Leaflet

Reed, D.W. 1993c. Operational control of land drainage pumping stations. Summary leaflet, Institute of Hydrology, 4pp.

INTRODUCTION

Context

The primary objective of pumping station operation is to avoid flooding, usually of high-grade agricultural land. Public safety and security for food production remain watchwords for the operation of land drainage pumping stations. However, to these must be added the need for economy of operation and it is in this context that the Institute of Hydrology has been investigating the operational control of land drainage pumping stations.

Scope

Electricity Boards encourage organizations with large but sporadic power requirements to be flexible in their use of energy, particularly on winter weekdays. Many low-lying catchments reliant on pumped drainage have significant storage capacity in their main drain. Coupled with the relatively slow response of such catchments - slow in comparison to the time-scale of a few hours over which Electricity Boards seek to limit peak demands - this storage provides the flexibility for Internal Drainage Boards, and other drainage authorities, to manipulate pumping schedules to minimize energy costs.

The need for control rules

In extreme flood events, the storage available in the main drain is required to absorb periods where the runoff rate exceeds the capacity of the pumping station, with conditions in the receiving watercourse sometimes imposing an additional restraint. Thus there is a potential conflict between using storage in the main drain to economize on pumping costs in minor events, and using it to minimize inundation in major events. A further factor in some applications is a requirement to maintain a relatively high water level in the growing season. Institute of Hydrology research suggests that the conflict can be resolved by adopting specific control rules for pump operation.

Existing practice

At present, most land drainage pumping stations operate in a semi-automatic fashion, whereby pump control is achieved by local water level sensors and time-clocks. A pump-run is triggered when water rises to a pre-set level and continues until it falls below a lower threshold. The time-clocks override the water level sensors to "disable" pumping in tariff periods where high unit energy costs apply.

In a major flood it may be necessary to "enable" reserve pumps and suppress the time-clock override. Generally these adjustments have to be done manually.

As manning levels are cut to economize on staff costs, the possibility arises of failing to respond adequately to a major flood. Thus there is a requirement to monitor conditions remotely, to provide some form of alert or warning.

Telemetry

Many national and regional organizations have turned to telemetry to monitor conditions at remote sites and, in some cases, to introduce centralized control. When economies in staffing costs are sought through increased automation it is evident that the introduction of telemetry has applications in land drainage also. But can the requirement for reliability in responding to major flood events be met by a telemetry system alone?

SPECIAL FEATURES OF FLOOD WARNING IN PUMPED CATCHMENTS

Introduction

Two related factors distinguish flood warning in pumped catchments from that in natural rivers. Firstly, the pattern of pumping exerts a strong influence on water levels in the main drain. To establish the possible significance of a high water level, it is necessary to know the extent of recent pumping. The second factor is that the very low gradients found in pumped catchments generally prohibit satisfactory gauging of flow rates.

The need to know the inflow

In order to represent the hydrological demand for pumping in real time, the key requirement is to monitor - and, perhaps, forecast - the catchment runoff which forms the inflow to the main drain. The inability to measure flows directly suggests that it may be helpful to use a rainfall-runoff model to simulate and forecast inflows from telemetered rainfall data. However, unless accurately calibrated on historic data, or updated in real time, a rainfall-runoff model is likely to be too inaccurate to give reliable warning of a major flood. So some other way is needed of estimating the rate of inflow to the main drain.

A budget method for calculating inflows

As part of strategic hydrological research for the Ministry of Agriculture, Fisheries and Food, a technique has been developed for deducing inflows to a main drain by a budget method. The inflow is calculated as:

(inflow) = (pumped outflow) + (increase in storage)

where the change in storage is evaluated from water level data and a simple representation of the main drain geometry.

The drain geometry model represents the drain as a tapering trapezoid in which the water width, W, at distance x, and depth, h, is given by:

W(h,x) = a0 + a1h - a2x

The parameters a0, a1 and a2 are derived from survey or design drawings showing the longitudinal section and representative cross-sections. If required, the parameters can be refined by observing the "before" and "after" water levels for isolated pump-runs of known discharge volume, carried out at times of low natural inflow. In such experiments the "after" water level reading is taken after the transient effect of the pump-run has subsided.

Determining the stock of water at time "now"

In applying the budget method in real time it is desirable to use water level data from both local and distant recorders. The local recorder is generally situated close to the weedscreen at the pump intake. The distant (or remote) recorder is sited sufficiently far up the main drain to be representative of conditions beyond the direct influence of the pumps.

The stock of water in the drain at time "now" is calculated using the drain geometry model, under the assumption of a uniform water level gradient between the local and distant recorders, and of a horizontal water profile upstream of the remote recorder. It has been found that the "drawdown" effect of pump operation is less pronounced in major floods, making a linear approximation tenable. Where the main drain has unusual features it may be helpful to position additional water level recorders to achieve a good estimate of stock.

In using the budget method to evaluate the average inflow rate over the last few hours, it is of course necessary to know the recent history of pumping. Thus, to the basic requirement for information from two water level recorders, one must add a need to telemeter pump-run information.

Conclusion

Telemetering water level at the pump intake is not in itself sufficient to provide effective warning of flood conditions. It is suggested that, if economies in staffing preclude traditional ways of maintaining high reliability of operation in flood conditions, flood warning in pumped catchments be met instead by: provision of a telemetry system to gather water level and pump-run data, derivation of a drain geometry model, implementation of the budget method, and generation and communication of warning messages.

The system now introduced goes an important step further. Significant additional benefits can be realized if the telemetry system embraces the operational computer control of pumps.

OCOPO - A SYSTEM FOR COMPUTER CONTROL OF PUMPING STATIONS

Introduction and acknowledgements

McMillan Cordell Associates and the Institute of Hydrology have collaborated in the development of a system for the Optimum Control Of Pump Operations. OCOPO is dedicated to maintaining the high safety record of land drainage pumping while at the same time improving operational performance by increasing energy efficiency and reducing manning costs.

The system draws on strategic research into the hydrology of low-lying catchments funded by the Ministry of Agriculture, Fisheries and Food. With the cooperation of the North Level Internal Drainage Board, the prototype has been working at Thorney since August 1986, controlling the Newborough pumping station.

The system

OCOPO considers the storage available in the drainage system and the flexibility this provides for pump operation. Control rules determine the pumping required in the coming operational period, taking account of water levels, runoff rates and electricity tariff structure.

OCOPO is supplied as a set of routines which run on an industry standard PC. Components include:

The role of flood forecasting

The forecasting of inflows is an integral part of the system. OCOPO uses a nonlinear storage model to forecast catchment runoff from telemetered rainfall data. In some pumped catchments, the natural response to rainfall is relatively sluggish and the rainfall-runoff model plays only a supporting role; an adequate inflow forecast can in some instances be based on the average inflow rate observed in the recent past. Except in quickly responding urban catchments, improvements in pumping station performance are likely to be achieved more through the introduction of control rules - linked to tariff manipulation, close monitoring of water levels in the main drain, and timely action - than through hydrological forecasting per se.

Status of system

After extensive trials on North Level IDB's Newborough pumping station, the system has been generalized to provide the following additional features:

An additional option - as yet not standard - is for the control decision to take account of forecast tidal influences on water levels in the receiving watercourse. Applications of OCOPO to barrage control in tidal rivers are also envisaged.

Summary of benefits

Where pumping stations are already operated to avoid pumping at peak tariff periods, large savings in electricity costs are unlikely. The major benefits of introducing operational computer control of pumping stations will accrue from reduced staff costs. The system enables the level of service to be maintained and possibly improved - ie reduced incidence, and more timely warning, of inundation - while reducing manning requirements. It is also a valuable source of management information.

Further information

The Institute of Hydrology (0491-38800, contact: Duncan Reed) will be pleased to discuss the hydrological aspects of pumped catchments and the method adopted for optimum control of pump operations. Specific enquiries about the OCOPO system and its purchase should be directed to McMillan Cordell Associates (0787-76532, contact: John McMillan).