Placed Rock as Protection against Erosion by Flow down Steep Slopes

The additional resistance to erosion due to flows down steep slopes that can be achieved by placing rock instead of dumping randomly has been quantified during a large-scale flume investigation. Testing was undertaken for slopes of 0.2, 0.3, and 0.4 with two layers of armor overlying a filter, consistent with conventional rock protection designs. Two sizes of sandstone with mean diameters of 76 and $109\mathrm{mm}$ and basalt with a mean diameter of $94\mathrm{mm}$ were tested. Placing rock to achieve maximum bulk density (mass per unit in situ volume) increased failure flow (flow at exposure of the filter) by 30% of that achieved with the same type of randomly dumped material but the total armor mass per unit surface area increased by 35%. A damage approach to steep erosion protection that is similar to that used for coastal breakwater design is presented. An improved method of partition between the aerated flow over as well as flow through both random and placed rock has also been quantified. Better characterization of aerated surface flow descriptions for random and placed crushed rock is presented. A non-Darcy coefficient of $3.3\pm 1.1$ best characterized the interflow component through random and placed rock.