Optimised Triangular Weir Design for Assessing the Full-Scale Performance of Green Infrastructure

Mohamad H El Hattab,Ana Mijic,Dejan Vernon

doi:10.3390/w11040773

Abstract

Conventional triangular weirs have been originally developed to measure, divert, and control surface water. However, a special application of these weirs, such as for low flow measurements in full-scale monitoring of Green Infrastructure (GI), is not well investigated. Available head-discharge relationships for triangular sharp-crested weirs are only valid under a free-flow regime. Literature focusing on the V-notch weir usage for GI assessment suggests that it is necessary to calibrate the head-discharge relationship before its use. This study focuses on understanding the effects of site constraints on the measurement performance of a V-notch weir at low flow rates, and the validity of equations derived for similar applications that can be found in the literature. The variation of discharge coefficient in various flow regimes was investigated experimentally based on calibration runs covering flow rates between 0.054 l/s and 7 l/s. The results show that for 30° and 45° V-notch weirs, three flow regimes can be identified. It was observed that literature equations to calculate the discharge coefficient are valid for partially-contracted triangular weirs only at heads greater than vertex distance from the channel. However, for low flows that are expected to occur when estimating the full-scale performance of GI, the equations available from the literature for similar site conditions underestimated the flow rate between 85% and 17%. This emphasises the need for accurate calibration of a V-notch device under the site conditions to achieve the necessary level of accuracy in GI performance estimation. The procedure outlined in this work can be easily replicated to determine the optimal monitoring system configuration. Alternatively, if the site conditions would match those described in this study, the computed discharge using the proposed relations, in combination with the general V-notch weir equation, provides a significant improvement in the accuracy of measurements, expands the head applicability range of V-notch weirs, and enables better understanding of GI performance at the full scale.

Highlights

Most of the urban areas in the world are drained through an old combined sewerage system constructed more than 50 years ago, with some water infrastructure being more than 100 years old, such as the combined sewerage system draining central London [1]
The observed differences become negligible for flow rates higher than 2 l/s
This study focused on the calibration process for a V-notch weir when the device is used for the assessment of the Green Infrastructure (GI) full-scale performance

Summary

Introduction

Most of the urban areas in the world are drained through an old combined sewerage system constructed more than 50 years ago, with some water infrastructure being more than 100 years old, such as the combined sewerage system draining central London [1] This type of system proved to be adequate in the past; due to the more rapid urbanisation in the last few decades and greater interest in managing stormwater flows to maintain environmental quality [2], the standards of performance expected to be delivered by the conventional drainage systems have become much higher than when they were first built [3]. Another limitation related to the hydrological monitoring programs of GI components is site constraints, for which conventional monitoring instruments become inapplicable [10]

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