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OSP Update & Proposed Regional Improvements

** Study is in Progress**  | Content will be added as it is completed

City of Englewood

Floodproofing

Guidance

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Outfall systems plan (OSP) UPDATE

Along with floodproofing techniques that can be applied at individual residences, this study aims to propose regional drainage improvements that can reduce flooding in the most critical areas of the City. In order to determine the best and most cost-effective drainage solutions, a study must be conducted to determine the extent and physical properties of the Probable Areas of Flooding for varying storm event intensities.

 

Calibre Engineering is currently in the process of updating the Outfall Systems Plan (OSP) as the basis for proposing regional improvements for the City of Englewood.

what is aN OUTFALL SYSTEMs PLAN (OSP)?

An Outfall Systems Planning study generally consists of three elements:

1. Hydrologic Analysis: Storm events of different intensities (2-year, 5-year, 10-year, etc.) are modeled in the study area to determine the peak flow and other flow characteristics of runoff entering major drainageways. 

2. Hydraulic Analysis: Using data from the hydrologic analysis, the flow within the major drainageways is modeled to approximate the flooding extents. Areas of Probable Flooding and flood depths are then identified.​

3. Identification of Projects for Proposal: Projects to mitigate flooding in the problem areas are proposed.

previous studies

The current city-wide Outfall Systems Plan (OSP) was completed for the City of Englewood in 1999 by Turner Collie & Braden Inc. and included study of the Probable Areas Affected by Flooding. These studies included hydrologic and hydraulic analysis and resulted in the delineation of an approximate flood boundary along the main drainageways. However, this study did not go into the detail that is now required from a typical OSP, thus limiting the accuracy of the flood boundary.

More recently, Matrix Design Group completed a Master Drainage Plan and Flood Hazard Area Delineation (FHAD) study of Dry Gulch in 2017. The Dry Gulch study did not encompass the basins or Areas of Probable Flooding included in the subject study.

A complete list of previous stormwater studies within the City of Englewood can be found on the Flood History page.​

goals of osp update

The purpose of updating the Outfall Systems Plan for the City of Englewood is to more accurately estimate the location and intensity of flooding and propose new projects to mitigate this flooding. This includes reanalyzing the hydrologic characteristics of various storm event intensities with more accurate rainfall and surface topography data. A more in-depth hydraulic model of the study areas is capable of more accurately predicting the flood extents and depths at different locations. A survey of local citizens helps identify problem areas that may not have been previously considered. 

AREAs OF PROBABLE FLOODING in the 100 year event

1998 study vs Updated study

This graphic compares the Areas of Probable Flooding in the 100-Year Event from the 1998 study to today's most up-to-date hydrology and hydraulic analysis.

Hydrologic analysis

Calibre performed a hydrologic analysis using data obtained in the 1998 OSP study by Turner, Collie & Braden, including basin delineation and the physical characteristics of each basin. A new hydrologic model was built which incorporates this data into more modern and accurate calculation methods for runoff timing and peak flows. Having found this data at the accumulation point of each local basin, as well as at the outfall into their respective principal drainageways, hydraulic modeling was performed along these drainageways.

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The study area for this project, as delineated in the 1998 OSP Study, includes five major basins and 48 subbasins, encompassing a total area of approximately 3.4 square miles. It is roughly bounded by Santa Fe Dr to the east, Belleview Ave to the south, Yale Ave to the north and Franklin St to the East. The study area does not, however, include the Little Dry Creek Basin (see the SWMM Model Elements map provided to the right for a visual representation of the project area and basins). Subbasin areas range from 14.7 acres to 128 acres, with an average area of 44.7 acres. Due to the city being almost entirely developed, the overall imperviousness for the project area is 48.9%. The weighted average slope along the principal drainageway of each major basin is 1.6%. 

 

Using the most recent Colorado Urban Hydrograph Procedure (CUHP Version 2.0.0, released September 9, 2016), Calibre developed updated hydrographs for each of these 48 subbasins for the 5, 10, 25, 50 and 100-year return period storms events.* The parameters required for this procedure, including point rainfall depths, watershed area, percentage imperviousness, length and slope along with Horton's infiltration parameters, were obtained from the 1998 OSP study. 

The flows from the resulting output CUHP hydrographs were then routed through the major basins using the Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) Version 5.1.013. Pipe sizes, lengths, slopes, and roughness coefficients as well as overflow channel characteristics were taken directly from the data provided in the 1998 OSP report. These routing characteristics were used to determine the timing of peak flow between design points and throughout the entirety of the project area. Two detention ponds were included in Calibre's updated hydrologic model: Rotolo Park Detention Facility and Bates Logan Park Detention Facility. The stage-storage-discharge relationship for these detention facilities were also obtained directly from the 1998 OSP study. 

Calibre used flow data from the SWMM model to evaluate the existing drainage infrastructure and find Areas of Potential Flood Hazard.

Information on the hydrologic model is provided in the links below.

Rainfall Depths
Taken from the 1998 OSP, this table demonstrates one-hour rainfall depths in various storm events.
SWMM Routing Map
This PDF shows how stormwater is being conveyed with current infrastructure and conditions.
SWMM Routing Schematic

This PDF shows how stormwater is being conveyed with current infrastructure and conditions.

Overview

This graphic shows the SWMM Model and the major basins included in this study.

Due to the updated and more accurate calculation method within CUHP, the resulting peak flows in Calibre's study are consistently lower than the peak flows found in the 1998 OSP study. The following link provides a table comparing Calibre's 100-year peak flows at key design points within the project area to the corresponding flows calculated in the 1998 OSP study. Please note that some areas that have previously experienced flooding or areas that residents reported flooding may not be shown as having risk of flooding on these maps. This is potentially because of several reasons. This analysis done by Calibre uses the best information available, including the most recent topographic data available. Between the release of the topographic data and the present, construction activities may have adjusted the topography, or the topographic data could be flawed. Also, the modeling software used in this study did not take into account certain obstructions to flow, such as cars, temporary fences, post fences, or debris piles. These items will change the blow of floodwaters, but it is nearly impossible to model these obstructions completely and accurately.

Previous Study Reconciliation Tables

These tables show how the storm event flows differ from the 1998 Study to the current hydrology and hydraulics. 

Further results of the updated hydrologic model are included in the following links:

Peak Flow and Total Volume Tables
These tables show the peak flows by design point location throughout the basins included in this area of study.
Flow Profiles
These PDFS demonstrate the flow profiles of the drainageways included in this area of study.
Hydrographs
These are the hydrographs for various design points in the 100-year event with existing infrastructure compared to proposed 10-year storm infrastructure expansion.
Based on the results of the hydrologic model, the capacity of the existing storm sewer along the principal drainageways was determined. In the map at right and the exhibit "Existing Storm Sewer Capaity below", pipe segments are broken up into 5 categories: pipes that convey less than flows resulting from the 5, 10, 25 and 100-year storm events as well as pipes that convey more than the 100-year storm event flows.
Existing Storm Sewer Capacity
This graphic shows the conveyance capacity of the current City of Englewood infrastructure as well as existing  pipe sizes. Note, however, that this does NOT account for the current condition or status of the infrastructure.

REsults

This graphic shows the existing storm sewer system and the capacity of each segment according to existing data, reports, and studies. Please note that the condition of the storm sewer system must be analyzed to verify pipe capacities in today's condition. This map can be used to see where critically undersized infrastructure is located throughout the basins of study.

hydraulic analysis

Calibre examined five major drainageways in the hydraulic analysis portion of this study. Utilizing the runoff flows at various points along each drainageway as well as local topography and surface roughness, hydraulic models were created to define Probable Areas of Flood boundaries.

Two-dimensional, unsteady flow HEC-RAS (Version 5.0.7) models were developed for this study. Approximately 7.9 miles of major drainageway were modeled in order to define approximate area of flood boundaries and determine the number and location of structures within them. Much of this drainageway length travels through residential areas and runoff not captured in the storm sewer system flows through private residential and commercial properties in many cases due to the natural topography. 

The 2-D hydraulic models utilize a 1-foot LIDAR (2013) surface for topographic data and a planimetrics layer (2014) including roadways, parking lots, driveways, sidewalks and roofprints in order to specify the roughness coefficients throughout the basins. Boundary conditions were inserted to represent flow entering from subbasins into the major drainageways (with data from the updated hydrologic model) and downstream normal depths for drainageway outfalls.

Hydraulic models were developed for the 5, 10, 25 and 100-year storm events for the entire project area. The resulting Areas of Probable Flooding consider the effect of the existing storm sewer infrastructure, which was analyzed in Calibre’s SWMM model. A total of 2,151 buildings were found to be within this Area of Probable Flooding for the 100-year storm event. 1,104 of these were habitable buildings (e.g. houses). Boundaries for the Areas of Probable Flooding can be found below. A PDF map of the 100-year storm event flood boundary, its approximate maximum depth at different locations, and the structures within it can be found in the 100-Yr Flood Depths PDF provided in the link provided below.

100-Yr Flood Depths
This map shows approximate depths within the 100-year flood boundary with existing storm infrastructure.

Understanding a

"STorm Event"

What does it mean to experience an

"X-Year Storm Event"?

An Analogy:

Think of a 5-year storm event as a cup with four white marbles and one red marble. If you randomly pick one marble out of five, the odds of you picking out the red marble is one out of five. If you replace the red marble and pick again, the odds of picking the red marble are still one out of five. But, you could in fact pick the red marble twice in a row. This is the same with the 5-year storm event. The odds of experiencing the 5-year storm are one out of five in a given year, but it could happen twice in a row, even in the same week.

The same is true for a 25-year, 50-year, or 100-year storm event. A 100-year storm event is like having 99 white marbles and one red marble. The odds of picking a red marble are one out of 100, but you could draw that red marble at any time, even twice in a row.

Flood maps

100-year Flood Boundaries w Expanded Infrastructure Layers: Central and South-Central Englewood Basins (Click here to expand)*

This map shows the extents of the Probable Areas of Flooding in the 100-year event in the Central and South-Central Englewood Basins, including an outline of structures that are inundated during the 100-year flood event. In addition, this map shows how various levels of infrastructure expansion will impact the flood levels. For example, there is a layer (which can be checked on and off) that shows where the flood levels will be if  City infrastructure is constructed to carry the 25-year storm. The map can be used to compare have various levels of infrastructure build-out will reduce or impact the extent of the Areas of Probable Flood in the 100-Year Event.

100-year Flood Boundaries w Expanded Infrastructure Layers:       North Englewood and Yale Basins (Click here to expand)*

This map shows the extents of the Probable Areas of Flooding in the 100-year event in the North Englewood and Yale Basins, including an outline of structures that are inundated during the 100-year flood event. In addition, this map shows how various levels of infrastructure construction will impact the flood levels. For example, there is a layer (which can be checked on and off) that shows where the flood levels will be if  City infrastructure is constructed to carry the 25-year storm. The map can be used to compare have various levels of infrastructure build-out will reduce or impact the extent of the Areas of Probable Flood in the 100-Year Event.

100-year Flood Boundaries w Expanded Infrastructure Layers:    South Englewood Basin (Click here to expand)*

This map shows the extents of the Probable Areas of Flooding in the 100-year event in the South Englewood Basin, including an outline of structures that are inundated during the 100-year flood event. In addition, this map shows how various levels of infrastructure construction will impact the flood levels. For example, there is a layer (which can be checked on and off) that shows where the flood levels will be if  City infrastructure is constructed to carry the 25-year storm. The map can be used to compare have various levels of infrastructure build-out will reduce or impact the extent of the Areas of Probable Flood in the 100-Year Event.

This interactive map shows the Areas of Probable Flooding by storm event in the North Englewood and Yale Basins with the current City of Englewood infrastructure. This map can be used to view properties that are within the Areas of Probable Flooding in different storm events, which can be used to help quantify risk of inundation.

This interactive map shows the Areas of Probable Flooding by storm event in the Central Basin with the current City of Englewood infrastructure. This map can be used to view properties that are within the Areas of Probable Flooding in different storm events, which can be used to help quantify risk of inundation.

This interactive map shows the Areas of Probable Flooding by storm event in the South Basin with the current City of Englewood infrastructure. This map can be used to view properties that are within the Areas of Probable Flooding in different storm events, which can be used to help quantify risk of inundation.

This interactive map shows the approximate depth of flood waters at buildings within the study area in the 5-year storm event. If you are a citizen or business owner of a building impacted by the 5-year storm, your risk of flood inundation is very high, and your property is sure to be impacted by flooding. You should obtain flood insurance.

This interactive map shows the approximate depth of flood waters at buildings within the study area in the 10-year storm event. If you are a citizen or business owner of a building impacted by the 10-year storm, your risk of flood inundation is high, and your property is very likely to be impacted by flooding. You should obtain flood insurance.

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This interactive map shows the approximate depth of flood waters at buildings within the study area in the 25-year storm event. If you are a citizen or business owner of a building impacted by the 25-year storm, your risk of flood inundation is moderate, and your property is moderately likely to be impacted by flooding. You should obtain flood insurance.

This interactive map shows the approximate depth of flood waters at buildings within the study area in the 100-year storm event. If you are a citizen or business owner of a building impacted by the 100-year storm, your risk of flood inundation is lower, and your property is less likely to be impacted by flooding. However, your property is still at risk, and you should obtain flood insurance.

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PROPOSED PROJECTS

Having evaluated the capacity of the existing storm sewer system and determining an approximate flood boundary, several alternative project plans were proposed in order to mitigate the damages caused by flooding. Both potential storm sewer upsizing and potential detention sites were analyzed along major drainageways throughout the project area in order to reduce the number of structures within the 100-year flood boundary or remove them entirely. Analysis of existing storm sewer sections offline from the major drainageways (i.e. storm sewer sections not included in the SWMM model) was not performed. The scope of this study also did not include analysis of storm sewer inlet capacity.

 

Eight potential sites, all located within parks or other publicly-owned land, were proposed to provide temporary detention storage and reduce peak flows downstream in their corresponding drainageways. Approximate stage-storage-discharge relationships were developed for these detention facilities from a conceptual design using Urban Drainage and Flood Control District's UD-Detention software version 3.07 (released February 2017).

This analysis resulted in seven alternative plans. Storm sewers throughout the project area were sized to convey the 5, 10, 25 and 100-year storm events, both including and excluding the effect of the aforementioned proposed detention facilities on the peak flows. The seven alternatives are as follows:

​1A:  5-year Storm Sewer Design - No Detention

1B:  5-year Storm Sewer Design - With Detention

2A:  10-year Storm Sewer Design - No Detention

2B:  10-year Storm Sewer Design - With Detention

3A:  25-year Storm Sewer Design - No Detention

3B:  25-year Storm Sewer Design - With Detention

4:    100-year Storm Sewer Design - No Detention

A complete table of pipe sizes for these alternatives, as well as the potential detention area locations can be found below. 

Alternative Storm Sewer Sizes
This table shows pipe sizes for each program alternative by basin.
Potential Detention Sites
This PDF shows potential detention sites throughout the City that could be utilized or expanded to help with flood mitigation.
Construction costs were estimated for the 10-year and 25-year storm event designs, as they were deemed the most cost effective for removing structures from within the flood boundary. Each alternative was broken up into 13 segments in order to provide projects with a more manageable scope. The flood boundary maps on this page show the effect of these alternative designs on the flood boundary. Below are links for a map of the 13 project segments, project cost estimates and a graphic showing a comparison of project costs versus the number of structures removed from within the flood boundary.
Capital Project Drainageway Segments
This PDF shows major Capital Project segments in the drainageways.
Capital Project Drainageway Segments - With Potential Detention Locations
This PDF shows major Capital Project segments in the drainageways with potential detention locations.
Cost Summary Table
This table shows the cost of each capital project with 10-year and 25-year capacity increases and both with and without detention for each basin area.
Cost Estimates - No Detention
This table demonstrates a more detailed cost of each project with 10-year and 25-year capacity increases without detention. 
Cost Estimates - With Detention
This table demonstrates a more detailed cost of each project with 10-year and 25-year capacity increases with detention
Graphic: Structures Removed from the Flood Boundary vs. Cost Comparison
This bar chart shows the number of buildings within the Areas of Probable Flooding in the 100-year event in various storm event situations. It also demonstrates how many buildings are removed from the Areas of Probable Flooding with the addition and expansion of infrastructure in each program. 

This interactive map shows the CIP projects analyzed in this study, along with details about the cost, location, elements, etc. of each project. 

prioritization

In order to prioritize the proposed projects, Calibre used three principal criteria:

  1. The first is that, in any given drainageway, storm sewer upsizing should generally be completed from downstream to upstream. This is due to the fact that upsizing upstream pipes will likely increase flow downstream, potentially causing increased flooding.

  2. The second criterion used in determining project priority is the capacity of the existing storm sewer. Areas where the capacity of the existing storm sewer is lower have higher priority.

  3. Finally, projects are prioritized based on the number of structures, especially residential structures, that are removed from within the flood boundary as result of the infrastructure improvements.

According to these criteria, Calibre prioritized the proposed project segments as follows:

  1. South Englewood Basin Segment 1 (S1)

  2. South Englewood Basin Segment 2 (S2)

  3. South Englewood Basin Segment 3 (S3)

  4. South Englewood Basin Segment 4 (S4)

  5. South-Central Englewood Basin Segment 1 (SC1)

  6. South-Central Englewood Basin Segment 2 (SC2)

  7. Central Englewood Basin Segment 1 (C1)

  8. Yale Avenue Basin Segment 1 (Y1)

  9. North Englewood Basin Segment 1 (N1)

  10. North Englewood Basin Segment 2 (N2)

  11. North Englewood Basin Segment 3 (N3)

  12. North Englewood Basin Segment 5 (N5)

  13. North Englewood Basin Segment 4 (N4)

Published June 11th, 2019

Modified August 5th, 2019