Red River Basin Flood Damage Reduction Framework

Transcription

1 Red River Basin Flood Damage Reduction Framework Red River Basin Flood Damage Reduction Work Group Technical and Scientific Advisory Committee Technical Paper No. 11 Principal Authors: Charles Anderson, JOR Engineering, Inc. Al Kean, Minnesota Board of Water and Soil Resources May 2004 Executive Summary Flooding is a major problem within much of the Red River Basin. This problem is primarily related to geology, topography, weather, and land use. The Flood Damage Reduction Work Group in Minnesota seeks to provide Project Teams, Watershed Districts and others with science-based and consensus-based tools to enable more effective flood damage reduction within the basin. A fundamental premise of this technical paper is that flood damage reduction (FDR) along the main stem of the Red River and the lower reaches of its major tributaries (glacial lakebed region) is substantially dependent on the types and locations of FDR and related measures implemented upstream. Flooding in the glacial lakebed region of the basin is substantially affected by runoff timing and volume from upstream areas. Runoff timing and volume are, in turn, substantially affected by the topography, soils, precipitation and land use within different regions of the basin, as well as by the types and locations of FDR and natural resource enhancement (NRE) measures that may be implemented. A basin-wide FDR framework will better enable a coordinated approach to integrate various FDR and associated NRE measures that are most effective for achieving the overall goals envisioned by the Red River Basin Mediation Agreement adopted in December The goal of this framework is to implement various types of FDR measures individually, or in concert, at locations for which they are best suited to achieve FDR benefits locally and in the watershed, while also contributing to reduction of main stem flooding risk. This framework includes FDR measures that are also NRE measures, and promotes multi-purpose projects. This technical paper presents critical concepts about runoff timing and volume in relation to flood peaks on the main stem of the Red River, and facts about variations in topography, soils, precipitation and evaporation within the Minnesota portion of the basin, as foundations for defining the expected peak flow reduction effects of implementing various FDR measures within different areas of the basin. Available geologic, topographic, meteorologic and historical flood data, as well as computed runoff travel times, are used to illustrate these concepts and to define RRB FDR Framework Final.doc 1

2 early, middle, and late runoff areas within the basin relative to the downstream limit of the Red River Basin in Minnesota at the U.S./Canada border. A wide array of alternative FDR measures are identified, categorized and discussed, including pros, cons, and general recommendations for the best areas in which to implement these measures to optimize overall FDR benefits. A summary table is presented for the identified array of FDR measures with ratings of potential for peak flow reduction on the main stem when these measures are implemented in early, middle, or late runoff areas relative to the main stem. This technical paper stresses the importance of using multiple types of FDR measures in a strategic manner to achieve local, watershed, and main stem flood damage reduction. It presents a framework for creating policies and trends that will help to achieve basin-wide FDR goals, as well as NRE goals outlined in the Red River Basin Mediation Agreement. This technical paper includes a multi-measure example for the Red River Basin, utilizing various types of flood volume reduction and temporary storage measures to reduce local, watershed and main stem flood peaks, and to provide NRE benefits. For this example, it is estimated that the collective effects could reduce the 100-year peak flood flow at the U.S./Canada border by approximately 20%. A spreadsheet method is provided to assess and document the expected peak flow reductions on the Red River main stem at the U.S./Canada border of flood volume reduction and temporary storage measures implemented upstream. This method uses ratios of implemented storage (at a project location) to ideal storage (effect on main stem peak flood volume and flow) for different types of flood volume reduction and temporary storage measures located in early, middle and late areas relative to the main stem. These effectiveness ratios are based on flood routing and other experience of Technical and Scientific Advisory Committee (TSAC) members, including TSAC Technical Paper No. 10, Basin Strategy Hydrologic Analysis, and other previous studies. This method could be used to track progress toward achieving long-term FDR and NRE goals. It is intended that this technical paper be used in conjunction with other TSAC technical papers and the User s Guide to Natural Resource Efforts in the Red River Basin, published in 2001, to give decision makers additional tools to assess and achieve basin-wide FDR and NRE goals. RRB FDR Framework Final.doc 2

3 Red River Basin Flood Damage Reduction Framework Red River Basin Flood Damage Reduction Work Group Technical and Scientific Advisory Committee Technical Paper No. 11 Table of Contents Page Purpose and Overview... 1 Background... 1 Need for a Basin-Wide FDR Framework... 2 Critical Factors and Types of Information Considered... 3 Topography and Geologic Landforms of the Basin... 4 Climate... 8 Streamflow Characteristics Defining and Quantifying the Flooding Problem Hydrographs Runoff Volume and Timing Historic Floods Runoff Travel Time Definition of Early, Middle, and Late Areas Relative to the Red River Main Stem Flood Damage Reduction Measures RRB FDR Framework Final.doc i

4 Table of Contents (Continued) Page 1) Reduce Flood Volume a) Construction or Restoration of Depressional Wetlands b) Cropland BMPs c) Conversion of Cropland to Perennial Grassland d) Conversion of Land Use to Forest e) Other Beneficial Uses of Stored Water 2) Increase Conveyance Capacity a) Channelization b) Agricultural Drainage c) Diversions d) Setting Back Existing Levees e) Increasing Road Crossing Capacity 3) Increase Temporary Flood Storage a) Impoundments 1) On-Channel Impoundments 2) Off-Channel Impoundments b) Restored or Created Wetlands c) Drainage d) Culvert Sizing e) Setting Back Existing Levees f) Overtopping Levees 4) Protection/Avoidance a) Urban Levees b) Farmstead Levees c) Agricultural Levees d) Evacuation of the Floodplain e) Floodproofing f) Flood Warning and Emergency Response Planning Summary Table of Flood Damage Reduction Measures and Effects A Basin FDR Framework Multi-Measure Example Temporary Storage and Flood Volume Reduction Components a) Wetland Restoration / Creation b) Culvert Sizing c) Overtopping Levees d) Impoundments RRB FDR Framework Final.doc ii

5 Table of Contents (Continued) Page Summary for Temporary Storage and Flood Volume Reduction Measures Non-Storage Components a) Runoff Volume Reduction b) Protection / Avoidance Effects of Implementing Example Measures Flood Storage Assessment and Tracking Method Summary of FDR Framework References Table 1. Expected Peak Flow Reduction Effects on the Red River Main Stem of FDR Measures Applied in Early, Middle, and Late Areas Upstream Figures 1. Shaded Relief Map of the Red River Basin in Minnesota Major Landforms of the Red River Basin in Minnesota Profiles of the Red River Normal Annual Precipitation Average Annual Runoff Average Annual Temperature Average Annual Lake Evaporation Average Temperature, Precipitation, and Evaporation at Roseau and Wheaton, MN Average Date of Last Snow Cover RRB FDR Framework Final.doc iii

6 Table of Contents (Continued) Figures (Continued) Page Year Snow Water Equivalent Red River Mean Daily Discharge at Grand Forks, ND Red River Annual Mean Discharge at Grand Forks, ND Annual Peak Flows at Grand Forks, ND Flood Flows at Cities Along the Red River Flooded Areas Along the Main Stem of the Red River in Approximate 100-Year Flood Hydrograph at Emerson, Manitoba Hydrographs of the 1997 Red River Flood Example of Tributary Contribution to Main Stem Flood Timing of Tributary Inflows (E=Early, M=Middle, L=Late Relative to the Main Stem) Tributary Contributions to the 1997 Flood Hydrograph at Emerson Comparison of a Tributary Contribution to Main Stem Flood Hydrograph in 1966 and Area-Weighted Contribution to Main Stem Flooding Computed Runoff Travel Time to the U.S./Canadian Border Early, Middle, and Late Runoff Timing Zones in the Red River Basin Volume of Ideal Storage and/or Flood Volume Reduction Required to Reduce the 100-Year Peak Flow at Emerson Summary of Example for Temporary Storage and Flood Volume Reduction Measures and Estimated Effectiveness to Reduce 100-Year Flood Peak at Emerson Estimate of Modified 100-Year Flood Hydrograph at Emerson RRB FDR Framework Final.doc iv

7 Red River Basin Flood Damage Reduction Framework Red River Basin Flood Damage Reduction Work Group Technical and Scientific Advisory Committee Technical Paper No. 11 Purpose and Overview The purpose of this framework is to better enable a coordinated approach to flood damage reduction (FDR) within the Red River Basin that addresses local, watershed, and main stem FDR goals, so that efforts to reduce flood damages in one area will also consider the potential for negative or positive impacts in other areas. This basin level framework recognizes that FDR goals for the main stem of the Red River, and the lower reaches of its major tributaries, necessarily involve measures strategically implemented farther upstream in the tributary watersheds and subwatersheds, as well as local FDR measures. This technical paper provides evaluations of available geologic, topographic, and hydrologic information to help define what types of FDR measures, applied where, can help achieve both local and downstream FDR goals. It is recognized that a basin-wide FDR framework must consider many local, watershed, and basin-wide needs, opportunities, and constraints, as well as geologic, land use, and climatic variables. This technical paper includes critical concepts and provides general guidance for a wide variety of FDR measures for different areas of the Red River Basin in Minnesota. It also includes a multi-measure example for the Red River Basin upstream of the U.S./Canada border, utilizing various types of flood volume reduction and temporary storage measures to reduce local, watershed and main stem flood peaks, and to provide NRE benefits. It is intended that this technical paper be used in conjunction with other Technical and Scientific Advisory Committee (TSAC) technical papers and the User s Guide to Natural Resource Efforts in the Red River Basin published in 2001 to further develop and implement the FDR and NRE goals of the Red River Basin Mediation Agreement adopted in December The primary audience is expected to be Project Teams and others involved with watershed planning and development of FDR and natural resource enhancement (NRE) projects within the Red River Basin in Minnesota. Background Many floods have occurred in the Red River Basin in recorded history. The earliest accounts are found in journal entries of trappers and explorers beginning in Major floods have occurred on the main stem and tributaries both in the spring, due to snowmelt and rain, and in the summer, RRB FDR Framework Final.doc 1

8 due to more localized heavy rains. Damages due to flooding have, on occasion, been catastrophic. Although flooding has been a natural occurrence in the basin since glacial Lake Agassiz receded, the potential for damage increased with settlement and subsequent industrial, urban, and agricultural development. Flooding occurs when water from upstream enters an area at a rate exceeding the channel capacity to carry that water downstream. Flood damages occur when flood levels rise high enough, or remain long enough, to cause adverse impacts. Flood damages in the Red River Basin have included severe structural damage to private and public facilities and infrastructure, extensive crop loss, major environmental degradation, and loss of life. The Red River of the North is one of only a few major rivers in North America that flow north. This increases its spring flood potential, because snow in the southern headwaters of the basin often melts before snow in the northern areas, causing peak flows from downstream tributaries to coincide with the flood crest on the Red River. The northward flow of the river also results in more ice jam problems than most southward flowing rivers experience. In addition, the Red River is located within the broad, flat bottom of glacial Lake Agassiz, which has only a mild northward slope. As a result, the main stem and tributary rivers in the glacial lake plain area of the basin frequently overflow onto broad floodplains. Need for a Basin-Wide FDR Framework There are many alternative measures that can be implemented to reduce flood damages. These include structural measures such as levees, channel modifications, and various types of floodwater impoundments, as well as nonstructural measures such as limiting floodplain development, changing floodplain use, and changing upstream land use to reduce runoff volumes and rates. A flood damage reduction measure is typically selected by those who benefit from, and pay for, its implementation. The choice may also be influenced by public policy, which can control some measures through regulation, or may support others by offering technical assistance or financial incentives. Affected people and policymakers base decisions on their understanding of the flood situation and priorities. Typically, there is good understanding of local flooding problems, because firsthand knowledge and experience are substantial at the local level. What may not be well understood, or adequately considered at the local and watershed levels, are the effects of a particular FDR measure at the watershed or Red River Basin level. A basin-wide FDR framework is needed to ensure a coordinated approach to achieve outcomes that are effective at the local, watershed, and basin levels. This framework should encourage the implementation of measures that reduce local, watershed, and main stem flood damage potential. It should discourage the implementation of measures that, while achieving local flood damage reduction, would increase flood damage potential at the watershed or basin level. RRB FDR Framework Final.doc 2

9 Individual watershed district overall plans address coordination of FDR measures to achieve local and watershed FDR goals. This basin-wide FDR framework provides additional guidance to improve consideration of watershed and main stem needs when selecting FDR measures at the local level. A basin-wide coordinated approach may utilize a variety of FDR and related NRE measures that, collectively, comprise a basin-wide FDR framework. This variety of measures may include small, dispersed measures, such as wetland restorations, watershed-wide culvert sizing, increased perennial vegetation and agricultural best management practices, as well as local protection / avoidance, increased conveyance capacity, and strategically located larger impoundments. Implementation of a basin-wide FDR framework requires long-term commitment. This requires policies and trends that achieve basin-wide FDR goals over the long term. Critical Factors and Types of Information Considered Development of a basin-wide FDR framework requires an understanding of critical factors that affect flooding, including runoff timing and volume. When implementing individual projects, it is necessary to know how water from any given area will affect downstream flooding. Will the peak runoff arrive ahead of, coincident with, or after, downstream flood peaks? Will the flood damage reduction measure significantly change the volume of floodwater from a specific area? Some areas contribute more water to downstream flood peaks than others. This generally relates to the area s location within the basin, topography, soils, watershed size, land use, distribution of precipitation, and other physical features that affect the rate and volume of runoff. On the other hand, meteorologic variability makes each flood event unique. Therefore, an area s contribution will vary from flood to flood. The greatest floods on the Red River occur when the spatial and temporal patterns of meteorologic events coincide with the physical features of the basin in a way that causes high volumes of runoff with coincident peak flood flows from many areas of the basin. This technical paper uses the following types of information and analyses to provide guidance for defining where different types of FDR measures work best to achieve basin-level FDR goals. 1) Definition of the effects of watershed topography, soils, location, size, and current land use on the timing and volume of runoff from different areas of the basin. This includes evaluation of runoff travel time to the U.S./Canadian border based on topography and land use. 2) Evaluation of timing and volume of historical flood events for major watersheds and the main stem of the Red River. These flood events reflect all of the physical characteristics of the basin, in combination with the temporal and spatial meteorologic characteristics of these flood events. RRB FDR Framework Final.doc 3

10 3) Discussion of many different types of FDR measures, including definitions, characteristics, and effects on flooding at, downstream, and upstream from the implementation site. 4) Identification of the effects of different FDR measures implemented in different areas of the basin on main stem flood damage reduction. Topography and Geologic Landforms of the Basin The Red River Basin upstream from Winnipeg has a drainage area of about 45,000 square miles. This includes 17,806 square miles in Minnesota, 20,820 square miles in North Dakota, 573 square miles in South Dakota, and the balance (about 5,800 square miles) in Manitoba. The Red River Basin in Minnesota is shown on the shaded relief map on Figure 1. Also shown are topographic cross-sections of the basin from west to east. To allow for direct comparison, the horizontal and vertical scales are consistent for all cross-sections. RRB FDR Framework Final.doc 4

11 Figure 1. Shaded Relief Map of the Red River Basin in Minnesota RRB FDR Framework Final.doc 5

12 Figure 2. Major Landforms of the Red River Basin in Minnesota RRB FDR Framework Final.doc 6

13 The Red River Basin has four general landform regions characterized by elevation, topography, soils, and stream characteristics, as indicated on Figure 2. The following descriptions of these landforms are from the highest to the lowest in elevation. 1) Glacial Moraine This headwaters region is characterized by rolling hills, lakes, depressional wetlands, and variable soils associated with glacial ground moraine. 2) Lake-Washed Till Plain This region of the basin is characterized by land of gradual slope with large areas of non-depressional wetlands and poorly developed stream networks. Surface soils include large areas of peat lands. 3) Beach Ridge Areas The beach ridge areas of glacial Lake Agassiz are characterized by sandy soils, multiple levels of beach ridges, relatively steep slopes, and incised rivers with relatively narrow floodplains. Wetland areas often exist on the upstream side of beach ridges. 4) Glacial Lake Plain The lowest elevations are within the lake plain of glacial Lake Agassiz. The land within the glacial lake plain region is extremely flat with very low surface and river channel gradients. This flat area originally included large areas of wetlands. The soils are dominated by relatively impervious lacustrine silts and clays. The highest land in the Minnesota portion of the Red River Basin is located in Clearwater County in the Wild Rice River watershed at an elevation of 2,010 feet above sea level. The lowest land, located in Kittson County along the Red River near the Canadian border, is 750 feet above sea level. The most flood-prone areas generally are those with the least slope and those downstream from areas of steep slopes. The gradient of the Red River ranges from a little over 1 foot per mile north of Breckenridge, to about 0.5 foot per mile in the vicinity of Grand Forks, to about 0.2 foot per mile at the Canadian border. Profiles of the Red River from Wahpeton to the Canadian border are shown on Figure 3. Figure 3. Profiles of the Red River Figure 3. Profiles of the Red River (data from U.S. Army Corps of Engineers, et al.) RRB FDR Framework Final.doc 7

14 Climate The Red River Basin is large, with significant climate differences from west to east and from south to north. Within the Minnesota portion of the basin, the average annual precipitation, as shown on Figure 4, varies from 20 inches in the west to 25 inches in the east. Similarly, the average annual runoff, shown on Figure 5, varies from 1 inch in the west to 5 inches in the east. Figure 4. Normal Annual Precipitation Figure 5. Average Annual Runoff RRB FDR Framework Final.doc 8

15 Significant temperature differences also exist. Average annual temperatures, shown on Figure 6, range from 44 degrees in the south to 36 degrees in the north. This creates a similar pattern in evaporation potential, depicted by lake evaporation shown on Figure 7, ranging from 31 inches in the south to 23 inches in the northeast. Figure 6. Average Annual Temperature (State Climatology Office, DNR Waters) Figure 7. Average Annual Lake Evaporation RRB FDR Framework Final.doc 9

16 Seasonal variability is also very important. The charts on Figure 8 show mean monthly temperatures and seasonal precipitation and evaporation patterns at Roseau, near the northern end of the basin, and at Wheaton, near the southern end. The differences in temperature and evaporation from south to north are modest but significant. The monthly evaporation shown is the expected loss from shallow lakes and reservoirs. The difference between evaporation and precipitation develops an annual deficit in shallow water bodies that is restored by runoff from upland areas. The average annual deficit is about 9 inches in the south and about 4½ inches in the north. This may help to explain why, for example, wetlands are considered more useful for flood control in southern areas of the basin than in northern areas. Figure 8. Average Temperature, Precipitation, and Evaporation at Roseau and Wheaton, MN RRB FDR Framework Final.doc 10

17 A very significant climatic factor that affects spring flooding is the timing of the snowmelt. Figure 9 is a map showing the average date of last snow cover for Minnesota. The south to north snowmelt trend tends to build greater peak flows during spring floods. A characteristic pattern in snow pack volume of available moisture also exists. Figure 10 illustrates the regional snow water equivalent pattern, which increases from southwest to northeast. A more local analysis may indicate a relationship to other variables such as land cover. Figure 9. Average Date of Last Snow Cover (State Climatology Office, DNR Waters) Figure Year Snow Water Equivalent (U.S. Weather Bureau Technical Paper No. 50, 1964) RRB FDR Framework Final.doc 11

18 Streamflow Characteristics Many tributary streams within the Red River Basin have only intermittent flows. The Red River itself is a perennial stream, but has high seasonal variability and extended periods of very low flow. Figure 11 shows the mean daily discharge of the Red River at Grand Forks. Figure 12 shows the annual mean discharge. Figure 11. Red River Mean Daily Discharge at Grand Forks, ND (USGS gage data) Figure 12. Red River Annual Mean Discharge at Grand Forks, ND (USGS) RRB FDR Framework Final.doc 12

19 Figure 13 shows a chart of annual peak flows at Grand Forks reported by the U.S. Geological Survey. Figure 13. Annual Peak Flows at Grand Forks, ND RRB FDR Framework Final.doc 13

20 Defining and Quantifying the Flooding Problem Red River main stem floods are characterized by long durations and widespread flooded areas. Flood damages to roads, farmsteads and urban areas are primarily related to peak stage and velocity of flooding. Agricultural damages are primarily related to time of year, peak flow, and flood duration. Peak flow defines the depth, velocity, and extent of the flooding, while time of year and duration of inundation influence the degree of agricultural damage caused by the flooding. Flood flows associated with a range of flood return periods are shown on Figure 14 as a plot of flow rate (in cubic feet per second or cfs ) vs. drainage area at points along the Red River. Note that peak flood flows increase relatively little between Grand Forks and Emerson. This is primarily due to the effect of floodplain storage. Substantial floodplain storage also limits the increase in peak flood flows between Fargo-Moorhead and Halstad. The extent of the floodplain along the Red River main stem is illustrated by the map on Figure 15, which shows flooded areas along the main stem during the 1997 spring flood. Figure 14. Flood Flows at Cities Along the Red River RRB FDR Framework Final.doc 14

21 Figure 15. Flooded Areas Along the Main Stem of the Red River in 1997 RRB FDR Framework Final.doc 15

22 All communities along the Red River are subject to varying levels of flooding risk. At many of these communities, levee systems have been installed that provide varying degrees of flood protection, generally from 50- to 100-year flood levels. The profiles on Figure 3 show levee elevations at a number of communities along the Red River. Emergency flood fight activities are often required to augment the protection provided by levees. Many farmsteads are also protected by levees (ring dikes). Agricultural levees along portions of the Minnesota side of the Red River provide spring flood protection equivalent to about a 5-year event. These levees provide a greater degree of protection against a summer flood event, probably about 25-year protection. The long durations of spring floods commonly cause crop production losses due to delayed planting. Understanding the relationship between flood damage reduction accomplished by these urban, farmstead and agricultural levees, and the additional reduction of flood damage, or risk, resulting from peak flow reduction measures and strategies, is critical to developing an FDR framework. The spring flood of 1997 may be typical of a 100-year flood in the basin. The basin average available moisture in the snow pack and spring rains was about 8 inches, which generated about 4 inches of runoff. The peak flow on the Red River at Emerson, Manitoba, was 129,000 cfs. The total runoff volume at Emerson was about 6,500,000 acre-feet. Hydrographs Flood flows are often depicted as a line chart called a hydrograph. Figure 16 is a hydrograph approximating the 100-year flood flow at Emerson, Manitoba. A discharge hydrograph is simply a plot of flow (typically in cfs) on the vertical axis vs. time (typically in days) on the horizontal axis. The peak of the hydrograph represents the highest flow rate at the applicable location. Adding up the average flows in cfs for each day would yield the total volume of the flood in cfs-days. That volume is graphically represented as the area under the hydrograph. A commonly used unit for flood volume is acre-foot, which is the Figure 16. Approximate 100-Year Flood Hydrograph at Emerson, Manitoba RRB FDR Framework Final.doc 16

23 volume equivalent of 1 foot of water over 1 acre. Volume in cfs-days can be converted to volume in acre-feet by multiplying by 1.98 (i.e., approximately 2). In this illustration, each grid block is 100,000 cfs-days, which is about 200,000 acre-feet. The shape of a tributary s hydrograph indicates its flood characteristics. Tributaries with high peaks and short durations are often referred to as flashy. Hydrographs showing low peaks and long durations are typical of tributaries having substantial temporary storage. Storage may be provided by lakes, reservoirs, and wetlands, or by extensive floodplain areas. Hydrographs with sustained base flows may reflect significant groundwater contributions. Hydrographs of the Red River 1997 spring flood at Wahpeton, Fargo, Halstad, Grand Forks, and Emerson are shown on Figure 17. Figure 17. Hydrographs of the 1997 Red River Flood (data from USGS, et al.) Runoff Volume and Timing The relationship between watershed runoff and downstream flooding is defined by volume and timing. This is illustrated by the example hydrographs on Figure 18. The large hydrograph represents the flow at a downstream location on the Red River main stem. The small hydrograph within represents the contribution of a subwatershed area to the flow on the main stem. The volume from the subwatershed area that arrives during the peak flood period on the main stem RRB FDR Framework Final.doc 17

24 directly contributes to the peak flow and amount of flooding. Knowledge of relative runoff volume from tributaries is important for defining and prioritizing runoff reduction and storage strategies at the basin and watershed levels. This knowledge must be used in combination with knowledge of the opportunities and limitations to manage runoff within individual watersheds. Key variables affecting the volume of runoff from a watershed include drainage area, precipitation, soils, topography, and land use. Timing of the subwatershed hydrograph peak relative to the main stem peak is also very important. The peak contributions from some areas, typically in the lower part of the basin, tend to arrive early, ahead of the main stem peak. The peak contributions from other areas, typically in the upper part of the basin, tend to arrive late, after the main stem peak. The peak contributions from the remaining areas, typically in the central part of the basin, tend to arrive in the middle of the flood on the main stem, coincident with the main stem peak. Therefore, tributary subwatershed areas, and their flow contributions to the main stem hydrograph, can be referred to as early, middle, and late. Figure 19 illustrates this concept using example hydrographs. Everything else being equal, the middle areas contribute the most to downstream main stem flood peaks. The relationship between tributary and main stem flooding is also easiest to understand in the middle areas. Activities that decrease the peak flow from these areas will decrease peak flows on the main stem. Conversely, activities that increase the peak flow from these areas will increase peak flows on the main stem. Figure 18. Example of Tributary Contribution to Main Stem Flood (McCombs) The flow contribution from an early subwatershed area to the main stem flood peak comes from the falling limb of that area s hydrograph. Therefore, activities that reduce local peak flows likely will not reduce main stem peak flows. For example, water stored during an area s flood peak and released immediately thereafter might add flow during the main stem peak. Conversely, conveyance improvement projects may move more of an area s water out ahead of the main stem flood and thereby reduce main stem flood peaks. Figure 19. Timing of Tributary Inflows (E=Early, M=Middle, L=Late, Relative to the Main Stem) RRB FDR Framework Final.doc 18