Improving Land Use in the Lake Erie Basin through Better Planning, Improved Regulations and Stormwater Modeling:

Transcription

1 Improving Land Use in the Lake Erie Basin through Better Planning, Improved Regulations and Stormwater Modeling: Part 2 Watershed Scale Impacts of Stormwater and Stream Corridor Management Chagrin River Watershed Partners, Inc. P.O. Box 229 Willoughby, Ohio (440) Ohio Department of Natural Resources Division of Soil and Water Conservation 2045 Morse Road Columbus, OH Ohio Non point source Education for Municipal Officials (NEMO) Program 590 Woody Hayes Columbus, OH

2 Project Partners This report was prepared by the Chagrin River Watershed Partners, Inc. (CRWP), Ohio Nonpoint Source Education for Municipal Officials (NEMO) program, and Ohio Department of Natural Resources (ODNR). Funding and support for this project was provided by CICEET, the Cooperative Institute for Coastal and Estuarine Environmental Technology. A partnership of the National Oceanic and Atmospheric Administration and the University of New Hampshire, CICEET develops tools for clean water and healthy coasts nationwide. Additional support for this report was provided by the Members of CRWP through their annual Member dues. CRWP is a non profit technical organization formed by the townships, villages, cities, counties, and park districts of the Chagrin watershed to develop and implement innovative solutions to address current, and minimize new, flooding, erosion, and water quality costs and to control the increasing infrastructure costs associated with urban/suburban development. CRWP provides Members with advice and assistance on zoning and subdivision codes, implementation of these codes, development plan review, and other best practice implementation at Member direction. ODNR was formed by the Ohio Legislature in 1949 and is charged with developing and implementing long term planning and programming related to the development and conservation of the states natural resources and participating in regulatory matters associated with environmental hazards. ODNR has many diverse programs and owns and manages 590,000 acres including 74 state parks, 20 state forests, 133 nature preserves, and 138 wildlife areas. The ODNR Division of Soil and Water Conservation (DSWC) provides leadership and services to conserve, protect, and enhance soil, water, and land resources. The Ohio NEMO program was started in 1999 to help local governments better manage nonpoint source pollution while accommodating growth. The program is built on the belief that well informed land use planning is critical for the protection of environmental quality. Since land use planning takes place primarily at the local level, Ohio NEMO focuses their efforts on working with local decision makers and their support personnel. More recently, Ohio NEMO has focused on technical issues and developing local capacity to use GIS and modeling tools to provide quantitative analyses of potential environmental impacts due to land use decisions. 2

3 Cover Photographs: Stream Restoration Project, Fosters Run, Mayfield Village. Table of Content Project Partners... 2 Abstract Project Overview Rational, Goals, and Objectives Overview of Modeling Approach Site Scale Modeling (Project Objective #1) Watershed Scale Modeling (Project Objective #2)... 7 Selection of a Watershed Scale Modeling Approach Mocnet River Network Spreadsheet Tool Theory used in the Mocnet River Network Spreadsheet Channel Dimensions Channel Roughness Channel Slope Channel Length Methods and Tools for Developing Drainage Network Relationships Land Use, Stormwater, and Floodplain Management Modeling Scenarios Chagrin River Relationships and Parameter Values Used in Modeling Results Effects of Land Use, Drainage System Type, Connected Impervious Area, and Stormwater Detention Criteria on Peak Discharge Effects of Floodplain Management Scenarios on Peak Discharge Rates Summary and Conclusion Future Work References APPENDIX A: Watershed Scale Modeling Results

4 Funding and support for this project was provided by CICEET, the Cooperative Institute for Coastal and Estuarine Environmental Technology. A partnership of the National Oceanic and Atmospheric Administration and the University of New Hampshire, CICEET develops tools for clean water and healthy coasts nationwide. Abstract An approach was developed to quickly evaluate the benefits of increasing floodplain storage (e.g. stream restoration in incised channel systems) along streams for reducing flooding in small watersheds in the Chagrin River Watershed in northeast Ohio. The approach utilized site scale hydrologic modeling results and a hydraulic model to route flow through drainage networks with a small amount of floodplain, a medium amount of floodplain, and a large amount of floodplain. A spreadsheet tool was developed to aid in the development of a representative drainage network. The spreadsheet tool greatly reduces the time and cost required to develop this type of model and can easily be used for similar studies in other watersheds. Combinations of four storm durations (3, 6, 12, and 24 hours), three storm distributions (Huff Quartile 1, Huff Quartile Four, and SCS Type 2), and 11 recurrence interval events (0.1, 0.2, 0.4, 0.8, 1.6, 3.1, 6.2, 12.5, 25, 50, and 100 year) were modeled to evaluate a range of factors and their influence on peak discharge and stage in a third order watershed. Results were variable depending on the size of storm, the storm distribution, and the amount of floodplain available for the storage of flood flows. In general, the flood peak, or peak discharge, was reduced by 5 30% or more which indicates that management of stream corridors and floodplains in a watershed context can be a beneficial practice for reducing flooding and managing stormwater. 1 Project Overview Rational, Goals, and Objectives Development decisions that impact flooding, erosion, and water quality in the Lake Erie basin are made on a daily basis. In Ohio, regulations that guide development are made at the local level and many local governments lack the technical capacity to effectively manage land use and stormwater in a watershed context. Furthermore, local governments often lack the resources and technical capacity to evaluate and implement innovative stormwater best management practices that could be used to more effectively mitigate the impacts of development. Throughout the Lake Erie basin communities are looking to improve comprehensive plans, enact forward thinking regulations, require effective best management practices, and make the watershed based decisions necessary to ensure that the Lake Erie coastal watersheds develop with reasonable growth; lowest long term infrastructure costs; and the least impacts to wetlands, streams, floodplains, and open spaces. In 2006, the Chagrin River Watershed Partners (CRWP), Ohio Non point Source Education for Municipal Officials (NEMO) program, and Ohio Department of Natural Resources (ODNR) applied for and received a grant from the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET) to begin to address these needs. The overall project goals outlined in the proposal were to: Project Goal 1: Utilize modeling technologies to enhance and inform stormwater, land use and comprehensive planning at the community level. 4

5 Project Goal 2: Promote the broad dissemination of project outcomes and findings to other communities in the Great Lakes region. To address the technical assistance needs of local communities in the Chagrin River watershed the following project objectives were identified: Project Objective 1: Use computer simulation models to evaluate the effects of innovative and traditional stormwater management strategies at the scale of individual developments. Project Objective 2: Model the watershed scale hydrologic impacts of stormwater management strategies and assess the potential of floodplain management strategies to meet stormwater management objectives. 2 Overview of Modeling Approach Over the past several decades knowledge of the impacts of poorly managed stormwater runoff has increased dramatically. As a result, stormwater management regulations and requirements have become more stringent in order to better protect water resources, public health, and public safety. Currently, communities across Ohio are grappling with the development and implementation of plans and programs to meet stormwater management goals which may include flood and erosion control, water quality protection, channel stability, and biological integrity. In many communities some objectives are more important than others, but clearly there is movement towards more holistic, multi objective stormwater management and regulation. As the complexity of stormwater management objectives increases, so does the number of solutions being developed to address problems. Unfortunately, the planning and regulatory framework in Ohio can make it somewhat difficult to implement innovative stormwater management strategies and encourages management on a site by site basis. Generally, little to no consideration is given to the impacts beyond the stormwater system outlet of an individual development. To better meet the goals of multi objective stormwater management requires a watershed approach which considers the cumulative impacts of land use, development, and floodplain management decisions. Multi objective stormwater management is not a simple task and little quantitative analysis specific to the Lake Erie basin exists to support local governments as they attempt to improve stormwater management plans and select best management practices. Evaluating the many factors that influence water quantity, water quality, and stream stability is not easily accomplished by direct measurements. Predicting changes that might occur over a range of climatic conditions, management scenarios, and land use changes is best accomplished with decision tools such as computer simulation models. For this project, a two step modeling process was used to: 1) model the effects of land use change and stormwater management on stormwater runoff quantity and quality at the site scale ( ~20 acres; the size of a typical development in the Chagrin River watershed), 2) model the cumulative (i.e. watershed scale) 5

6 impacts of land use change and stormwater management decisions made at the site scale, and 3) model the watershed scale impacts of several floodplain management scenarios. A conceptual diagram of the modeling approach used to undertake Project Objectives #1 and #2 is provided in Figure 1. First, rainfall data and site data were used to parameterize a rainfallrunoff model. The rainfall runoff model was used to predict runoff quantity and quality from a hypothetical site for a series of rainfall events ranging from small, frequent events to rare, extreme events. Site characteristics incorporated into the hypothetical site model included parameters used to describe the topography, land use, and soils properties. The parameter and variable values used in the rainfall runoff model of the hypothetical site were determined using GIS datasets (e.g. National Elevation Dataset) and tools, zoning overlays, and input from representatives of local communities and stormwater programs in the Chagrin River watershed. After an initial screening of the GIS data, twenty actual sites in the watershed were selected for further study. Each site was then visited, evaluated, and characterized using a standardized data collection procedure. Data from these twenty sites were used to create the hypothetical site scenarios used throughout the modeling exercise. These sites covered a range of development scenarios from low density residential with swale drainage systems to commercial developments with curb and gutter drainage systems. Specific details on the site scale hydrology modeling and data collection methods are provided in an accompanying document entitled Modeling the Effectiveness of Traditional and Innovative Stormwater Management Strategies in the Chagrin River Watershed: Development Site Scale. Site(s) Data Drainage Network Data Rainfall- Runoff Modeling Runoff from Site(s) Hydraulic Modeling Watershed Discharge Rainfall Data Project Objective #1 Project Objective #2 Figure 1 Conceptual diagram of the modeling process used in this study. Objective #1 involves the site scale modeling of land use and stormwater management practices. Objective #2 involves the watershed scale modeling of the cumulative impacts of stormwater and floodplain management practices. 6

7 In the next step of the modeling study, predicted runoff hydrographs from the hypothetical site scale development scenarios were used as inputs to a hydraulic routing model. A hydraulic model uses measured or predicted runoff from source areas (i.e. the sites) in conjunction with a physical model of the receiving stream network to simulate the movement of flows through the drainage network. Variables used to describe the physical drainage network model included: 1) channel cross sectional geometry, 2) channel and floodplain roughness, and 3) channel slope. A spreadsheet tool was developed and used to simplify the creation of the hydraulic model input text files which represent the drainage network in the hydraulic routing model. The spreadsheet tool allows for quick and simple manipulations of various aspects of the drainage network model and is described in greater depth later in this report. Throughout the project, many approximations were made to simplify the modeling study. Several examples of approximations that were made to simplify the modeling exercise include: 1) channel reaches with uniform bed slopes, 2) channel dimensions estimated with regional hydraulic geometry relationships, and 3) a simplified 8 point two stage channel cross sectional geometry. Due to these simplifications, the modeling results are not specific to a particular place or channel system, but are representative of many subwatersheds and channels within the Chagrin River Basin. Results presented later in this report may not be applicable if the simplifying assumptions used in the study make the model a poor representation of reality. In this case, a more detailed study should be conducted. The simplified approach was used because: 1) the study was meant to be an educational activity to teach and inform local officials, planners, and engineers engaged in planning processes about the potential site and watershed scale impacts of various stormwater and floodplain best management practices, and 2) it required less time and resources to develop and evaluate a range of stormwater management practices and development scenarios. More details on the rational and implementation of each objective are provided in the following sections. 3 Site Scale Modeling (Project Objective #1) Details on the site scale modeling objectives, methods, and results are presented in the document: Modeling the Effectiveness of Traditional and Innovative Stormwater Management Strategies in the Chagrin River Watershed: Development Site Scale. 4 Watershed Scale Modeling (Project Objective #2) As discussed in Section 2, little consideration is currently given to the watershed scale impacts of development and land use decisions in most communities around Ohio. Furthermore, most communities do not evaluate or explicitly consider the role of floodplains and their management in an overall stormwater management strategy. Therefore, a watershed scale modeling study was undertaken to evaluate the affects of various stormwater management practices and to quantify the flood attenuation benefits of maintaining, protecting, or restoring active floodplains. Active floodplains are areas that are flooded regularly, sometimes several times per year, and provide space for the temporary storage of floodwaters during overbank flow events. This temporary storage of floodwaters is often called floodplain storage. The term floodplain storage can refer to both long and short term retention of floodwaters on 7

8 floodplains. In a stream system with attached, active floodplains larger flows will fill the streams main channel and occasionally spill over into the adjacent floodplain. A portion of the floodwaters may fill surface depressions or infiltrate into floodplain soils and be slowly released back to the stream over time as baseflow or enters into groundwater reserves. These pathways are considered long term storage and would typically account for a few percent or less of the total flow volume. Short term floodplain storage, the focus of this study, occurs as flood flows spread across floodplains which are vegetated with woody or herbaceous plants. Vegetation on floodplains impedes the flow reducing its velocity and delaying downstream conveyance resulting in a temporary storage of a portion of the flow on the floodplain. An example of attenuating affects of floodplains can be seen in Figure 2. In this example, discharge at the outlet of a small watershed (~10 square miles at the outlet) is shown for two scenarios that differ only by the amount of floodplain contained within the upstream drainage network. The first scenario (solid line) has narrow floodplains throughout the upstream drainage network while the second scenario (dotted line) has broad floodplains throughout the drainage network. A hydrograph routed through the narrow floodplains drainage network scenario results in a high, rapidly rising discharge rate at the watershed outlet relative to the same hydrograph routed through a drainage network with broad floodplains. The figure shows a shift in the delivery of discharge as a portion of the runoff volume (red hatch lines) is delayed and discharged later in time (green hatch lines). The reduction in discharge rate, delay in the downstream conveyance of flow, and greater storage volume within the broad floodplain scenario also results in lower maximum water surface elevations (Figure 3) at the watershed outlet. Discharge Narrow floodplain scenario Broad floodplain scenario Time (hours) Figure 2 Two hydrographs at the outlet of a watershed. One scenario has an upstream drainage network with narrow attached floodplains while the second scenario has broad floodplains that provide temporary floodplain storage reducing and delaying peak discharge. 8

9 Figure 3 The broad floodplain scenario (gray, dashed line) has a much lower water surface elevation at the peak discharge rate relative to the water surface elevation at the peak discharge rate for the narrow floodplain scenario (solid black line). High quality, self sustaining streams are generally in a state of dynamic equilibrium where water and sediment discharges are in balance and the stream neither aggrades nor degrades over time (Rosgen, 1996). Dynamic equilibrium depends on an attachment to an active floodplain which floods frequently and dissipates the potentially erosive energy of higher flows across a floodplain and concentrates lower flows within a main channel. An attached floodplain is a function of fluvial processes and associated with channel forming discharges (Ward et al., 2009). The bankfull discharge, a type of channel forming discharge, is the amount of discharge that just fills the main channel before spilling onto the floodplain. The channel that contains this discharge is called the bankfull channel and the cross sectional dimensions of the bankfull channel are called the bankfull width, bankfull depth, and bankfull cross sectional area. Some streams in the Chagrin River watershed have become incised or entrenched as a result of poor stormwater management practices and have lost connection to an active floodplain. Many of these streams could be improved through some form of channel enhancement project which may provide stormwater management benefits and potentially mitigate downstream flooding. Many streams have lost their connection to an active floodplain either unintentionally as a channel adjusts to shifting climatic, hydrologic, or sediment regimes or intentionally through management activities (e.g. channelization, levees). Figure 4 shows an example of an incised channel that has become disconnected with its floodplain (i.e. the lawn). The stream has also widened at the current bed elevation as it attempts to reduce its energy and create the space needed to build a floodplain at the new bed elevation. This process of degradation and aggradation is described by many of the well known channel evolution models (Shumm et al., 1984; Simon and Hupp, 1986) which outline the failure and recovery processes in disturbed channels. A discussion of several channel evolution models is provided at the USEPA Watershed Assessment of River Stability & Sediment Supply (WARSSS) website ( The example in Figure 4 illustrates a stream that is adjusting to upstream land use change and encroachment, and riparian vegetation removal. The contributing watershed has been urbanized, which increases the amount of impervious surfaces and increases the stormwater 9

10 runoff rates and volumes. The addition of impervious surfaces on the landscape has reduced sediment supply to the channel creating a sediment imbalance as sediment entering the system does not offset the sediment being transported out of the system. The channel is in a failing mode and now requires larger and larger flows to fill the channel before accessing the floodplain. This creates a domino effect leading to bed scour and bank erosion until the energy of the system has been reduced to a point that the stream can begin to rebuild its floodplains (i.e. until the channel has widened enough to slow flows and deposit materials). Furthermore, the failing channel has little vegetation within the channel to provide resistance and impede flows. For most flow events this lower resistance to flow and simplified cross sectional geometry translates to higher flow velocities and downstream discharge rates compared to a channel system in dynamic equilibrium with well attached, vegetated floodplains. Figure 4 A stream in an urbanizing watershed that is failing due to altered hydrology and sediment regimes. In incised stream systems several management practices can be implemented to reconnect floodplains and increase floodplain storage. Several common practices include: stream restoration, two stage channel designs, and self forming channel designs. Stream restoration, sometimes referred to as natural channel design, involves the construction of floodplains and instream features such as riffles, pools, and a sinuous main channel as seen in Figure 5A. Various restoration techniques that involve building and shaping of floodplain and instream features are considered constructed stream systems. The two stage channel design involves expanding floodplain benches at an elevation associated with the channel forming discharge within an incised channel. No earthwork is performed within the confines of the main channel as floodplains are excavated and expanded at the bankfull elevation (Figure 5B). This channel modification and enhancement technique can also be considered a type of constructed stream system. The self forming channel design is similar to the two stage channel concept except that the floodplain terrace and benches are completely removed down to the channel bed elevation as seen in Figure 5C. Removal of benches creates a space within which the stream can build floodplains through instream processes (e.g. deposition, accretion) to form a new channel floodplain system (Figure 5D). The result of this channel modification practice is a selfforming stream system. Further descriptions of each practice are provided in the literature (Rosgen,1998; Jayakaran et al., 2009). 10

11 A B C D Figure 5 Examples of stream rehabilitation strategies: A) a natural channel design with an active floodplain, B) a two stage channel design, C) a self forming channel design after initial construction, and D) the same self forming channel two years after construction. Selection of a Watershed Scale Modeling Approach A common approach to evaluate the impact of development and stream corridor management at the watershed scale is to use a hydraulic routing model, such as the Army Corps of Engineers Hydrologic Engineer Center River Analysis System (HEC RAS). A hydraulic model simulates the movement of flows through a channel system given a physical model of the channel network and information on the location and magnitude of runoff from source areas to the receiving channel system. One of the most common uses of HEC RAS is to conduct Federal Emergency Management Agency (FEMA) flood studies. FEMA flood studies are undertaken to identify areas that are likely to be inundated during flood events of a certain probability such as the 1% (i.e. 100 year) or 0.2% (i.e. 500 year) floods. The Committee on FEMA Flood Maps et al. (2009) recently released a document, entitled Mapping the Zone: Improving Flood Map Accuracy, which provides details on several common approaches for conducting hydraulic routing studies used in flood studies. This document provided valuable background information regarding the level of detail and analysis commonly used in hydraulic modeling analyses. It, also, identifies the advantages, disadvantages, costs, and uncertainties associated with each approach. The three approaches described in that document are: (1) detailed analysis, (2) limited detailed analysis, and (3) approximate analysis. 11

12 Detailed analysis: As the name suggests, detailed analysis incorporates highly detailed information into the hydraulic routing model. High resolution Light Detection and Ranging (LiDAR) digital elevation models (DEMs) are used to generate physical models of the drainage network which are typically supplemented with bathymetric survey data to better describe stream subsurface topography which is not described well by most DEMs. Field surveys and measurements are made to characterize hydraulic structures, identify obstructions to flow, and assign channel and floodplain roughness values to cross sections within the channel network. Detailed hydrologic rainfall runoff analysis is conducted to generate peak discharge estimates and/or hydrograph inflows from the landscape to the receiving drainage network. The cost for a detailed analysis is approximately $13,000 per mile (Table 2.1 from Committee on FEMA Flood Maps et al., 2009). Limited detailed analysis: Limited detailed FEMA flood study analysis is similar to detailed analysis except less effort and resources are spent to collect information on instream topography, structures (e.g. culverts, bridges, weirs, etc.), and channel and floodplain roughness. In addition, simpler methods such as regional regression equations may be used to predict stormwater runoff from contributing source areas. This approach is generally limited to steady state analysis of stream discharge and average cost is approximately $9,300 per mile (Table 2.1 from Committee on FEMA Flood Maps et al., 2009). Approximate analysis: An approximate analysis is conducted with lower resolution data than the more detailed methods described previously. In many places LiDAR DEMs are not available and National Elevation Data (NED; 30 meter pixel resolution) DEMs are used instead. No supplemental survey or instream structure data generally are collected to support an approximate analysis. Regional regression equations are used to predict peak discharges and subsequently used in a steady state flow analysis. An approximate analysis costs approximately $900 per mile (Table 2.1 from Committee on FEMA Flood Maps et al., 2009). During the initial planning stages of the project, many modeling strategies including those described above were evaluated for their ability or inability to meet the stated project objectives within the resource constraints of the project. Several limitations with the approaches described above became clear during the evaluation. First, the detailed modeling methods were too expensive given the available resources. The level of resources available would support modeling for only a couple of small subwatersheds if the more detailed approaches were used. Although lower in cost the approximate modeling methods were similar to the detailed methods in that they focused on modeling a specific watershed and drainage network. For the purposes of this project a more flexible approach that could be easily adapted to model a range of watershed conditions and management scenarios was preferred and, therefore, other modeling strategies were evaluated. Ultimately a synthetic watershed modeling approach was selected. The approach relies on a conceptual or hypothetical watershed and drainage network to simplify the modeling exercise. The creation of synthetic watershed physical models was semi automated by a spreadsheet tool, called Mocnet River Network, that was developed to allow a user to quickly generate the physical model of the receiving channel network by providing some basic information about the system. The results using this approach may not be applicable to a specific place, but should be 12

13 representative of watersheds and drainage networks with similar attributes. The methods and tools developed here are meant to be educational aids that facilitate approximate, rapid, and inexpensive assessments of the potential impacts of land use change and stormwater and floodplain management strategies at the watershed scale. Watershed scale studies are usually not conducted during planning efforts because they are too time consuming and expensive to undertake. The methods and tools developed help to overcome some of these issues. Other important considerations and limitations of the methods and tools used in this study include the following: 1) The methods and tools should be used by trained professionals with experience in water resources and computer simulation modeling. 2) The methods should not be used for any purpose that requires detailed engineering design. 3) The results should be used as general guidance only. The results are approximations because the model inputs used to generate the results are approximated and not detailed representations of a particular place within the study watershed. 4) The results do not consider instream hydraulic structures which may have a significant influence on flow routing and peak discharge. Mocnet River Network Spreadsheet Tool One of the most time consuming and resource intensive aspects of a hydraulic routing study is the development of a physical model of the drainage network. The Mocnet River Network spreadsheet tool was developed to address these issues and provide the user a quick and simple means to create and manipulate HEC RAS input data files. The spreadsheet uses several well accepted relationships that describe attributes of the drainage network such as channel length, slope, and channel dimensions as functions of drainage area. This version of the spreadsheet generates a physical model of a third order drainage network (Figure 6). Spreadsheets for other watershed configurations may be developed in the future. The current version of the Mocnet River Network spreadsheet generates channel cross sections with a twostage channel geometry (Figure 7A) which includes an inset channel and floodplain. Future versions of the spreadsheet will incorporate a three stage channel geometry (Figure 7B) to better represent the form of an incised channel system by including the valley floor (i.e. a terrace) and valley side slopes. The use of a three stage geometry may provide more precise modeling results for some rainfall runoff scenarios resulting in flows that exceed the capacity of the incised channel; however, this added detail will present additional modeling challenges that were not addressed in the current study. A detailed two stage cross section which includes terminology used to describe various aspects of the channel cross section is provided in Figure 8. This terminology will be used throughout the remainder of this report. 13

14 1 st Order Stream 2 nd Order Stream 3 rd Order Stream Junction Figure 6 A conceptual diagram of the stream network created by the Mocnet River Network spreadsheet. 14

15 Valley Floor and Side Slopes 3 rd Stage Floodplains Inset Channel 2 nd Stage 1 st Stage a) Two-Stage Channel Cross Section b) Three-Stage Channel Cross Section Figure 7 A cross sectional view of a) a two stage and b) three stage channel geometry. Channel Boundary Channel Side Slope (Horizontal:Vertical) Floodplain Width Bankfull Width Inset Channel Side Slope (Horizontal:Vertical) Bankfull Depth Figure 8 A conceptual diagram of the two stage cross section created in the Mocnet River Network spreadsheet and used in HEC RAS. 15

16 Theory used in the Mocnet River Network Spreadsheet Channel Dimensions The dimensions (i.e. cross sectional geometry) of the inset bankfull channel are calculated in the Mocnet River Network spreadsheet using bankfull hydraulic geometry relationships relating bankfull channel dimensions to the contributing drainage area. Bankfull hydraulic geometry relationships are generally expressed power function equations as follows: W D bkf bkf CSA = aa = ca bkf b d = ea f Equations 1 to 3 where W bkf is bankfull width (ft), D bkf is mean bankfull depth (ft), CSA bkf is bankfull cross sectional area (ft 2 ), and A is drainage area (square miles). Coefficients a, c, and e and exponents b, d, and f are specific to a reach, watershed, or region and determined by plotting a power function regression line through field measured bankfull dimensions and drainage area data. Several bankfull hydraulic geometry data relationships have been developed for watersheds and regions throughout Ohio (Sherwood, 2005; Witter, 2006; Mecklenburg and Ward, 2009, Chang et al., 2004). In the Mocnet River Network spreadsheet the inset channel side slope ratio and channel side slope ratio can be specified by the user. In addition the floodplain width or extent of the second stage floodplain can be manipulated by adjusting the flooded width ratio (FWR) value in the Mocnet River Network spreadsheet to automatically adjust the width of the second stage floodplain. The FWR is: Floodplain Width (ft) FWR= Bankfull Width (ft) Equation 4 FWR is expressed as a multiple of the bankfull channel width and can be varied independently for each stream segment in the drainage network. Examples of four channel cross sections with floodplain width ratios 1.5, 3, 5 and 10 are provided in Figure 9. This feature is useful for manipulating floodplain geometries which provides a rapid means to simulate floodplain management scenarios. The flooded width of the second stage then is equal to the product of FWR and the bankfull channel width at a particular cross section. 16

17 FWR = 1.5 FWR = 3 FWR = 5 FWR = Figure 9 An example of four cross sections with different flooded width ratios (FWR). Flooded width ratios depicted vary from 1.5 times the bankfull width (i.e. a narrow floodplain) to 10 times the bankfull channel width (i.e. a broad floodplain). Channel Roughness In the Mocnet River Network spreadsheet, channel roughness values are specified separately for the main channel and floodplain areas. The user has two options to provide roughness values for the channel bed. The first option is to allow the spreadsheet to estimate a Manning s n value as a function of the bankfull channel depth, mean bed material size, and hydraulic radius. Manning s value (n) is calculated as (Simons and Senturk, 1992): * R n = Equation 5 8g f where R is the hydraulic radius (ft), g is the gravitational constant (32.2 ft/sec 2 ) and f is the Darcy Weisbach friction factor. The Darcy Weisbach friction factor can be estimated using the empirical relationship given by Griffiths (1981). Griffiths equation is: f d = 5.6*log(2.42* ) Equation 6 d 50 where d is depth in feet and d 50 is mean particle size of bed material in millimeters. The second option is to provide user defined values of channel bed roughness to the spreadsheet based on the best judgment of the user. Similarly in the current version of the spreadsheet the floodplain roughness values are always determined by user input values and 17

18 can be varied in each segment of the drainage network. There are several common methods to aid the user in selecting channel and floodplain roughness values including: 1) estimates based on descriptions of channels from lookup tables in textbooks (e.g. Chow, 1959; Ward and Trimble, 2004), 2) by comparing a site to photographs of streams with predefined Manning s values (e.g. and 3) other empirical methods or standard procedures (e.g. Newbury and Gabory, 1981; Cowan, 1956). In addition, the spreadsheet tool and modeling procedure has the capability to deal with floodplain roughness values that vary as the water surface elevation above the floodplain increases or decreases. Research has shown that Manning s n values vary vertically as a function of flow depth over the floodplain, the type of vegetation, the depth of flow relative to the height of vegetation, and the time of year. A total of three user defined Manning s roughness values that vary as a function of the depth of flow above the floodplain can be entered into the spreadsheet. This allows for an approximate representation of a roughness curve to describe roughness values as a function of flow depth. Channel Slope In the Mocnet River Network spreadsheet, channel slope is calculated for each reach using a relationship between bankfull discharge and slope (Leopold, 1994). That relationship is: s Q z = bkf Equation 7 where s is slope (ft/ft), Q bkf is the bankfull discharge (ft 3 /sec), and z is an exponent that describes the concavity of a watershed. A classical model of a watershed and channel network describes the headwaters of a stream system beginning in more steeply sloped terrain and progressing downstream to valleys with gentler slopes. A longitudinal plot of the channel bed elevation progressing from headwaters to valley takes the form of a concave surface. The shape of the concave surface is represented by the parameter concavity (z) with higher z values representing a higher degree of concavity (i.e. strong gradient difference between upstream and downstream areas) and lower z values representing a less concave surface (i.e. gentler slope transition from headwaters to valley). Leopold (1994) suggests an average z value of ~0.75 which is the default value provided in the spreadsheet; however, z can be adjusted with a user defined value to more closely approximate the transition of slopes throughout a study watershed. The bankfull discharge term in the equation is calculated using a bankfull discharge drainage area relationship. A bankfull discharge drainage area relationship for Ohio is expressed as a power function and the spreadsheet default values are based on analysis of Ohio streams and USGS gage data (Sherwood and Huitger, 2005). That relationship is: Qbkf = 93.3A Equation 8 where A is the contributing drainage area in square miles. The coefficient and exponent of the discharge drainage area relationship can be adjusted by the user to better represent conditions of a study watershed, region, or other states. These values can be determined by plotting 18

19 estimated bankfull discharge from several sites in a region across a range of drainage areas. The resulting power function regression equation provides the necessary coefficient and exponent to parameterize the equation in spreadsheet. Another method that could be used to improve the current relationship in the spreadsheet is to take a subset of data from sites in the Sherwood and Huitger (2005) study and develop a new relationship more specific to the region (i.e. northeast Ohio). Additionally, the user also has the option to manually overwrite calculated slope values with user defined values for each group of stream order segments in the drainage network. Channel Length In the spreadsheet, channel length is estimated with a power function relationship between drainage area and channel length. Hack (1957) proposed the following channel length drainage area relationship based on measurements from 11 streams in Virginia and Maryland, USA: L 0.6 = 1.4A Equation 9 where L is stream length (miles) and A is drainage area (square miles). Stream length is calculated for each stream order segment in the drainage network model. The user has the option to modify the default coefficient and exponent of the power function to better fit a study watershed or overwrite the calculated length values with user defined values. Methods and Tools for Developing Drainage Network Relationships Several readily available methods and tools can be used to generate the values needed to populate the Mocnet River Network spreadsheet. Hard copy maps, GIS methods and data, and interactive websites are just some sources of information that can be used to delineate drainage areas, measure stream lengths, and determine elevations needed to calculate drainage network relationships (some examples are provided in Table 1). With these tools measurements can be made quickly at many sites throughout a region and plotted on a graph. A regression trend line fit through empirical data and the resulting regression equation coefficients and exponents of the trend line can be determined and used to populate the spreadsheet with values needed to create a synthetic watershed physical model. 19

20 Table 1 Examples of data sources and providers that can be used to develop drainage basin characteristic estimates needed to populate the Mocnet River Network spreadsheet tool. Tools Data name Source Notes Hard copy maps GIS data Webbased tools USGS Topographic USGS Maps ODNR ODNR Division of Geological Survey 2045 Morse Rd. Building C Columbus, OH Digital Elevation Models Hydrography Data Digital Watersheds Ohio Geographically Referenced Information Program USGS National Elevation Dataset United States Dept. of Agriculture Geospatial Gateway USGS National Hydrography Dataset Institute for Water Research Michigan State University Streamstats USGS Land Use, Stormwater, and Floodplain Management Modeling Scenarios For this study, nineteen development scenarios were generated to model the range of land use intensities and stormwater management practices common to the Chagrin River watershed. Land uses that were modeled included undeveloped (UND), low density residential (LDR), medium density residential (MDR), and commercial/industrial (COMM) land uses. Two types of drainage systems were modeled including curb and gutter and swale drainage systems. Swale drainage systems consisted of grass swales on native soils designed to meet most common local regulations in the Chagrin River watershed. Two scenarios of 0% and 100% connected impervious area were modeled. Stormwater detention methods that were evaluated included no detention (ND), the water quality volume method (WQV), critical storm method (CSM), and the water quality volume with critical storm method (WQV+CSM). A list of all land use and stormwater management scenarios that were modeled is provided in Table 2. In addition, four floodplain management scenarios (FWRs of 1.5, 3, 5, and 10) were modeled to better understand the affects of floodplain on the routing of flows and attenuation of peak discharge throughout the watershed drainage network. In each of these modeling scenarios the FWR was held constant throughout the entire drainage network; however, models with FWRs that vary by stream segment can be accommodated with the Mocnet River Network spreadsheet tool. 20

21 Each combination of land use, stormwater management, and floodplain management scenario was modeled for storm events with durations of 3, 6, 12, and 24 hrs. Additionally, eleven different rainfall events from small, frequent storms to rare, large events (recurrence intervals of 0.1, 0.2, 0.4, 0.8, 1.6, 3.1, 6.2, 12.5, 25, 50, and 100 years) were modeled. The size of various rainfall events were taken or extrapolated from published maps or tables of rainfall depths (Huff and Angel, 1992; Bonnin et al., 2004). This modeling effort produced a total of 3,344 results which collectively describe watershed scale impacts of several land use, stormwater management, and floodplain management strategies for a typical third order watershed in the Chagrin River Basin. Table 2 Watershed Scenarios: Land use, stormwater management, and floodplain scenarios that were modeled at the watershed scale. Land Use 1 Soil Type Drainage System Connected Impervious (%) Stormwater Detention 2 UND Clay Swale 0 ND LDR Clay Swale 0 ND LDR Clay Curb and gutter 0 ND LDR Clay Swale 100 ND LDR Clay Curb and gutter 100 ND MDR Clay Swale 0 ND MDR Clay Curb and gutter 0 ND MDR Clay Swale 100 ND MDR Clay Curb and gutter 100 ND MDR Clay Curb and gutter 100 WQV MDR Clay Curb and gutter 100 CSM MDR Clay Curb and gutter 100 WQV+CSM COMM Clay Swale 0 No Detention COMM Clay Curb and gutter 0 No Detention COMM Clay Swale 100 No Detention COMM Clay Curb and gutter 100 No Detention COMM Clay Curb and gutter 100 WQV COMM Clay Curb and gutter 100 CSM COMM Clay Curb and gutter 100 WQV+CSM 1 UND undeveloped; LDR low density residential; MDR medium density residential; COMM commercial. 2 ND no detention; WQV water quality volume method; CSM critical storm method; WQV+CSM water quality volume and critical storm method. Historically, Ohio stormwater programs have been most interested in more extreme events such as short duration, high intensity storms used to size bridges, culverts, and detention practices. More recently, state regulations requiring stormwater treatment of more frequent storms (i.e. <0.75 inches), called the water quality volume, has broadened the range of storms 21

22 that need to be considered in the design of a stormwater treatment facility. In this study, rainfall events commonly used by stormwater engineers to design infrastructure or protect against rare flood events would equate to the 3 hour or 6 hour events with 12.5, 25, 50, or 100 year recurrence interval. However, in this study a range of rainfall durations and recurrence intervals were modeled to better understand the effects of more common storms and their impact on downstream flooding. Furthermore, sediment transport and stream stability issues are affected by the entire range of small, medium, and larger flows. An effort was made to model stream bed sediment transport and bank stability as part of this effort; however, the results are not presented here as additional modeling and critical evaluation of the results are needed. Chagrin River Relationships and Parameter Values Used in Modeling A GIS analysis was conducted using ArcGIS software, the ArcHydro extension, and several GIS datasets provided by the Chagrin River Watershed Partners to characterize a typical watershed and drainage network in the Chagrin River Basin. Using the GIS tools and datasets each stream reach in the drainage network was assigned a stream order (i.e. 1 st order, 2 nd order, etc.). Next, a random subsample from each group of stream orders was selected and the channel length, channel slope, and contributing drainage area were measured and recorded. These measured characteristics were then used to develop an empirical relationship between channel length and drainage area and channel slope and drainage area. The resulting coefficients and exponents of the Chagrin specific empirical relationships are provided in Table 3. The default parameter values in the Mocnet River Network spreadsheet are also provided to show that significant differences in parameter values may be expected from the default values that are provided in the spreadsheet. Additionally, an average bifurcation ratio for the Chagrin River Basin was determined by comparing the number of stream reaches of successive orders (e.g. ratio of the number of 1st order streams relative to the number of 2 nd order streams). A bifurcation ratio of 4 was typical for the Chagrin River watershed and used in the analysis. This bifurcation ratio describes the shape or branchiness of the drainage network. For example, much of the Chagrin River watershed is characterized by rolling and steep hillslopes which results in a landscape highly dissected by the drainage network. In general, there are four 1st order streams for each 2 nd order stream system, four 2 nd order streams for each 3 rd order stream, and so on. Incorporating the bifurcation ratio along with the channel length relationships into the physical model of the drainage network allows the user to represent the amount and distribution of channel and floodplain within the watershed. The spatial arrangement of channel and floodplain within the drainage network affects the routing of flows through the drainage network. Bankfull hydraulic geometry relationships (see Equations 1 to 3) specific to the Chagrin River Basin were developed from field measurements of bankfull channel dimensions and site drainage area values determined using the USGS StreamStats tool ( The bankfull channel measurements were made by ODNR and USGS personnel. The resulting coefficients and exponents of the empirical hydraulic geometry relationships are reported in Table 3. 22

23 Table 3 A comparison of the default parameter values in the Mocnet River Network spreadsheet to the Chagrin specific values derived from field measurements and the GIS analysis. Parameter Default value Chagrin specific value Bifurcation ratio 4 4 Bankfull Width Coefficient (a) Exponent Bankfull Cross Sectional Area Coefficient Exponent Bankfull Depth Coefficient Exponent Channel Length Coefficient Exponent Concavity (z value) Results Effects of Land Use, Drainage System Type, Connected Impervious Area, and Stormwater Detention Criteria on Peak Discharge The results of the modeling study suggest that higher intensity land uses without adequate stormwater controls are likely to lead to a large increase in peak discharge rates throughout the drainage network. Figures 10, 11, and 12 provide a comparison of the peak discharge rates for the 19 land use and stormwater management scenarios at the outlet of the 3 rd order (synthetic) Chagrin River watershed model. The duration of the rainfall event for each of the scenarios presented in Figures 10 through 12 is 3 hours and the floodplain width scenario was held constant at 5 in these examples in order to isolate the effects of land use and stormwater management on peak discharge rates at the watershed outlet. The effect of floodplain management scenarios will be examined in the next section of this report. Figure 10 shows the modeled peak discharge rates for the 0.8 year recurrence interval, 3 hour duration rainfall event. Figure 11 shows the modeled peak discharge rates for the 12.5 year recurrence interval, 3 hour duration rainfall event. And, Figure 12 shows the modeled peak discharge rates for the 100 year recurrence interval, 3 hour duration rainfall event. In each of the bar graph figures the results for four land use scenarios are presented including: undeveloped (blue bar), low density residential (green bars), medium density residential (orange bars), and commercial/industrial (red bars) land uses. Management scenarios include combinations of swale drainage systems, curb and gutter drainage systems, connected and disconnected impervious areas, and several stormwater detention criteria. The drainage system and 23