G. Discharge Measurement Structures

G.1 Introduction

This section is intended to serve as a guide to the selection and installation of permanent and semi-permanent structures with a predetermined rating. The sampling of common pre-fabricated units included, each of which has been rated in laboratory test programs. In addition, all have effective discharge coefficients of less than 3% error when operated within their modular range and the other limits of application have been satisfied. With the exception of larger rectangular weirs, the capacities of the structures described do not exceed 4 m³/s.

Larger units are available, but the cost of engineering and construction place them beyond routine applications. The need for rating verification for these structures becomes more important for the larger units or where the design is modified in any way. Reference to standard texts is recommended, as no details on the hydraulics or modular limits are provided. (Discharge Measurement Structures. Edited by M.G. BOS. Published by International Institute for Land Reclamation and Improvement/ILRI PO Box 45, 6700 AA Wageningen, The Netherlands.)

G.2 Purpose

The measurement of flow in natural streams is hampered by many factors, which undermine the reliability of metering equipment. These conditions include:

• Remote locations interfering with regular and timely visits.
• Access difficulty, steep banks, seasonal or poorly maintained roads.
• Movable bed and high sediment transport conditions resulting in control shifts, perhaps several during a single season.
• Metering trips must be repeated frequently and not all are productive.
• Poor metering sites, for example steep rocky gradients.
• Shallow flows where normal metering equipment is not acceptable.

Rated structures can be used in such situations to minimize or eliminate many of these problems. The initial installations costs can be high for major installations, but must be evaluated against repeated metering which can also be expensive and all too frequently wasted. The cost of buying and installing prefabricated structures can vary from about $2000 to $20 000, but can easily provide high quality data for many years with little or no metering.

G.3 Design

The selection of alternate flow measurement methods, particularly if involving larger streams with potentially destructive flows, should only be made after consultation with an hydraulic engineer or hydrometric specialist. These together with any larger measuring structures require the services of a structural engineer for final design.

G.3.1 Design Considerations

The selection of a streamflow measuring device, its location and installation should be based on sound information acquired before the design is finalized. Much or all of the following information should be acquired:

1. Duration of the proposed project
2. Expected range of discharge to be measured
3. Likelihood of flows above the expected range should be assessed in terms of:
• device withstanding the stress of being over-topped
• extreme events being measured
4. Geometry of the channel reach
5. Streambed gradient above and below the proposed site
6. Presence and extent of bedload and/or sediment transport
7. Presence of any downstream constriction which may cause elevated tailwater levels at higher flows
8. Permeability of streambanks, streambed and underlying strata
9. Frost line depth
10. Proximity of vehicle access (this may possibly include access by redi-mix truck or small load mix-trailer and travel time from depot)
11. Construction window for in-stream work

G.3.2 Selecting the Measuring Device

Initially, the choice of a measuring device for a particular location will depend on the conditions set out in the items 2 to 7 in the previous sub-section. Table G-1 enumerates the maximum and minimum discharges for the various sizes of devices together with their ability to prevent sedimentation and pass bedload and debris. With reference to Table G-1, and Figures G-1 and G-2, the placement of weir crests has an important dimension, p that is defined as the depth of the approach channel below the crest on the upstream side of the weir. It also is required in the ratio h¹/p, listed with p in right hand column, and must be used to specify the placement of the various weir crests when these weirs are operating at capacity.