1984 - robertson - surface water drainage and sedimental

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Proceedings of the 8thAnnual British Columbia Mine Reclamation Symposiumin Victoria, BC, 1984.The Technical and Research Committee on Reclamation

SURFACE WATER DRAINAGE AND SEDIMENTAL CONTROL PRACTICES AT THEBULLMOOSE COAL MINE

by J.D. Robertson, P.Eng.

IntroductionThe Bullmoose Mine is located 87 kilo-metres south of Chetwynd, B.C. within whatis generally termed "the Northeast CoalBlock." The mine, which commenced pro-duction in October 1983, has an annual pro-duction rate of 2.3 million tonnes of highquality metallurgical coal.

As part of the B.C. Coal DevelopmentGuidelines and as part of the mine recla-mation plan, a comprehensive water manage-ment plan was developed.

This plan was integrated into the total

mine design to ensure environmental pro-tection of the natural aquatic resources andto provide protection for the minesite facil-ities during the flood events. The generalcomponents of this management plan are thesubject of this paper.

Planning StrategyA preliminary assessment of the short

and long term aspects of the surface watermanagement problem resulted in adoption ofa staged approach in the development ofseveral specific solutions which collectively

satisfied a wide variety of mining, land recla-mation, and environmental objectives.The first stage consisted of adopting a

strategy directly at the outset of the projectto prevent damages to the existing aquaticresources during the construction period.

The second stage of the plan consistedof engineering design and subsequent con-struction of drainage facilities, flood controlstructures and sedimentation ponds for long-term control of surface drainage. The thirdand final phase consists of the ongoing de-velopment of the drainage system outlined in

the water management plan during the life ofthe mine, and the compliance with severalWaste Management Permits and the Recla-mation Permit issued by the Ministry of

Environment and the Ministry of Energy,Mines and Petroleum Resources respectively.The main impact of the overall drainageplan on the reclamation plan is thereduction in surface erosion which resultsfrom the proper management of surfacewater. In addition the retention of surfacesoils in the sedimentation ponds allows futureuse of this material when the ponds arereclaimed for maintenance or ultimate aban-donment. The planning and design work com-pleted in each stage of the plan is discussedin the following sections.

Preservation of Aquatic ResourcesThe primary action implemented to

prevent damages to the local creeks con-sisted of restricting construction activities,wherever practical, to within 30 metres ofthe major creeks. This green belt of pre-served, undisturbed vegetation prevented di-rect erosion of this area, and also preventedthe disturbance of the principal spawning,rearing and overwintering habitats of a resi-dent population of Dolly Varden char.

A second action taken was to di rect

site drainage from the plantsite area, whichexperienced extensive site grading work, toan existing natural swamp adjacent to theplantsite. This practice was effective duringlow precipitation periods, and preventedmajor sediment losses during significantstorm events. This interim sedimentation/swamp area was ultimately included in thearea of Sedimentation Pond 2.

Drainage System DesignA conceptual minesite drainage system

was laid out for the mine plan to demonsrate

drainage feasibility, to allow calculation ofsedimentation pond design flows, and to pro-vide guidance for the development of a de-tailed mine drainage plan (Figure I).

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Figure I

SEDIMENTATION

POND No 1

SEDIMENTATIONPOND No 2

NORTHDUMP

EAST

DUMP

BULLMOOSE PROJECT

SURFACE

WATER

DRAINAGEO 200 400 600 800 1000

meters

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The fully developed mine plan (to beachieved in approximately 1996) was dividedinto three drainage basins with areas roughlyin the proportion to the surface areas of thethree corresponding sedimentation ponds.The drainage ditch system was laid out to

divert runoff away from steep terrain to thewest, south and east of the minesite wherecollection and settling of runoff were notfeasible. Existing natural stable channelswere identified and used in the plan to eco-nomically convey the runoff down steep grad-ients to the sedimentation ponds. Flexibilitywas incorporated in the drainage layout byallowing runoff from the upper mine area tobe routed to either of the two larger ponds(Nos. I and 3).

Geotechnical studies conducted duringthe early field assessment period revealed a

problem of constructing the planned wastedumps on steep terrain adjacent to the pitarea. This problem was resolved in con-

junction with the drainage problems by em-ploying a major ravine located on the westperimeter of the mine for both purposes. Thisdesign involved installation of a rock drainwhich utilized competent coarse waste ma-terial as the foundation material for thedump.

The location of the dumps in this areawill result in a significantly shorter hauldistance of waste material while maintaining

a virtually level haulage route as the minedevelopment progresses up the mountain. Afurther major benefit of this structure will bethe smoothing effect on the storm hydro-graph (Figure 2) which results from the waterretention capability of the drain. This re-duction in peak flow allowed a correspondingreduction in the cost of the spillway for PondNo. 3. In addition, the storm flows, which arerouted to Pond No. 3 for sediment removal,will have a longer retention time and conse-quently more effective treatment.

Sedimentation Pond DesignThe hydrologic design required in stage2 of the plan was complicated by the lack oflong term local meteorologic and streamflowdata. Consequently, a network of rain gauges

and stream gauges was set up in the earlyphase of the project. Estimations of minesiterunoff flows were achieved by a careful re-view of available regional rainfall intensitydata and integration of local data collectedduring 1981 and 1982. A rainfall runoff model

was then used to estimate the sedimentationpond inflow hydrographs. The sizes of thesedimentation ponds were determined by theten year return period flow estimates, andthe designs of the spillways were based onestimates of 200 year return period flows.

The surface areas of the three pondsmeet the British Columbia Government de-sign guidelines for treatment of runoff fromthe entire area resulting from the 13 yearlife of the South Fork deposit. The designcriteria are outlined in Table I. In addition,the ponds incorporated baffle dykes to maxi-

mize the actual retention time and improvesettling, and roughing ponds to facilitatecleanout maintenance procedures. The finaldischarge structures for Ponds I and 3 consistof economical rock weir spillways, con-structed of competent coarse rock, while thedischarge structure for Pond 2 consists ofcorrugated steel pipe arches with an outflowstilling basin composed of large riprap.

Available areas for sedimentation pondswere restricted to the West Bullmoose CreekValley bottom immediately northwest of theplantsite. The pond dykes were built on a

foundation of dense, silty sands and gravelsand were constructed as compacted, im-permeable, earthfill structures. The generalpond areas also consist of glaciofluvial de-posits, and as such are expected to initiallyexfiltrate varying portions of the pond in-flows to the groundwater table.

Flood ControlThe data derived from the hydrologic

surveys employed in the mine drainage designwas also utilized to predict the 200 yearflood flow rates in the adjacent Bullmoose

and West Bullmoose Creeks. These flowrates, which occurred during spring freshetand summer storm events, were used to pre-dict the corresponding flood levels in thearea of the plantsite, the access bridge, and

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Figure 2

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Table I

Sedimentation and Pond Design

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the main Bullmoose valley. Consequentlydykes were designed to protect the plantsite,the sedimentation ponds, and the tailingspond. These dykes, constructed of compactednative materials, were armoured withquarried rock riprap which was installed to a

level below the scour depths of the creeks.Similarly the plantsite bridge across Bull-moose Creek was designed as a single spanwith adequate clearance for the predictedfloodwaters and floating debris. The bridgeabutments were also extensively armouredwith riprap material.

The construction work required for allof these facilities was performed during lowflow periods and specified "time windows"which avoided seasonal fish spawning periods.Thus direct contact with the creek was pre-vented, and any impact on the adjacent

fishery resources was minimized.

Operation and Performance of the PondsPonds No. I and 2 were completed and

operational in the fall of 1982. It must beemphasized that the design criteria adoptedis firstly intended to treat runoff from theultimate area of disturbance (1996 mine area)and secondly, is intended to accommodatelarge storm events which may occur duringthe spring snowmelt period or early summer.The flows during 1983 were low due to thelimited area draining to the ponds and the

absence of a significant freshet or majorstorm event. Hence no positive dischargeoccurred from either pond, as the exfiltrationcapacity was sufficient to process the entire

inflow volume.Pond No. 3 was completed in the late

fall of 1983 and hence has limited operatingdata. Similar to the other two ponds, theentire inflow is initially expected to exfil-trate to the adjacent ground water table.

ConclusionsThe Bullmoose Mine Surface Water

Management plan was successfully imple-mented by first following a strategy of pre-serving the natural stream conditions andsecondly, designing appropriate structures forthe control and treatment of surface drainage prior to discharging water to the adja-cent creeks. The design of the components ofthis system was based on a limited knowledgeof post startup conditions with respect to

detailed mine layout, the runoff responsefrom developed areas, expected sedimentyields and sediment settling characteristics.Downslope drainage is economically conveyedto sediment ponds either through stable nat-ural channels or through a rock drain locatedin the base of a major waste dump. Floodprotection dykes were installed to controlflood waters in Bullmoose and West Bull-moose Creeks and to protect the adjacentplantsite facilities.

Implementation of this water manage-ment plan will ensure that erosion losses are

minimized and sediment discharges are con-trolled, thereby preventing removal of thebasic resource material necessary for a suc-cessful land reclamation program.

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1984 - robertson - surface water drainage and sedimental

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