City of Elmhurst City of Elmhurst Storm Sewer System Workshop November 22, 2010 1
City of Elmhurst Watershed divide (green dashed line) through Elmhurst Area east of divide drains to Addison Creek Area west of divide drains to Salt Creek 2
Elmhurst I-290 Topography North Avenue Salt Creek Butterfield Road 3
Storm Sewer System Overview What is a storm sewer system? A storm drain, storm sewer, stormwater drain or simply a drain or drain system is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems. They are fed by street gutters on most roadways, as well as surface waters during heavy rainfalls. 4
Storm Sewer System Overview There are over 702,000 linear feet (133 miles) of storm sewer within Elmhurst. The storm sewer system also includes eight stormwater lift stations, which are required to convey stormwater to Salt Creek once the creek reaches a certain level (with the exception of #7). 5
Storm Sewer Summary Table Storm Sewer Size Range (in) Linear Feet Miles 6-10 40,173 7.6 12-18 368,579 69.8 21-30 148,437 28.1 33-42 60,864 11.5 48-57 46,247 8.8 60-72 33,628 6.4 84-96 4,527 0.9 TOTAL 702,455 133.0 6
Storm Sewer System Overview The major storm sewers are highlighted in orange. These sewers drain to either a lift station, a detention basin, or directly to Salt Creek. As shown to the right, there are a limited number of stormwater management facilities (detention). Elmhurst, much like any other community established at the time, did not include stormwater detention as part of its development regulations. Stormwater detention was not a requirement of development in DuPage County until 1992. However, Elmhurst required detention for commercial developments prior to 1992. 7
Storm Pumping Stations Before the Elmhurst Salt Creek levee was constructed, the creek would overtop its banks and flood some neighborhoods of Elmhurst on the east side of the creek. When the creek is at a high flood level, the internal drainage (stormwater collected behind the berm) has to be pumped. The pumping rates from the lift stations are restricted to the rates that existed at the time the levee was being permitted. 8
Storm Pumping Stations Station ID Location Number of Pumps Total Capacity (cfs) Backup Power 1 Berkeley & Adams 2 147 Standby Generator Diesel Fuel 2 Graue Woods 2 6 None 3 Harrison 1 45 Dual ComEd Feeds 4 Jackson 1 134 Dual ComEd Feeds 5 McKinley 2 134 Dual ComEd Feeds 6 Randolph & West 2 182 Dual ComEd Feeds 7 Underpass 2 8 8 Utley 2 203 Standby Generator Diesel Fuel Standby Generator Diesel Fuel 9
Example: Harrison Street Lift Station Note: Vertical scale is not the same as the horizontal scale 10
Flood Insurance Rate Map Although the levee protects southwest Elmhurst from Salt Creek overbank flooding, several low-lying lying areas were determined to be at flood risk and were mapped as 100-year floodplain. During any storm that exceeds the design of the sewer, the capacity of the storm sewers is exceeded, which results in stormwater runoff draining overland to these low-lying lying areas. The storm sewer inlets in the low areas may see stormwater runoff from areas that are 10-20 times the size of the tributary area that they were designed to handle. 11
Example Washington Street Although it is not mapped as 100-year floodplain, Washington Street is an example of a low- lying area that receives overland flow from other areas. As shown to the right, there are overland flow paths that direct stormwater runoff toward Washington Street. Because there is no overland outlet for the Washington Street area, the only outlet is the already surcharged storm sewer. The low-lying lying area fills up, resulting in flooded yards, streets, and eventually homes. 12
Storm Sewer Design Storm sewers in the 1960 s, just like today, were designed based on a 10-year storm event. A 10-year storm event has a 10% chance of being exceeded in any given year. The 10-year storm was based on the appropriate rainfall data at the time (TP-40), but rainfall data has been updated since the 1960 s, and the rainfall depths have increased (Bulletin 70). 13
Storm Sewer Design The storm sewer system in Elmhurst is similar to that of other communities in northeast Illinois. The storm sewer system was designed in accordance with typical design standards at the local, county, and state levels. There is not a storm sewer system in northeast Illinois that has the capacity to handle the June and July 2010 storm events. 14
Storm Sewer Design This newspaper article from January 22, 1959 discusses the design capacity of the storm sewer system. It was estimated that the storm sewer could handle a rainfall intensity of 2 inches/hour, which translated to a 10-year storm event at the time. Because the rainfall depths that we use today are higher, the design intensity of 2 in/hr only translates to about an 8-year 8 storm event. 15
June & July 2010 Storm Events June 23, 2010 Storm Event 4.5 inches in 30 minutes Approximately a 500-year storm event July 23 24, 2010 Storm Event 6.84 inches in 12 hours (6.96 inches total) Approximately a 150-year storm event 16
July 23-24, 24, 2010 Storm Event 17
Recent Storms vs. Design Storms July 23-24, 2010 6.84 in. - 12 hrs. 18
July 2010 Rainfall 19
Rear Yard Drains To reduce nuisance flooding, the City has developed a cost share program for the installation of rear yard drains. The cost share covers 50% of the installation cost, up to a maximum of $1,000. Over 1,000 rear yard drains have been installed as part of the program since 1991. Although the drains help alleviate nuisance flooding, they do not provide flood protection for the residence. 20
New Homes vs. Old Homes A common misconception is that new construction increases the amount of impervious area, thereby increasing stormwater runoff. In fact, the amount of impervious area associated with new homes is similar or less than the impervious area associated with older homes. Newer homes provide additional stormwater benefits, including: The removal of all illegal connections to the sanitary sewer Instead of discharging stormwater toward neighboring homes, sump pumps are directly connected to the storm sewer 21
Flood Reduction Improvements Flood reduction improvements will offer flood protection up to a certain design event, and this protection will no longer be provided during large storm events that exceed the design event. Because some flooding problems are unique to individual residences, it is unlikely that public flood reduction improvements will solve every resident s problems. Flood reduction improvements will not only require significant costs, but also the cooperation of various private and public entities, such as schools, parks, and forest preserve districts. 22
Green Infrastructure The use of natural areas, open spaces, and native plant vegetation to naturally manage stormwater runoff and improve water quality. Examples of green infrastructure include: Rain gardens Pervious pavement Green roofs Swales Rain barrels Although the incorporation of green infrastructure will be a component of flood-reduction alternatives and these measures help reduce stormwater runoff and improve water quality, the major problem in Elmhurst is water quantity. As an example, if Elmhurst experiences a 10-year, 24-hour storm event, the stormwater runoff volume would be the equivalent of 8.78 million 55-gallon rain barrels. 23
Possible Solutions - Pumps The pump station capacity can be best utilized by installing variable frequency drives (VFD). VFDs would increase the efficiency of the pumps without increasing the pump capacity. The pumping station capacity cannot be increased because of permit stipulation and the threat of downstream flooding. The only way to increase pump capacity is to purchase or create off-site flood storage. The storage would mitigate any increases in downstream flood storage resulting from increased pumping. 24
Possible Solutions - Internal Storage Internal storage can be provided in green and open spaces. Three sites are shown below for illustrative purposes only. If vertical l side slopes were used and the sites were dropped five feet, a total volume v of about 38 acre-feet could be obtained. The internal storage volumes necessary to keep flood levels to an acceptable level would be considerable. These facilities would be dry and stormwater would only be diverted during significant storm events. POTENTIAL DETENTION SITES 25
Possible Solutions Overland Flow Routes Because many of the flood-prone areas in Elmhurst are located in low areas with a storm sewer outlet, overland flow routes could be installed which provide a positive gravity outlet for the stormwater prior to water levels reaching the lowest building entry point. The overland flow route is created by regrading streets, backyards, and areas between houses. The elevation of the overland flow route is set below the elevation of nearby homes, protecting the homes from flooding. 26
Possible Solutions - Increase Pipe Sizes Regulations prohibit communities from sending their flooding problem to the next downstream community. Therefore, we cannot increase storm sewer sizes without mitigating for the increases in flows downstream. For flood-prone areas where local detention can potentially be provided, a solution may involve a combination of larger pipe sizes with the local flood storage. The larger pipe is routed to the detention basin, where the provided storage offsets the increases in flow. 27
Flood Enhancements Because flood reduction improvements will likely require significant expenditures and time to develop, design and construct, flood proofing alternatives for individual residences will be reviewed. Some of the flood proofing options that will be assessed include: Glass block windows Raised window wells Creation of side or rear yard swales Driveway modification **Enhancement projects require professional review and approval to ensure no negative impacts to structure or neighboring properties. 28
Future Flood Protection The City will strive to identify and implement flood reduction improvements for its residents, but it is unlikely that the improvements will solve every resident s s flooding problems. Any flood reduction improvement will offer flood protection up to a certain design event. This protection is not provided during storm events that exceed the design event. Private property improvements may offer additional protection and require City permits and inspections. 29