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HomeMy WebLinkAbout2021-08-16-I10 Sugar Creek Watershed AssessmentAGENDA ITEM: CITY OF WAUKEE, IOWA CITY COUNCIL MEETING COMMUNICATION MEETING DATE: August 16, 2021 AGENDA ITEM:Consideration of approval of a resolution approving the City of Waukee Sugar Creek Watershed Assessment FORMAT:Resolution SYNOPSIS INCLUDING PRO & CON: The Sugar Creek Watershed Assessment was completed by RDG Planning & Design, in coordination with City staff. It is a detailed study of the conditions within the 15.5 square miles of land area that drains to Sugar Creek and its tributaries north of Interstate 80. The purpose of the study is to understand past, current and projected conditions. This analysis will be used as a basis for decisions on projects and policies needed to address existing challenges and reduce the potential for future negative impacts on Sugar Creek and its tributaries. The executive summary is included in the meeting packet however due to them being large file sizes, the Assessment and Stream Condition Reports are not included. These are available upon request from staff. FISCAL IMPACT INCLUDING COST/BENEFIT ANALYSIS: COMMISSION/BOARD/COMMITTEE COMMENT: STAFF REVIEW AND COMMENT: RECOMMENDATION: Approve the Resolution. ATTACHMENTS: I. Resolution II. Sugar Creek Watershed Assessment Executive Summary PREPARED BY: Beth Richardson REVIEWED BY: Rudy Koester RK PUBLIC NOTICE INFORMATION – NAME OF PUBLICATION: DATE OF PUBLICATION: I10 THE CITY OF WAUKEE, IOWA RESOLUTION 2021- APPROVING CITY OF WAUKEE SUGAR CREEK WATERSHED ASSESSMENT IN THE NAME AND BY THE AUTHORITY OF THE CITY OF WAUKEE, IOWA WHEREAS, the City of Waukee, Iowa is a duly organized municipality within Dallas County; AND, WHEREAS, the Sugar Creek Watershed Assessment was completed by RDG Planning & Design, in coordination with the City of Waukee; AND, WHEREAS, it is a detailed study of the conditions within the 15.5 miles of land area that drains to Sugar Creek and its tributaries north of I-80; AND, WHEREAS, the purpose of the study is to understand past, current and projected conditions; AND, WHEREAS, this analysis will be used as a basis for decisions on projects and policies needed to address existing challenges and reduce the potential for future negative impacts on Sugar Creek and its tributaries; AND, NOW THEREFORE BE IT RESOLVED by the City of Waukee City Council in session this 16th day of August, 2021, that it hereby approves the City of Waukee Sugar Creek Watershed Assessment. ____________________________ Courtney Clarke, Mayor Attest: ___________________________________ Rebecca D. Schuett, City Clerk RESULTS OF VOTE: AYE NAY ABSENT ABSTAIN Anna Bergman Pierce R. Charles Bottenberg Chris Crone Larry R. Lyon Ben Sinclair EXECUTIVE SUMMARY August 2021 The Sugar Creek Watershed Assessment was completed by RDG Planning & Design, in coordination with the City of Waukee. It is a detailed study of the conditions within the 15.5 square miles of land area that drains to Sugar Creek and its tributaries north of Interstate 80. The purpose of this study is to understand past, current and projected conditions. This analysis will be used as a basis for decisions on projects and policies needed to address existing challenges and reduce the potential for future negative impacts on Sugar Creek and its tributaries. There are five primary goals of this study: 1. Assess Conditions Within the Watershed This study identified the boundaries of the watershed area to Sugar Creek and its tributaries and subdivided the watershed into smaller areas for analysis. Past, present and expected future properties within each of these smaller areas were evaluated. This report identifies issues that could influence stormwater runoff rates and volumes or create other impacts that could affect the stability and quality of the stream network. 2. Evaluate Conditions Along Key Stream Segments Almost 13 miles of Sugar Creek and some of its larger tributaries were assessed in detail. Conditions were documented using photos and video from both the ground and air. 3. Identify Stormwater Management Opportunities Stormwater management practices are used to reduce negative impacts caused by increases in stormwater runoff. As urban growth extends further into this watershed, stormwater management features can be incorporated into new development plans and integrated within recreational features such as ponds, lakes and wetlands. This study reviews the requirements of the recently updated City post-construction stormwater management ordinance and identifies opportunities to create recreational amenities that could improve the quantity and quality of stormwater runoff. 4. Delineate Stream Buffer Protection Areas The City desires to establish safe distances for new buildings, infrastructure and even some types of site grading to be set back from streams and significant water overflow paths. This study identifies criteria that could be used to establish these standards to reduce the potential of flood damage and streambank erosion. From these recommended standards, maps of these buffer areas have been created. 5. Plan for Stormwater  Management Within the Future Civic Campus Area A 220-acre parcel at the southwest corner of the future extension of University Avenue and 6th Street has been identified as a potential Civic Campus site. In advance of planning for this area, the City wished to identify areas that may be needed for stormwater management. The goal is to use water as a resource to create opportunities for fishing, trails, stream walks and other amenities. This report establishes a framework to plan surrounding land uses for this site and budget for these future improvements. INTRODUCTION This report consists of three documents: this Executive Summary (0verview); the Watershed Assessment (primary document); and the Stream Conditions Report (appendix) Summary Assessment Conditions 3 Esri, HERE, Garmin, © OpenStreetMap contributors, and the GIS user community, Iowa State University GIS Facility Legend (100) Sugar Creek - Lower (200) Sugar Creek - Upper (300) North - Northwestern Tributary (400) Northeastern Tributary (500) Western Tributary 1 inch = 5,000 feet 5,000 02,500 Feet . Sugar Creek watershed area Sugar Creek flows generally south- southeasterly towards Interstate 80 within the study area. Eventually Sugar Creek flows to the Raccoon River. For the purpose of this study, the Sugar Creek watershed has been divided into five primary subwatersheds (table, right), as shown on the map on the previous page. These larger subwatersheds were subdivided into a total of 239 smaller “micro-watersheds” for detailed analysis. Based on these, the past, present and future conditions were  reviewed, including: • Historic maps of streams and vegetation • Historic aerial photographs (1930s– present) • City population growth data • Surface topography (slopes) • Soil properties • Existing natural areas • City comprehensive plan (Imagine Waukee 2040) • Impervious cover (current and future) Soil conditions and impervious cover  greatly influence how much rainfall  becomes runoff. By reviewing these properties, this study estimates the volume of water that will run off the landscape during various storm intensities. These estimates of runoff volume are based on both current conditions and a projection of how those will change if the community develops based on the land uses outlined in the Imagine Waukee 2040 plan. This analysis identifies where the largest changes are expected to occur across the watershed. Currently, only about 8% of the study area is covered by impervious surfaces (such as buildings, driveways, parking lots, sidewalks, trails and roads). Based on projected land uses, impervious cover may increase to 38% of the study area over the next 20 years. The increases in runoff volume are  expected to be greatest during the  smaller storm events. Since smaller storms happen more frequently, increases in runoff rates and volumes from these types of storms can have a greater impact on stream stability and increase the potential for bank erosion. These impacts can be reduced to a great extent by effective stormwater management, as discussed later in this summary. WATERSHED CONDITIONS WATERSHED ID#NAME AREA (ACRES)AREA (SQ.MI.) 500 Western Tributary 1,425 2.2 400 Northeast Tributary 1,301 2.0 300 North - Northwest Tributary 3,392 5.3 200 Sugar Creek (Upper) 3,869 6.0 100 Sugar Creek (Lower) 9,937 15.5 RUNOFF VOLUME (WATERSHED-INCHES) CHANGE IN VOLUME Storm Event Rainfall Current Future %Gallons 1-year (24-hour duration)2.67”0.99 1.28 +29%78,000,000 100-year (24-hour duration)7.12”4.76 5.30 +11%145,000,000 5 Legend > 80% 60 < 80% 40 < 60% 20 < 40% 5 < 20% LESS THAN 5% CHANGE 1 inch = 4,000 feet 4,000 02,000 Feet . Change in potential runoff volume during a 1-year storm event (2.67”) An assessment of watershed conditions would be incomplete without an evaluation of conditions along important stream corridors. The scope of these evaluations was directed at select stream segments (“reaches”), designated by City staff for a more detailed inspection. These study reaches were typically selected for one or more of the following reasons: • The reach is within City property or land that may be acquired at a future date as part of a park or greenbelt • The reach is on private property but has a watershed area of significant size • The reach has City infrastructure (such as trails, sanitary sewer or water mains) running along or across it In total, 18 stream reaches totaling 13 miles in length were identified to be assessed in detail as part of this plan. The goals of these assessments were to: • Evaluate existing conditions along each reach • Identify and highlight areas of instability, impairments, safety concerns and threats to infrastructure • Develop methods and metrics to compare conditions among reaches and prioritize identified improvements These assessments were completed in two ways: • Ground assessments Two-person teams walked along each reach, collecting key metrics to evaluate erosion potential and documenting conditions with video and photos • Aerial assessments Video was collected by a remote- controlled aerial vehicle operated by City staff The collected data was analyzed by RDG Planning & Design and input into Geographic Information System (GIS) mapping software. The conditions along each reach were mapped for both the left and right bank (direction as seen when looking downstream), resulting in a “heat map” showing where streambanks are more or less stable. This information can be used to target where stabilization measures are most needed and to determine where stream corridor projects should be prioritized. The stream assessments used two different measurements to evaluate streambank stability: • RASCAL (Rapid Assessment of Stream Conditions Along Length) An assessment protocol developed by the Iowa Department of Natural Resources, where the observer categorizes streams to into four categories ranging from stable to unstable. RDG has modified this approach to assign a score on a scale from 1–10, where 1 is most stable and 10 is most unstable. • BEHI (Bank Erosion Hazard Index) This method is less subjective than RASCAL, where measurements of key parameters like bank height, bank angle and protective cover are entered. Mathematical formulas calculate a numeric score that assigns each bank segment into a category that determines how fast the bank is expected to move due to erosion. This method allows for the rate of bank movement (in feet per year) and the total volume of soil loss (in tons per year) to be calculated. STREAM ASSESSMENTS 7 Sample reach assessment map from the Stream Conditions Report EROSION POTENTIAL: REACH H Esri, HERE, Garmin, © OpenStreetMap contributors, and the GIS user community, Iowa State University GIS Facility Legend Parcels STABLE/MODERATELY STABLE (10-19) BEHI LOW (20-24) BEHI MODERATE (25-29) BEHI MODERATE (30-34) BEHI HIGH (35-39) BEHI HIGH (40-45) BEHI VERY HIGH (46-50) BEHI EXTREME (51-60) BEHI EXTREME 1 inch = 300 feet 300 0150 Feet . RASCAL and BEHI results were used by RDG and City staff to identify where work was most needed. From these discussions, six project priorities were identified: 1. Reach K – Part of the northwest  tributary to Sugar Creek, north of future extension of University Avenue (300th Street) and east of T Avenue.  This project would address erosion along the existing stream corridor. A restored stream would be created along the upper segment that is currently drained by a large agricultural tile. This would provide a better storm sewer outfall condition for future developments in the surrounding area. Part of the upper segment could also be developed into a stormwater wetland or a pond that could function as a recreational feature and also provide stormwater management for about 110 acres of land area. 2. Reach M – Part of the northeast  tributary to Sugar Creek, from the outfall from Warrior Lane to the northern part of the Sugar Creek Golf Course.  This segment was ranked highest in streambank erosion potential. Some portion of the soil materials eroded from this segment likely end up deposited in the pond within the golf course. A potential project could stabilize streambanks and reduce erosion potential through grading and bank protection. The lower part of the reach could be developed into a stormwater pond, which could be incorporated into a future expansion of the golf course. 3. Reaches E and F – Sugar Creek,  immediately downstream of T Avenue:  These reaches extend more than two miles through an undeveloped area, which could be considered for acquisition as a publicly owned greenbelt in the future. The stream has lowered over time, leading to significant bank erosion. A restored stream could reduce both erosion potential and flood risk by shaping banks back to stable slopes and better connecting the stream to the adjacent flood plain. 4. Reach J – Civic Campus site:  Project alternatives include stormwater ponds or a restored stream through the future Civic Campus site. 5. Reach Q – Western tributary to  Sugar Creek, west of U Place:  This is a small stream near the upstream end of the western tributary to Sugar Creek. This area could be incorporated into a publicly owned greenbelt in the future. This segment has less erosion today, so improvements would address some minor erosion, restore stream functions and improve habitat. 6. Reaches I and L – Stream  segments within Sugar Creek Golf Course:  These projects would address streambank erosion within the golf course. Projects that are designed to restore  stream health and function could be  eligible to be treated as streambank mitigation bank sites. There is a review and approval process for these banking locations, but if approved, these projects could be created and then credits sold to offset stream impacts created by other projects. The City could “sell” credits to themselves to offset their own project impacts, or could sell to other landowners, cities or state agencies that need to buy credits. Chapter 3 of the Sugar Creek Watershed Assessment summarizes the results of the stream assessments in greater detail.  It includes more information about these priority projects and the financial needs to implement them. It also includes the next steps that need to be taken to implement these measures. A separate document (Stream Conditions Report) has also been prepared and gives much more extensive information on how the assessments were conducted and the conditions observed along each reach. A separate report on each of the 18 stream reaches assessed is included in that document. 9 Larger Stream Restoration Section (Reaches E/F/J/K) Existing Grade Future Grade - Reconnected to Floodplain RDG conducts a stream assessment of Reach C, November 23, 2020 Chapter 4 of the Sugar Creek Watershed Assessment starts with background information about  stormwater management. It answers questions such as: • What factors are considered in planning for stormwater management? • How do factors like soils, impervious cover and rainfall impact these changes? • What increases in runoff VOLUMES and RATES are expected from developed areas? • How are stormwater management practices used to mitigate these increases? This chapter also reviews the City’s  recently updated post-construction  stormwater ordinance and how it  is used to implement the guidance  within the Iowa Stormwater Management Manual. As mentioned previously, significant changes in runoff VOLUME are expected to occur in the study area over the next 20 years. Effective implementation of new City policies will reduce negative effects, by using stormwater best management practices (BMPs). In short, even though runoff  VOLUMES may increase over time,  properly designed stormwater BMPs can be used to control  runoff RATES, leading to reduced  risk of downstream flooding and  streambank erosion. Most of these management practices will likely be implemented at the development scale. Private developers will construct new stormwater practices that comply with these updated requirements. Individual landowners or homeowners’ associations would be responsible for long-term maintenance. However, in some cases it may be preferable for a larger-scale practice to be built that would meet the stormwater management needs for a larger area. These practices could be publicly owned and maintained and integrated into park or greenbelt areas. These features would be seen by most citizens as a recreational amenity—a place to fish, a wetland to explore or a pond to walk around. However, the primary purpose of each would be to reduce the quantity of runoff and improve water quality downstream. Chapter 4 of the assessment report identifies 15 locations that  have the greatest potential for “regional” stormwater management areas. Each of these could meet the management requirements for portions of multiple parcels, serving watersheds from 60 to 320 acres in area (depending on site location). This information can be used by City staff, in discussions with property owners, to explore opportunities for private or public-private partnerships to create unique features that would benefit the community in a variety of ways. STORMWATER MANAGEMENT 11 Pracht Wetlands Park, Wichita, KS (photo courtesy RDG Planning & Design) Many streams and paths of concentrated flow pass through areas where growth is projected in the City’s comprehensive plan. The corridors  along these paths can serve several  critical functions: • Reserve a safe path of conveyance for water flow • Prevent additional construction of structures and other infrastructure which could be harmed by flooding • Protect a buffer along streams to improve habitat, biology, water quality and stream health • Improve connectivity via shared-use paths and trails • Provide access for vehicles and equipment to perform ongoing maintenance and repairs by private or public groups • Create, support or connect to recreational opportunities and park spaces along the corridors Chapter 5 of the Sugar Creek  Watershed Assessment reviews areas identified as having known  flood risk, based on Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs). These maps are based on studies that identified areas along major streams which may be flooded by events having as little as a 0.2–1.0% chance of happening in any given year. These maps are based on current conditions and do not consider impacts from future developments or changes in rainfall patterns. These flood maps are one piece of information used in draft ordinance language prepared by the members of the Walnut Creek Management Authority (WMA). This organization is a collection of cities and counties that fall partially or totally within the area that drains to Walnut Creek. Waukee is a member of this organization, as much of the northern and eastern part of the community flows to Walnut Creek. The draft language considers the mapped flood risk, patterns of stream meandering and drainage area to define buffer widths along assigned streams into three categories: • Type 1 Streams have areas of mapped risk of being inundated by either the 1.0% or 0.2% annual recurrence chance flood event on the FIRMs • Type 2 Streams do not meet the requirements of Type 1 streams, but are identified on the United States Geological Survey (USGS) topographical maps as either a solid blue or dashed blue line • Type 3 Streams don’t meet the requirements of either a Type 1 or Type 2 stream, but have a minimum drainage area of 50 acres Chapter 5 explores how the  proposed ordinance defines the  required buffer width for each  stream type. Those parameters were used to map the required buffer zones across the entire Sugar Creek watershed study area. These maps can be used by City staff to discuss buffer requirements with landowners and designers as new developments are proposed. That chapter also includes a discussion about establishment and maintenance of vegetation within buffer zones. Some of these buffer zones could be candidates to become publicly owned greenbelts. The future costs to establish and maintain vegetation along several priority corridors has been calculated. STREAM BUFFERS 13 Esri, HERE, Garmin, © OpenStreetMap contributors, and the GIS user community Legend ZONE A - 1% AR ZONE X - 0.2% AR Type 1 2 3 Study Watershed Area Parcels 1 inch = 2,000 feet 2,000 01,000 Feet . Sample buffer map This assessment includes a review of potential stormwater management strategies that could be employed within the 220-acre plot where a future Civic Campus is planned. The intent is to understand where stormwater practices could be best located so they can be integrated into future planning efforts with the proper size and location reserved. The desire is to create public amenities,  using water as a resource, to provide  recreational opportunities and add  value to the surrounding area. In this way, the surrounding development can reserve adequate space and be planned to properly relate to these water features. Chapter 6 of the Sugar Creek Watershed Assessment details the design considerations for several  alternatives to manage stormwater at this site. The preferred alternative is a feature pond which would have a water surface area of approximately 12 acres. A pond of this size could manage runoff from all parts of the site area which would be expected to drain to it. A few areas downstream of the pond would need to have smaller stormwater management areas that could be developed as stormwater wetlands or detention practices. The chapter also reviews two other design alternatives. One is creation of a smaller 5-acre pond with multiple additional features for stormwater management. The third option would not construct a pond along the stream itself, but would instead create several smaller water features to manage water before it enters the stream. The large pond was selected as  the preferred option through discussions with City staff, based  on the desire to create a large-scale  public amenity as a key feature  within the Civic Campus. This option would require relocation of a section of sanitary sewer and purchase of mitigation credits for impacts to the stream and adjacent wetlands. However, even with those costs included, initial projections of total project cost indicate that the preferred option would be less expensive than constructing multiple stormwater practices along the edges of a stream buffer through the site. The large pond option may require a longer review period for permitting through the IDNR and the U.S. Army Corps of Engineers, but a design could be advanced early enough for permitting to occur to allow construction concurrent with or in advance of the Civic Campus. Chapter 6 includes a detailed list of the next steps required for implementation. If other stream restoration projects outlined in this report are pursued and approved as a mitigation site, the City could potentially purchase the required stream mitigation credits from itself, further reducing total implementation costs. CIVIC CAMPUS AREA 15 POND STORMWATER WETLAND or POND DETENTION / BIORETENTION HIGH WATER ELEVATION (100-year) RESTORED STREAM SEGMENT EXISTING TRUNK SEWER RELOCATED TRUNK SEWER N Preferred “big pond” option To continue to address stormwater issues in Waukee, this plan will need to be frequently updated. Implementing this plan will also require levels of study that go beyond the scope of this effort. This planning effort is based on the most current information available from various sources in 2021 and includes evaluation of past and current conditions, along with future land use projections within the City’s comprehensive plan. However, the community’s rapid rate of growth  may require adjustments to this plan  to ensure its objectives are achieved. The subwatershed and microwatershed boundaries can be retained in City GIS libraries, allowing ongoing comparisons between assumptions of future conditions made in this current plan and how development actually occurs. This will also reduce time and effort required to re-delineate boundaries or create a new watershed ID system. The GIS analyses related to runoff analysis and stormwater management should be refreshed at least every five years, or more frequently as driven by rates of development. These analyses should also be updated as part of reviewing any future amendments to the City’s comprehensive plan, to update microwatershed properties based on current conditions at that time and to consider any changes in projected future land uses. Organizing watershed GIS data in the same fashion as this report will streamline analysis calculations,  allowing currently assembled  analysis spreadsheets to be updated more efficiently, reducing cost. Assessments of stream reaches  located on publicly owned parcels  should also be refreshed at least every ten years. Methods similar to this study (employing GoPro cameras to record field and aerial assessments) should be followed, allowing more direct comparisons of changes in conditions over time. Stream movement and changes in stream stability can be evaluated, allowing more accurate determination of erosion rates and adjustment of work priorities. When the City prepares to acquire  a greenbelt or stream corridor,  those reaches should be assessed  to document conditions before  development impacts occur.  Updates of those assessments can be incorporated into the program of updates of other reaches. Alternatively, reach updates can be done on a rotation to reduce the cost of assessments to be completed in any given year. However they are scheduled, using the current assessment protocols, GIS information and analysis tools as a template should help to reduce the cost of conducting and updating assessments in the future. RECOMMENDATIONS AND UPDATES Conclusion This executive summary is one of three documents prepared as part of this planning effort. For additional details about the information included in this summary, please refer to the Sugar Creek Watershed Assessment and the Stream Conditions Report, prepared as part of this study. 17