BASIN STRATEGY: Hydrologic Analysis

Transcription

1 BASIN STRATEGY: Hydrologic Analysis March 2003 Prepared for: The Red River Basin Flood Damage Reduction Work Group Prepared by: Technical and Scientific Advisory Committee (TSAC) Primary author: Brent Johnson

2 BASIN STRATEGY: Hydrologic Analysis March 2003 Prepared for: The Red River Basin Flood Damage Reduction Work Group Prepared by: Technical and Scientific Advisory Committee (TSAC) Primary author: Brent Johnson I hereby certify that this plan, specification, or report was prepared by me or under my direct supervision, and that I am a duly Registered Professional Engineer under the laws of the State of Minnesota. Brent H. Johnson MN. Reg. No Date: Houston Engineering, Inc rd Avenue N. Ste. 106 Maple Grove, MN Phone (763) Fax (763) HE Project #

3 TABLE OF CONTENTS for Basin Strategy: Hydrologic Analysis March 2003 Chapter 1: Introduction 1-1 Chapter 2: Hydrologic Model Study General Modeling Method Gate-Controlled Reservoir Simulation Local Inflow Hydrographs Detention Time Reservoir Storage Period Limitations Hydrologic Modeling Results Ideal Storage 2-25 Chapter 3: Storage and Channel Capacity for 10-Year Ag Drainage Design Year Precipitation and Runoff Runoff Volume Versus Drainage Area SCS Design Channel Capacity Versus Drainage Area Storage Versus Channel Capacity Summary 3-9 Chapter 4: Timing of Tributary Peak Flow Contributions to Mainstem Floods Tributary Timing Comparison Comparison of Tributary Timing By Flood Tributary Timing and Runoff Volumes Within 8-Day Window Summary 4-7 Chapter 5: Study Summary How Long Should Flood Water Be Stored? How Much Flood Storage is Needed and What Will Be the FDR Impact? 5-6 Page List of Tables Chapter 2 Table 2-1 Tributary Areas Studied 2-3 Table 2-2 Example of Hydrograph Diversion 2-5 Table 2-3 Detention Time of the Red River Floods 2-12 Table 2-4 Volumes Diverted to Virtual Storage From Tributary Areas 2-22 Table 2-5 Observed Flow and Virtual Flows Reductions 2-23 ( ) -i-

4 Chapter 3 Table 3-1 Design Precipitation and Runoff Depths 3-3 Table 3-2 Drainage Area vs. Runoff Volume 3-4 Table 3-3 Storage Requirements vs. Drainage Area for Typical Ditch Capacity 3-7 Chapter 4 Table 4-1 Floods Studied by the Corps and RRWMB 4-1 Table 4-2 Highest Peak Discharges Observed at Emerson, Manitoba 4-2 Table 4-3 Time Interval Between Peaks of Routed Tributary and Mainstem Hydrograph 4-6 Table 4-4 Time Interval Between Peaks of Routed Tributary and Mainstem Hydrograph (Second Peaks for Buffalo, Goose, and Turtle Rivers) 4-8 Table 4-5 Tributary Timing and 8-Day Volumes for 1997 Flood 4-9 List of Figures Chapter 2 Figure 2-1 Modeling Example 2-6 Figure Flood Observed and Routed Through a Virtual Storage Reservoir 2-7 Figure 2-3 Detention Time of Red River Floods 2-12 Figure 2-4 Detention Time of 1997 Flood at Emerson, Manitoba 2-13 Figure 2-5 Red River Spring Floods at Grand Forks (1948 to 1997) 2-13 Figure Flood Observed at Wahpeton, ND and Virtual Flow due to 50% Tributary Reduction 2-15 Figure Flood Observed at West Fargo, ND and Virtual Flow due to 50% Tributary Reduction 2-15 Figure 2-8 Ottertail River 1997 Flood Observed and Routed Through a Virtual Storage Reservoir 2-16 Figure 2-9 Triangular Hydrograph Volumes 2-17 Figure Flood Observed at Emerson, Manitoba and Virtual Flow due to 50% Tributary Reduction 2-19 Figure Flood Observed at Emerson, Manitoba and Virtual Flow due to 25% Tributary Reduction 2-20 Figure 2-12 Mainstem Peak Flow Reduction vs. Volume Stored Within Virtual Dams 2-24 Figure 2-13 Tributary Peak Flow Reduction vs. Volume Stored Within Virtual Dams 2-24 Figure 2-14 Ideal Volume Removed From Flood for Peak Flow Reduction Virtual Effects of Storage of Upper 25% of Tributary Flows 2-26 Figure 2-15 Ideal Volume Removed From Flood for Peak Flow Reduction Virtual Effects of Storage of Upper 50% of Tributary Flows 2-26 ( ) -ii-

5 Chapter 3 Figure 3-1 Storage Capacity vs. Channel Capacity; 1, 10, and 100 Square Miles 3-7 Figure 3-2 Storage Capacity vs. Channel Capacity; 1 Square Mile 3-8 Figure 3-3 Storage Capacity vs. Channel Capacity; 10 Square Miles 3-8 Figure 3-4 Storage Capacity vs. Channel Capacity; 100 Square Miles 3-9 Chapter 4 Figure 4-1 Timing Between Routed Tributary and Red River Peak Flow 4-3 Figure 4-2 Number of Routed Tributary Peak Flows Occurring Within Successive Eight-Day Windows of the Mainstem Peak 4-4 Appendix A Appendix B HEC-1 Input Data CD Containing Digital Copies of Model Cover map prepared by: Ecological Research Division Environment, Canada ( ) -iii-

6 CHAPTER 1 INTRODUCTION The Red River Basin Flood Damage Reduction Work Group (Work Group) requested its Technical and Scientific Advisory Committee (TSAC) to develop specific basin-wide strategies for achieving flood damage reduction and natural resource goals for use in preliminary planning for basin flow management efforts. The natural resource problems and opportunities document has been completed. This report provides the technical basis of the storage goals component of the hydrologic study. This report includes several components: -Hydrologic Model Study of the Effects of Tributary Storage -Analysis of Storage and Capacity Relations in 10-year Ag Drainage Design -Analysis of Timing of Tributary Contributions to Mainstem Floods ( ) 1-1

7 CHAPTER 2 HYDROLOGIC MODEL STUDY This study analyzes flood damage reduction within the 36,000 square mile contributing watershed of the Red River Basin of Minnesota, North Dakota and South Dakota. Goals of the study are to determine target flood storage volumes, flood storage locations, and to reduce flood damages on the Red River and its tributaries. This work included hydrologic routing within a 430-mile river reach extending from Lake Traverse to Emerson, Manitoba, data analysis, and preparation of the study report. This portion of the study deals with several what if scenarios. For example: What effect would there be upon Red River flood flows if peak tributary inflow was reduced by 25 and 50 percent? How much storage would be required to achieve these reductions? How long should floodwater be stored? 2.1 General Modeling Method The Red River Watershed Management Board s (RRWMB) hydrologic model of the 1997 Red River Flood 1 was modified for use in this flood storage analysis. The RRWMB model is a HEC-1 2 hydrologic model using the Straddle-Stagger empirical flood routing method. The storage model simulates the virtual condition where hypothetical reservoirs temporarily store part of the tributary inflow during the 1997 Red River flood. The model allows a determination of the effects that additional storage would have had upon flow in the Red River downstream. The effects of the virtual storage are compared to the 1 Red River Watershed Management Board, Hydrologic Routing of the 1997 Red River Flood, US Army Corps of Engineers, HEC-1 Flood Hydrograph Package, Hydrologic Engineering Center, Davis California, ( ) 2-1

8 CHAPTER 2 observed flood conditions. The model allows the determination of the theoretical impact that virtual reservoirs might have had upon the 1997 Red River Flood. The study analyzes the Red River upstream from Emerson, Manitoba. Virtual reservoirs are simulated on many of the rivers that are tributary to the Red River as well as within areas of local inflow (ungauged areas that contribute directly to the Red River). The Roseau River was not included within the model since it is not upstream from Emerson, but volume calculations for the Roseau River at Caribou have been made and are included in the summary tables. Table 2-1 lists the 39 U.S. Geological Survey stream flow gauging stations and tributary areas included in the study. ( ) 2-2

9 CHAPTER 2 Table 2-1 Tributary Areas Studied SITE DRAINAGE AREA (sq. mi.) BOIS DE SIOUX R. NEAR WHITE ROCK, SD 1160 OTTERTAIL R. BELOW ORWELL DAM 1740 RED RIVER LOCAL ABOVE WAHPETON 1110 RED R. AT WAHPETON, ND 4010 RED R. LOCAL ABOVE HICKSON 290 RED R. AT HICKSON, ND 4300 WILD RICE R. NEAR RUTLAND, ND 546 WILD RICE R. LOCAL ABOVE ABERCROMBIE 1534 WILD RICE R. AT ABERCROMBIE, ND 2080 RED R. LOCAL ABOVE FARGO 420 RED R. AT FARGO, ND 6800 SHEYENNE R. NEAR KINDRED, ND 5000 SHEYENNE R. LOCAL ABOVE HORACE 40 SHEYENNE R. AT HORACE, ND 5040 SHEYENNE R. LOCAL ABOVE W. FARGO 30 SHEYENNE R. AT W. FARGO, ND 5070 MAPLE R. NEAR ENDERLIN, ND 843 MAPLE R. LOCAL ABOVE MAPLETON 637 MAPLE R. AT MAPLETON, ND 1480 RUSH R. AT AMENIA, ND 116 BUFFALO R. NEAR HAWLEY, MN 322 S. BRANCH OF THE BUFFALO R. AT SABIN, MN 522 BUFFALO R. LOCAL BELOW SABIN AND HAWLEY 196 BUFFALO R. AT DILWORTH, MN 1040 WILD RICE R. AT HENDRUM, MN 1600 RED R. LOCAL ABOVE HALSTAD 1894 RED R. AT HALSTAD, MN GOOSE R. AT HILLSBORO, ND 1203 MARSH R. NEAR SHELLY, MN 151 SANDHILL R. AT CLIMAX, MN 426 RED LAKE R. AT HIGHLANDING 2300 THIEF R. NEAR THIEF RIVER FALLS, MN 959 CLEARWATER R. AT PLUMMER, MN 512 LOST R. AT OKLEE, MN 266 CLEARWATER R. LOCAL BELOW PLUMMER AND OKLEE 592 CLEARWATER R. AT RED LAKE FALLS, MN 1370 RED LAKE R. LOCAL ABOVE CROOKSTON 641 RED LAKE R. AT CROOKSTON, MN 5270 RED R. LOCAL ABOVE GRAND FORKS 1250 RED R. AT GRAND FORKS, ND TURTLE R. NEAR ARVILLA, ND 311 FOREST R. AT MINTO, ND 740 MIDDLE R. AT ARGYLE, MN 265 PARK R. AT GRAFTON, ND 695 RED R. LOCAL ABOVE DRAYTON 2689 RED R. AT DRAYTON, ND S. BRANCH TWO RIVERS AT LAKE BRONSON, MN 444 PEMBINA R. NEAR WINDYGATES, MAN PEMBINA R. LOCAL BELOW WINDYGATES 390 PEMBINA R. AT NECHE, ND 3410 TONGUE R. AT AKRA, ND 160 RED R. LOCAL ABOVE EMERSON 1386 RED R. AT EMERSON, MAN ROSEAU R AT CARIBOU 1560 (Note: Roseau R is not in model) ( ) 2-3

10 CHAPTER 2 Specific reservoir locations have not been identified except as within each tributary drainage area. The modeling has been done as though the storage areas are located at the gauging sites, with the assumption that the storage is fully effective in reducing the flows at the gauging sites. The model determines the virtual storage removed from a hydrograph. Actual reservoirs will need to hold larger volumes to achieve the same effects since actual reservoirs are not totally efficient and timing of operation during a flood will not likely be as perfect as a computer simulation using hindsight. In addition, actual reservoirs will not be located ideally some storage will likely be used to reduce local floodplain storage without mainstem flow reductions. The model uses a diversion statement following each tributary inflow hydrograph to divide the inflow into two hydrographs. One undiverted hydrograph is routed downstream, while the other diverted hydrograph is modeled as though its flow is temporarily stored within a gate-controlled reservoir. The virtual reservoir stores all of the diverted flow for a selected time interval (20, 30 or 40 days) following the onset of diversion, and releases 90% of the stored volume at a uniform rate over the succeeding 30 days. The diversion statement splits the inflow hydrograph at a selected ratio of the observed peak flow (eg. 25%, 50%). The portion of flow less than or equal to the selected ratio (target ratio), of the observed peak flow, is arranged in an undiverted hydrograph and routed downstream. The portion of flow exceeding the selected ratio, of the observed ( ) 2-4

11 CHAPTER 2 peak flow, is virtually diverted recorded in a diversion hydrograph and routed through a virtual reservoir. Table 2-2 provides an example where flows in excess of 50% of the peak are diverted to a virtual reservoir. The model arranges the observed hydrograph flow values into a diverted hydrograph and an undiverted hydrograph as shown in Table 2-2. Figure 2-1 provides a graphical illustration of this example. Table 2-2 Example of Hydrograph Diversion Note: Flows exceeding 50% of the observed peak are diverted Observed Flow (cfs) Undiverted Flow (cfs) Diverted Flow (cfs) Time 1 Time 2 Time 3 Time 4 (peak flow) Time 5 Time 6 Time ( ) 2-5

12 CHAPTER 2 Figure 2-1 Modeling Example (for 50% of peak flow diverted) Flow (cfs) Time (days) Observed Flow (cfs) Undiverted Flow (cfs) Diverted Flow (cfs) The model was used to determine the effects of the temporary storage of a portion of the tributary inflow. Runoff was modeled as temporarily stored then gradually released to the Red River. The volume stored within the hypothetical reservoirs is temporarily detained within each contributing area, then released and routed to simulate the effects downstream. Both the undiverted flows and the temporarily stored flows were routed through the river system to determine the effects of the storage at all of the gaging stations downstream. Figure 2-2 includes the observed hydrograph for the Bois de Sioux River as well as the simulated hydrograph for the condition of virtual storage of the flow exceeding 50% of the peak flow. ( ) 2-6

13 CHAPTER 2 Figure Flood Observed and Routed Through A Virtual Storage Reservoir (Bois de Sioux, flow stored above 50% of peak flow, 40 day retention) Flow (cfs) Bois de Lake Traverse observed Bois de Lake Traverse virtual reservoir /30/1997 5/23/1997 5/16/1997 5/9/1997 5/2/1997 4/25/1997 4/18/1997 4/11/1997 4/4/1997 3/28/1997 Date 2.2 Gate-Controlled Reservoir Simulation The HEC-1 model is capable of simulating storage routing within reservoirs having automatic (ungated) reservoir outlets. An automatic reservoir outlet includes spillways or conduits with fixed crests and fixed gate openings. Water flows automatically through an ungated reservoir outlet as a function of the fixed spillway geometry and the changing reservoir water level. A gated reservoir outlet includes adjustable gates and spillways as well as an operating plan to guide the storage and release of water. Water flows out through a gated reservoir primarily as a function of the gate operation. In this study, hypothetical reservoirs were modeled as gate-controlled reservoirs. The assumed operating plan included an initial storage period with no outflow followed by a discharge period. The discharge period includes constant outflow at a uniform rate ( ) 2-7

14 CHAPTER 2 sufficient to evacuate 90% of the stored volume within a 30 day period. The reservoir simulation was done within HEC-1 by manually inputting the volume of flow stored within the reservoirs. The volume was input as a one-day runoff event, on the day corresponding to the end of a selected storage period (e.g. 20, 30, or 40 days). For example, 20 days following the first diversion of flow from the observed hydrograph, the simulated reservoir is filled with the volume of water diverted to it and begins to discharge. The reservoir outflow is set at a constant rate so that 90% of the stored volume will be discharged within the succeeding 30 days. Figure 2-2 shows the effect of a 40-day storage period on the Bois de Sioux River. This gate-controlled reservoir analysis requires that the model be run twice. The initial run is used to determine the volume of water removed from each inflow hydrograph. These volumes must next be manually input into the model before performing another run to compute the reservoir releases and the flood routing. Since HEC-1 is not explicitly set up for modeling gate-controlled reservoirs, a two-step process was used to accomplish the simulation. 2.3 Local Inflow Hydrographs Local Inflow Hydrographs require additional modeling steps. Local Inflow Hydrographs are defined as the residual between the observed hydrograph at a gauging station and the sum of the hydrographs routed to the site from upstream gauging stations. The residual hydrographs often include negative flow values at some time ordinates. The RRWMB 1997 Flood report states: ( ) 2-8

15 CHAPTER 2 The local inflow represents the difference in the flow routed to a gauging station and the flow observed at the gauging station. The local inflow was determined as the difference between the observed hydrograph and the sum of those hydrographs routed to the location of the observed hydrograph. These local inflow hydrographs include negative flow values at some hydrograph ordinates. A negative flow value indicates that the sum of the routed hydrographs exceeds the observed flow value at that ordinate. In essence, the model indicates more upstream flow routed to the downstream site, than was observed at the site on that date. This may indicate the effects of floodplain storage, diversion or interbasin flow transfers, or errors in either the flow measurements or the routing. Negative local inflow values most likely result from the modeling process which does not account for floodplain storage, or from interbasin flow transfers. 3 Several additional modeling steps are required to determine the diverted flows and stored volumes from these local inflow (residual) hydrographs. The HEC-1 modeling routines place all negative inflow values into the diverted hydrograph, regardless of the values entered in the input rating curves. The negative inflow values are important for further flood routing, so to avoid losing those data a slightly different modeling scenario is required: Set the model input so the diversion rating curves divert those flows below the target peak flow ratio and retain the flows above the target ratio, 3 Red River Watershed Management Board, Hydrologic Routing of the 1997 Red River Flood, ( ) 2-9

16 CHAPTER 2 Add an additional diversion step to empty the stack (divert all flow and retain zero flow), Manually input the volume of target storage to the virtual reservoir, Recall the diverted low (and negative) flows, add the reservoir outflow, and rout downstream. For examples of these model steps, see the HEC-1 input data included in Appendix A. A digital copy of the model is also included in the attached CD (Appendix B) and a printed copy of the input data deck for the Hec-1 model is included in Appendix A. 2.4 Detention Time The McCombs-Knutson report analyzed the runoff volume occurring within an eight-day window around the mainstem peak. 4 The eight-day window around the Red River flood peak was used as a planning target. Reservoir construction or operation which would reduce a tributary s volume of runoff within the target eight-day window would in turn reduce the Red River flood peak and associated damages. Similarly, reservoir construction or operation which altered a tributary s contribution in a way that increased the runoff in the eight-day window would be expected to increase the Red River flood peak and associated damages. 4 McCombs-Knutson Associates, Inc. Water Resources Engineering/Planning Program for the Red River of the North Basin in Minnesota, May 1984 ( ) 2-10

17 CHAPTER 2 As an outgrowth of the McCombs-Knutson study, the Red River Watershed Management Board generally favors gate-controlled flood storage reservoirs with potential for longduration flood storage. Un-gated flood control reservoirs generally receive less RRWMB funding, since un-gated reservoirs typically have short-term storage which is less beneficial for mainstem flood control, and could even increase a flood peak on the mainstem in certain situations. The detention time for storage of flood runoff, as a measure to reduce Red River flood peaks, is defined within the McComb-Knutson report as the interval between the start of the eight-day window to the point where the river flow recedes below flood stage of 35,000 cfs at Emerson. Ten floods occurring from 1948 through 1979 were analyzed in the McCombs-Knutson report. Table 2-3 and Figure 2-3 present the detention times calculated in the McCombs-Knutson report as well as the 1997 flood detention time (determined from the information in the RRWMB report). Detention time ranges from 11 days during the 1970 and 1974 floods to 31 days during the 1997 flood. Of the floods studied, the average detention time is 18 days and the standard deviation is 7 days. Figure 2-4 is a hydrograph of the Red River flood at Emerson in The 8-day window around the peak and the detention time are shown on the figure. Figure 2-5 includes hydrographs from 12 historic floods on the Red River at Grand Forks. Each flood hydrograph has been normalized so the peaks are aligned for easier viewing. This figure allows a visual comparison of various windows around the Red River peak (e.g. 8-day, 10-day, etc.) ( ) 2-11

18 CHAPTER 2 Table 2-3 Detention Time of the Red River Floods Detention Time Year (days) Figure 2-3 Detention Time of Red River Floods Detention Time Detention Time (days) Flood Detention Time (days) Average=18 Ave - SD Ave + SD ( ) 2-12

19 CHAPTER Figure 2-4 Detention Time of 1997 Flood 1997 Flood at Emerson, Man. Red River of the North at Emerson, Man. Peak day Window Flow (cfs) Detention Time Days Flood Stage 35, / 18/1997 4/ 20/1997 4/ 22/1997 4/ 24/1997 4/ 26/1997 4/ 28/1997 4/30/1997 5/2/1997 5/4/1997 5/6/1997 5/8/1997 5/10/1997 5/12/1997 5/14/1997 5/16/1997 5/18/1997 5/20/1997 5/22/1997 5/24/1997 5/26/1997 5/28/ /30/1997 Figure 2-5 Red River Spring Floods at Grand Forks (1948 to 1997) F 1948 F 1950 F 1965 F 1966 F 1969 F 1970 F 1975 F 1978 F 1979 F 1989 F 1996 F 1997 ( ) 2-13

20 CHAPTER Reservoir Storage Period In this study, hypothetical reservoirs were modeled as gate-controlled reservoirs. The assumed operating plan included an initial storage period with no outflow followed by a discharge period. Three different storage periods (with no outflow) were used in the hydrologic model analysis of the 1997 flood: 20, 30 and 40 days. A 20-day storage period was initially used. For the 1997 flood simulation the 20-day storage period was beneficial, but not nearly as beneficial as the 30 and 40-day storage periods. The 20 and 30 day periods were too short at some locations, so that flows released from storage caused both tributary and mainstem flow to be too high in some locations. Figure 2-6 is a hydrograph of the Red River at Wahpeton showing a comparison of 20-day and 40-day storage periods. Flow released following the 20-day storage period causes a peak of approximately 10,000 cfs about 1000 cfs higher than the peak flow resulting from 30- day and 40-day storage periods. The 20-day storage period showed particularly poor performance on tributary rivers with broad hydrographs. Figure 2-7 provides an example of flow discharged following a 20- day storage period on the Sheyenne River at West Fargo that would have resulted in an increase in peak flow on the Sheyenne. Extending the storage period to 40 days resolves that problem. The Otter Tail River at Orwell Dam has a very broad hydrograph due to the natural and regulated characteristics of the basin. The operating plan used in this study results in an increase in peak flow on the Otter Tail during the discharge period even following a 40-day storage period. Figure 2-8 is a hydrograph of the Otter Tail River showing the observed and modeled conditions. ( ) 2-14

21 CHAPTER 2 Figure FLOOD OBSERVED AT WAHPETON, ND AND VIRTUAL FLOW DUE TO REDUCTION OF TRIBUTARY PEAK FLOWS BY 50% RED RIVER OF THE NORTH OBSERVED AT WAHPETON, ND Red at Wahpeton w/virtual dams-20 day storage Red at Wahpeton w/virtual dams-40 day storage Mar-97 4-Apr Apr Apr Apr-97 2-May-97 9-May May May May-97 Figure FLOOD OBSERVED AT W. FARGO, ND OBSERVED AT W. FARGO, ND AND VIRTUAL FLOW DUE TO REDUCTION OF TRIBUTARY PEAK FLOWS BY 50% Sheyenne at West Fargo 7000 SHEYENNE RIVER w/virtual dams-20 day storage 6500 Sheyenne at West Fargo 6000 w/virtual dams-40 day storage Mar-97 4-Apr Apr Apr Apr-97 2-May-97 9-May May May May-97 ( ) 2-15

22 CHAPTER 2 Figure Flood Observed and Routed Through A Virtual Storage Reservoir (Otter Tail Orwell Dam, 50% of flow stored, 40 day retention) Flow (cfs) Otter Orwell Dam Observed Otter Orwell Dam virtual reservoir /30/1997 5/23/1997 5/16/1997 5/9/1997 5/2/1997 4/25/1997 4/18/1997 4/11/1997 4/4/1997 3/28/1997 Date No effort was made to time the tributary storage within the model to optimize the mainstem benefits. The timing aspect of the storage modeling is only a function of the occurrence of the flows equaling or exceeding 50% and 75% of the tributary peak flow. The volumes of runoff stored within each tributary were not individually adjusted to target select tributaries for greater or lesser storage. The same rule was applied to all tributaries to determine the volumes stored. This rule partitioned each tributary hydrograph at 50% and 75% of the peak flow and stored the hydrograph volumes equaling or exceeding these values. Figure 2-9 is a drawing of two triangular hydrographs partitioned at 50% and 75% of the peak flow. In a triangular hydrograph, the volume stored in these scenarios is ¼ and 1/16 of the total runoff, respectively. ( ) 2-16

23 CHAPTER 2 Figure 2-9 Triangular Hydrograph Volumes No volume was stored from the Sheyenne River local area between Kindred and Horace. The residual hydrograph from this area includes negative flow values at most of the ordinates--indicating substantial flood plain storage or inter-basin export of flow. Reservoir operating plans are needed to ensure that water is released when downstream conditions allow. It is possible that poorly timed reservoir releases could increase peak flows downstream or extend the duration of damaging flows. These scenarios are also a potential problem with automatic or fixed spillway reservoirs. 2.6 Limitations The HEC-1 model of the 1997 Red River Flood is an empirical routing model. Flood routing is performed by the Straddle-Stagger (average-lag) method. Empirical coefficients are used in the flood routing calculations. Major changes in inflow may ( ) 2-17

24 CHAPTER 2 change the flood characteristics of the river so that the empirical routing coefficients may no longer reflect the observed conditions. The HEC-1 routing model of the 1997 Red River flood was selected for use in this study as the best study effort that could be performed given an existing model and the time and funding constraints. The analyses could be improved by using a physically-based hydraulic model of unsteady flows. Hydraulic modeling will improve the analyses by providing more detailed flood routing, particularly by simulating changes in channel hydraulics and floodplain storage as a function of tributary flow reductions Hydrologic Modeling Results Figure 2-10 includes hydrographs of the 1997 Red River Flood at Emerson. The peak flow of 129,000 cfs was observed on April 26, The results of the simulated condition, where hypothetical reservoirs temporarily store flow exceeding 50% of the peak tributary inflow, are also shown in Figure Temporary storage periods of 20 and 40 days are shown. The Red River peak is reduced to approximately 89,000 cfs in both scenarios a peak reduction of over 30%. The variable storage period had little effect on the simulated peak flow at Emerson, but does effect the flow and duration of the drawdown period. Figure 2-11 includes hydrographs at Emerson for the modeling scenario of storage of tributary flows exceeding 75% of the peak inflow. The Red River peak is reduced to approximately 115,000 cfs in this scenario an 11% reduction in flow. ( ) 2-18

25 CHAPTER 2 Figure FLOOD OBSERVED AT EMERSON, MAN. AND VIRTUAL FLOW DUE TO REDUCTION OF TRIBUTARY PEAK FLOWS BY 50% RED RIVER OF THE NORTH OBSERVED AT EMERSON, MAN Red at Emerson w/virtual dams-20 day storage Red at Emerson w/virtual dams-40 day storage Mar-97 4-Apr Apr Apr Apr-97 2-May-97 9-May May May May-97 FLOW (cfs) DATE ( ) 2-19

26 CHAPTER 2 Figure 2-11 FLOW (cfs) FLOOD OBSERVED AT EMERSON, MAN. AND VIRTUAL FLOW DUE TO REDUCTION OF TRIBUTARY PEAK FLOWS BY 25% RED RIVER OF THE NORTH OBSERVED AT EMERSON, MAN Red at Emerson w/virtual dams-40 day storage 28-Mar-97 4-Apr Apr Apr Apr-97 2-May-97 9-May May May May-97 DATE ( ) 2-20

27 CHAPTER 2 Table 2-4 and Table 2-5 list the volumes virtually stored within each tributary and the associated peak flow reduction, respectively. The storage analysis indicates that: Observed runoff in March, April and May of 1997 = 6.9 Million ac-ft for the Red River at Emerson and Roseau River at Caribou o Volume of approximately 3.4 inches of runoff from the basin; To reduce tributary peaks by 25% requires approximately 460,000 acre-feet of floodwater to be removed from the flood; o Volume of approximately ¼-inch of runoff from the basin; o The mainstem flow is reduced by about 11%; To reduce tributary peaks by 50% requires approximately 1.55 million acrefeet of floodwater to be removed from the flood; o Volume of approximately ¾-inch of runoff from the basin; o The mainstem flow is reduced by about 31%. Figure 2-12 and Figure 2-13 are graphs of tributary storage versus peak flow reduction for the mainstem and tributaries, respectively. Two of the points on each curve were determined from the model study. The end points are known by definition storage of the total inflow will result in a 100% reduction in flow and zero storage will not have any effect on the flow. The CD Attachment includes Excel Spreadsheet files that include the numerical data and hydrographs for most of the sites modeled. The filenames are: Graphs 97 flood output impact virtual dams 40 days 50%.xls Graphs 97 flood output impact virtual dams 40 days 25%.xls Roseau Caribou 97 Spring from USGS.xls ( ) 2-21

28 CHAPTER 2 Drainage Area Observed Volume Table 2-4 Volume Observed Volume Diverted tovolume Diverted tovolume Diverted tovolume Diverted to Virtual StorageVirtual StorageVirtual StorageVirtual Storage (50% of peak flow) (50% of peak flow) (25% of peak flow) (25% of peak flow) (sq. miles) (acre-feet) (inches) (acre-feet) (inches) (acre-feet) (inches) 1Bois de Sioux at Lake Traverse Ottertail R at Orwell Dam Red R local below Traverse and Orwell Red R local below Wahpeton Wildrice R at Rutland Wild Rice local below Rutland Red R local below Abercrombie and Hickson Sheyenne R at Kindred Sheyenne R local below Kindred Sheyenne R local below Horace Maple R at Enderlin Maple R local below Enderlin Rush R at Amenia Buffalo R at Hawley S. Br. Buffalo at Sabin Buffalo R local below Hawley and Sabin Wild Rice at Hendrum Red R local below Hendrum, Dilworth, Amenia, 18Mapleton, W. Fargo and Fargo Goose R at Hillsboro Marsh R at Shelly Sandhill R at Climax Red Lake R at Highlanding Thief R at Thief River Falls Clearwater R at Plummer Lost R at Oklee Clearwater R local below Plummer and Oklee Red Lake R local below Red Lake Falls, Thief 27River Falls and Highlanding Red R local below Crookston, Climax, 28Hillsboro, Shelly and Halstad Turtle R at Arvilla Forest R at Minto Middle R at Argyle Park R at Grafton Red R local below Arvilla, Minto, Argyle, 33Grafton and Grand Forks Two R at Lake Bronson Pembina R at Windygates Pembina R local below Windygates Tongue R at Akra Red R local below Akra, Neche, Lake Bronson 38and Drayton Roseau R at Caribou Basin Total 37,960 6,912, ,550, , ( ) 2-22

29 CHAPTER 2 Table 2-5 Site Observed Peak Flow Virtual Peak Flow (tributary peaks reduced 50%--20 days storage) Peak Flow Reduction (tributary peaks reduced 50%--20 days storage) Virtual Peak Flow (tributary peaks reduced 50%--40 days storage) Peak Virtual Peak Flow Flow (tributary Reduction peaks reduced 25%--40 days storage) Peak Flow Reduction (cfs) (cfs) (percent) (cfs) (percent) (cfs) (percent) 1 Red R at Wahpeton % % % 2 Red R at Hickson % % % 3 Wild Rice R at Abercrombie % % % 4 Red R at Fargo % % % 5 Sheyenne R at Horace % % % 6 Sheyenne R at West Fargo % % % 7 Maple R at Mapleton % % % 8 Buffalo R at Dilworth % % % 9 Red R at Halstad % % % 10 Clearwater R at Red Lake Falls % % % 11 Red Lake R at Crookston % % % 12 Red R at Grand Forks % % % 13 Red R at Drayton % % % 14 Pembina R at Neche % % % 15 Red R at Emerson % % % Average Peak Reduction 26% 32% 12% ( ) 2-23

30 CHAPTER 2 Figure 2-12 Mainstem peak flow reduction versus volume stored within virtual dams Ratio of mainstem peak flow reduction to observed flow ,000,000 2,000,000 3,000,000 4,000,000 5,000,000 6,000,000 Volume stored in virtual dams (ac-ft) 7,000,000 Mainstem peak flow reduction through virtual dams Figure 2-13 Tributary peak flow reduction versus volume stored within virtual dams Ratio of tributary peak flow stored within virtual dams ,000,000 6,000,000 5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 0 Volume stored in virtual dams (ac-ft) Tributary peak flow reduction through virtual dams ( ) 2-24

31 CHAPTER Ideal Storage The model determines the virtual storage removed from a hydrograph. Actual reservoirs will need to hold larger volumes to achieve the same effects since an actual reservoir is not totally efficient and the timing of gate operations during a flood will not likely be as perfect as a computer simulation using hindsight. In addition, actual reservoirs will not be located ideally some storage will likely be used to reduce local floodplain storage without mainstem flow reductions. Figures 2-14 and 2-15 are hydrographs at Emerson showing the ideal volumes removed from the flood to achieve the simulated peak flow reductions. Table 2-6 lists the ideal volumes computed at Emerson in the virtual storage simulation of the 1997 flood. Table 2-6 IDEAL Volumes from Virtual Storage Simulation of the 1997 Flood Storage of Upper 25% of 1997 Tributary Flow Storage of Upper 50% of 1997 Tributary Flow Ideal Volume to Achieve Simulated Peak Flow Reduction Non-Ideal Volume Total Virtually Stored Volume (acft) 110, , , , ,000 1,440,000 ( ) 2-25

32 CHAPTER 2 Figure 2-14 Ideal Volume Removed From Flood For Peak Flow Reduction Red River at Emerson Virtual Effects of Storage of Upper 25% of Tributary Flows Flow (cfs) /1/1997 4/8/1997 4/15/1997 4/22/1997 4/29/1997 5/6/1997 5/13/1997 Date 5/20/1997 5/27/1997 OBSERVED AT EMERSON, MAN Ideal Volume Removed From Flood (110,000 ac-ft) Non-Ideal Volume Removed from Flood (320,000 ac-ft) Emerson Virtual Effect of Reducing Tributary Peaks 25% Figure 2-15 Ideal Volume Removed From Flood For Peak Flow Reduction Red River at Emerson Virtual Effects of Storage of Upper 50% of Tributary Flows Flow (cfs) /1/1997 4/8/1997 4/15/1997 4/22/1997 4/29/1997 5/6/1997 5/13/1997 Date 5/20/1997 5/27/1997 OBSERVED AT EMERSON, MAN Ideal Volume Removed From Flood (550,000 ac-ft) Non-Ideal Volume Removed From Flood (890,000 ac-ft) Emerson Virtual Effect of Reducing Tributary Peaks 50% ( ) 2-26

33 CHAPTER 3 STORAGE AND CHANNEL CAPACITY FOR 10-YEAR AG DRAINAGE DESIGN This study explores the relationships between runoff, channel capacity and storage in agricultural areas of the Red River Basin. The mediation agreement 1 lists the following goals: 1. Prevent loss of human life. a. Promote the development of community flood warning systems and emergency response plans. b. Promote the development of flood plain management plans and land use ordinance administration and enforcement. c. Ensure state oversight of project design and technical criteria. 2. Prevent damage to farm structures, homes and communities. a. Promote the construction of farmstead ring dikes built to a minimum of 2 feet of freeboard over the flood of record, or 1 foot above the administrative 100- year flood, whichever is greater. b. Promote the construction of community setback levees and floodwalls built to the flood of record plus uncertainty (3 feet) or the 100-year flood plus uncertainty, whichever is greater. c. Promote the acquisition and permanent removal of flood-prone structures and establishment of greenways within the 100-year flood plain. d. Accelerate flood insurance studies, flood plain remapping and hydraulic/hydrologic studies in poorly defined or unmapped areas. e. Accelerate comprehensive watershed and systems approaches to basin management. 1 Red River Basin Flood Damage Reduction Work Group, Agreement, December 9, ( ) 3-1

34 CHAPTER 3 f. Discourage the development of structures within the 100-year flood plain, with the exception of those approved in a community s flood plain ordinances. 3. Reduce damage to farmland by: a. Providing protection against a ten-year summer storm event for intensively farmed agricultural land; b. Maintaining existing levels of flood protection when consistent with a comprehensive watershed management plan; and c. Providing a higher level of protection, e.g., 25-year event, when feasible at a minimal incremental cost. 4. Reduce damage to transportation. 5. Reduce damage to water quality, including direct and chronic impacts, from floodwaters coming into contact with potential contaminants. 6. Reduce environmental damage caused by flood control projects. a. When advancing a project that requires a permit, select the least environmentally damaging (or most environmentally enhancing), feasible and prudent alternative that accomplishes the water management goals. b. Design projects or packages of projects that provide net natural resource enhancement. c. A planned response to a flooding problem should take into account natural resource benefits, as well as negative impacts, in a watershed context (beyond the immediate project site). 7. Reduce social and economic damage. 8. Reduce damage to natural resource systems caused by flooding. ( ) 3-2

35 CHAPTER 3 The third listed goal applies directly to agricultural flood damages: Reduce damage to farmland Providing protection against a 10-year summer storm event for intensively farmed ag land. Maintain existing levels of flood protection, when in accord w/plan. Provide a higher level of protection, e.g. 25-year event, when feasible at a minimal incremental cost. This study provides estimates of the channel capacity and storage required to meet the agricultural flood damage reduction goals Year Precipitation and Runoff For this analysis, the 10-year recurrence interval 24-hour duration precipitation depth has been estimated as 3.57 inches and the corresponding runoff volume have been estimated as 1.35 inches. Table 3-1 provides a comparison of several storms with varying recurrence intervals and durations. Table 3-1 Design Precipitation and Runoff Depths Recurrence Interval 24-hr Precipitation24-hr Runoff10-day Precipitation 10-day runoff (inches) (inches) (inches) (inches) 10 year year year ( ) 3-3

36 CHAPTER Runoff Volume Versus Drainage Area Table 3-2 includes the runoff volume generated by a 10-year 24-hour event for a range of drainage areas from 1 square mile to 100 square miles. Runoff of 1.35 inches is equivalent to a volume of 72 acre-feet per square mile. Table 3-2 Drainage Area vs. Runoff Volume Drainage Area 10-Year 24-Hour Runoff Volume (1.35 inches) (square miles) (acre-feet) SCS Design Channel Capacity Versus Drainage Area The USDA Soil Conservation Service (now the Natural Resources Conservation Service) has published design guidelines for agricultural drainage systems. The following equation has been recommended for sizing ag ditches in the Red River Valley 2. Q=20M (5/6) Where: Q is flowrate in cubic feet per second M is drainage area in square miles (for areas of 1 to 100 square miles) 2 USDA Soil Conservation Service, Drainage of Agricultural Land, National Engineering Handbook, Section 16, 1971 ( ) 3-4

37 CHAPTER 3 Many public drainage ditches in the Red River Valley have been designed using this equation or other similar design guidelines. Ditch design capacities vary but systems having a flow capacity given by this equation could be considered typical of many ag drainage systems in the Red River Valley. 3.4 Storage Versus Channel Capacity Drainage coefficients are often used in agricultural subsurface drainage (tile) design. A drainage coefficient is defined as the depth of runoff to be removed from the project drainage area within a 24-hour period. Typical drainage coefficients range from.5 to.75 inches for tile drains with surface inlets in Minnesota. 3 While drainage coefficients are less commonly used for surface drainage design, the design concept is the same channels must provide the capacity necessary to remove the design depth of runoff from the project area within a specified time period (typically within 24 or 48 hours). The time frame for removal of surface runoff is important since crop damages result if standing water remains longer than about 24 hours. To meet the design goals, the 10-year 24-hour runoff volume must be either carried by drainage channels or stored. The amount of storage required for a given area depends upon the channel capacity in that area. An area with large capacity channels will require relatively little storage for the design runoff volume while areas with lower capacity channels will require greater storage volumes. A comparison of storage and channel 3 USDA Soil Conservation Service, Minnesota Drainage Guide, St. Paul, MN ( ) 3-5

38 CHAPTER 3 requirements for given drainage areas are presented in Table 3-3. For example, a drainage area of 100 square miles will produce 7200 acre-feet of runoff during a 10-year 24-hour storm. Assuming that ditches in the drainage area have a typical capacity of 928 cfs, 1841 acre-feet of water will be removed by the ditches in a 24-hour period and 5359 acre-feet will remain beyond 24-hours. Graphical presentations of similar data are given in Figures 3-1, 3-2, 3-3, and 3-4. These graphs can be used to estimate the 10-year 24- hour storage volume required in conjunction with a given channel capacity. For example Figure 3-2 indicates that (for a 1 square mile drainage area) no storage is required if the channel capacity is 36 cfs, and 72 acre-feet of storage is required if the channel capacity is near zero. Figures 3-3 and 3-4 provide similar graphs for drainage areas of 10 and 100 square miles. Figures 3-1 through 3-4 also include points showing the capacity of a typical drainage ditch design for the Red River Basin (capacity as per Q=20M (5/6) ). This general method can be used to compare the ditch capacity and storage in a basin for the 10-year 24-hour runoff event. The following steps are required: Determine the drainage area. Determine the existing or proposed channel capacity (cfs) Estimate the runoff volume for the area (ac-ft). Convert channel capacity to acre-feet per day. Calculate required storage by subtracting channel capacity from runoff volume. ( ) 3-6

39 CHAPTER 3 Table 3-3 Storage Requirements Vs. Drainage Area for Typical Ditch Capacity Drainage Area (square miles) Storage Requirements versus Drainage Area For Typical Ditch Capacity Channel Channel Channel Capacity (cfs) Capacity Capacity Q=20A^.833 (cfs/sq. mi.) (ac-ft/sq. mi.) Runoff Volume (acft) for 1.35" 10yr 24 hour 24 hour channel capacity (ac-ft) Storage Required (ac-ft) Storage Required (inches) Figure 3-1 Storage Requirements vs Channel Capacity (for 10-year 24-hour runoff of 1.35 inches) Storage Requirements (ac-ft) Storage vs Channel Capacity-1 sq. mi. Storage vs Channel Capacity-10 sq. mi. Storage vs Channel Capacity-100 sq. mi. 0.1 SCS Ag Drainage Capacity Q=20A^ Channel Capacity (cfs) ( ) 3-7

40 CHAPTER 3 Figure 3-2 Storage Requirements vs Channel Capacity (for 10-year 24-hour runoff of 1.35 inches) Storage Requirements (ac-ft) Storage vs Channel Capacity-1 sq. mi. SCS Ag Drainage Q=20A^ Channel Capacity (cfs) Figure 3-3 Storage Requirements vs Channel Capacity (for 10-year 24-hour runoff of 1.35 inches) Storage Requirements (ac-ft) Storage vs Channel Capacity-10 sq. mi. SCS Ag Drainage Capacity Q=20A^ Channel Capacity (cfs) ( ) 3-8

41 CHAPTER 3 Figure 3-4 Storage Requirements vs Channel Capacity (for 10-year 24-hour runoff of 1.35 inches) Storage Requirements (ac-ft) Storage vs Channel Capacity-100 sq. mi. SCS Ag Drainage Capacity Q=20A^ Channel Capacity (cfs) 3.5 Summary This method can be used to estimate the storage and/or channel capacity required to convey or contain the runoff from storm events. If the design goal is to provide protection at the 10-year level to intensively farmed lands: o if done exclusively with channel enlargement, the result would be an increase from two to four times in the capacity of many ag ditches; o if done exclusively with storage added to complement existing ditches, would require the storage of about 6/10 to 1 inch of runoff; If the design goal is to provide protection at the 25-year level to intensively farmed lands: o if done exclusively with storage added to complement existing ditches, would require the storage of about 1.1 to 1.5 inches of runoff. ( ) 3-9

42 CHAPTER 4 TIMING OF TRIBUTARY PEAK FLOW CONTRIBUTIONS TO MAINSTEM FLOODS Three previous Red River hydrology reports by McCombs-Knutson 1, the Army Corps of Engineers 2, and the Red River Watershed Management Board 3 have analyzed the timing differences between flood peaks on the tributaries and the flood peak on the main stem. The Corps studied 10 Red River floods occurring within the period from 1948 to The Red River Watershed Management Board studied the 1997 flood. Table 4-1 lists the floods studied by the Corps and the Red River Watershed Management Board. Table 4-2 lists the 15 highest peak discharges observed at Emerson, Manitoba 4. Those floods studied by the Corps and the Red River Watershed Management Board are indicated by bold print within Table 4-2. Table 4-1 Floods Studies by Corps and RRWMB Flood Season Year 1997 Spring 1979 Spring 1978 Spring 1975 Spring 1975 Summer 1969 Spring 1966 Spring 1965 Spring 1950 Spring 1950 Summer 1948 Spring 1 McCombs-Knutson Associates, Inc. Water Resources Engineering/Planning Program for the Red River of the North Basin in Minnesota, May US Army Corps of Engineers, Technical Resource Service, Red River of the North, Volume I Timing Analysis, March Red River Watershed Management Board, Hydrologic Routing of the 1997 Red River Flood, Tara Williams-Sether, High-Streamflow Statistics of Selected Streams in the Red River of the North Basin, North Dakota, Minnesota, South Dakota, and Manitoba, U.S. Geological Survey Open-File Report , 2000 ( ) 4-1

43 CHAPTER 4 Table 4-2 Highest Peak Discharges Observed at Emerson, Manitoba Rank Year of Flood Month of Peak Date of Peak Gage Height (feet) Flow at Emerson (cfs) April , May , May , April , April , April , April , April , April , April , April , May , April , April , April , Tributary Timing Comparison Figure 4-1 presents the timing difference between the peak of observed Red River floods and the peak of routed tributary hydrographs. This figure indicates how frequently a given tributary contributes its peak flow to an 8-day window around the Red River peak and indicates whether the tributary s peak flow arrives before, after or during the mainstem peak. The Wild Rice (Minnesota), Marsh, Sandhill, Red Lake and Park Rivers have fairly consistent timing between their routed peak flows and the eight-day window around the mainstem peak. The Maple, Goose, Turtle, Tamarack and Pembina Rivers have mixed timing results with about half of their peak flows routed within the mainstem eight-day window. The Sheyenne River routed peak flows arrive after the mainstem eight-day window in 10 of 11 floods studied, with the 1997 flood being the one notable exception. ( ) 4-2

44 CHAPTER 4 Figure 4-1 Timing difference between routed tributary peak flow and Red River peak (days) Timing between routed tributary and Red River peak flow. Floods plotted include: 1948, '50, '50 summer, '65, '66, '69, '75, '75 summer, '78, '79, and '97. References: Table 3 Time Interval Between Tributary Peak and Total Peak (in days) US Army Corps of Engineers, Technical Resource Service, Red River of the North, Volume I Timing Analysis, March 1988 Hydrologic Routing of the 1997 Red River Flood, Red River Watershed Management Board ( ) Wild Rice, ND Sheyenne Maple Rush Buffalo Wild Rice, MN Goose Marsh Sandhill Tributary River 4.2 Comparison Of Tributary Timing By Flood Red Lake Turtle Forest Snake Park Tamarac Two Rivers Pembina Figure 4-2 is a set of histograms for nine of the Red River floods. Each histogram shows the number of tributaries with routed peak flows occurring within successive eight-day windows of the mainstem peak. The histograms are arranged from top to bottom by flood magnitude. These histograms indicate that during large floods on the Red River, most of the tributary peak flows contribute to the eight-day window around the mainstem peak. This is an important observation since it means that efforts for flood control on the tributaries targeted on reducing the tributary peak flow are also likely to be beneficial in reducing mainstem peak flows. Tributary flood peak reduction methods will be beneficial to mainstem flood peak reduction provided that the tributary efforts remove the flood contribution from the ( ) 4-3

45 Figure 4-2 The Number Of Routed Tributary Peak Flows Occurring Within Successive Eight-Day Windows Of the Mainstem Peak 1997 Histogram 1950 Histogram Number of Routed Tributary Peaks Occuring Within Time Increment of Mainstem Peak More 20 Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Time From Peak (Days) Time From Peak (Days) 1979 Histogram 1966 Histogram Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Time From Peak (Days) Time From Peak (Days) 1969 Histogram 1948 Histogram Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More 20 Time From Peak (Days) Time From Peak (Days) 1978 Histogram 1965 Histogram Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Time From Peak (Days) Time From Peak (Days) 1975 Histogram Number of Routed Tributary Peaks Occurring Within Time Increment of Mainstem Peak More Time From Peak (Days) ( ) 4-4

46 CHAPTER 4 mainstem peak. In a tributary storage scenario, the detention time needs to be long enough for the mainstem flood to recede so it is important that tributary storage efforts include potential for long-duration flood storage. 4.3 Tributary Timing and Runoff Volumes Within 8-Day Window Table 4-3 includes data from the Corps of Engineers and the Red River Watershed Management Board studies on the timing difference between peak flows on the tributaries and the mainstem. Table 4-3 provides a listing by flood of the time interval between the peak of routed tributary hydrographs and the peak of the mainsteam hydrograph. Negative values listed in the table indicate that the routed tributary peak occurred before the mainstem peak. The table also includes a summation, by tributary, of the number of floods when the tributary routed flood peak occurred within an interval of four days before or after the mainstem peak. For example routed peak flows during 10 of 11 floods studied on the Sandhill River arrived within the 8-day window. In contrast, only 1 of 11 floods on the Sheyenne River arrived within the 8-day window. Table 4-4 is similar to Table 4-3 except that routed peak flows for the Buffalo, Goose and Turtle Rivers in 1997 were revised to show the timing of the second flood peak on these rivers. The tributary peaks all coincide closely with the mainstem peak in with the exception of the Buffalo River at Dilworth, the Goose River at Hillsboro, the Turtle River at Arvilla and the Otter Tail River at Orwell Dam. The Buffalo, Goose and Turtle Rivers all had double peaks in The first peaks routed downstream arrived about 10 to 15 days prior to the Mainstem Peak, but the second peaks aligned much closer to the mainstem peak. The second peaks on the Buffalo, Goose and Turtle Rivers were 6950 cfs, 4320 cfs, and 842 ( ) 4-5

47

48 CHAPTER 4 cfs; 83%, 54% and 91% of the first peak magnitude, respectively. If the second peaks are considered on the Buffalo, Goose and Turtle Rivers, seventeen of the eighteen tributaries studied coincide with the eight-day window around the mainstem peak. The peak flow on the Otter Tail River at Orwell Dam is about 5 or 6 weeks after the mainstem peak. Table 4-4 also provides the average (and standard deviation) timing of routed tributary peak flows for each flood. Table 4-5 provides a listing by tributary of the time interval between the 1997 peak of routed tributary hydrographs and the peak of the mainstem hydrograph. Table 4-5 also provides the tributary runoff volume contribution to the 8-day window around the mainstem peak. 4.4 Summary Several previous reports have analyzed the timing differences between flood peaks on the tributaries and the flood peak on the main stem. These studies present the timing difference between the peak of observed Red River floods and the peak of routed tributary hydrographs. Information is available on the number of times the routed flood peak of each tributary occurred within an interval of four days before or after the mainstem peak. The Wild Rice (Minnesota), Marsh, Sandhill, Red Lake and Park Rivers have fairly consistent timing between their routed peak flows and the eight-day window around the mainstem peak. The Maple, Goose, Turtle, Tamarack and Pembina Rivers have mixed timing ( ) 4-7

49