Section 1 Executive Summary

 

In February 1999, Johnson County Public Works retained Camp Dresser & McKee Inc. (CDM) to complete the Blue River Watershed Study. The Blue River Watershed Study is the first in a series of watershed studies conducted by Johnson County’s Stormwater Management Program (SMP). This watershed was given the highest priority, because a majority of the basin is undeveloped and expected to experience significant growth in upcoming years. This study, funded entirely by the SMP, has a scope of work based on the Tomahawk Creek Watershed Study, completed by Overland Park in 1997.

 

The study area includes that portion of the Blue River Watershed defined by the contributing drainage area to the point where the Blue River exits Johnson County, approximately 4,600 feet north of the south Jackson County boundary line as shown in Figure ES-i. The study area within the County is divided into six major subwatersheds, including Blue River Main, Blue River North, Negro Creek, Coffee Creek, Wolf Creek, and Camp Branch. The entire drainage area is approximately 86 square miles, with approximately 76 square miles located in Johnson County. As shown in Figure ES-2, the drainage area within the County encompasses portions of four major government entities, including Johnson County and the cities of Leawood, Olathe, and Overland Park. The study had the following multiple objectives:

 

•   Develop 100-year floodplain boundaries for current and future conditions.

 

Floodplain boundaries have been developed, up to 160-acre tributary area, for both current development conditions and conditions that may occur in the future as the watershed continues to develop. The current conditions 100-year floodplain boundaries have been developed with the intent to submit to FEMA to replace outdated FEMA maps for Johnson County. Future conditions 100-year floodplain boundaries, also known as the planning condition scenario, have been developed to provide an effective and proactive planning tool to manage future development. With floodplain boundaries established prior to development, the County and cities can significantly reduce flooding potential and need for reactive stormwater improvements as the watershed continues to develop.

 

•   Identify stormwater conveyance system inadequacies and develop improvements.

 

The criteria used to evaluate performance of the stormwater conveyance system components were based ori existing regulations adopted by the County, cities, and the Kansas Department of Transportation. Conveyance systems beneath roads were analyzed for adequacy based on roadway classification — and for all roadway classifications, the maximum allowable depth of stormwater overtopping the road was 7-inches during the 100-year design storm. A total of 120 stormwater conveyance structures, including 87 beneath arterial roads and 33 beneath residential streets, were evaluated. Based on study criteria, 71 conveyance

structures beneath arterial roads were identified as inadequate while 21 conveyance structures beneath residential streets were identified as inadequate.

 

A building was considered to be in the floodplain if the existing or planning 100-year floodplain intersected the building. This requirement was applied to habitable buildings, which was defined as buildings with regular long-term occupancy and sanitary sewer facilities. Based on the above criteria, 65 habitable buildings were determined to be in the 100-year existing floodplain and an additional 15 buildings were determined to be in the 100-year planning conditions floodplain.

 

Because of the watershed’s rural nature, a significant number of the conveyance system components beneath unimproved roads were identified to not meet the study criteria. As development occurs, it is anticipated that the majority of undersized conveyance structures will be improved at the same time the road is improved and widened to handle the increase in traffic flow. Therefore, for the purposes of this study, improvement projects were not developed for the majority of these undersized conveyance structures since detailed designs would be completed at the time the roadway is upgraded. However, improvements were developed for those undersized conveyance structures that were either causing habitable building flooding, posed an immediate safety concern, or were identified by the County or cities as high-priority projects. Additionally, improvements were developed for locations where habitable buildings flooded, and flooding was not caused by inadequate conveyance system components beneath roads. The objective of the improvements was to develop a feasible planning level conceptual solution.

 

Based on input from the County and cities, the flooding problems identified as part of this study were grouped into 19 improvement projects at a total estimated probable cost of approximately $20 million. Figure ES-3 shows the general location of each project. Table ES-i summarizes the number of projects and costs for each subwatershed.

 

Table ES-i

___________________                                                                                               Probable Costs

  Subwatershed  Number of Projects    Total Probable Project Costs

  Negro Creek        4                      $10,730,000

  Coffee Creek        2                      $2,755,000

  Wolf Creek        4         $870,000

  Camp Branch        1         $65,000

  Blue River Main               8         $5,580,000

  Blue River North               0            $0

  Total            19        $20,000,000

•   Develop hydrologic and hydraulic models and GIS datasets.

 

Strict requirements for the computer models and GIS data sets were to provide a deliverable that can be regularly maintained and updated to keep current development conditions model “current.”

 

A computer model review evaluation was conducted to identify appropriate stormwater computer models to be used for this study, and for future County evaluations. The evaluation resulted in selection of U.S. Army Corp of Engineers (USACE) HEC-1 (version 4.0, September 1990) and HEC-RAS (version 2.2, September 1998) computer models. Using these two models, a detailed hydrologic and hydraulic analysis was conducted of the study area. The HEC-1 model was used to estimate stormwater runoff flow rates from each subarea, and to route flows downstream. The HEC-RAS model was used to simulate stormwater flowing in the channels and drainage structures, and to estimate floodplain water surface elevations based On existing and potential future landuse conditions. Detailed floodplain maps at a scale of 1 inch = 200 feet, were submitted under separate cover to the County and cities.

 

The Environmental Systems Research Institute’s (ESRI) software Arclnfo (version 7.2.x) and ArcView (version 3.2) applications were required to utilize Johnson County’s extensive Geographic Information System (GIS) datasets provided by the Automated Information Mapping System (AIMS) Department. The GIS datasets were used to develop data inputs for the HEC-1 and HEC-RAS models and automate the floodplain delineation process. The floodplain automation process was accomplished using HEC-GeoRAS (version 1.0), which is a software application that works directly with Arclnfo. Graphic and attribute data collected and developed for the project were organized and stored in a database structure designed as part of the project. The graphic data was stored in Arclnfo while the attribute data was stored in Microsoft’s SQLServer (version 7.0) relational database.

 

•   Conduct a public input/education program.

 

The intent of the public involvement program was to encourage residents in the watershed, particularly those who live along the channels, to become actively involved in the study. A questionnaire was mailed out that described the prOject and requested comments and input from landowners regarding existing flooding problems. A total of 1,328 questionnaires were mailed to residents located adjacent to the river and 147 responses were received, representing a return rate of 11 percent. As a follow-up to the stormwater questionnaire, a public meeting was held to inform and educate residents about the study and to gather information about existing problems. It is expected an additional public meeting will be held in the future to present final study results.

 

•   Include additional reference points to the Johnson County horizontal and vertical control network and perform detailed system surveying.

CDM retained Cook Flatt & Strobel Engineers (CFSE) to conduct a field survey of the stormwater conveyance system as defined in Section 8.3 — Primary Stream Network. The survey consisted of obtaining necessary data regarding physical characteristics for each drainage structure, establishing benchmarks along major stream crossings, and obtaining the lowest elevation along the perimeter of habitable buildings adjacent to the channel where water could enter the structure. The survey information was used to develop the computer model of the stormwater conveyance system.

 

•   Identify known/mapped outfall locations and evaluate best management practice~

 

These tasks were completed to assist Johnson County and the cities with compliance of EPA’s pending Phase II Stormwater National Pollutant Discharge Elimination System (NPDES) Regulations. Although compliance is not officially required until March 8, 2003, the work completed as part of this study will move the County and cities closer towards meeting the requirements of the regulations.