Assessing the Potential of Reservoir Outflow Management to Reduce
Total Page:16
File Type:pdf, Size:1020Kb
HYDROLOGICAL PROCESSES Hydrol. Process. 27, 1426–1439 (2013) Published online 23 April 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9284 Assessing the potential of reservoir outflow management to reduce sedimentation using continuous turbidity monitoring and reservoir modelling† Casey Lee* and Guy Foster US Geological Survey, Lawrence, KS, 66049, USA Abstract: In-stream sensors are increasingly deployed as part of ambient water quality-monitoring networks. Temporally dense data from these networks can be used to better understand the transport of constituents through streams, lakes or reservoirs. Data from existing, continuously recording in-stream flow and water quality monitoring stations were coupled with the two-dimensional hydrodynamic CE-QUAL-W2 model to assess the potential of altered reservoir outflow management to reduce sediment trapping in John Redmond Reservoir, located in east-central Kansas. Monitoring stations upstream and downstream from the reservoir were used to estimate 5.6 million metric tons of sediment transported to John Redmond Reservoir from 2007 through 2010, 88% of which was trapped within the reservoir. The two-dimensional model was used to estimate the residence time of 55 equal- volume releases from the reservoir; sediment trapping for these releases varied from 48% to 97%. Smaller trapping efficiencies were observed when the reservoir was maintained near the normal operating capacity (relative to higher flood pool levels) and when average residence times were relatively short. An idealized, alternative outflow management scenario was constructed, which minimized reservoir elevations and the length of time water was in the reservoir, while continuing to meet downstream flood control end points identified in the reservoir water control manual. The alternative scenario is projected to reduce sediment trapping in the reservoir by approximately 3%, preventing approximately 45 000 metric tons of sediment from being deposited within the reservoir annually. This article presents an approach to quantify the potential of reservoir management using existing in-stream data; actual management decisions need to consider the effects on other reservoir benefits, such as downstream flood control and aquatic life. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS suspended sediment; turbidity; sediment trapping efficiency; reservoir modelling Received 8 August 2011; Accepted 17 February 2012 INTRODUCTION immediacy of the problem and the expense and difficulty of dredging, decommissioning or building new reservoirs, In addition to flood control, communities are reliant on the management of reservoir outflows has been altered to reservoir storage for drinking water, agricultural use and decrease or arrest sediment accumulation (Fan and Morris, industrial use. In Kansas, reservoir storage is the source of 1992a, 1992b; Morris and Fan, 1998; White, 2001; Palmieri drinking water for more than two thirds of the state et al., 2003; Morris et al., 2008). These reservoir population; studies project that a severe drought will result management strategies use the velocity of incoming flood- in water supply shortages in multiple basins (Kansas Water waters to transport incoming and previously deposited Office, 2008). Water supply shortages will become more sediments through reservoirs but require varied levels of likely as human populations grow and as sediment reservoir drawdown to maximize effectiveness. The accumulation continues to decrease available reservoir feasibility of reservoir management to reduce sediment storage. Solutions to maintaining reservoir storage are deposition varies depending on reservoir, watershed and limited because (i) sediment is naturally transported in economic considerations (White, 2001; Palmieri et al., streams and rivers; (ii) improved erosion controls may 2003; Morris et al., 2008). Compared with reservoirs not affect sedimentation for decades because of field, worldwide, the percentage of storage loss in large reservoirs floodplain and in-stream sediment storage of previously in the United States has been limited because these eroded sediments (Trimble, 1999; Evans et al., 2000); and reservoirs typically have large storage capacities relative (iii) dredging of large reservoirs such as in Kansas has, thus to incoming inflow volumes (G. Morris, written communi- far, been cost-prohibitive and disposal of sediments is cation, 2009). difficult (Kansas Water Office, 2008). Numerical models are improving the ability to simulate Internationally, the effects of sediment accumulation in the movement of turbidity currents (Gelda and Effler, 2007; reservoirs have long been realized. Because of the Chung et al., 2009) and cohesive sediment through reservoirs (Simões and Yang, 2008; Yang and Simões, *Correspondence to: Casey Lee, US Geological Survey, 4821 Quail Crest 2008). However, the episodic nature of sediment transport Place, Lawrence, KS, 66049, USA. E-mail: [email protected] to reservoirs and the spatial complexity of sediment within †This article is a US Government work and is in the public domain in the USA. reservoirs often make it difficult to test model simulations. Copyright © 2012 John Wiley & Sons, Ltd. CONTINUOUS TURBIDITY TO ASSESS RESERVOIR SEDIMENTATION 1427 Water quality sensors are increasingly being deployed 40% or more clay and 40% or more silt) is the dominant soil continuously in streams and rivers as part of ambient water type in the upstream part of the basin (US Department of quality programs (e.g. see http://waterwatch.usgs.gov/ Agriculture, 1994). wqwatch/). These data can accurately represent the flux of John Redmond Reservoir was completed in 1964 and suspended sediment at fine temporal scales (Rasmussen had a capacity of approximately 101 million m3 in the et al., 2009) and when collected upstream and downstream conservation pool, and with approximately 650 million from a lake or reservoir can be used quantify sediment m3 acre-feet of capacity including the flood control pool. trapping efficiency more accurately than through periodic The deepest point of the reservoir is approximately 312.7 sample collection (Lee et al., 2008). Further, when coupled m above mean sea level (NGVD29; Kansas Biological with an understanding of reservoir hydrodynamics, these Survey, 2010), and the top of the flood pool is 325.5 m data can be used to characterize how short-term processes, above mean sea level (Figure 2). The primary outlet such as how variation in reservoir outflow management structure is a 170.7-m-wide ogee weir, and the crest of the affects sediment flux through reservoirs. This study coupled spillway is located at 314.9 m above mean sea level. The a CE-QUAL W2 hydrodynamic reservoir model with outlet structure has a maximum discharge capacity of existing continuous turbidity data at US Geological Survey 16 400 m3/s at maximum pool level; two additional low- (USGS) streamgage sites upstream and downstream from flow outlet pipes exist at an elevation of 309.5 m above John Redmond Reservoir to assess the potential of altered mean sea level with a maximum discharge capacity of reservoir management to reduce reservoir sedimentation. 3.7 m3/s. These pipes are typically used for improving downstream water quality during low-flow conditions (USACE, 1996) but were not incorporated into the Study area reservoir model because specific information regarding John Redmond Reservoir was constructed on the their use was not made available and because their size Neosho River from 1959 through 1964 for purposes of relative to the larger gates precludes them from having a flood control, water supply and recreation [US Army Corps substantial effect on sediment flux from the reservoir. The of Engineers (USACE), 1996]. Since the dam was maximum bankfull capacity of the channel downstream completed in 1964, sediment deposition has reduced water from the dam is 340 m3/s; releases are typically kept storage at the normal operational level (termed the below this value (USACE, 1996). conservation pool) by 42%, which is among the largest The USGS streamgages located on the Neosho River percentage loss of reservoirs owned by the USACE in the near Americus (Americus) and on the Cottonwood River State of Kansas (Kansas Water Office, 2010). Approximate near Plymouth (Plymouth) were the farthest downstream sedimentation rates in John Redmond Reservoir from 1964 gages before stream entry to John Redmond Reservoir to 2006 (~910 000 m3/year) are nearly double the expected from February 2007 to May 2009 (Table I). These gages sedimentation rate expected at the time of reservoir design cover 6118 km2 (78%) of the 7809 km2 that drains to the (~500 000 m3/year; Kansas Water Office, 2010). Seventy- reservoir (USACE, 2002). A streamgage was installed on one percent of water rights downstream from John the Neosho River at Neosho Rapids (Neosho Rapids) in Redmond Reservoir are allocated for cooling of the Wolf August 2009, which better quantified the amount and Creek Nuclear Power Plant (USACE, 2002). Most of this timing of sediment transport to the reservoir (draining water is lost to evaporation after cooling (Barfield, 2010). 7130 km2 of the basin upstream from John Redmond In addition, 14% of the water rights are allocated to Reservoir). The downstream gage is located on the Neosho municipalities, 10% to irrigation and recreational uses and River at Burlington (Burlington), approximately 5 miles 5% for other industrial uses (USACE, 2002). John downstream from John Redmond Reservoir (with 70 km2 Redmond Reservoir typically