Increasing Development & Urbanization Leads to Frequent Flooding
Since the 1980s, increasing development and urbanization of northeast Des Moines and central Polk County have led to recurring and frequent flooding within the Hamilton Drain Watershed. Residents of areas experiencing the most frequent flooding report it occurring during even relatively small, one-year storm events. Prior to flood control improvements now in place, the issue caused damage to residential and commercial properties as well as roadways and other public infrastructure.
A significant portion of the Hamilton Drain Watershed includes industrial and commercial sites. By their nature, industrial and commercial land uses involve large areas of impervious structures that prevent stormwater infiltration, such as parking lots and buildings. Without infiltration, stormwater runoff increases. Runoff also occurs more quickly on impervious surfaces than it would on natural areas. The combination of increased runoff volume and decreased runoff time results in significantly higher flow rates into the conveyance system that receives storm runoff.
The topography of the Hamilton Drain Watershed is also flat, so there are few areas to store and detain higher flows. High stormwater runoff combined with a lack of ability to store and detain it will overwhelm the capacity of a conveyance system, causing rivers, lakes, and streams to overflow.
To better understand the problem and determine potential solutions, Polk County commissioned studies in the mid-1980s. Recommendations of the studies included the construction of numerous storm sewer and channel improvements, along with the construction of 11 regional stormwater detention basins.
Since completion of the report, six detention basins and many storm sewer conveyance upgrades were constructed, which have eased flooding in portions of the Hamilton Drain Watershed.
To further mitigate flooding in the watershed, Snyder & Associates began assisting Polk County on a FEMA Hazard Mitigation Grant Program project in 2009. The project began with a benefit-cost analysis of flood damages and proposed improvements to justify the project, leading FEMA and the State of Iowa to fund 85 percent of this $9.9 million project.
Documented Flood Damages Guide Flood Mitigation Efforts
To determine viable damage for mitigation, Polk County provided us with documented flood damage data dating back to 1998. Damages were defined as economic losses associated with road closures during flood events.
Six areas were targeted for flood mitigation within the Hamilton Drain Watershed:
NE 54th Avenue
This area flooded five times in 10 years. Flood mitigation at this location consists of a detention basin located directly upstream and a second detention basin located farther upstream.
NE 3rd Street
This area flooded six times in 10 years. Flood mitigation consists of two detention basins located upstream, as well as storm sewer improvements to divert flow traveling south to a third detention basin.
NE 5th Street
This area flooded 18 times in 10 years. Mitigation includes storm sewer improvements to reduce flow to this portion of the watershed and improve the efficiency of conveyance within the existing storm sewer.
NE 50th Avenue
This area flooded 17 times in 10 years. Mitigation for this was achieved through diversion on NE 3rd street to reduce flow through a shared outlet from NE 3rd and NE 50th, which will improve the efficiency of flow. This effort is part of the mitigation planned for NE 5th Street.
NW 2nd Avenue
This area flooded 10 times in 10 years. Mitigation consists primarily of attenuating flow and decreasing peak flow rates using four upstream detention basins.
NW 6th Drive
This area flooded 18 times in 10 years. The detention basins installed for mitigation of NW 2nd Avenue would mitigate flooding in this area.
Benefit-Cost Analysis for Hazardous Mitigation Grant Program (HMGP) Funding
The benefit-cost analysis is a crucial component of the Hazard Mitigation Grant Program and is where many other mitigation projects fall short and fail to qualify for FEMA funding assistance. Through close collaboration with the Iowa Department of Homeland Security and Polk County Public Works, we documented flood damages and completed a successful benefit-cost analysis, which was essential in obtaining HMGP funding from FEMA.
The estimated project costs included in the HMGP application were based partially on estimated costs included in the reports prepared in the 1980s. The benefit-cost analysis was based on the damages from recurring road closures detailed above. The total benefit of constructing the project in 2012 dollars was calculated at $16,340.097. Total costs amounted to $12,058.528. Based on these values, the original benefit-cost ratio was 1.36.
Through preliminary hydraulic modeling, we determined several of the improvements recommended in the original report would be ineffective at reducing roadway flooding at documented road closure locations. Taking this into account, we were able to eliminate several of the improvements; thereby reducing the project costs and improving the benefit-cost ratio to 1.42.
On behalf of Polk County, we submitted the HMGP funding application to FEMA. Due to the size of the project, FEMA split it into two phases, so preliminary design work could be completed to verify the project’s feasibility.
In January 2011, FEMA granted us Phase 1 approval of the project, which included preliminary design, an environmental assessment, and Phase 1 cultural resources investigation.
Preliminary Design: Proposed Hazard Mitigation Plan
The primary objective of the preliminary design phase was to update the conceptual designs proposed in the 1980s studies and verify the proposed hazard mitigation plan would provide valuable relief for recurring flood events in the Hamilton Drain Watershed. To demonstrate this, we developed a two-dimensional hydraulic model of the watershed utilizing XPSWMM software. Modeling allowed for an evaluation of overall flood control benefits due to the interconnectivity of the basins. Using the model, we were able to analyze and size four regional detention basins and confirm that they would accomplish the flood relief desired within the watershed.
Another fundamental objective of Phase 1 was to develop a comprehensive cost estimate for all aspects of the project including design, construction, property acquisition, environmental reviews, permitting, and wetland mitigation. With updated costs, the benefit-cost analysis was again calculated and confirmed that the project provided a substantial cost benefit.
A detailed technical report that explained the modeling, proposed mitigation efforts, and related details was submitted to FEMA. After some follow-up to answer questions from FEMA, Phase 2 was awarded, which included final design, environmental permitting, survey, property acquisition, and construction.
Flood Mitigation Final Design & Project Success
The final design phase began with wetland delineation, a Phase 1 Environmental Site Assessment, subsurface investigations, and identification of protected species habitat. As the lead for this project, we worked closely with the USACE to obtain both nationwide and individual wetland permits and to identify a source for wetland mitigation.
Acquisition of approximately 30 parcels along with 14 temporary and permanent easements was required to secure the land needed for the proposed infrastructure. Public meetings were held with affected property owners while acquisition and easement plats were prepared. Once all the necessary properties and easement plats were acquired, contract documents were finalized and the project was let for public bidding.
Construction began in fall 2014 and was completed in fall 2015 followed by FEMA closeout in early 2016. Major storm events in 2016 resulted in the first test of the improvements. Unsolicited comments from area residents indicated that they witnessed significantly less flooding, if any, compared to similar storms they experienced in the past.