Laserfiche WebLink
<br />-3- <br />2. The amendment'proposes to retain the water treatment pond for wildlife <br />use and to reduce sediment loads. The operator predicted that the dam <br />would last over 400 years based on anticipated sediment inflows and <br />available sediment storage. It might be appropriate for the operator to <br />predict the life expectancy of the dam from a hydrologic and stability <br />standpoint. This could be a general statement based on past experienced, <br />dam design, spillway design, past maintenance requirements at the dam, <br />etc. It would also be appropriate to discuss the worst case consequence <br />of a dam failure downstream. <br />3. Page 7 of the Drainage and Erosion Control Plan states that the upland <br />diversions are not conducive to maintenance-free operation. Therefore, <br />it is proposed to breach the diversions at existing drainages. First, <br />since the diversions will be very useful early on in minimizing runoff <br />on reclaimed sites, could the diversions be breached only partly? This <br />would allow the diversions to function during smaller events, but breach <br />at the appropriate point during a large event. Secondly, are there <br />reasons that the diversions are not maintenance-free that would be <br />analogous to the other drainage channels proposed for the mine? <br />Drainage Control <br />1. The peak flow calculations for the mine assume an SCS runoff-curve <br />number of 60 for the mine. This appears somewhat low. Under what <br />conditions is this curve number representative of? (i.e. vegetation <br />type, soils, vegetation cover, etc.). What curve number might we expect <br />during the early states of reclamation? <br />2. A cascade drop structure is proposed above drainage point 4. The <br />amendment should address how durable and competent the rock slope is in <br />this area. Also, would it be appropriate to include an energy <br />dissipator at the base of this structure? <br />3. Several ramps are to retained in the North Pit (ex. 475, 550, 575). It <br />was not made clear how drainage is to be handled off of and down these <br />ramps. The amendment should address runoff in this pit. Items we would <br />want to consider are which way ramps are pitched to drain, are roadside <br />ditches to be utilized, are water bars to be used on ramps, and other <br />control practices to handle runoff without scouring vegetation or <br />causing gullies. How will this drainage plan function under the <br />movement expected in this pit? Awell-planned drainage plan for the <br />North Pit needs to be provided. <br />4. The ditch from the South Pit converges with another ditch above the Tie <br />Camp dump; is this intersection designed (armored) to dissipate energy <br />and prevent breaching? <br />5. Hydrologic investigations at the mine have used rainfall models to <br />predict impacts and design structures. No discussion on snowmelt <br />characteristics was presented, but should be discussed to the extent <br />possible. Is streamflow data available that would indicate peak <br />snowmelt conditions, and how snowmelt compares with rainfall runoff? <br />This type of information is needed to assess the reliability of your <br />runoff calculations, erosion potential, life expectancy of structures, <br />and reclamation success. <br />