Flood Flooding is a key concern within this watershed. A flood event occurred during the planning process and intensified the focus on this issue. Urban development has occurred within many flood-prone areas. Using the new NOAA data (mentioned above), this study updated hydrologic and hydraulic models as well as inundation/flood mapping for Walnut and North Walnut Creeks. The chart (right) demonstrates how much additional land is expected to fall within flood- prone areas and how much wider these flood plains are now compared to previous years.
It’s important to note flash flooding can occur outside of areas with mapped flood risk. Flash flooding can be caused by clogged inlets, storm sewers and culverts; overloaded storm sewer systems; blocked overflow Changes in Flood Risk Due to Increases in Rainfall paths and urban small stream flooding.
Stream conditions Walnut Creek is always in motion. In some areas, there is evidence of past stream meanders (curving stream segments) that were more than 500 feet from where the stream flows today. In other areas, the stream has moved several feet in only a few years. Streams are getting wider and lower. Nearly three quarters (71%) of the streams in the watershed have become incised or deeply incised—downcut over time. More than half (57%) of all field-assessed streams had moderate to severe erosion. Streams in the watershed are now 4–10x wider than they were prior to pioneer settlement. Improved stream buffers are needed. Nearly half of the smallest streams (48%) have no stream buffer or have a buffer less than 50 feet wide. Changes in land use and sources of increased sediment loads (such as cropland, gullies and construction sites with insufficient controls) can accelerate the cycle of stream evolution.
Pollution The lowest 7.6 miles of Walnut Creek are listed by the State of Iowa as an impaired waterway. E.coli bacteria are often measured at levels several times higher than water quality standards set by the State of Iowa. This poses a potential risk to health when people fish, wade, canoe or participate in other recreational activities that would put them in contact with the water.
- Length Modeled
- Added Area of Flood Risk
- Average Increase
- Flood Plain Width
- North Walnut Creek 6 miles 16 acres 24 feet
- Walnut Creek 18 miles 73 acres 34 feet
- Pollutant Sources by Land Use
N P Sediment
- Urban 14% 26% 7%
- Cropland 81% 49% 10%
- Pastureland 2% 2% 0%
- Forest 0% 1% 0%
- Grasslands 0% 0% 0%
- Gully 1% 5% 19%
- Streambank 2% 10% 38%
- Construction Site 1% 8% 25%
Construction sites, making up less than 0.1% of the overall watershed area are likely large contributors of sediment to Walnut Creek.
Use of subwatersheds To help this plan provide meaningful information for recommendations to the whole of the watershed, planners focused their attention on three subwatersheds, representing the primary conditions found in Walnut Creek.
By focusing scientific study on these three subwatershed types, the planners have been able to gain the most information from stream assessment field work and computer modeling. The recommendations for these three subwatersheds serve as a “template” of sorts for the balance of the watershed under similar conditions.
Three "Case Study" Subwatersheds
A developing area in parts of Waukee, Clive
- Rural and Urbandale along Little Walnut Creek expected to see rapid urban growth over the next ten years. An agricultural area draining to Walnut Creek
- Developing in rural Dallas County. A largely developed urban area within parts
- Urban of Waukee, Clive and West Des Moines that drains to South Walnut Creek (which flows through Country Club Lake)
Improved stream buffers are needed. Nearly half of the smallest streams (48%) have no Nitrates E-Coli.
Stream buffer or have a buffer less than 50 feet wide. Changes in land use and sources of increased sediment loads (such as cropland, gullies and construction sites with insufficient controls) can accelerate the cycle of stream evolution.
Pollution The lowest 7.6 miles of Walnut Creek are listed by the State of Iowa as an impaired waterway. E.coli bacteria are often measured at levels several times higher than water quality standards set by the State of Iowa. This poses a potential risk to health when people fish, wade, canoe or participate in other recreational activities that
Source “Monitoring Data from Iowa Soybean Association, IDNR Snapshot and Iowater” would put them in contact with the water. 57% of all field assessed streams had moderate or severe erosion.
Approaches for Developing Case Study improvements include:
1. Adopt use of the ISWMM manual for stormwater management design - Use its Unified Sizing
Criteria to manage runoff from both small and large storm events, to better mimic natural runoff conditions. For rainfall events that typically happen about once a year (2.67” in 24 hours), peak rates of runoff from developing areas would be expected to be reduced by over 95%, compared to traditional detention methods. Significant rate reductions would also be expected during larger events.
2. Restore healthy soil layers to open spaces in developing areas - Healthy soils have the ability
to absorb rainfall closest to where it first lands. Their absence can significantly increase surface runoff volume and rates—increases that need to be considered in the design of downstream storm infrastructure. Using techniques described in ISWMM, healthy soils can be preserved or restored.
3. Consider using low-impact design techniques - Developments can be designed to reduce
their impact on the landscape. Practices such as bioswales, wetlands and wet ponds can be incorporated into public greenbelts and private open spaces—creating a network of aesthetic features which also serve a stormwater management function. These practices are known to reduce delivery of key pollutants, such as bacteria and sediments.