1.0 Introduction

1.1 Authorization

This report was authorized by the Town of Lyons in the Disaster Recovery Services Agreement of February 16, 2016. A fully executed agreement was received by ICON Engineering Inc. on March 28, 2016. This study was supported by the grant funding from the Community Development Block Grant Disaster Recovery program.

1.2 Purpose and Scope

Stormwater Master Plans are commonly used by local and regional governments in a manner similar to land use, parks, or other master plans. These plans become part of the local community's comprehensive and capital improvement plans. Stormwater master plans help community leaders, planners, and engineers work with developers and private land owners. Stormwater drainage is unaffected by administrative or paper boundaries and a stormwater master plan helps all parties understand the natural conditions, constraints, and opportunities to manage stormwater in a safe, compliant, and sustainable manner. Like most master plans, this report provides the Town a starting point for ensuring the public safety and welfare of its citizens, businesses, and visitors.

The purpose of this study is to develop a comprehensive stormwater master plan for the Town of Lyons. The focus of the study is to:

  • Quantify stormwater runoff and quantity

  • Identify potential flood hazards and problem areas for subbasins tributary to North St. Vrain Creek, South St. Vrain Creek, and St. Vrain Creek, as they flow through the Town

  • Identify and prioritize capital improvement projects to reduce flood frequency and flood risk

  • Recommended improvement to enhance water quality and meet other sustainable objectives

  • Provide a guide for increasing stormwater resilience within the Town of Lyons.

The original drainage master plan for the Town of Lyons was completed by BRW, Inc. in 1998. This study will provide and update to the 1998 plan. Today, the Town has grown beyond the limits of the 1998 study. Furthermore, updates to the means and methods of engineering analysis provide additional information to determine risk and benefits. This Stormwater Master Plan utilizes current topographic, property, and engineering data collected between 2013 and 2016. Like its predecessor, this report is likely to serve the community for many years to come.

This study initially identified and inventoried stormwater infrastructure throughout town. A geospatial database of over 250 features was collected and is being incorporated into the Town's larger GIS inventory. From that point, hydrologic analysis was completed to determine the range of stormwater runoff from each of the Town's drainage basins. A hydraulic analysis used the stormwater runoff values to model how the surface waters drained through town. The existing storm sewer system was evaluated to determine how capture and conveyance systems operated during the various design storms. The resulting flood depth maps were used to identify or confirm problem areas and structures at risk of flood damages. Alternative solutions to mitigate the potential damages were developed. Each alternative was considered for resulting benefits (i.e. averted damages) and costs. A recommended plan was developed to guide the town through future infrastructure, land use, and stormwater management decisions. Additional information was developed for System Maintenance, Storm Water Utility, Adjacent Watersheds, and Water Quality.

The following is a summary of the scope of work completed for the Town of Lyons Storm Drainage Master Planning study:

  • Data Collection and Review

  • Field inventory of existing stormwater features

  • Baseline Hydrology and Hydraulics

    • Document rainfall using NOAA Atlas 14

    • Develop a basin-wide two-dimensional hydrodynamic model to estimate general flow paths within the watershed.

    • Define individual subwatershed boundaries

    • Develop hydrologic models for the 2-, 5-, 10-, 50-, and 100-year return period storms subject to the following guidelines:

      • Use the Colorado Urban Hydrograph Procedure (CUHP) to generate basin runoff hydrographs

      • Use the Environment Protection Agency Storm Water Management Model (EPA SWMM) to route the individual hydrographs

      • Evaluate the performance of existing storm drain infrastructure 24 inches or greater in size

    • Evaluate hydrologic condition for 120% of the 100-year return period

    • Identify residual flood potential within the basin using a two-dimensional hydrodynamic model, such as FLO-2D

  • Alternatives Analysis

    • Identify existing and future potential drainage and water quality problems along tributary paths to North St. Vrain Creek, South St. Vrain Creek, and St. Vrain Creek, as they flow through the Town

    • Develop and evaluate alternative plans on an outfall-by-outfall basis to reduce flooding potential on insurable structures and to provide water quality treatment

    • Estimate benefits of flood reduction

  • Capital Improvements Plan

    • Development of a recommended approach to stormwater management within the Town Limits

    • Identify a Phasing and Prioritization Plan for improvements

1.3 Mapping and Survey

One foot interval contours were generated from LiDAR project mapping. Project mapping was based on Federal Emergency Management Agency (FEMA) 2013 Post-flood LiDAR mapping with the following attributes and is equivalent to 1-foot contour interval topographic mapping:

Name: 2013 South Platte River Flood Area 1

Collection Date: Fall 2013 – Spring 2014

Vertical Accuracy: 9.25 cm RMSE

Point Spacing: 0.7 m

Vertical Datum: NAVD88

Horizontal Datum: NAD83

Survey data at existing storm drains and sewers were collected by ICON Engineering and associated sub-consultants as part of this contract. All survey was also gathered on NAD83 horizontal datum and the NAVD88 vertical datum.

An inventory of existing storm sewer infrastructure was completed between May and July 2016. A photo, description, condition, and location of each structure was recorded in the field and stored in a geospatial database. The geospatial data has been utilized within the Town's larger GIS database. A link to the online repository for the data is available for review here. This online repository link will expire in the near future, but the data is preserved in perpetuity through the Town's GIS database.

1.4 Data Collection

A summary of reports reviewed alongside the preparation of this report are presented below:

Table 1-1: Data Collected

Document Title Date Author
Town of Lyons, Boulder County, Colorado, Drainage Master Plan Final Report April 1998 BRW, Inc.
Zoning District Map of the Town of Lyons, Colorado January 2009 King Surveyors, Inc.
2010 Lyons Planning Area Map 2010 Civil Resources
Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual UDFCD

1.5 Data Delivery

This report is produced for the Town in a variety of formats. Hardcopy reports are provided to the Town as interim and final deliverables. A web based version of the entire report and appendices are provided to the Town for integration into the Town web portal. A downloadable PDF is also available from the web link version. And, the supporting map information is provided on a web-based mapping program that can be hosted on the Town's website now and in the future.

GIS data collected during the stormwater inventory was transferred to the town in September 2016 to complement the separate GIS database project undertaken by the town.