The proposed Post-Construction Stormwater Management Ordinance includes new stormwater design standards that were developed during 2019 and 2020 by the Capital Crossroads (CXR) Local Governance Collaborative (LGC) which consisted of representatives from the development community and local governments in the Des Moines metro area, including Ankeny. The purpose of the Post-Construction Stormwater Management Ordinance is to protect, maintain, and enhance the public's health, safety, and welfare by establishing minimum requirements to control the adverse impacts associated with increased stormwater runoff. Proper management of stormwater runoff will minimize damage to public and private property; control stream channel erosion; reduce local flooding; and help protect our natural resources. This is achieved by maintaining, as much as possible, the pre-development runoff characteristics of the land after it is developed.
In February 2018, CXR met with mayors and city managers of the Des Moines metro cities and determined that the next topic for CXR LGC would be stormwater management because of its regional importance and the need for improvement. There was consensus among cities that the current approach to stormwater management was no longer sufficient and a new approach was needed. Since 2018, numerous educational sessions on stormwater have been held for interested parties, and presentations have been made to various city councils. Representatives from the CXR LGC, development community, and local governments met over the course of several months in 2019 and 2020 to develop a template for the new stormwater design standards in the Des Moines metro area. The Central Iowa Taxpayers Association, Des Moines Area Home Builder's Association, and the Iowa Commercial Real Estate Association were also involved in the process. Several key lessons were learned during the process, including the following:
1) Since 2008, Ankeny has spent approximately $11 Million on stream corridor stabilization and repair projects in various watersheds.
2) Since 2004 and projected through 2024, local governments in the Des Moines metro area have spent and are projected to spend more than $415 Million on stormwater projects and operational costs.
3) The amount projected to be spent annually on stormwater is increasing for most cities.
4) Current stormwater design standards are no longer sufficient and new standards are needed to avoid the costs associated with continuing to use standards that are known to cause problems.
5) Compared to current standards, the new stormwater standards are expected to result in:
a) A 90% reduction in flow rates from the 1-year storm event due to extended detention.
b) A 10 to 40% reduction in flow rates from the 100-year storm event.
6) Proactive stormwater design is more economical and less disruptive in the long term.
7) There are best management practices (BMPs) available in the Iowa Stormwater Management Manual that provide real long-term benefit to cities and their taxpayers.
8) Watershed Management Authorities (WMAs) in Central Iowa are proactively working upstream with agricultural interests to address stormwater before it reaches metro communities.
9) For every 100 acres of land developed, changing from the current stormwater design standards to those proposed by CXR LGC would likely require an additional 0.4 to 1.5 acres of the land to be set aside for stormwater management. The variance is based on local soil conditions and proposed land uses.
With the Post-Construction Stormwater Management Ordinance, the City will be adopting the Iowa Stormwater Management Manual (ISWMM) for managing stormwater quantity and quality. The Iowa Statewide Urban Design and Specifications (SUDAS) will continue to be used for the design of storm sewer systems (pipes, culverts, intakes). Included within the ISWMM is the adoption of the Unified Sizing Criteria (USC) to better control smaller storm events. All development and redevelopment sites will need to meet the requirements of the USC, as described within ISWMM and as noted in the Post-Construction Stormwater Management Ordinance.
The primary goals of the Post-Construction Stormwater Management Ordinance are to more effectively manage stormwater within the City; maximize benefits and minimize future maintenance costs; and have stormwater design standards that are consistent with other local governments in the Des Moines metro area.
To date, the cities of Clive, Johnston, Urbandale, Waukee, and Windsor Heights and Polk County have adopted their updated stormwater ordinances which include the recommendations from the CXR LGC. Also, the city of Des Moines is planning to adopt their updated stormwater ordinance in May or June of 2021, and the cities of West Des Moines and Pleasant Hill anticipate adopting their updated stormwater ordinances by July 1, 2021.