Blue Lakes Stream Gauge Improvement and Lake Level Stabilization Project:Summary of Dye Tracer TestingCity of Twin FallsSeptember 12, 2011

Brief BackgroundBlue Lakes pumps operational 1994Lower lake weir installed 1997 in an effort to stabilize lake level fluctuationsMembrane liner placed upstream of weir, and later extended to about 50Excessive seepage from bottom of lake prevents weir from operating effectively:The orifice large holeFissure(s)Seepage around boulders upstream of weir

Brief BackgroundExtensive computer modeling: must have at least 130 to 140 cfs net inflow to lower lake. Below this flow, level control is compromisedNet inflow = Natural Spring Inflow City Pumping Lake control problem aggravated by continuing declines in natural spring flow

Where is it going? Statistically, the trend is still downward.

Brief BackgroundCurrent maximum day pumping: 25 cfsDown from a peak of 31 cfs in 2003Maximum day typically occurs in JulySprings have a seasonal pattern, low point typically in May, and have not recovered by JulyNatural spring inflow must be 155 to 165 cfs to achieve effective lower lake level control at current maximum day demand.July natural flow, last 5 years: 120 to 140 cfs

Proposed SolutionComponent 1: Replace USGS gauging station with measuring deviceRated section is highly inaccurate+/-30 cfs is not adequate to properly manage the lake systemTrend in natural spring inflow (not the total Blue Lakes supply at the BL Trout weir)Daily value of spring inflow guides how much can be pumped

Proposed SolutionReduce seepage from Lower LakeAttempt to find more cost effective ways other than controlling the orificeOther flow pathsMore flow passes over the lake control weir and sinks

Question: Will changing the seepage patternReduce the flow in Alpheus Creek?Change the distribution of outflow from the various springs in the Alpheus Spring complex?

Dye Tracer Test: ObjectivesIdentify high seepage areas in lake bottom (other than orifice)Define flow paths, travel times, and mass balanceDetermine whether all water reaches Alpheus Springs, or if some bypasses the systemDetermine effect of seepage reduction actions on distribution of flow from Alpheus Springs

BLCC allowed testing and permit applications

Preliminary WorkInitial reconnaissanceThree divesSmall amounts of dye used to identify seepage areasIdentified another major seepage location (fissure)Measured outflow of fissure and the orifice using current meterOrifice: 24 cfsFissure: 14 cfsNo appreciable seepage from upper lakeVideo

Dye Tracer TestFour injection locations, slug injectionsMeasured dye concentration curve at four locations:Alpheus Springs, Alpheus Creek at BL Trout weirMeasured presence/absence of dye in Snake River Dye: Rhodamine WTAcknowledgementsIDWR and Idaho Power Co. fluorometer equipment and expertiseIDEQ discrete samplerOzark Underground Laboratories, Missouri

Overall map

Map of lake and injection points

Map of sample locations

Breakthrough CurveElapsed TimeConcentrationTime to peakArrival time

Test 1:OrificeTest 2:FissureTest 3:Boulder FieldTest 4:Pool below WeirTest 5: Orifice

Overall Mass Balance at BL Trout WeirIntegrate Q x C under breakthrough curveAll tests - mass injected: 30 lbMass recovered: 25.7 lb (86%)Some adsorption and other degradation of dye is expectedNo dye detected in any charcoal packet in Snake RiverMargin of error in mass balance may be +/- 10 to 15%Conclude: no significant mass loss

Mass Balance Individual Springs

Seepage Reduction Action ScenariosScenario 1: 15 cfs eliminated from fissure, infiltrates in pool below weirScenario 2: 20 cfs eliminated from boulder field, infiltrates in pool below weirScenario 3 (Preferred): 15 cfs eliminated from fissure, 20 cfs eliminated from boulder field, infiltrates in pool below weirScenario 4: 20 cfs eliminated from orifice, infiltrates in pool below weirScenario 5: 15 cfs eliminated from fissue, 20 cfs from boulder field, 20 cfs from orifice, infiltrates in pool below weir

Predicted Flow ChangesFlow change in absolute CFS:

Basic Findings and ConclusionsAll water seeping from the Lower Lake emerges at one or more springs in the Alpheus complex81% of the flow emerges from the City Spring / Main Spring complex; 19% of the flow emerges from the Clubhouse SpringSignificantly different response curves depending on seepage locationProposed seepage reduction actions will not decrease the flow in Alpheus CreekProposed seepage reduction actions will not significantly change spring flow distribution.

Proposed PlanInstall measuring device at USGS gaugeReduce or eliminate seepage in fissure and boulder fieldNatural gravel packPotential for membrane liner if neededTarget 35 cfs seepage reductionTarget minimum inflow to lower lake would be reduced from 130-140 cfs to about 95-105 cfsSpring inflow would need to be:120-130 cfs to allow City to pump current 25 cfs max day147-157 cfs to allow City to pump full 52 cfs water right

Proposed PlanMitigate for roadside creek flowHydraulic fixOptions for pumping