Floccing ESC myths: Rock filter check dam - what the? (Part 1)

One of the most commonly misunderstood and incorrectly referenced controls within our ESC toolkit is that of a rock filter dam and a rock check dam. Sure they sound the same, which no doubt leads to confusion as to which one to use, where to install, and importantly how do they work?

Rock filter dams are a Type 2 sediment control measure. They do NOT filter, rather the rock structure should allow (if installed correctly) temporary ponding of runoff against the upslope face providing opportunity for coarse sediment to settle out via gravity. The height of the rock filter dam, rock size and slope of rock placement are all key factors which influence the hydraulic capacity and ponding area and volume achieved by the control. Installation should include a spillway, ensuring overtopping occurs over the rock structure and onto a stable surface.

Rock check dams are a temporary drainage control measure. Flow velocities greater than 0.5m/s will likely cause erosion (depending on soil type) to unstabilised surfaces and drainage lines or flow paths. Rock check dams effectively reduce the grade of the drain and therefore velocity of water within the drain. Key aspects include the spacing of the check dams (based on the slope of the drain, with the toe of the upslope dam to be level with the top of the downslope dam), cross section of check dam (installed so as to not overtop the drain or reduce flow capacity) and rock size. Alternatives to rock include sand bags, coir logs, silt socks or similar so long as the installation achieves the principle of reducing flow velocity. The result of check dams reducing flow velocity is often they will capture heavy sediment particles upslope of the dam structure, which gives the impression that we have achieved ‘sediment control’. The quantity of sediment captured by check dams is very minor and secondary to reducing flow velocities to reduce/avoid scour within drains.

Typical engineering plans do not highlight the difference between these two controls…

Keep an eye out over the next few months as we will try and clear up (floc out you could say) some of the common myths and failures seen on ESC plans and their implementation onsite. If you have any suggestions please let us know.