Tech Talk: Sizing Type 2 Sediment Controls

You may be aware that under IECA (2008) sediment control measures are grouped into four categories (namely Type 1, Type 2, Type 3 and Supplementary) based on their ability to trap a specified particle size. Type 1 controls typically include sediment basins, which are designed to capture both coarse and suspended sediments for a range of design events. Technically, Type 1 controls are those which will theoretically capture sediment particles smaller than 0.045mm (i.e. clays and fine silts). The most common Type 3 sediment control is by far and away sediment fence and to a lesser degree some stormwater inlet protection measures. Type 3’s are limited to capturing large particles, such as coarse sediment (i.e. sands). But what about Type 2 controls… What are they, how effective are they and do they require sizing?

Well as you probably know (or can guess), Type 2 controls fit between Type 1 and Type 3. That being they are more effective than a Type 3, but ultimately are still limited in effectiveness to capturing predominantly coarse sediments, and are therefore not as effective as a Type 1 controls and offer little improvement to turbidity of water discharging.

A range of Type 2 control measures may be utilised onsite to suit conditions and construction works. Typical Type 2 measures include rock filter dams, excavated sediment traps, mulch bunds or some other ponding structure. Type 2 controls can be an effective sediment control measure for small catchments or constrained sites, especially when combined with appropriate drainage and erosion controls. One aspect of Type 2 controls which isn’t well understood is the requirement to size devices and construct, to provide a minimum surface area of ponding for a specific flow rate. To highlight this, we will look at rock filter dams as an example…

Rock filter dams are typically sized for a design event equivalent to half the Q1 critical storm. In sizing a rock filter dam, firstly nominate the critical sediment size. As you can see in the table below, classification as a Type 2 control can be by achieving a critical sediment size of 0.1mm or 0.05mm. Adopting the smaller sediment size significantly increases the required surface area to achieve settling, however we should always strive for the most effective option. Applying simple hydrology principles, namely the Rational Method determine your peak flow (Q in m3/sec) for your given catchment area (m2), time of concentration (Tc in mins) and design rainfall intensity (mm/hr). Using the ‘minimum settling pond surface area per unit inflow rate’ (see table below) for your adopted critical sediment size, noting water temperature (one benefit of been in the northern half of Australia is warmer water will result in slightly more effective settling) multiple by your calculated peak flow. The result will be a minimum surface area (m2) for your rock filter dam. Other aspects to consider is application of a blockage factor, spillway/weir sizing and noting key levels, being base of rock, top of embankment and weir invert to ensure designed parameters will not only suit site conditions but be practical in terms of application onsite.

Improvements to current Type 2 controls are currently being trialled (in particular by our colleagues in NZ) to improve performance and sediment capture efficiency. In some instances, coagulants or flocculants are being integrated into Type 2 controls to capture a higher proportion of fine and suspended sediments, which ultimately will make a big difference to reducing impacts to downstream waters.  It is hoped that improvements and innovation continues within this area, as Type 2 controls are an essential part of our design toolkit for low risk, small and constrained sites.

Source: Catchments and Creeks