We are pleased to enclose our Report on Hydrodynamic Modelling of the Lower Pembina River.

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2 WMCI-7012 March 31, SHELLBRIDGE WAY RICHMOND, B.C. V6X 2W8, CANADA International Joint Commission Red River Basin Task Force c/o R. Halliday & Associates 717 Sixth Avenue North Saskatoon, SK, S7K 2S8 TELEPHONE: (604) TELEFAX: (604) Attention: Bob Halliday Dear Sir: Re: Hydrodynamic Modelling of the Lower Pembina River We are pleased to enclose our Report on Hydrodynamic Modelling of the Lower Pembina River. Thank you for the opportunity to work on this interesting study. Yours truly, CDN WATER MANAGEMENT CONSULTANTS INC. C. David Sellars, P.Eng. Project Manager

3 INTERNATIONAL JOINT COMMISSION HYDRODYNAMIC MODELLING OF THE LOWER PEMBINA RIVER March /R3 Prepared for: International Joint Commission 22 nd Floor, 234 Laurier Street West Ottawa, Ontario K1P 6K6 Prepared by: Water Management Consultants Shellbridge Way Richmond, British Columbia V6X 2W8

4 CONTENTS Page 1 INTRODUCTION Geographical Setting Flood History Study Objectives 4 2 MODEL DEVELOPMENT Hydrodynamic Modelling Model Layout Hydrologic and Topographic Data Model Parameters Model Limitations 12 3 RESULTS Existing Conditions Flow Combinations Removal of Dikes No Dikes, No Roads Condition Removal of County Road 55 and the Border Road Boundary Floodway Set Back Dikes Controlled Breakouts Pembelier Dam Bridges and Dikes at Pembina and Emerson Raising the Border Road West of Emerson 21 4 CONCLUSIONS 22 FIGURES Figure 1.1 Figure 1.2 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Pembina River Basin Study Area Computational Scheme Schematic Model Layout Pembina River Hydrographs at Neche Lower Pembina River Digital Elevation Model Preliminary Digital Elevation Model Section Four Miles East of Neche

5 Figure 2.6 Air Photo Mosaic April 23, 1997 Figure Peak Flow Distribution for Existing Conditions Figure 3.2 Air Photo of Aux Marais River Crossing Figure 3.3 Slope of the Peak Water Surface Profile in Branch C2 Figure Peak Flow Distribution for Existing Conditions Figure 3.5 Major Dike Systems Upstream of Neche Figure Peak Flow Distribution Without Dikes Figure Peak Flow Distribution Without Dikes Figure Peak Flow Distribution Without Dikes and Roads Figure Peak Flow Distribution Without Dikes and Roads Figure Peak Flow Distribution Without Dikes and County Road 55 Figure Peak Flow Distribution Without Dikes and Border Road Figure Flow Distribution with Boundary Floodway Figure 3.13 Conceptual Set Back Dike Alignment Figure 3.14 Set Back Dike Section Four Miles Downstream of Neche Figure year Peak Flow Distribution- with Setback Dikes and Controlled Breakouts Figure 3.16 Water Surface Profile of the Red River in the Vicinity of Pembina and Emerson

6 1 1 INTRODUCTION 1.1 Geographical Setting The Pembina River rises in Manitoba and flows in a generally southeasterly direction crossing the International Boundary about 15 miles northwest of Walhalla, North Dakota. The river joins the Red River at Pembina, North Dakota, two miles south of the International Boundary. Figure 1.1 is a map of the Pembina River Basin. The total area of the watershed is about 3950 square miles, divided nearly equally between Manitoba and North Dakota. The length of the Pembina River basin from west to east is about 130 miles as the crow flies, although the meandering river channel itself is about 310 miles long. The river drops from about 2000 ft above sea level at its source on the Turtle Mountain to about 790 ft at its confluence with the Red River. There are two distinct topographical types in the watershed separated by a prominent ridge known as the Pembina Escarpment. Relief along the escarpment in the Walhalla area rises abruptly 500 to 600 feet above the plain to an altitude of about 1500 ft at its crest. During the most recent Ice Age until about 12,000 years ago, the entire watershed was covered by a continental glacier. As the glacier melted and receded to the north, glacial Lake Agassiz formed along the glacier s leading edge, covering the Red River Valley. The Pembina Escarpment formed the western margin of Lake Agassiz. Evidence of beaches created during this period are still recognizable today. Over several hundred years, rich layers of silts and clays up to 200 feet thick accumulated on the lake bed. When the lake finally receded, an extremely flat, smooth plain emerged. The area west of the escarpment is an undulating glacial moraine upland of potholes, ridges and knolls. Surface soils are relatively shallow and characteristic of glacial till. The entire Manitoba portion of the Pembina River watershed lies above the Pembina Escarpment. The river leaves the Turtle Mountain, flowing northeastward through an increasingly deep valley until it reaches the eastern end of Pelican Lake. From this point, the river flows through a broad valley eastward -- the remnant of a massive glacial meltwater channel formed as the continental glacier receded. In places, the valley is 200 feet deep and two miles wide. WATER MANAGEMENT CONSULTANTS

7 Introduction 2 Several streams and deep coulees entering the valley along this reach of the river have deposited sediments along the valley bottom, creating natural dams behind which several lakes have formed, including Pelican, Rock and Swan lakes, as well as some smaller water bodies. Downstream from Swan Lake, the river turns to the southeast, occupying an ever-deepening channel as much as 400 feet deep and a mile or more wide. Near Walhalla, the river emerges from the uplands and onto the former Lake Agassiz lakebed. Over the next 15 miles downstream from the escarpment, the valley gradually disappears. The Pembina River crosses County Road 55 about six miles west of Neche, North Dakota, and downstream of this point the river is higher than the surrounding plain. This is a result of sediment deposition over centuries of flooding gradually building up the elevation of the floodplain. The Lower Pembina River flows along the height of land and any flows that spill from the main channel do not return to the river unless redirected by roads. There are no tributary inflows in this reach until the Tongue River enters the Pembina a few miles upstream of its junction with the Red River. The climate of the basin is characterized by wide variations in temperatures and precipitation. Average monthly temperatures range from 68 0 F in July to 2 0 F in January. However, extremes of F and F have been recorded. Average annual precipitation is about 20 inches, of which 21 percent falls as snow. Heavy summer rains are not uncommon, although they are generally localized and occasionally result in short-term flooding on smaller tributaries. The heaviest 24-hour rainfall recorded in the basin is 8.76 inches at Boissevain in August, Land use above the escarpment consists mainly of grain farming and mixed farming interspersed with pasture, forage production, wetlands and wooded valley slopes. Below the escarpment, land use is almost entirely a continuous-cropping agricultural monoculture. Major crops include wheat and other grains, oilseeds, corn, sugar beets and potatoes. Much of the land in this area owes its productivity to an extensive agricultural drainage network which has claimed land from marshes and bogs over the course of time. Drainage activities continue today, although generally on a more local scale. 1.2 Flood History Floods are a natural and common occurrence along the entire length of the Pembina River. But of the 310 miles of river channel, the most significant and devastating flooding occurs along the 35- mile reach between the Pembina Escarpment at Walhalla and the Red River. This area is highly susceptible to flooding because of the extremely flat terrain. All of the significant floods in this reach of the river have occurred in the spring as a result of snowmelt or heavy rain either combined with, or immediately following, snowmelt. Because of the well defined channel and associated valley, floods above the escarpment are generally limited to the valley floor. Localized flooding may also occur along the tributaries to the Pembina as a result of rapid snowmelt or heavy spring or summer rains. WATER MANAGEMENT CONSULTANTS

8 Introduction 3 Historic accounts mention major floods in 1882, 1897, 1904 and Since 1940, several other significant floods have occurred on the Pembina River downstream of the escarpment including those of 1950, 1974, 1979, 1996 and Flood damages result from direct economic losses such as damage to buildings and infrastructure, flood fighting costs, and so on. In addition, many intangibles may be considered as indirect costs, such the threat to human life, human misery, community disruption and threats to the quality of water supplies. Agriculture may suffer losses through flood losses of buildings, stored grains and other products, loss or damage to stored agriculture input products and loss of valuable topsoil through erosion and sedimentation. In addition, crop seeding may be delayed (or lost altogether), generally resulting in later crop maturity and subsequent lower yields at harvest time. Crop management techniques following a flood may require additional inputs for weed control, or to compensate for fall-applied crop management inputs (fertilizers and herbicides) washed away or lost to the flood waters. For more than 50 years, local farmers and governments, as well as provincial, state and federal governments on both sides of the border have been trying to solve the problems of flooding from the Pembina River. Efforts have ranged from local, unilateral efforts to some attempts at cooperation in fighting a common problem. Unilateral flood control efforts on both sides of the Boundary have created tension between landowners on either side. Over time, more local drainage and flood control activities undertaken in North Dakota have led Canadian farmers to believe that American farmers were attempting to divert flood water into Canada. In response, Canadian farmers constructed a road/dike along the International Boundary in an attempt to prevent this water from entering Canada. Culverts installed in the road/dike cannot handle the substantial overland flows when the Pembina spills over its banks. Discussions relating to the amount of water that should be allowed to pass along the various drainage ways crossing the International Boundary have been underway for more than 20 years. While some agreement has been reached on design flows into Canada from North Dakota at each crossing, the issue of sharing the costs of the necessary works has delayed progress. In addition, decisions on the amount of water that will be allowed to cross the border have been somewhat influenced by the general agreement that a major flood control project on the Pembina River is necessary to fully alleviate the problem. A number of flood control projects proposed by the International Joint Commission, the U.S. Army Corps of Engineers and other international and local groups have all been proven to be economically unfeasible or unacceptable to the local people. WATER MANAGEMENT CONSULTANTS

9 Introduction Study Objectives To assist in resolution of the flooding issues in the Lower Pembina River, the International joint Commission contracted with Water Management Consultants to prepare a detailed hydrodynamic model of the Lower Pembina River. The objectives of the study were as follows: Develop a hydrodynamic model of the Lower Pembina River that simulates conditions during the floods of 1997 and Model the effects of the 1997 Pembina River flood combined with a two year flood on the Red River. The 1997 flood on the Red River would also be combined with a two year flood on the Pembina River. Model the effect of removing dikes. Model the effects of removing dikes and roads acting as dikes. This scenario was called natural conditions though land use changes were not made. Model the effects of removing dikes and roads acting as dikes but leaving the road along the international boundary and County Road 55 in place Model a floodway along the international boundary. Model set-back dike alternatives for 2, 10 and 25 year floods. Model controlled break-outs with the set-back dikes. Determine the hydraulic characteristics of the road bridge at Pembina and the rail bridge at Emerson. Determine the effects of the dikes at Emerson and Pembina on water levels. Determine the impact on water levels if the boundary road west of Emerson is raised above 1997 flood levels. Assess the potential impact of an upstream storage project constructed for flood control. The model was developed from the County Road 55 crossing of the Pembina River upstream of Neche to the confluence of the Pembina River with the Red River. The study area is shown in Figure 1.2. WATER MANAGEMENT CONSULTANTS

10 5 2 MODEL DEVELOPMENT 2.1 Hydrodynamic Modelling The most common method of calculating water levels in a river is to carry out a steady state water surface profile analysis using the US Army Corps of Engineers backwater analysis program, HEC-2. As this method assumes steady state gradually varied flow, it cannot account for changes in discharge that are dependent on the storage effects in the river channel. This is not usually a disadvantage because in a well-defined river reach the storage effects are negligible and the discharge in the main channel and from the tributaries can be assumed constant. For most applications, calculation of the water level at a given discharge is required and the HEC-2 program is ideally suited for this purpose. For modelling flood flows in a river with a complex floodplain, the storage effects cannot be neglected because the floodplain areas provide significant volumes of storage. Disregarding tributary inflows for the moment, the peak discharge through the system would not be constant because the peak flow is attenuated as the flood wave moves down the river. This is a similar phenomenon to the flood routing effects of a reservoir. For calculating water levels for unsteady flow, a hydrodynamic model is required. A hydrodynamic model provides the time dimension to a water surface profile analysis. Thus, instead of constant discharges, inflows can be represented by complete hydrographs. The model will explicitly calculate the storage effects throughout the river and floodplain system and provide not only peak water levels and discharges at any location but also complete water level and discharge hydrographs throughout the modelling time period. The movement of a flood wave advancing into a river with a large floodplain like the Pembina River is a typical example of unsteady non-uniform flow, i.e. flow, depth and velocities varying both spatially and temporally. Since vertical accelerations are negligible for most of the area the hydrodynamics of the Pembina River flood are best represented with a two-dimensional depthaveraged hydrodynamic approach. However, the floodplain complexities, extent of the area to be covered and computer time required pose a real challenge for this type of model. To overcome these difficulties a quasi two-dimensional model was set up for the International Joint Commission Red River Task Force using the MIKE 11 model, developed by the Danish Hydraulic Institute (DHI). MIKE 11 is essentially a one-dimensional model, which has special capabilities to simulate two-dimensional depth-averaged hydrodynamic cases. This approach proved to be very effective for the Pembina River model for the following reasons: WATER MANAGEMENT CONSULTANTS

11 Model Development 6 large area to be covered; complexity of the flow over the floodplains particularly breakout flows; practicality of the model to adjust to new case scenarios; flexibility of the model to analyze results; very low computer time; model accuracy. MIKE 11 is a computer model for the simulation of flows, water quality and sediment transport in estuaries, rivers, irrigation systems, channels and other water bodies. MIKE 11 has a modular structure which allows flexibility in module integration according to the needs of the user. The main modules are: Hydrodynamics Hydrology Sediment Transport Water Quality Flood Forecasting GIS. The hydrodynamic module is the core module of MIKE 11. All the other modules need the hydrodynamic module to run except Hydrology which can be run as a stand-alone module or integrated with other modules. The hydrodynamic module was used for modelling the Lower Pembina River. The laws of mechanics on which the movements of a flood wave are based are conservation of mass, momentum and energy. Since the thermodynamic effect can generally be neglected for river flows, the laws of conservation of momentum and energy become identical. The complete set of equations of the conservation of mass and momentum laws for the three-dimensional case are referred to as the Navier-Stokes equations. The simplification of these equations to the two and one-dimensional cases are referred to as the Barre de Saint Venant equations. MIKE 11 uses the implicit direct method for solution of the finite difference approximations of the Saint Venant flow equations. The first step consists of representing the partial differential equations by a corresponding set of difference equations. The second step is the solution of a system of algebraic equations in conformance with the initial and boundary conditions. In order to perform the first step a numerical algorithm has to be defined. The numerical algorithm is a set of algebraic equations defining the spatial and temporal relationship of the variables between defined points or nodes, in the river reach. Numerous numerical algorithms have been proposed for the implicit direct method. The most commonly used ones are the WATER MANAGEMENT CONSULTANTS

12 Model Development 7 Priesman scheme, developed by Dr. Priesman in France and the Abbott scheme, developed originally by Dr. Abbott in Holland and later improved in Denmark. MIKE 11 uses the Abbott numerical scheme. The Abbott algorithm solves the algebraic system of equations in a computational grid consisting of alternating Q, discharge, and h, water level, points where the discharge and water levels are computed respectively. The h points in the computational grid are defined by the user and the model generates automatically the Q points which are always placed midway between neighbouring h points, as shown in Figure 2.1. Figure 2.1 Computational Scheme Q Q h h Q h Flow One of the advantages of this scheme is that it can solve subcritical and supercritical flow. Although in the Pembina River subcritical flow is normally encountered, supercritical flow may also occur at localized sites such as dike and road overtopping. The flood propagation in the Pembina River is complicated by the existence of a large floodplain, which includes numerous roads and dikes. MIKE 11 is especially designed to treat complex floodplains like the Pembina River. The model allows the modeller to simulate separately the main stem from the floodplain. This is done by using a unique feature called link channels. When using this feature the main channel and the floodplain are first defined as separate channels by appropriate cross sections. They are then connected using the link channel feature. The link feature defines a broad crested weir which the user can customize with regards to size, crest elevation and discharge coefficient to reflect natural conditions. The broad crested weirs in the links are very flexible structures allowing the flow to go in either direction. The hydrodynamics of the broad crested weirs are accounted for automatically in the model depending on the upstream and downstream water levels at the current time step. The model can differentiate from free flow or submerged broad crested weir flow. Usually the links or broad crested weirs are applied to particular topographical features which divide the main stem from the floodplain. These features can be roads, railway tracks, natural WATER MANAGEMENT CONSULTANTS

13 Model Development 8 high ground or watershed divides. However, links are not restricted to these topographical features. In fact a link can be applied anywhere in the model by appropriate sizing of the broad crested weir to represent the physical conditions of the area. This unique flexibility allows the model to branch and expand laterally approximating a two-dimensional model. Then, any number of additional nodes can be defined where computer outputs, i.e. water levels and discharges, are desired. 2.2 Model Layout The area of interest in the Lower Pembina River is the 23 mile section of river where frequent flooding occurs from the County Road 55 bridge located about six miles west of Neche to the confluence with the Red River. The Pembina River joins the Red River near the town of Pembina and flooding in this area is a result of both Pembina River flows, backwater from high stages in the Red River causing flooding in the Pembina River and direct flooding from the Red River. The flow processes in this area are very complex and to develop a model that would simulate these processes it was necessary to link the Pembina River model to a model of the Red River. The Red River model would then form a dynamic downstream boundary condition for the Pembina River model. A Red River model was developed by Klohn-Crippen Ltd. for the IJC and the study report was issued on May 31, The upstream boundary condition for the Red River model was the recorded discharge by the United States Geological Survey (USGS) at Grand Forks plus the tributary inflows downstream. The Red River model downstream boundary condition was originally at Selkirk north of Winnipeg. For the current study, this was moved upstream to St. Jean Baptiste in Manitoba to save computation time as the model downstream of this location does not affect the Pembina River model. The upstream boundary condition for the Pembina River model was the USGS recorded data at Neche transposed to the County Road 55 bridge. No modifications to the hydrographs were made as it is believed there were no major inflows or outflows in this reach in 1996 or Some minor breakouts upstream of Neche were observed by residents of the area but these were not of sufficient magnitude to be evident on air photo and satellite coverage. Recent information, however, indicates that a road acting as a dike failed and flow by-passed the gauge at Neche to the north. This breakout was modelled, as discussed in Section 3.1, and it was estimated that about of the peak recorded flow at Neche by-passed the gauge. For the 1997 and 1996 simulations the recorded inflows were used. For the 2, 10 and 25 year floods, concurrent floods of the same return period were used on the Pembina and Red Rivers by scaling the 1997 inflow hydrographs in proportion to the peak flows. Simulations were also carried out for floods of different magnitude to investigate the relative flooding contributions of the two rivers. A 2 year flood on the Pembina River was combined with the 1997 flood on the Red River and the 1997 flood on the Pembina River was combined with a 2 year flood on the Red River. WATER MANAGEMENT CONSULTANTS

14 Model Development 9 The model layout is shown on Figure 2.2. Six major overland flow branches were defined from west to east along the major overland flow paths. These flow branches were labelled as follows: Pembina River Branch between the dikes and natural levees Branches OBN, OBS-1 and OBS-2 representing overbank flows adjacent to the Pembina River Branch Man1 north of the International Boundary Branch A immediately south of the International Boundary Branch B1, B2 and B3 south of Branch A Branch C1 and C2 south of Branch B and north of County Road 55 Branch D1 and D2 south of County Road 55 Branch E south of Branch D Although the slope of the land is towards the northeast and southeast, the road system tends to redirect the flow due east. To allow for the flow to also move north and south, links were defined at the ends of each branch cross section as shown in Figure 2.2. These links allow flow to transfer from one branch to another depending on the relative water level in adjacent cross sections. Highway 18 is the only north-south road included in the model cross sections. The other northsouth roads, as observed from air photos, are a less dominant flow control feature than the eastwest roads as they are not significantly elevated above prairie levels. The north-south roads have a local effect on water levels but do not significantly redirect the flow. The Pembina model flow branches were joined to the Red River model branches by direct branch connections or via links if the connections were at a road. A number of links between the Pembina and Red River models were defined at Interstate 29. In summary, the basic modelling concept was to introduce a flow hydrograph in the Pembina River at the upstream end of the model at County Road 55 and model the flow distribution throughout the floodplain based only on the topography and the roughness factors in the channels and floodplain. The topography was defined by branch cross sections and link elevations between branches. At the downstream end of the model the flows in the Red River create a backwater effect in the Pembina River which tends to locally raise water levels in the Pembina River. 2.3 Hydrologic and Topographic Data The hydrologic data for the model were obtained from the USGS for all gauges in North Dakota and Minnesota. Tributary inflows to the Red River in Canada were obtained from Water Survey of Canada. To account for ungauged areas, the tributary inflows were adjusted in proportion to area as described in Klohn-Crippen (1999). Estimates of the 25, 10 and 2 year floods at Neche WATER MANAGEMENT CONSULTANTS

15 Model Development 10 were obtained from the United States Army Corps of Engineers (USACE). To develop inflow hydrographs for the upstream boundary conditions for the Pembina Model, the 1997 inflow hydrograph at Neche was factored down in relation to the peak 25, 10 and 2 year flows. The 1997, 1996, 25, 10 and 2-year return period hydrographs for Neche are shown in Figure 2.3. The IJC recognised that there is a need for accurate topographic data throughout the Red River basin as such data forms the basis for floodplain definition and floodplain management. The lower Pembina basin served as a test area for preparation of a Digital Elevation Model (DEM). A 130,000-acre study area largely in Pembina County, ND but extending across the Red River into Minnesota and north to Highway 243 in Manitoba was selected. Three different technologies were used to collect topographic data with the intent of fusing the data into one DEM. The US Army Topographic Engineering Center (TEC) managed the project. In the fall of 1998 a differential global positioning system (DGPS) survey of the centerline of paved and gravel roads was carried out. The work was curtailed because of a snowstorm so spot heights on levees and fields were not obtained. The data were processed to provide a complete set of elevations to an accuracy of 5 to 10 cm. A 50,000-acre section of the study area along the River from Neche to the Red River was flown in October 1998 using LIDAR (LIght Detection And Ranging) technology. Essentially a laser mounted in an aircraft whose position and attitude are known fires several hundred thousand bursts a minute and measures the return signals. The data can be processed to produce a highly accurate 'bare earth' DEM, that is buildings and trees are removed in the processing. The result was a DEM with absolute accuracies in the order of 15 cm. Accurate LIDAR surveys can be conducted rapidly but the cost is high. Another more experimental airborne technology known as IFSAR (Inferometric Synthetic Aperture Radar) was also used to map the study area in October The stated aim of the system is to collect data for processing into a DEM at the rate of 100 km 2 /minute with 3-metre accuracy. The system therefore has potential to map large areas at medium accuracy at a reasonable cost. It was anticipated that, by fusing the DGPS, LIDAR, and IFSAR data, a more accurate IFSAR product would be possible. The flooding in the Lower Pembina River is very complex and overland flow is shallow and therefore the DEM has to be very accurate for flood mapping. A preliminary DEM was forwarded from TEC to Water Management Consultants in August It was found that there were inconsistencies in the data, particularly in the area covered by the IFSAR data that could not be resolved with other data sources. Nevertheless it was found that the preliminary DEM was useful for a general portrayal of the topography of the area and demonstrated the flow along the height of land that is characteristic of the Pembina River. Figure 2.4 is a three-dimensional portrayal of the preliminary DEM and Figure 2.5 is a cross section of the DEM about 4 miles downstream of Neche. It was found, however, that the preliminary DEM was not sufficiently accurate for flood mapping as errors of 1-2m were observed when comparing the DEM with other data sources. The LIDAR WATER MANAGEMENT CONSULTANTS

16 Model Development 11 data alone could not be used to generate a DEM suitable for flood mapping because it did not cover the complete area to be modelled. The LIDAR data were used however for verifying cross sections where the LIDAR data were available. The topographic data that were used to develop cross sections for the MIKE 11 model came from three sources: HEC 2 cross sections provided by the USACE from the 1982 Channel Capacity Study cross section survey conducted in 1980 GPS data for road crests 5 Contours and spot heights at road intersections from the USGS 1:24,000 topographic map series based on 1972 aerial photography The vertical datum used for the model was NAVD1929. The North Dakota State Water commission provided a set of air photos flown on April 23, 1997 at a scale of 1:17,000. These photographs were joined together into an uncontrolled photo-mosaic of the Lower Pembina River as shown in Figure Model Parameters Except for the USGS gauge at Neche, there are no water level data in the Lower Pembina River available for calibration. The roughness parameters developed for the model were therefore based on experience from calibration of the Red River model. For the Red River model a Manning s n of provided the best results in the floodplain areas. This value was therefore use for the floodplain areas surrounding the Pembina River. For the main channel of the Pembina River, a slightly smaller roughness factor of provided water levels that best simulated breakout flows along the river banks for The roughness factor in the main channel is relatively high to account for vegetation and debris along the river. Once the model was operational, the results were compared with the detailed April 23 rd air photos and anecdotal information to confirm the direction of flows and extent of flooding. To compare flooded areas with the model a simple DEM was set up using the USGS topographic series mapping. While this DEM is not suitable for detailed flood mapping it assisted the process of model calibration. Calibration results are discussed in Section 3.1. Because of the complex flow conditions with shallow overland flow, stability problems with the model were encountered. These stability problems were overcome by setting narrow slots in the overland flow sections, which maintained water in the flow branches but below ground surface level unless, of course, there was flooding along the flow branches. The slots were V shaped and up to 5 m deep and 1 to 2m wide at the crest. The slots can be thought of as simulating minor drainage ditches throughout the model area. WATER MANAGEMENT CONSULTANTS

17 Model Development Model Limitations The hydrodynamic model of the Lower Pembina River provides information on the magnitude of flows in the designated flow branches and over the flow links. Because a detailed, accurate DEM was not available, some of the topographic data used may be in error, particularly the crest elevation of dikes. Therefore, the flows that overtop dikes would be overstated if the dikes have been raised since the topographic data were collected. Dyke elevations were based on the HEC- 2 cross sections. The road crest data that were used were more accurate as they were based on recently collected GPS data. Bridges across the Pembina River were not explicitly modeled. Head losses at the bridges were incorporated into the Manning s n value for the channel. The bridge decks in the Lower Pembina river area are generally constructed above prairie levels and so overtopping of bridges did not occur. The water levels in the Pembina River are frequently affected by ice and debris jams but the model does not include these effects. It is possible, therefore, that there were short periods when higher water levels occurred than were predicted by the model, due to ice and debris temporarily retarding downstream flow. Therefore brief periods of overtopping of some roads may have occurred that were not simulated by the model. As it would be expected that these phenomena would be of short duration they would not influence the major conclusions of the modelling study. The model that was used for this study was a fixed bed model, which means that sediment transport and deposition were not included. MIKE 11 has the capability to model sediment transport processes and this could be considered for subsequent modelling studies. WATER MANAGEMENT CONSULTANTS

18 13 3 RESULTS 3.1 Existing Conditions Before modelling alternative dike removal scenarios, the hydrodynamic model was used to simulate existing conditions in 1997 and This provided a way of verifying whether the model was providing a reasonable simulation of the hydrodynamics of the Lower Pembina River. Peak water level data at the Neche gauge were compared to the modelled 1997 results. The Neche gauge, located between cross sections and 19000, had a peak water level reading of 833.7ft. The cross sections had modeled water levels of ft and ft respectively indicating a reasonable calibration for this area of the model as the recorded high water level was between the two modelled water levels upstream and downstream. At Pembina on the Red River, the peak recorded water level was feet and the modelled water level was feet, which is within 0.1 feet of the recorded level indicating a very good calibration. Figure 3.1 shows the peak flow distribution in 1997 as a percentage of the 1997 peak instantaneous flow of 15,100 cfs. The percentage flow distribution is expressed to the nearest, which reflects the accuracy of the topographic data used in the model. This figure can be compared with the air photo mosaic (Figure 2.6) and it can be seen that the model simulated flow patterns indicated on the air photos very well, indicating a good calibration. The air photo mosaic provided a much higher level of detail than the 1:150,000 scale satellite imagery. It was not possible to produce a flood map for a comparison because of the lack of data of sufficient accuracy to produce a DEM as discussed in Section 2.3. The flow distribution also was in agreement with anecdotal information provided by Randy Wagner, a local resident and other residents of the area. They confirmed that the major breakouts occurred downstream of Neche. It was found that using the peak mean daily flow of 14,300 cfs resulted in no breakouts upstream of Neche. However, using an inflow hydrograph with the peak instantaneous flow of 15,100 cfs resulted in two minor breakouts in this area. The first was near a small drainage course that flows into Hyde Park Coulee. The breakout was to the north and most of the flow was redirected east by an east-west road. A very small quantity of flow overtopped the road and crossed the international boundary. At a location about two miles upstream of Neche a breakout to the north had been reported by Randy Wagner. The model indicated that the water level was only slightly higher than the road crest but no significant overflow was indicated by the model unless the road had been washed out. Randy Wagner subsequently consulted local township officials and was advised that the road had been washed out to ground level. The washout was incorporated in the model and it was found that about of the peak flow passed through the breach. This flow WATER MANAGEMENT CONSULTANTS

19 Results 14 overtopped Highway 18 and continued to the east. A small quantity overtopped the border road north of Neche. Apart from these two relatively small breakouts it can be seen in Figure 3.1 that the diking system and roads contain the flow upstream of Neche. Downstream of Neche major breakouts occur both north and south of the river. The flow remaining in the main channel of the Pembina River, contained by existing dikes and natural levees, is 40% of the peak flow or about 6000 cfs. The breakout to the north reaches the flow branch along the international boundary and the model indicated that about 15 % of the peak flow moved along this branch. About of the peak flow moves along the branch to the south and 3 between the Pembina River and County Road 55. County Road 55 was sufficiently high in this area that the 1997 flood did not overtop the road. East of the County Road 55 bridge, the flow along the south side of the Pembina River crosses to the north side because County Road 55 prevents flow moving south. The flow remaining in the channel is 4 of the peak flow or about 6800 cfs. The flow along the border continues at 1 of the peak flow (2300 cfs) and floods an area of about a square mile in the vicinity of a transmission tower as shown in Figure 3.2. This area is contained by the road along the International Boundary to the north and an area of higher land to the east known locally as Switzer Ridge. Some flow (about 130 cfs) leaves this area to the north through culverts in the road along the International Boundary into the Aux Marais River. Most of the flow moves east over Switzer Ridge. In 1996 the model indicated zero flow into the Aux Marais River originating from the Pembina River. In the flow branches to the south, about 3 of the peak Pembina River flow ( 5300 cfs) flows overland north of the Pembina River. East of this area the Pembina flow begins to merge with flooding from the Red River. The slope of the water surface profile significantly reduces due to the influence of backwater of the Red River. The location of the change in slope of the water surface profile will change depending on the relative magnitude of the Pembina River and Red River floods. Figure 3.3 shows the slope of the water surface profile for peak water levels in 1997 and 1996 along Branch C2 from the County Road 55 crossing to near the town of Pembina. It can be seen that in 1997 the water surface profile becomes horizontal at chainage and has a slight negative gradient between and because of back flow from the Red River. In 1996, however, the slope is still quite steep up to and remains positive throughout. The flood in the Red River in 1996 was relatively smaller than the 1996 flood in the Pembina River. To compare water levels for different scenarios, two locations were selected downstream of Neche and four locations were selected in the area where Pembina River overland flows meet the overland flow from the Red River northwest of the town of Pembina. Water levels were also compared in the Red River at the town of Pembina. The locations are shown in Figure 2.2 and the modelled water levels for different scenarios are listed in Table 3.1. It is evident from Table 3.1 that water levels in the lower area were very flat in 1997 indicating the influence of Red River flooding. Flows across the International Boundary north of this area occurred in 1996 and 1997 and were primarily caused by the flood in the Red River. The Pembina River made only a minor contribution to this overflow as the peak flow in the Red River is about 10 times the peak Pembina River flow. WATER MANAGEMENT CONSULTANTS

20 Results 15 Table 3.1: Water level comparisons for Pembina River Scenarios Peak water elevation at each location (ft) P1 P2 P3 P4 P5 P6 P7 Existing No Dikes Natural Floodway Existing No Dikes Natural Floodway Existing No Dikes Year Natural Floodway Setback Dikes Setback Dikes with Breakouts - - Existing No Dikes Year Natural Floodway Setback Dikes Figure 3.4 shows the flow distribution for the 1996 flood with a peak flow at Neche of 8500 cfs. As would be expected, the flow is contained by the dikes upstream of Neche and breakouts occur at the same locations downstream of Neche as occurred in The flow remaining in the channel up to the dike crests is 70% of the peak or about 6000 cfs, similar to the flow remaining in the channel in The flow along the border of of the peak is much less than in Downstream of the County Road 55 bridge, the flow remaining in the channel reduces to 60% of the peak or about 5100 cfs. From this analysis it is apparent that the capacity of the main channel of the Pembina River downstream of Neche, with the existing dikes, is in the range of 5100 to 6800 cfs. Any flows larger than this will spill from the channel and will generally not return to the main channel because of the slope of the land away from the river. The exception is where flow is redirected back to the main channel by roads acting as dikes. WATER MANAGEMENT CONSULTANTS

21 Results Flow Combinations To investigate the relative importance of the Pembina and Red River floods in the area west of Pembina, the 1997 Pembina River flood was combined with a two-year flood on the Red River. This scenario lowered the water level west of Pembina as shown in Table 3.2. As would be expected, the backwater effect of the Red River was significantly reduced as shown in Figure 3.3. Nevertheless, the slope of the surface water profile still flattens due to the lower river gradient in this area. To examine the other extreme, the 1997 flood on the Red River was combined with a two-year flood on the Pembina River. This shows the backwater effect of the Red River in the area west of Pembina as shown in Figure 3.3 and Table 3.2. It can be seen in Figure 3.3 that the 1997 Pembina River flood raised water levels in this area only slightly above Red River levels. There is a slight negative slope to the water surface profile because the flow is from the Red River into the Pembina floodplain. Table 3.2: Water level comparisons for Red River Scenarios Peak water elevation at each location (ft) P1 P2 P3 P4 P5 P6 P Existing Red - 2 yr Pembina yr Red Pembina with Raised Border Road with Bridges Removed with Pembina & Emerson Dikes Removed Removal of Dikes Model runs were carried out to simulate removal of the dike systems in the Lower Pembina River. The dikes are discontinuous but generally follow the boundaries of the Pembina River Branch shown in Figure 2.2. Major dikes upstream of Neche are shown in Figure 3.5. These dikes were removed from the model together with all dikes either side of the Pembina River. Dikes were not removed upstream of the County Road 55 bridge as this was outside the study area. Roads acting as dikes were not removed from the model for this scenario. Figure 3.6 shows the flow distribution for the 1997 flow with the dikes removed. The simulation indicated that, upstream of Neche, breakout flows occurred both north and south of the Pembina River. The flows to the north were constrained by roads acting as dikes about one mile south of the International Boundary though a small quantity of flow crossed the border at Hyde Park Coulee. Flows to the south were constrained by County Road 55 though some flow overtopped County Road 55 because of the blockage effect of the north-south road near Neche. The largest proportion of the breakout flows occurred in the corridor between the Pembina River and County WATER MANAGEMENT CONSULTANTS

22 Results 17 Road 55. The flows in the branch were about 50% of the peak flow, reducing to 4 east of Neche. At the location of a dike system north of the river just west of Neche, breakout flows might have occurred if less flow had not already spilled from the main channel upstream to the south and east. It was found that these major upstream breakouts eased the pressure in the vicinity of Neche as only 40% of the peak flow remained in the channel, which is 6000 cfs. It is apparent that the existing dike system upstream of Neche is effective at containing flows of the magnitude of the 1997 flood. The major effect of dike removal was to move the major breakouts from a point downstream of Neche to upstream of Neche. Because of the unusual topographic conditions of the Lower Pembina River, with the main channel flowing along a ridge of higher land, changing the location of the first major breakout in the Lower Pembina River results in a completely different pattern of overland flooding. In particular as shown in Figure 3.6, there is no flooding along the International Boundary east of Neche if dikes are removed. The roads acting as dikes west of Neche prevent more flow moving northwards and the breakout to the north downstream of Neche that caused the flooding along the International Boundary in 1997, is significantly reduced from natural conditions because of the reduced flow in the main channel. Figure 3.7 shows the flow distribution without dikes for the 1996 flood. The flow distribution is similar to 1997 except flow does not cross County Road 55. About 50 to 70% of the flow remains in the main channel. It can be seen in Table 3.1 that, for major flood events, removing the dikes has no impact on water levels in the area north of County Road 55 and west of the town of Pembina and no influence on water levels in the town of Pembina. If the dikes are removed there is a slight decrease of 0.2 to 0.3 ft at P5 and P6 for the 10 year flood. This is because there is less flow along Branch C2 and the influence of the Red River flood is less during a 10-year event. 3.4 No Dikes, No Roads Condition The final step to simulate natural conditions was to remove all the roads in addition to the dikes. Figure 3.8 shows the 1997 peak flow distribution for this scenario. It was found that about 1 of the peak flow spills northwards from the main channel towards Hyde Park Coulee and into the South Buffalo drainage in Manitoba. A further 10% of the peak flow crosses the International Boundary about a mile west of Neche and east of Neche. This total flow of 30% of the peak (4500cfs) to the north is matched by 30% (4500 cfs) flowing south across County Road 55. The model is set up so that flow was not modelled south of channel D1. All of the flow to the south of the Pembina River flows toward the Tongue River and ponds in the area of the confluence with the Pembina. With the outflows from the Pembina River, there is only 30 % of the flow left in the main channel. This flow of 4500 cfs could be considered the natural capacity of the Lower Pembina River. Under this scenario, there is no flow along the south side of the International Boundary east of WATER MANAGEMENT CONSULTANTS

23 Results 18 Neche because there are no spills from the main channel to the north as occurred in This is because upstream breakouts reduce the flow in the main channel in this area. It appears that the peak flow distribution in Figure 3.8 indicates a total flow of 10. This is because it represents the distribution of the peak flows and not the distribution of total flows. The peak flows in the branches did not occur at the same time and some overflows contained spikes of flow with relatively small volumes of flow. This resulted in the total of the peak flows being slightly greater than the inflow peak. Figure 3.9 shows the peak flow distribution for the 1996 flood for the No dikes, No Roads Condition. The results are similar to the 1997 flow distribution except that only 1 flows to the north with 30% still flowing to the south. This is because there is a broad valley to the south, which forms the headwaters of Louden Coulee south of Neche. Once County Road 55 is removed from the model, the wide slope of this area is efficient at removing water that spills over the right bank of the Pembina River. The simulations of this condition demonstrate that roads in the area have a significant influence on the distribution of flood water. Removal of the roads allows water to move efficiently away from the main channel and the flow remaining in the main channel is less than with only the dikes removed. It appears that the natural maximum capacity of the main channel of the Lower Pembina River ranges from 5300 cfs upstream of Neche to 4500 cfs downstream. It is of interest to note that the modelled flow in the main channel downstream of Neche under natural conditions was a similar value for both 1996 and The natural maximum capacity of the main channel of the Lower Pembina River corresponds to a flood with a return period of less than I in 10 years. In Table 3.1 it can be seen that water levels in the area north of County Road 55 and west of Pembina are lower for the No Dikes, No Roads Condition for all flood events. This is because local roads in the area tend to increase water levels in this extremely flat terrain. 3.5 Removal of County Road 55 and the Border Road The effect of County Road 55 on overland flow south of the Pembina River was investigated by removing the dikes and County Road 55 but leaving other roads in place. The results are shown in Figure This figure should be compared with Figure 3.6, which shows the peak flow distribution without dikes, but County Road 55 in place. In Figure 3.10 there is additional flow crossing County Road 55. The flow in the branch south of this road increases to 5 of the peak flow compared with 10% of the peak with County Road 55 in place. There is correspondingly less flow in the branches to the north of County Road 55. The effect of removing the boundary road in combination with removal of the dikes but not other roads is demonstrated in Figure The results are the same as in Figure 3.6 because there is no flow along the south side of the border once the dikes are removed. This is because the removal of dikes increases the flow to the south and east just downstream of the County Road 55 Bridge and there is a relatively small flow left in the channel for potential breakouts to the north. WATER MANAGEMENT CONSULTANTS

24 Results Boundary Floodway In 1976 the USACE completed a report on the Pembina River called Feasibility Report for Flood Control and Related Purposes. This report was updated in 1983 and one of the alternatives considered was a floodway along the international Boundary with a capacity of 2000 cfs. The diversion point was upstream of Neche and the floodway alignment would be along the south side of the International Boundary to the Red River just upstream of Emerson. it was planned that the main channel would carry 3500 cfs for a total design flow of 5500 cfs. The overall design was for a flood of return period of about 1 in 10 years. The floodway was inserted into the model and the influence of the floodway investigated during a flood of the magnitude of 1997 for existing conditions. Figure 3.12 shows the alignment of the floodway and the 1997 peak flow distribution. It can be seen that the floodway would not significantly reduce the extent of flooding in It is important to note that the floodway could impede overland flow from the Red River crossing the border west of Emerson. Unless significant flow passages are provided under the floodway, it would raise water levels in this area. The model did not include provisions for by-pass flows and thus modelled water levels were higher in 1997 south of the border west of Emerson and at the town of Pembina. This can be seen in the water level comparison points in Table Set Back Dikes Set back dikes were modelled for 2, 10 and 25 year flows. Figure 3.13 shows a conceptual alignment for a set back dike system. It would start at the County Road 55 crossing of the Pembina River upstream of Neche and terminate near the town of Pembina. The dike tie-ins were not investigated as part of the modelling scope of work. While the tie-in at the upstream end appears to be feasible because the Pembina River is emerging from a shallow valley into the reach where the channel is elevated, the tie-in at the Red River will probably be more challenging. There are no dikes along the Red River at this location but it may be possible to tiein to the Interstate Highway and this was assumed for the modelling. However, the Tongue River enters the Pembina River just upstream of the Interstate Highway so the set back dikes would have to either extend up the Tongue River or the south set back dike would have to terminate upstream of the Pembina/Tongue confluence. The set back dikes would create a flood corridor about 2700 feet wide. The set back dike system was modelled for two conditions; Allowing farming (grazing) in the flood corridor Allowing the flood corridor to revert to floodplain forest For the first condition the roughness values in the flood corridor were assumed to be the same as the rest of the Pembina River floodplain. The Manning s n flood values were assumed to be for this condition. For the second condition the trees and brush that would constitute the floodplain forest would have a much higher roughness and a Manning s n of was assumed. WATER MANAGEMENT CONSULTANTS

25 Results 20 Figure 3.14 shows a cross section of the set back dike system for a section about four miles downstream of Neche. It can be seen that the water level for the floodplain forest condition would be about two feet higher than if farming was allowed in the flood corridor. Therefore the crest of the set back dikes would have to be about two feet higher if the planned land use in the flood corridor was floodplain forest. It can be seen in Table 3.1 that the set back dikes would lower water levels in the area north of County Road 55 and west of Pembina by about a foot for the 10 year and 25-year floods. This is because the set back dikes would eliminate overland flow from the Pembina River during the design flood. This conclusion is only valid for a set-back dike system that is continuous on the north side of the Pembina River. 3.8 Controlled Breakouts The set-back dike system was also modelled with controlled breakouts as proposed by local landowners and communicated by Randy Gjestvang of the North Dakota State Water Commission. The controlled breakouts would to a certain extent, simulate natural conditions. The first breakout would be located at Hyde Park coulee and would flow north. The next would be to the south about 2 to 3 miles upstream of Neche and the third would also be to the south about one mile downstream of Neche. Each breakout would have a capacity of about 500 cfs at the peak of the 25-year flood and would begin to operate around the magnitude of the 10-year flood. The three breakouts were modelled in combination with the set-back dike system. Each breakout was modelled as a 330-foot wide overflow section with the invert set at about the elevation of the 10-year flood. The breakout flows would therefore not occur for floods smaller than a ten year flood. Downstream of the breakouts, the flows were not modelled through the existing topography as it was envisaged that a designed channel or designated flood corridor would convey the flows safely away from the Pembina River. Design of such a channel or flood corridor system was beyond the scope of this study so the flows were modelled as links leaving the Pembina River system and joining the Red River. The breakout flows to the south would flow into the Tongue River and would join the Red River near the Pembina confluence. The breakout to the north would flow into the South Buffalo River in Manitoba and join the Red River 20 miles north of Emerson. The flow distributions for the controlled breakout alternative is shown in Figure 3.15 for the 25 year flood. The flow in the Pembina River downstream of the breakouts is about 1500 cfs less than with the standard set-back dike alternative. This reduces the height of the required dike by about 0.6 feet as shown in Figure The effect on water levels in the area west of Pembina is shown in Table Pembelier Dam A multipurpose dam has been proposed on the Pembina River just upstream of Walhalla. As described in the Phase 1 General Design Memorandum, U.S. Army Corps of Engineers( 1983), the dam would be a 145 foot high earthfill structure and provide 147,000 acre feet of controlled storage. A total of 128,000 acre feet would be exclusively for flood control. The outlet works WATER MANAGEMENT CONSULTANTS

26 Results 21 would comprise a 12 foot diameter conduit through the south abutment of the dam and discharges would be limited to 4000 cfs up to about a 40 year flood event. For larger floods, additional flows would be released over the spillway and at the peak of the Probable Maximum Flood, the total outflow would be 6340 cfs. As described in Section 3.4, the natural capacity of the Pembina River downstream of Neche is about 4500 cfs. The proposed Pembelier Dam would limit releases to 4000 cfs up to a 40 year flood, and therefore the dam would provide total flood control for floods up to a 40 year flood. For the Probable Maximum Flood the peak outflows from the dam would be 6340 cfs which corresponds to an existing downstream flood of about 1 in 10 years. The peak outflow during a Probable Maximum Flood would be less than the 1996 flood of 8500 cfs Bridges and Dikes at Pembina and Emerson The influence of the road bridge at Pembina and the rail bridge at Emerson was investigated by detailed modelling of this reach of the Red River. Additional sections were added to the Red River model to ensure that the detail of the bridges was represented. The bridges were modelled as sections with all the piers represented. It was found that the water level did not intersect the underside of the bridge in 1996 or The 1997 flood was then modelled both with and without the bridges in place. It was found that there was no difference in the upstream water levels between Pembina and Emerson though there is a small difference (less than 0.1 feet) upstream with these two conditions. It was concluded that the bridges do not impact upstream water levels. However, there is a noticeable change in the water surface profile in this reach. Figure 3.16 shows a plot of the water surface profile and there is a steepening of the slope through the area where the bridges are located and an apparent flattening of the slope upstream. This indicates that there is a constriction in flow in this area which is raising upstream water levels. To investigate the effect of the town dikes these were also removed from the model and the profile is shown in Figure Removal of the dikes reduces the upstream water level by about 0.5 feet. Additional water level comparisons are shown in Table Raising the Border Road West of Emerson The impact of raising the road along the border west of Emerson above 1997 flood levels was investigated. It was found that cutting off the overland flow west of Emerson would increase water levels near Pembina about 0.5 feet for the 1997 flood. However, in the floodplain west of Pembina, water levels would be raised by about a foot. Water level comparisons are shown in Table 3.2. WATER MANAGEMENT CONSULTANTS

27 22 4 CONCLUSIONS The lower Pembina River flows on the height of land from the County Road 55 crossing upstream of Neche to near the confluence with the Red River. As a result of this unusual topography, flood flows in excess of the main channel capacity spill from the channel as breakouts and do not return to the Pembina River unless redirected by roads acting as dikes. Breakouts tend to occur where there are weaknesses or discontinuities in the existing diking system along the main channel. Under current conditions, dikes upstream of Neche prevent breakouts. Downstream of Neche, breakouts occurred in 1996 and 1997 both to the north and to the south of the channel and continued as overland flows in a generally easterly direction directed by the road system and occasionally spilling to the north and south over east-west roads. Roads acting as dikes prevented major overland flows occurring across the International Boundary and south of County Road 55. Hydrodynamic modelling of the Lower Pembina River was carried out using the one-dimensional model, MIKE 11, developed by the Danish Hydraulics Institute. Because of the complexity of the flow patterns at the confluence with the Red River, the Lower Pembina River model used the Red River model as the downstream boundary condition. Recorded hydrographs at Neche, transposed to the County Road 55 crossing were used as the upstream boundary condition for the Pembina River model. The preliminary Digital Elevation model (DEM) provided by the US Army Corps of Engineers was used to develop an understanding of the topography of the Lower Pembina River. The DEM has not yet been finalized and could not be used for flood mapping because of data discrepancies that have yet to be resolved. The GPS data collected by the US Army Corps of Engineers was used to develop the model cross sections in conjunction with HEC-2 sections and the USGS 1:24000 topographic maps. The 1997 and 1996 floods were simulated for the existing conditions and the flow distributions compared with data from air photographs taken during the 1997 flood to ensure that the model was providing a reasonable simulation. The hydrodynamic model provided information on the distribution of flows throughout the Lower Pembina River floodplain. It was found that the breakouts that occurred downstream of Neche constituted about 5 of the peak 1997 flow of 15,100 cfs. About 1 of the peak flow (2300 cfs) was conveyed along the south side of the International Boundary. About 130 cfs passes through the culverts at the International Boundary into the Aux Marais River. Further to the south, about 3 of WATER MANAGEMENT CONSULTANTS

28 Conclusions 23 the peak flow (5300 cfs) flows overland north of the Pembina River. Similar flow distributions were found for the 1996 flood. Simulations were carried out with the dikes removed but leaving the roads acting as dikes in place. It was found that upstream of Neche, breakout flows occur both north and south of the river. The flows to the north were constrained by roads acting as dikes and flows to the south were constrained by the embankment of County Road 55. Because the breakouts occur upstream of Neche, the flow in the main channel is reduced and much smaller breakouts occur downstream of Neche. It was these breakouts that caused the flooding along the south side of the International Boundary in 1997 in the vicinity of the Aux Marais River. With the dikes removed there is no flooding in this area. Simulations were also carried out with all dikes and roads removed. This simulated natural conditions. It was found that about 1 of the peak 1997 flow would spill north into Hyde Park Coulee and across the border into the South Buffalo drainage. An additional 1 spills across the border further east, for a total of 30% flowing north. A similar quantity flows south across County Road 55. The flow remaining in the main channel is 4500 cfs, less than the magnitude of the 10 year flood. Simulations were carried out with the dikes and County Road 55 removed but leaving all other roads in place. It was found that there was additional flow across County Road 55 and correspondingly less flow in the branches to the north. A scenario with the dikes and only the Border Road removed results in the same flow distribution as the scenario with just dikes removed. This is because the removal of dikes increases the flow to the south and east just downstream of the County Road 55 bridge and there is a relatively small flow left in the channel for potential breakouts to the north. A simulation was carried out with the boundary floodway proposed by the US Army Corps of Engineers in This floodway was designed to convey the balance of the 10 year flood in the Pembina River and had a capacity of 2000 cfs. It was found that the floodway would have a minor impact on the extent of flooding in a major flood such as Set back dikes were investigated and a 2700 ft wide flood corridor was modelled. The details of dike tie-ins at the upstream and downstream ends would need to be resolved for feasibility. It was found that the set-back dike system would reduce water levels west of Pembina by about a foot for the 10 year and 25-year floods. This assumes continuous dikes on the north side of the Pembina River. If the flood corridor was allowed to revert to floodplain forest, it was found that the dikes would have to be about two feet higher. If controlled breakouts are incorporated in the setback dike system the required height of the setback dike for a 25-year flood could be reduced by 0.6 feet. The proposed Pembelier Dam would have a significant impact on downstream flooding. There would be no flooding up to about a 10-year flood and for the Probable Maximum Flood, the peak downstream flow would be less than occurred in An analysis of the impact of the various scenarios on water levels in the flat area north of County Road 55 and west of the town of Pembina was carried out. It was found that, for major floods, eliminating the dikes on the Pembina River would have no effect on water levels in this area because the flooding is dominated by Red River flows. For the 10-year event, however, there is a slight decrease in water WATER MANAGEMENT CONSULTANTS

29 Conclusions 24 levels at the comparison points in Branch C2. For natural conditions there is a decrease in water levels throughout this area and in the Red River at the town of Pembina. This is because local removal of the roads reduces local flood levels. It was found that the set back dikes would decrease water levels in this area because they would eliminate overland flow during the design flood. The detailed hydraulics of the road bridge at Pembina and the rail bridge at Emerson were investigated with the Red River model. A simulation was first carried out with the bridges in place and then the bridges were removed from the model for comparison. It was found that the bridges had no effect on upstream water levels at Pembina and a very small effect (less than 0.1 foot) upstream of Pembina. However, the town dikes at Pembina and Emerson increase water levels by about 0.5 feet for the 1997 flood event. If the Border Road west of Emerson were raised above 1997 flood levels, water levels at Pembina would have increased by about 0.5 feet in 1997 and would have increased by about a foot in the floodplain west of Pembina. CDN WATER MANAGEMENT CONSULTANTS INC. Morgan Garrett, P. Eng. Project Engineer C. David Sellars, P.Eng. Project Manager WATER MANAGEMENT CONSULTANTS

30 fig1-1 locplan.apr WATER ~ ~ MANAGEMENT CONSULTANTS Figure 1.1: Pembina River Basin River Red River Pembina CANADA UNITED STATES # Walhalla Study Area # Neche # Emerson # Pembina N Miles

31 Fig1-2 studyarea.apr Red River River Neche Emerson Pembina Red River River Tongue Aux Marais River Pembina WATER ~ ~ MANAGEMENT CONSULTANTS Buffalo South River CANADA UNITED STATES County Road 55 N Figure 1.2: Study Area Miles

32 Pembina.apr WATER ~ ~ MANAGEMENT CONSULTANTS Figure 2.2: Schematic Model Layout & # # # # # # # # # # # # ## # # # # # P1 # r A P3r P4 # # # # r# # # # B1 # # B2 # # # B3 # # # # # # # # # # OBN # # # OBS-1 # C1 P2r C2 P5r P6r # # # # # # # # # # # # # D2 # D1 OBS-2 # # # # # # # # # Inflow Hydrograph Man E # # # # # # r P7 # # r Water Level Comparison Point # Link Section N

33 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 2.3: Pembina River Hydrographs at Neche Discharge (tousands of cfs) yr 10 yr 2 yr Apr Apr Apr-97 1-May May May May-97 Date 7012 Inflow Hydrographs.xls - Hydrographs-Chart

34 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 2.4: Lower Pembina River Preliminary Digital Elevation Model Pembina-Surface.srf

35 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 2.5: Preliminary Digital Elevation Model Section Four Miles East of Neche Section.srf

36 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 2.6: Air Photo Mosaic April 23, 1997 CANADA US Neche County Road 55 Pembina N Miles

37 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.1: 1997 Peak Flow Distribution for Existing Conditions Peak Flow: 15,100 cfs % % 100% 4 Breakout noted in report text 3 Major flow path ( percentage of peak inflow)

38 fig3-2airphoto.apr WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.2: Air Photo of Aux Marais River Crossing April 23, 1997 N Aux Marais River Culverts Switzer Ridge Transmission Tower

39 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.3: Slope of the Peak Water Surface Profile in Branch C2 for 1997 and Elevation (ft) Pembina - 2 yr Red 2 yr Pembina - 97 Red Ground Chainage (m) 7012 Figures-March.xls - C2-Prof-Grph-March

40 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.4: 1996 Peak Flow Distribution for Existing Conditions Peak Flow: 8,500 cfs 70% 20% 30% 100% 60%

41 air photo - dyke system.apr WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.5: Major Dike Systems Upstream of Neche CANADA US Neche County Road 55

42 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.6: 1997 Peak Flow Distribution Without Dikes Peak Flow: 15,100 cfs 3 10% 4 40% 4 50% 4 30% 100% 10% 4

43 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.7: 1996 Peak Flow Distribution Without Dikes Peak Flow: 8,500 cfs 7 50% 50% 20% 50% 4 30% 100% 70%

44 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.8: 1997 Peak Flow Distribution Without Dikes and Roads Peak Flow: 15,100 cfs 1 10% 70% 10% 3 30% 10% 100% 3 30% 30% 40%

45 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.9: 1996 Peak Flow Distribution Without Dikes and Roads Peak Flow: 8,500 cfs % 10% 10% 100% 30% 30% 50% 3

46 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.10: 1997 Peak Flow Distribution Without Dikes & County Road 55 Peak Flow: 15,100 cfs % % 5 40% 30% 60%

47 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.11: 1997 Peak Flow Distribution Without Dikes and Border Road Peak Flow: 15,100 cfs 3 10% 4 40% 4 50% 4 30% 100% 10% 4

48 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.12: 1997 Peak Flow Distribution Boundary Floodway Peak Flow: 15,100 cfs % 30% 2 100% 4

49 Fig3-13setbackdykes.apr WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.13: Conceptual Set-Back Dike Alignment CANADA UNITED STATES County Road 55 N Miles

50 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.14: Set-Back Dike Section Four Miles Downstream of Neche Elevation (ft) yr Rough 25 yr Normal 25 yr Breakout Normal 10 yr Rough 10 yr Normal Section Distance (ft) 7012 Setback Water Levels.xls - March-Section

51 WATER ~ ~ MANAGEMENT CONSULTANTS Figure 3.15: 25 year Flow Distribution With Setback Dikes & Controlled Breakouts Peak Flow: 10,000 cfs 90% % 8