Brannen Lake Storage Feasibility Potential Effects on Water Levels

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1 Brannen Lake Storage Feasibility Potential Effects on Water Levels

2 Brannen Lake Storage Feasibility Potential Effects on Water Levels Prepared for: BC Conservation Foundation #3, 1200 Princess Royal Avenue Nanaimo, BC V9S 3Z7 Prepared by: Fisheries and Oceans Canada, Resource Restoration Unit 1965 Island Diesel Way Nanaimo, BC V9S 5W8 October, 2010 Brannen Lake Storage Feasibility Potential Effects on Water Levels i

3 CREDITS AND AKNOWLEDGEMENTS Topographic surveys conducted to establish elevations and spatial relationships in support of the numerical modeling were conducted by Mr. Greg Keel of Parallel Geo-Services Inc. Rik Norgan of DFO processed the survey information to create a digital terrain model of the Millstone River downstream of Brannen Lake. Graham Hill of Northwest Hydraulic Consultants provided technical information and advice in the initial phases of the investigation. Michelle Kehler of the BC Conservation Foundation provided water-level data and logistical support for the topographic surveys. Orthophotos were provided by the City of Nanaimo. Report prepared by: Richard Powley, M. Sc., P. Eng. Resource Restoration Engineer Fisheries and Oceans Canada Brannen Lake Storage Feasibility Potential Effects on Water Levels ii

4 Table of Contents Introduction...1 Flood Hydrology...1 Effect of Proposed Storage on Winter Lake Levels...7 Effect of Proposed Storage on Summer Lake Levels...9 Summary...14 Brannen Lake Storage Feasibility Potential Effects on Water Levels iii

5 Brannen Lake Storage Feasibility Potential Effects on Water Levels Introduction The BC Conservation Foundation (BCCF) is working with the BC Ministry of Environment, Fisheries and Oceans Canada (DFO) and the Snuneymuxw First Nation to improve summer rearing conditions for salmon, trout and other aquatic species in the Millstone River ecosystem. Owing both to climate change in the Pacific Northwest and longer, more severe droughts in southern BC watersheds, stream rearing trout and salmon stocks experience reduced productivity due to declines in both habitat quantity and quality. Urban development and agricultural withdrawals exacerbate these issues in the Millstone Watershed. One of the most effective ways to sustain or enhance a river s fish stocks is to provide adequate flows during the summer months, which is often a critical factor that limits fish production in small streams. To improve summer stream flow, the possibility of installing a water-control structure (e.g. weir) at the outlet of Brannen Lake to annually store a small portion of spring runoff and release it gradually through the summer and early fall is being considered. Brannen Lake naturally fluctuates up to 2.0 m (6.5 feet) during winter high water events. The amount of storage being considered for this project is approximately 30 cm (12 inches) of water acquired in the late spring and managed through the summer low period. Storage can be achieved through top or bottom strategies or a combination of the two. Top storage is water stored immediately above a lake s natural lowest level. Bottom storage is water stored below the lake s natural lowest level, made accessible by lowering (i.e., excavating) the lake outlet, or by installing a pipe that can release water. In both cases, a small weir would start to store water in the spring, and water would be released through summer and early fall. Often, a combination of top and bottom storage has the least impact on lakeshore properties and the lake and foreshore ecology. In all three situations the configuration of the weir would be such that the impact on winter lake levels would be negligible. To evaluate the potential effect that such a proposal would have on water levels in Brannen Lake, two approaches were taken; in one case, a hydrodynamic model of the Millstone River downstream of Brannen Lake was used to simulate water levels in Brannen Lake with and without a weir for a range of flows. In the second approach, historic summer water levels in Brannen Lake were simulated and compared with a case where about 0.3 m of storage was developed, and the stored water was gradually released. Flood Hydrology To establish quantitative limits on the range of flows to be modeled, two sources of information were used: hydrometric records from gauging stations in the Millstone River watershed, and a report prepared for the City of Nanaimo by Water Management Consultants 1 that documents the results of an investigation of the incremental damages associated with failure of Westwood Lake Dam during a probable maximum flood (PMF), vs. damages associated only with a PMF event. There are hydrometric flow records from two gauging stations along the Millstone River downstream of Brannen Lake: Millstone River Near Wellington (08HB027) and Millstone River at Nanaimo (08HB032), as well as records of water levels in Brannen Lake (08HB026). 1 Westwood Lake Dam Inundation Study, October, 2004, Water Management Consultants Ltd. Brannen Lake Storage Feasibility Potential Effects on Water Levels 1

6 Station 08HB032 was established in 1961, but only has continuous records from 1987 to the present. Station 08HB027 has records from 1961 to 1974, and station 08HB026 has fairly continuous lake level records for the period April 5th, 1961 to December 31st, The PMF study conducted by Water Management consultants produced quasi-deterministic flow values at various locations throughout the watershed based on rainfall-runoff modeling and hydrodynamic routing. The flow values produced through such a process are the maximum flows that could reasonably be expected to occur, given the physical characteristics of the watershed, and the meteorologic characteristics of the region. The following figure, created by compiling information contained in the WMC report, illustrates the estimated PMF flow at the outlet of Brannen Lake: Brannen Lake Storage Feasibility Potential Effects on Water Levels 2

7 82 Brannen Lake - W/S Elev.-Storage-Outflow Outflow (m 3 /s) Water Surface Elevation (m) PMF Outflow ~37 cms Storage Outflow Water Surface Elevation (m) ,000 2,000 3,000 4,000 5,000 6,000 7,000 Storage Above FSL (dam 3 ) According to this analysis, the PMF outflow from Brannen Lake would be about 37 m 3 /s. However, a PMF event is a fairly extreme event, and probably isn t appropriate for setting an upper limit on the range of flows to be modeled. Nevertheless, it does provide some context and a sense of the order of magnitude of flood flows at this location. While PMF determination is a quasi-deterministic method of evaluating flood flows, analysis of hydrometric records typically uses stochastic methods. The length of record associated with each of the gauging stations on their own is inadequate for estimating anything larger than about a 1:20 event, but by combining the records from the two stations, it is possible to conduct a more meaningful analysis. Utilizing mean daily flow records from the two stations available from overlapping operational periods, a relationship between mean daily flows at 08HB027 and 08HB032 was developed using regression analyses, as illustrated in the following figure: Brannen Lake Storage Feasibility Potential Effects on Water Levels 3

8 100 Millstone River Mean Daily Flows - Station 08HB027 vs. Station 08HB Millstone River Near Wellington (m 3 /s) Q Well = ( Q ) N 2 ( Q.00248Q ) N N Millstone River at Nanaimo (m 3 /s) Within the limits of its range of applicability, this relationship was used to adjust the recorded maximum annual mean daily peak flows from 08HB032 to reflect associated values that would have occurred at 08HB027 for the years 1987 to 2007, to augment the maximum mean daily flow data available at 08HB027 for the period 1962 to Values outside of the range of applicability of the regression relationship were adjusted by a factor of the ratio of the drainage areas of the two stations raised to the power. A relationship between instantaneous peak flows and mean daily peak flows was developed using data from station 08HB032, as illustrated in the following plot. Brannen Lake Storage Feasibility Potential Effects on Water Levels 4

9 60 Millstone River at Nanaimo - WSC Gauging Station 08HB032 Instantaneous Peak Flow vs. Mean Daily Peak Flow 50 Instantaneous Peak Flow (m 3 /s) Q i = *Q m Mean Daily Peak Flow (m 3 /s) Using this relationship, the array of maximum annual mean daily peak flows developed through the methodology previously described was adjusted to generate an array of annual maximum instantaneous peak flows that would have occurred in the vicinity of the outlet of Brannen Lake: Year Maximum Instantaneous Peak Flow (m 3 /s) Year Maximum Instantaneous Peak Flow (m 3 /s) Brannen Lake Storage Feasibility Potential Effects on Water Levels 5

10 A frequency analysis 2 of these data yielded the following plot: 100 Flood Frequency - Millstone River Near Brannen Lake PMF LP3 Distribution Discharge (m 3 /s) ,000 10,000 Return Period (Years) While the PMF, by definition, has no probability of occurrence associated with it, it has arbitrarily been assigned a return period of 10,000 years for the purposes of plotting on the graph above to provide a visual comparison to the stochastic analysis of recorded flow data. Based on the foregoing information, the upper limit on the range of flows to be modeled should probably be something in the order of about 30 m 3 /s. While it can be seen that fairly high peak flows occur with some regularity on an annual basis, the duration of high flows is relatively low. Using the augmented data array of mean daily peak flows developed for station 08HB027, an exceedance curve was developed for mean daily peak flows at this location for the October-to-June high-flow period (see following plot): 2 Because standard frequency analyses are based on the assumption that each event is discrete and independent, the discontinuous nature of the data (missing records from ; ) should not affect the result. However, the effect of any short-term climatic trends that may have influenced flows during the periods where data are missing would not be reflected. Brannen Lake Storage Feasibility Potential Effects on Water Levels 6

11 100 Millstone River Near Wellington - 08HB027 October-to-June Flow Exceedance 10 1 Flow (m 3 /s) Percent of Time Flow Equalled or Exceeded (%) As can be seen from this plot, flows are less than about 5 m 3 /s about 90% of the time (or, conversely, are greater than 5 m 3 /s 10% of the time). The median flow is about 1.5 m 3 /s. Effect of Proposed Storage on Winter Lake Levels It is proposed that the full supply level (FSL) of Brannen Lake be raised by up to 0.3 m above the level currently considered to be FSL. The purpose of increasing the lake level would be to store water that can be released during the summer months to augment flows in the Millstone River for enhancing aquatic habitat. The concept is to potentially develop both positive and negative storage that is, develop infrastructure that would allow the water level in the lake to be maintained at a level up to 0.3 m higher than existing conditions allow, and to potentially be able to draw the level of the lake down below the existing zero-outflow elevation. The proposed infrastructure associated with raising the full supply level of the lake would be an adjustable-crest structure (ie. permanent abutments and foundation, but with a removable weir section). Incorporating a removable weir section would allow Brannen Lake to rise and fall naturally during the winter time, as it currently does. Nevertheless, it is desireable to determine the impact that the proposed structure would have on water levels associated with high-flow events, should the removable section be left in place for any reason during the high-flow events. To evaluate the effect that an adjustable-crest structure would have on water levels in Brannen Lake, a numerical hydraulic model of the Millstone River downstream of Brannen Lake was developed. The model used for this study was HEC-RAS version 4.1.0, a one-dimensional hydrodynamic model developed by the US Army Corp of Engineers. It uses a backwater, step iteration process and estimated channel roughness to calculate water surface elevations for a variety of flows through a series of model sections and structures. Brannen Lake Storage Feasibility Potential Effects on Water Levels 7

12 The model was run at a variety of flows for the situation that simulates existing conditions, and for a situation where a small weir, with a removable crest, was placed upstream of the Biggs Road bridge. The following image illustrates the profile of the Millstone River downstream of Brannen Lake as it was abstracted for the modeling exercise: 79 Brannen Millstone Plan: 1) Existing -High n 2) PanWeir Modif ied Millstone River Main Legend G round LO B ROB Elevation (m ) D/S Bi... Biggs R... U/S Big... Wier Log Bridge D/S Biggs Road U/S Biggs Road Main Channel Distance (m) Prison Bridge The following image illustrates, in an elevation view looking downstream, the proposed weir structure as it was abstracted for the modeling exercise: 82 Brannen Millstone Plan: 1) Existing -High n 2) PanWeir Modif ied Riv er = Millstone River Reach = Main RS = 435 IS Wier Legend G round Bank Sta Elevation (m ) Station (m) Brannen Lake Storage Feasibility Potential Effects on Water Levels 8

13 The modeled configuration is one where 15 cm of top storage is developed that is, with the weir in the raised position, the lake level would be 15 cm higher than the current Full Supply Level. The image on the following page illustrates the calculated water surface profiles for a variety of flows, with and without the proposed weir structure. As can be seen from these plots, an adjustable-crest structure would have a negligible impact on high-flow water levels in Brannen Lake. The following chart illustrates, more concisely, the relationship between water levels in Brannen Lake and flows in the Millstone River for both the existing and proposed conditions: 78.5 Brannen Lake Water Surface Elevation vs. Outflow 78.0 Water Surface Elevation (m) Existing Conditions With 3 m Wide, 0.3 m High Removable Weir m Top/Bottom Storage With 3 m Wide, 0.3 m High Removable Weir m Top Storage Outflow (m 3 /s) Effect of Proposed Storage on Summer Lake Levels It is proposed that the full supply level (FSL) of Brannen Lake be raised by up to 0.3 m above the level currently considered to be FSL. To examine the effect that such a proposal might have on summer water levels, an historic simulation was conducted. It was assumed, for the purposes of the simulation, that the existing FSL would be about El m (consistent with the assumption used in the WMC report, and with the findings of the surveys conducted for the hydrodynamic modeling exercise). It was further assumed that the FSL would be raised by either 0.15 m to El m, or by 0.3 m to El m, and that water would be continuously released at a rate of 25 L/s throughout the months of July, August and September in both cases. Recorded lake levels from station 08HB026 for the period April 5 th, 1961 to December 31 st, 1978 were used in the simulation. As the recorded lake levels were relative to a local datum, the surveys conducted for the hydrodynamic modeling included the establishment of a geodetic elevation for the WSC benchmark associated with water-level monitoring station. Additional guidance was provided by relating recorded lake levels to the corresponding flows recorded at the WSC station nearby (08HB027), as illustrated in the following chart: Brannen Lake Storage Feasibility Potential Effects on Water Levels 9

14 Brannen Millstone Plan: 1) Existing -High n 2) PanWeir Modif ied Millstone River Main Legend WS 30 cms - Existing -High n WS 30 cms - PanWeir Modified WS 20 cms - Existing -High n WS 20 cms - PanWeir Modified WS 10 cms - PanWeir Modified WS 10 cms - Existing -High n WS 5 cms - PanWeir Modified WS 5 cms - Existing -High n WS 1 cms - PanWeir Modified WS 1 cms - Existing -High n WS 0.2 cms - Ex ist ing -High n WS 0.2 cms - PanWeir Modified Ground LO B ROB Elevation (m) Log Bridge D/S Biggs Road U/S Biggs Road Main Channel Distance ( m) Prison Bridge Brannen Lake Storage Feasibility Potential Effects on Water Levels 10

15 25 Mean Daily Millstone River Flow vs. Mean Daily Brannen Lake Level ( ) 20 Mean Daily Flow (m 3 /s) Millstone River Near Wellington (08HB027) Brannen Lake - Mean Daily Water Level (m) (08HB026) Bathymetric information for Brannen Lake was used to develop a relationship between water surface elevation and surface area and storage in Brannen Lake. The following two figures show the bathymetric map, and the area-capacity curve, respectively: Brannen Lake Storage Feasibility Potential Effects on Water Levels 11

16 Brannen Lake Flooded Area-Storage-Elevation Curves Flooded Area (ha) Existing Full Supply Level Elevation (m) 69 Storage Surface Area 69 Elevation (m) ,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 Storage Volume (dam 3 ) To conduct the simulation, net inflows under existing conditions to the lake had to be estimated. Net inflows would consist of an aggregation of surface inflows from runoff and base-flow in the surrounding and upstream watershed, precipitation on the lake surface, outflows at the lake outlet, losses to groundwater, and evaporation from the open-water surface of the lake. The aggregate effect of these various inflows and outflows would be manifested in a change in the amount of water stored in the lake. Therefore, daily net inflows for the period were estimated by using the recorded mean daily lake levels to estimate the amount of water stored in the lake for the given lake level on a given day. The daily change in water level was used to estimate the corresponding daily change in storage, and hence, the daily net inflow. To estimate the effect that the proposed changes would have had on the historic water levels, lake levels were set at either El m or El m on July 1 st (unless the recorded historic lake level on that day was higher), and the array of net inflows, plus an additional outflow of 25 L/s, was added to the storage at the start of the simulation to estimate the amount of water stored in the lake the following day. This storage estimate was used to estimate a new corresponding lake level, and so on throughout the period of the simulation 3. The figure on the following page illustrates the historic difference in lake levels between existing conditions, and what would have occurred with the proposed modifications: 3 Because simulated lake levels, and hence surface area of the lake, differed from those used to develop the array of net inflows, it can intuitively be said that the simulation would have over- or under-estimated evaporation in the simulation. However, the maximum difference in lake levels between the simulation and the historic values would only have resulted in about a 2% difference in surface area. This should be considered insignificant relative to the accuracy of the information on which the simulation is based. Brannen Lake Storage Feasibility Potential Effects on Water Levels 12

17 77 Brannen Lake July-September Simulated Water Levels Un-modified FSL Increased by 0.3 m; 25 L/s Flow Augmentation FSL Increased by 0.15 m; 25 L/s Flow Augmentation 76.7 Lake Level (m) Note: In periods where it appears that there is only a green line, the light blue and green lines are coincident Year Brannen Lake Storage Feasibility Potential Effects on Water Levels 13

18 Another way of illustrating the effect that the proposed physical and operational changes would have had on historic lake levels is through an exceedance plot: Brannen Lake July-September Lake Level Exceedance Lake Level (m) Un-modified Increased FSL by 0.3 m; 25 L/s Flow Augmentation Increased FSL by 0.15 m; 25 L/s Flow Augmentation Although the maximum difference between historic and proposed lake levels would have been about 0.3 m, it can be seen that for the vast majority of the time, the difference between historic and proposed lake levels would have been less than about 50 mm, or two inches for the top-storage scenario, whereas for the top/bottom-storage scenario, summer lake levels would have been about mm (3-4 inches) lower than those without any modifications. Summary Percent of Time Lake Level Equalled or Exceeded It is proposed that the full supply level (FSL) of Brannen Lake be raised by up to 0.3 m above the level currently considered to be FSL. The purpose of increasing the lake level would be to store water that can be released during the summer months to augment flows in the Millstone River for enhancing aquatic habitat. The concept is to potentially develop both positive and negative storage that is, develop infrastructure that would allow the water level in the lake to be maintained at a level up to 0.3 m higher than existing conditions allow, and to potentially be able to draw the level of the lake down below the existing zero-outflow elevation. Installation of an adjustable-crest structure to develop this storage would have a negligible effect on water levels in Brannen Lake for outflows greater than about 10 m 3 /s, a situation that would occur about 2% of the time during the winter period (October to June). About one-third of the time during this period, lake levels would be within about 100 mm (4 inches) of lake levels under existing conditions. Under a scenario where the full supply level of Brannen Lake was raised by 0.3 m and flows in the Millstone River were augmented at a rate of 25 L/s during the summer (July to September), water levels in Brannen Lake would be within about 50 mm (two inches) of the lake levels under existing conditions. Brannen Lake Storage Feasibility Potential Effects on Water Levels 14