details on the proposed device are reported in Fiorentino et al. (1998).

 

Hydraulic design

Fig.1 shows a definition sketch with h = flow depth, Q = discharge, D = diameter, l = length, c = height, w = width, subscripts o, e, and d referring to approach, end, and downstream sections, respectively, and subscripts sw, and bo referring to sideweir, and bottom opening, respectively. The end section is so-called in relation to the end section of a pipe discharging into the atmosphere. For subcritical approach flow h_o will denote the critical depth.

 

Fig.1 Definition of flow: (a) plan; (b) side view

 

Designing the proposed overflow structure is controlled by two discharges: (a) critical discharge to sewage treatment station for which there is just no lateral outflow; (b) maximum discharge usually much larger than (a). Case (a) determines the structure geometry, and case (b) has to be verified for excess treatment discharge.

 

Geometric characteristics

Referring to the critical treatment discharge, the structure geometry may be determined as follows:

The weir length l_sw should be assumed equal to 4D. In fact, this length provides a good compromise between efficiency and compactness of sideweirs of low weir height;

The weir height c_sw should be equal to the approach flow depth h_o;

The width w_bo of the bottom opening should be equal to the approach surface width;

The length l_bo of the bottom opening should be determined from the following equation (Oliveto et al., 1997a)

 

(1)

 

where F_o is the approach Froude number (equal to 1 for subcritical approach flow).

Thus, for a given weir height the bottom opening geometry is completely defined.

 

Overflow discharge

Assuming that the influence of the bottom outlet on the outflow process along the weir is negligible, the overflow discharge may be predicted by applying equations relative to sideweir and bottom opening. Thus, referring to the maximum discharge and for given both approach flow conditions and overflow structure geometry, the following procedure may be applied:

The lateral outflow Q_o - Q_d,sw should be determined by the equation (Oliveto et al., 1998)

 

(2)

 

in which the coefficients and are given by

 

(3)

(4)

 

Eq. (2) may be used provided that: 1£ l_sw/D £ 6, 0.1£ c_sw/D £ 0.25, and 0.35£ h_o/D £ 0.70. All these conditions assure supercritical sideweir flow, i.e. no hydraulic jump occurs along the weir (Sassoli, 1963).

Consequently, the downstream discharge Q_d,bo should be determined by using the average discharge equation (Biggiero, 1969; Oliveto et al., 1997b)

 

(5)

 

In particular, the approach flow depth h, assumed equal to h_d,sw, can be determined by the following theoretical equation (Oliveto et al., 1998)

 

(6)

 

and the average discharge coefficient C_da can be determined by either the equation (Biggiero, 1969)

 

(7)

 

or the equation (Oliveto et al., 1997b)

 

(8)

 

where the approach flow depth h and the approach Froude number F can be assumed equal to the flow depth h_d,sw and the Froude number F_d,sw at the end of the weir, respectively.

 

EXPeriments

 

experimental SETUP

The tests were conducted with a Plexiglas pipe of internal diameter D = 192 mm inserted in a rectangular channel 500 mm wide and 500 mm high. The system observation was thus readily available from the existing channel. The pipe length from the inlet section to the beginning of the sideweir was 2,100 mm. The pipe lengths from the end of the bottom opening to the outlet section ranged between 4,155 and 4,100 mm according to the bottom opening length. The invert of the pipe was 200 mm above the channel bottom. In the stilling zone between the upstream channel end and the pipe inlet, pressure heads were up to 480 mm. The approach

flow in the pipe was stabilized by a flow straightner inserted at the pipe inlet section and made of pulled PVC pipes with 14 mm internal diameter and length 192 mm. In order to further regularize the flow in the pipe, the inlet flow depth was adjusted by a Polystyrene sheet.

Four configurations were used. The transverse section of the outflow device was U -shaped. The relative length l_sw/D of the sideweir was always equal to 4 and the relative weir heights c_sw/D were 0.10, 0.15, 0.20, and 0.25. The weir crests were sharp and horizontal, and one-sided outflows were considered. The beginning of the outlet was located 50 mm downstream the end of the sideweir.

 

experimental measurements

For each experiment the surface profile was taken to ± 1 mm accuracy. The approach discharge was measured by an orifice meter provided Q > 10 ls^-1. Deviations were smaller than ± 2%. The discharge across the bottom opening was rated with a mobile Venturi flume (Ueberl and Hager, 1994) inserted below the tailwater pipe. Its accuracy was ± 2% - 3% due to some wave action set up by the plunging jet.

 

experimental observations

Some photographs are added to provide a description of the flow characteristics.

The relative weir height c_sw/D of sideweir is 0.10, the relative length l_sw/D of bottom opening is 1.45·c_sw/D, and the relative width b_bo/D is 0.55.

Fig.2a shows a top view on the outflow structure for a case in which all discharge flows through the opening. Figs. 2b to 2d refer to h_o/D nearly 50%. As it can be noted only a portion of the approach discharge passes through the outlet. The flow is tranquil, and no surface waves may be seen except downstream from the bottom opening. Although the sideweir is one-sided, the feeble asymmetry of the flow can be confidently neglected.

 

 

 

Fig.2 Photographs of experimental stand.

 

(a) view on outflow structure for h₀ < cSW;

(b) downstream view to the sideweir flow (the stability of the approach flow surface an be recognized;

(c) view on bottom opening;

(d) Plexiglas sheet to separate the lateral outflow from the outlet discharge.

 

Analysis of data

 

sideweir End depth

From the experiments, all characterised by values of h_o/D ranging between 0.40 and 0.60, the following equation was obtained

 

(9)

 

according to observations made by Biggiero et al. (1994) on one-sided weirs with low or medium weir height.

 

sideweir DOWNSTREAM depth

For given parameters h_o, h_d,sw, Q_o, and Q_d,sw one may define the downstream depth ratio Y_d,sw = h_d,sw/h_o and the discharge ratio R_sw = Q_d,sw/Q_o. Compared to the data obtained determining R_sw by Eq. (2) and Y_d,sw by observations, the theoretical curve Y_d,sw (R_sw) from Eq. (6) was found somewhat low (-5%) which must be attributed to the simplified approach. Eqs. (2) and (6) give thus an acceptable estimate of Y_d,sw, although referred to sideweir alone. Based on the foregoing results, the assumption that the interaction between the sideweir and the outlet is negligible can be retained reasonable.

 

discharge across the bottom opening

Regarding to the outlet discharge Q_d,sw - Q_d,bo, the following empirical curve was found

 

(10)

 

where R_sw is given by Eq. (2). Thus, based on Eqs. (10) and (2) the hydraulic performance of the proposed overflow structure can be easily determined. To this purpose, the procedure based on Eqs. (2) to (8) can also be used with reasonable accuracy.

Defined by [(Q_o -Q_d,sw) + Q_d,bo]/Q_o the efficiency of the proposed overflow structure and by (Q_o -Q_d)/Q_o the efficiency of sideweir alone, one finds, by applying the above equations, that the efficiency of the proposed overflow structure is much higher than that of sideweirs with a low or medium weir height, and comparable with that of sideweirs with a high weir height.

 

Conclusions

A storm sewage overflow constituted by a sideweir of low weir height and a bottom outlet just downstream the weir was studied. This structure is suitable for combined sewer systems with subcritical approach flows and requires a limited maintenance.

By using empirical and theoretical equations referred to sideweir or bottom opening the hydraulic design criteria are specified.

Moreover, based on model observations the following information were deduced:

- the influence of the bottom outlet on the outflow process along the weir is negligible;

- the discharge that passes through the outlet and continues towards the sewage treatment station may be easily determined by Eq. (10).

By applying Eq. (10) one can also demonstrate that the efficiency of the proposed outflow structure is much higher than that of sideweirs of low weir height and comparable with that of sideweirs of high weir height.

 

References

Biggiero, V. (1969). Scaricatori di piena per fognature. Criteri di progettazione. Ingegneri, Naples, Italy, 57: 1-36 (in Italian).

Biggiero, V., Longobardi, D., and Pianese, D. (1994). Analisi sperimentale del comportamento idraulico degli sfioratori laterali a bassa soglia. Giornale del Genio Civile, 132(3): 183-199 (in Italian).

Fiorentino, M., Oliveto, G., and Mininni, G. (1998). A storm overflow structure constituted by a sideweir and a bottom opening. Submitted to Journal of Irrigation and Drainage Engineering, ASCE.

Oliveto, G., Biggiero, V., Fiorentino, M., and Hager, W.H. (1997a). Sul processo d'efflusso da luci di fondo in reti drenaggio urbano. Atti delle Giornate di studio in onore del prof. Edoardo Orabona, Bari, Italy, (in Italian).

Oliveto, G., Biggiero, V., and Hager, W.H. (1997b). Bottom outlet for sewers. Journal of Irrigation and Drainage Engineering, ASCE, 123(4): 246-252.

Oliveto, G., Biggiero, V., and Fiorentino, M. (1998). Hydraulic features of supercritical flow along prismatic side weirs. Submitted to Journal of Hydraulic Research.

Sassoli, F. (1963). Ricerca sperimentale sugli sfioratori laterali in canali a sezione circolare. Parte seconda: andamento e tracciamento dei profili liquidi. Atti VIII Convegno di Idraulica e Costruzioni Idrauliche, Mem. A-12, Pisa, Italy: 1-18 (in Italian).

Ueberl, J., and Hager, W.H. (1994). Mobiler Venturikanal im Rechteckprofil. Gas - Wasser - Abwasser, 74(9): 761- 768 (in German).