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Indian River Basin Study

Phase I Report
Indian River Basin Hydrologic Model

Table of Contents

1.0
Introduction........................................................... 1

2.0 Background and Level of
Effort.................................................................... 1

3.0 Technical Assumptions and Data
Sources................................................................. 2

4.0 Hydrologic Analysis and
Results.................................................................. 4

5.0 Description of Control
Structures.............................................................. 5



List of Figures

Figure 1 Major Watersheds, Indian River Basin
Figure 2 Subwatersheds, Indian River Basin
Figure 3 Land Use, Indian River Basin
Figure 4 Soils, Indian River Basin
Figure 5 Roadway Culvert Restriction Using a Manhole and Small Culvert Primarily for Flood Protection
Figure 6 Roadway Culvert Restriction Using a Manhole and Small Culvert for Extended Detention and Water Quality Treatment
Figure 7 Bridge or Large Diameter Culvert Restriction Using a Ring Dike or Small Restricted Flow Culvert for Flood Detection


List of Tables

Table 1 Range of Cost Estimates for Roadway Modifications

1.0 Introduction
This report summarizes the Phase I Water Management Plan that was completed for the Lake Pelican Water Project District (LPWPD). The report describes the level of effort for this study, relates the information gathered as input to the hydrologic analysis, details the methodology used for the analysis, and summarizes the results. The report concludes by providing a description of some potential types of control structures that could be used to reduce the peak flows throughout the watershed, along with a range of costs that might be expected to implement these structures.

2.0 Background and Level of Effort
In January 1999, the LPWPD contracted with Barr Engineering Company to begin work on a comprehensive surface water management plan (SWMP) to effectively manage the water quantity and quality within the Upper Big Sioux River Watershed. The SWMP will focus on detaining the flow of water in the upper portions of the basin, by using small low-impact flood and water quality control structures and promoting the use of watershed best management practices (BMPs). These measures will reduce flooding in the City of Watertown and reduce the high sediment load from the watershed.

This Phase I study was completed as an effort to identify the potential reduction in peak flows that could be achieved through installation of small flood control structures in the upper watershed of the Upper Big Sioux River. The Phase I study was limited to evaluation of the peak flow reductions in only a portion of the watershed because of limited funding. The potential impacts to the water quality of the Upper Big Sioux River will be analyzed as part of future phases of the project. The Indian River Basin was selected for the Phase I analysis for several reasons: it is located in the upper part of the watershed, the topography is steeply sloped, digital information was readily available, and Grant County had offered assistance with obtaining data.

The Indian River basin has an area of approximately 39 square miles. It extends east from the Big Sioux River in Grant County. There is considerable relief in this watershed that has created a high density of creeks and rivers; there are no large surface water bodies within this basin. The soils are primarily sandy loam with moderate infiltration capacity. The land use in this basin is agricultural, with about one-third of the basin in CRP or fallow.

Hydrologic analyses were completed to estimate the timing and amount of runoff that would be expected to occur under various precipitation events. These models are most accurate when they can be calibrated to existing gaged information. The Corps of Engineers did an extensive hydrologic analysis of the gage data for the near Watertown gage (Flood Control for Watertown and Vicinity, South Dakota, Corps of Engineers, August 1994). The near Watertown gage is located just downstream of the confluence with Mahoney Creek, and has the longest period of record of gages in this area (27 years). This gaged data will be used to calibrate the watershed model when it is developed to include the entire tributary area to the gage (not just the Indian River model). However, prior to completion of the model of the entire watershed tributary to the near Watertown gage, the results of partial watershed hydrologic models (such as the Indian River Basin model) should be considered preliminary.

The Phase I study examined a small portion of the entire watershed, and additional analyses are required to calibrate the model to the near Watertown gaging station. This study was also based on the limited information which was currently available, requiring estimates of various watershed and drainage system parameters that were not available or easily obtained. These parameters will require further detailed survey and study prior to implementation of flood control structures. As a result, this should be considered a reconnaissance level study, and throughout this report we emphasize that the results should be considered preliminary.

3.0     Technical Assumptions and Data Sources
The initial work for the Phase I study included obtaining existing information, maps, and reports from various agencies that could be used for the analysis. This section discusses the types and sources of information that were used as input to the hydrologic analysis.

The United States Geological Survey (U.S.G.S.) quadrangle maps were used for basin topography. The maps were obtained in a digital format and were used to delineate the watershed boundaries. The quadrangle maps in this basin provide contours at 10.foot intervals. The U.S.G.S. quadrangle maps were also used to estimate the invert elevations of culverts and bridges on Township roads. The invert elevations of culverts and bridges under Highways 29 and 81 were obtained from North Dakota Department of Transportation (ND DOT) as-built plans. Approximate storage volumes at road crossings and proposed flood control structures were also computed using the topography from the U.S.G.S. quadrangle maps. Information on all other culverts and bridges was gathered by Lake Pelican Water Project District staff, using approximate methods to measure sizes and heights to the overflow points where a road would be overtopped by flood waters. There are a total of approximately 100 road crossings within the Indian River Basin.

The Indian River basin was separated into six major watersheds: Upper, Middle, Lower, North, South, and East. These major watersheds define the major tributaries of the Indian River, and are shown on Figure 1. These six major watersheds were further subdivided into 141 subwatersheds, shown on Figure 2. The subwatersheds were delineated at nearly all of the road crossings, and areas larger than one square mile were subdivided. Subwatersheds are identified by three designations: basin name (IR for Indian River), major watershed (U.Upper, M.Middle, L.Lower, N.North, S.South, and E.East), and subwatershed number. For example, the 17th subwatershed in the Middle watershed of the Indian River basin is known as IR.M.17. This nomenclature will be used when developing the remaining sections of the Water Management Plan.

Land use for the Indian River basin was obtained from the Natural Resources Conservation Service (NRCS) by quarter quarter sections for 1996. The 1996 land use percentages includes: 14 percent CRP, 22 percent fallow, 16 percent pasture, 20 percent small grains, 10 percent gravel pits, 5 percent millet, 4 percent alfalfa, 3 percent grass, and the remaining 6 percent other crops. This information was translated into digital format for this study, and is shown on Figure 3.

The Grant County Soil Survey data was also obtained in a digital format, and used to estimate the soil types in the basin. Figure 4 shows the soils types within the basin. Soil types were classified using the hydrologic group given by the Soil Conservation Service (SCS.now known as the NRCS). The Indian River basin is composed of 80 percent Group B soils, 16 percent Group A soils, and 2 percent of both Groups C and D soils. Group A soils have a high infiltration rate and Group B soils have a moderate infiltration rate.

SCS curve numbers were used in the analyses to define the rate of infiltration. The estimated soil runoff curve numbers for each subwatershed within the Indian River basin were computed using a weighted average of the land use and soil types within each subwatershed. The runoff curve numbers for each land use and soil type were estimated using SCS recommendations for rural areas. The percent imperviousness for the watershed was estimated to be 5 percent, based on the roadways and farmsteads in the region. Although the runoff curve number and the percent imperviousness are computed based on the information available, they can be adjusted during calibration of the model to gaged data.

The SCS dimensionless unit hydrograph was used for the analysis, using an estimated lag time based on the reach length, slope and velocity. The Muskingum Cunge method was used for routing of channel flows, which is based on the channel routing length, slope, shape, and roughness. These watershed parameters were obtained from the U.S.G.S. quadrangle maps.

The hydrologic model was used to determine the flows for the 100.year frequency event. The 100.year frequency event is typically used to design flood control structures. The peak flows for other frequency events can be evaluated using the same model, although it was not completed for this Phase I study. The duration and shape of the precipitation event that was used for the hydrologic analysis was the SCS Type II 24.hour storm. This is a standard event that is widely accepted for hydrologic analyses. The event also correlates closely to the event that was modeled by the Corps of Engineers in their Feasibility Report, Flood Control for Watertown and Vicinity, August 1994. The 100.year 24.hour rainfall amount of 5.5 inches was obtained from Technical Paper 40 (T.P. 40).

        Hydrologic Analysis and Results
The Phase I hydrologic model was developed to reflect the existing Indian River basin drainage system during warm weather conditions (i.e., crops providing ground cover in farmed areas). A second model was also developed to determine the effects of small control structures in the upstream watershed on the peak flow rates from the watershed. This section describes the methodology used for these two analyses. The hydrologic models were completed using the HEC.HMS computer model developed by the Hydrologic Engineering Center of the U.S. Army Corps of Engineers (Corps). The model uses the inflow hyetograph, unit hydrograph, rate of infiltration and flood routing to estimate peak discharges and flood elevations.

In the absence of a gage station on the Indian River watershed, verification of existing Indian River flows was conducted by development of a large-scale model of the five watersheds that are tributary to the near Watertown gage. This large-scale model was calibrated to the gage data presented in the Feasibility Report Technical Appendix, Flood Control for Watertown and Vicinity, South Dakota, Corps of Engineers, June 1994. However, this large-scale model was based only on available watershed parameters and did not consider flow restrictions and storage. Therefore, the model may not accurately predict the peak flows from the upland watersheds and the results of the Indian River Basin should be considered as preliminary. It is important to remember that the goal of the Phase I model is to determine the relative reduction in peak flow rates that is possible by using small control structures.

The 100.year peak flow at the near Watertown gage was estimated by the Corps to be approximately 8,700 cubic feet per second (cfs), with a total runoff volume of about 14,000 acre-feet. The large-scale model indicated that the 100.year peak discharge from the Indian River basin would be approximately 2,200 cfs, with a total runoff volume of around 2,500 acre-feet. This Indian River data was used for comparison with the more detailed basin model that was developed for the Phase I study. Although this information is not valid for accurate calibration of the Indian River basin model, it can be used as a preliminary estimate of expected values and is quite valuable in demonstrating the percent of flow reduction that can be achieved by a network of small detention basins.

The first step in the hydrologic analysis of the Indian River basin was to use the HEC.HMS model to simulate the existing conditions in the watershed. This model uses information for the existing watershed, to simulate the effect of the existing natural channel system and road crossings. The model predicted a 100.year runoff volume of 2,500 acre-feet from the Indian River Basin, matching the large-scale model volume for the Indian River Basin. The 100.year peak discharge predicted by the Indian River Basin detailed model (1,700 cfs) was less than the peak discharge for Indian River that was predicted by the large-scale model (2,500 cfs). However, since this peak flow rate was within the range of the large-scale model, it was used as the preliminary estimate. This peak flow rate will need to be reviewed when the total watershed model is developed and, in turn, can be calibrated to gage data. The model results also indicated an area of approximately 640 acres is flooded behind the existing road crossings.

The second model analyzed the potential reduction in peak discharges that could be achieved through installation of small control structures. For this model, approximately 80 percent of the existing culverts and bridges were restricted with smaller openings, and about 70 percent of the existing road overflows were raised to determine the potential reduction in peak flows within the watershed. Small openings were assumed to further restrict the flow in the first two feet of elevation, to allow for water quality basins at these control structures. In upstream areas where few roads exist, four additional small flood control structures were assumed to be constructed across the channel to store runoff and reduce the peak flows. This model revealed that the peak flow rate from Indian River with these modifications could be reduced by about 85 percent, to 235 cfs, for the 100.year storm event. The flooded area behind these modified culverts was approximately 960 acres, an increase in temporarily flooded area of about 50 percent. These model results indicate that a significant reduction in peak flows can be achieved with only minor increases in the flooded area. Therefore, the proposed small control structures will be able to provide a large portion of the required reduction in flows from the watershed. The percent reduction in peak flows is the primary conclusion of this Phase I study, and the actual peak flow rates should be considered preliminary.

5.0         Description of Control Structures
The design of the small control structures will be dependent on the configuration of the existing culvert or bridge and on the proposed purpose of the structure. Control structures can be designed primarily for flood detention or can have a dual purpose of extended flood detention and water quality treatment. Figure 5 Figure 6 & Figure 7 show typical designs that could be used for these small control structures. Each basin analyzed for flow reduction will require a water quality analysis to determine the potential sites for water quality reduction for the basin. A more detailed investigation of each flood detention and water quality treatment site will also be required to determine the design that is best suited based the particular site conditions. Table 1 provides a range of estimated costs for these control structure designs and installation, and for road raises that may be required at these control structures. These estimated costs are provided as level of magnitude estimates, to be used only for preliminary planning purposes. Actual costs for any particular site will depend on site conditions, soils, length of roadway, etc.

For flood detention at culverts (Figure 5), a manhole could be constructed on the upstream end of the pipe with a small inlet pipe to reduce the flows until the flooding level rises to an unacceptable height, when large flows would enter the top of the manhole and be released in the existing culvert. To include water quality treatment at culverts, a similar manhole structure would be constructed with the addition of a flow restriction and skimmer in the manhole to further reduce the flows at lower levels (Figure 6). At a bridge or large-diameter culvert, one option for flood detention would be installation of a ring dike on the upstream side that restricts low flows through a small culvert, but allows high flows to pass over the dike (Figure 7).

Table 1
Range of Cost Estimates for Roadway Modifications

Description Minimum Estimated Cost Maximum Estimated Cost
Flood Detention Culvert Restriction $10,000 $30,000
Extended Detention and Water Quality Treatment $15,000 $35,000
Flood Detention Bridge Restriction $10,000 $50,000
Road Raise
(assuming no seepage or stability issues)
$3,000 $45,000
NOTE: Estimated costs are for construction only. Easement acquisitions, engineering costs (typically 15% to 25% of construction costs), and financing costs are not included in these costs. Also, this estimate assumes no utility relocations will be necessary (such as power lines, gas line, telephone lines, etc.).