City of Mission Hills, Kansas

2021

WRS analyzed and developed two proposed stormwater-related fees for the City of Mission Hills, Kansas:

• A proportionally assessed development fee necessary to fund installation of a new behind-the-curb drainage system. The proposed new system would be designed to intercept rainwater from residential roof gutters and deliver it to the existing stormwater drainage system.
• A proportional development impact fee necessary to pay for anticipated additional burden on the existing system created by future increases in the amount of impervious area within residential and commercial properties.

The development fee would help cover costs of installing a proposed behind-the-curb drain system to comprehensively replace the existing roof-runoff collection system. The current system consists of a patchwork of connections to the existing under-curb drain system which currently exists to drain portions of the city’s roadway system. Over the years, the City of Mission Hills has typically permitted residents to connect roof downspouts to this system of curb underdrains along the streets where they exist. However, those underdrains are designed and sized only to remove the ponding water within the base under the roadways and not necessarily to collect runoff from houses or other additional impervious areas. 

The development fee calculated by this study is intended to provide a cost-based and equitable estimated fee that would be required by the city to connect the roof drainage from each residential property to such a future new system.

This analysis first estimated the necessary size for that proposed new system to adequately accommodate all impervious areas (rooftops) in the city, based on several factors. It then calculated the engineer’s opinion of probable costs to build those improvements. Finally, it allocated the anticipated cost of the new system to the residences it would serve based on each residence’s calculated contribution of runoff to the system. The final result is a suggested development fee structure to proportionally and equitably allocate the costs to the end users.

For this analysis, the hydrology of each drainage area was calculated using the TR-55 method and the WinTR–55 modeling program. WinTR–55 is a single-event rainfall-runoff, small watershed hydrologic model originated by the U.S. Department of Agriculture’s Natural Resources Conservation Service. For selected points along a stream system, the model can create a hydrograph, a graph or plot that shows the amount of water flowing past that specific point or cross section over a given time window. Those graphs help predict how much runoff will flow downstream for sub-areas within a watershed based on any given precipitation return interval. A generalized range of street-slope categories was used to calculate necessary pipe size at the known Manning’s n for each slope category: 0.5%, 1%, 2%, 3%, 4%, and 5%. Once the capacities of the various pipe sizes at different slopes were calculated, a desktop geographic information systems (GIS) exercise was conducted to assign appropriate pipe sizing for each segment of the proposed system based on its slope, its drainage area, and the percent of impervious area within that drainage area.

The GIS slope analysis sized pipes for all streets where the system is likely to be implemented. The pipes were sized to supply sufficient capacity to carry the run-off generated by the 10-year storm from the top of the slope to the closest downhill inlet into the city stormwater management system. The impervious area for each system segment was determined using Johnson County Automated Information Mapping System building footprint shapefile imagery, increased by a 1.25 multiplier in order to account for covered patios and other impervious area not always included in the shapefile area calculations. Once recommended system sizing was completed for each segment, an engineer’s opinion of probable cost was then calculated based on the length and size of pipe determined by the GIS analysis. Unit costs were based upon actual known costs from previous Mission Hills stormwater construction projects. The cost estimate was individualized to each development zone in Mission Hills. Finally, the estimated cost was divided by the number of houses in each development zone to arrive at a per-residence development fee. 

The city also requested that WRS determine a proposed stormwater system impact fee. Impact fees are a one-time assessment on new or expanded development intended to pay for the cost of additional infrastructure required to accommodate that specific development. An impact fee is designed to accurately capture the portion of existing system and future capital improvements that will provide capacity to manage the new burden on the system.

As a part of a cost-based methodology, the first step in establishing impact fees is determining the system planning criteria. For a stormwater system, total impervious area can be divided by the average impervious square footage of a residential lot to determine total system equivalent residential units (ERUs). This important calculation provides the linkage between the amount of sormwater infrastructure necessary to provide service and a set number of customers. Once the total number of equivalent units has been determined, an impact fee stated in dollars per ERU can be calculated. For this analysis, system improvement costs were based on the engineer’s opinion of probable costs developed from the Stormwater Pipe System Masterplan Update, which WRS updated for the city previously.

The gross impact fee was then calculated based on the impact of one square foot of impervious area within each of the pipe systems of the stormwater system evaluated in that study. Next, the total lotted area, the existing impervious area and the maximum allowable impervious area as determined by the city were calculated for each development zone. From these figures, the possible increase in impervious area for each zone was determined. One plus the possible increase in impervious area was used to define the Ultimate Impervious Factor. Finally multiplying the cost per unit of impervious area by the ultimate impervious factor was used to calculate the gross impact fee per square-foot unit of impervious area added.