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HomeMy WebLinkAbout05212024 Planning & Zoning Laydown - Leirer 1 SCHEFFLER CREEK FLOOD MITIGATION NOTES By Steve Leirer March 29, 2024 Flooding history-- Reference the attached Scheffler Creek map marked in Zones below. Scheffler Creek in past years has had flooding issues, with two recent, significant flood events. Most years, the flooding is uneventful, with smaller rain storm flooding. Its flood water is contained in the stream banks with some movement of gravels downstream, and some minor changes in its channel in the lower areas (Zone E and F on map drawing). The flooding usually occurs between mid-September and mid-October and is due to greater rain fall (5 to 11 inches in a twenty-four-hour period) brought in by south moving warm moist currents from the Pacific. Usually prior to flooding, over weeks, smaller rain falls (3 to 5 inch) saturate and load up the soils thus allowing for a fast runoff by a main storm event. Of the most serious flood event, caused the washout of a portion of Phoenix Rd on its west side, significant washout of the ditch on the East side of Phoenix Road, eroding into private property, and a wash out of the Culverts in Zones B and C. Contributing to the seriousness of this flood was a possible landslide, (in Zone A), bank erosion, excessive stream bed erosion, and material matter like trees and other wood being carried down. It was said that the creek bed where it entered the residential area at Barwell, was dammed high with debris, with mud and water behind it. When this gave way, the water and material spilled out onto Barwell Street and onto Phoenix Road. The first culvert, at Dunaway’s driveway, plugged and forced the water onto Barwell. This material also plugged the downstream culverts in zones B and C, damming the culverts, allowing most of the water to spilled out onto Phoenix Road, into its East side. During the cleanup of this major flood, I was involved in the cleanup area of the creek in Zone D and Zone E. In Zone D, the creek channel was filled to about 70% of capacity up to its banks. Some of this material was removed by excavator from the creek channel and stacked on the creeks east side. Scheffler Creek in Zone D, cleaned itself out over the next three to four years. In Zone E, the process took about 8 to 9 years to clear itself back to equilibrium condition. During this stabilization period, the channel randomly shifted left to right. In this self-cleaning process, most of the excessive gravel material was washed into the Lagoon (Zone F). It has been suggested that the material in the lagoon should be cleaned out. I feel this is not necessary, and a waste of funding to do so. The filling has added habitat and wetland for birds and other wildlife, and is an asset. The lesser major flood event causes problems at most of the culverts where they became plugged with wooden drift, then becoming dammed, the water flows over the top of the culverts washing them out. In this level of event, little water crosses Phoenix Road. Most of the flow remained in the channel. 2 CURRENT ACTIONS BY SBCFB The Seward-Bear Creek Flood Service Area Board is currently planning to take mediation actions on the upstream portion of Scheffler Creek in the area on the Creek Map shown as Zone A. This should be a positive first step to controlling extreme flooding. I endorse positively this effort. Scheffler Creek, its layout Scheffler Creek can be subdivided into six zones; each zone with characteristics that separate each, according to its tolerance to resist major flooding and the possible commercial damages. How much flood water can each of these Zone sections tolerate before going out of its channel to damage surrounding property? This question varies according to Zone. In addition to the depth and the force of the flow of flooding water several factors influence flooding: 1) the influence is foreign debris, like trees and limbs that plug culverts, or cause damming; and 2) new gravels washing in to raise the stream bed. In the big flood event, the plugging and damming debris came from Zone A. The raising of the streambed was caused by new gravels moving down, from Zone A, and a washout of gravel from Phoenix Road. ZONE F, the Lagoon Starting with the least flood prone section, Zone F, at the Lagoon; we observe a delta, that has adequate room to meander with any negative effect. It building different channels enhances the building of marsh areas for wildlife and bird habitat. The one, three-foot diameter, concrete Culvert at Dairy Hill Lane is a problem in that it cannot handle any volume of water flow in flood conditions, backing up floodwater that has on occasion crossed the road at its lowest point to the East. The water at this culvert slows and will not allow gravel to wash downstream. At times, this backup of washed in gravel fills this culvert, with only 6 to 12 inches for flowing water. In 1950 a bridge existed at this location. The Dairy Hill Lane culvert needs to be expanded to possibly three larger culverts. Possibly a one five-foot diameter culvert to handle the normal flow, with two lesser three-foot culverts on each side for flooding conditions. When this culvert is replaced, the street and curve at this point on Dairy Hill Lane needs to be widened and redesigned to proper road/curve standards. Currently it is not engineered properly and it is too narrow. Two automobiles meeting on this curve have difficulty passing, not mentioning a problem with a pedestrian walking on the side. The City of Seward should immediately get this problem on its agenda, being a safety and flood issue. I think it has priority over other items I see on the agenda. ZONE E Scheffler Creek exits from Zone D, upstream, with its defined and more permanent channel, into Zone E. In Zone E, the channels have spread laterally into a flat area. Since the major big flood, the channels have shifted from the east side to the west side. Sloping ground on the west side 3 acts as a barrier, and on the east side is a wooded, barrier berm, that define the creek’s limits. Between these channel limits, the creek is free to change channels, as new gravels move down stream and older gravel is washed into the Lagoon area. ZONE D Downstream from the first culvert, (at the Tract 5A driveway), Scheffler Creek is contained by four to six feet banks. In this zone, the creek is stable with rocky bottom, and is at its stable depth. In this zone there is little risk of overtopping its banks; however, if wood, trees, and migrating gravels were to cause damming it may go over its banks. ZONE C, ZONE B These two zones are similar in characterizes and flood risk as described in Zone D, with several additional flood risks factors. There is an additional risk of the river overtopping because of the culverts becoming plugged with wood debris and gravels, then allowing property-damaging flooding. Plugged culverts are the main risk of the creek overtopping its banks. The four culverts in zones C &D are: Tract 5 Driveway/Street Culvert, a single, 3 feet. Bayview Place curve to Phoenix, a double, 3 feet. Olsen Driveway, a single, 3 feet Dunaway Driveway, a single, 2 feet The culverts outlined above in the past two major flood events were causes of flooding and damage to Phoenix Rd on each side. When these culverts plug and the stream overtopped, it then flows across Phoenix Road into residential property on the east side. The east side of Phoenix Road was washed out to a depth three to five feet. This gravel filled the lower river in zones C and D to three to four feet above normal. Even a small flood, with the culverts plugging, is a concern. ZONE A In Zone A, west and upriver from the streets and residential areas, the Seward Flood Board has requested remediation for cleanup and channel design, with a riprap barrier, to help keep the creek in a channel. This action should minimize the potential for debris and gravels washing down during flooding to cause the waters to flood onto the streets. How should remediation in Zone A be done and to what extent? How wide should the channel be? How much gravel if any should be removed? Could scarring of the land off the stream bed and in its channel have a disturbing impact? Can the channel on its sides and bottom be reinforced by a heavy rock riprap amour to limit erosion? How far upstream should any mediation go, and is it possible to go too far up? What is the effect of natural trees and willows to hold back erosion? Are landsides possible that allow strong currents to move gravels downstream to fill and plug the culverts? 4 GENERAL CONCEPTS, possible SOLUTIONS The following concepts would need considerable design and planning debate by the stakeholders and engineers. Likely the solution would be a balanced approach, combining some parts of each option presented. Some options are easy, some more difficult and with sacrifice. 1). In Zone B, deepening the channel from the Dunaway drive culvert west, upstream (as proposed by the SBCFB); and east, downstream to Phoenix Road by a combination of raising the street, digging the channel deeper, or building a concrete a barrier wall along Barwell St. 2). Making the Dunaway culvert larger, or constructing a wide bridge. (There are shallow utility lines under the Dunaway drive culvert, which limits deeper digging) 3). Making Barwell St a single lane, one-way to allow for the widening of the stream bed on both sides; into Barwell St. and possibly into the stream bank on the south side. 4). Looking at the possibility of eliminating culverts in Zones B and C. The removed culverts possibly replaced by bridges reducing the possibility of damming. 5). In Zone C and Zone D, where permissible, widening the channel on its west side by three feet. 6). The Contract Planners suggested deepening the ditch on the east side of Phoenix Road. This conclusion and suggestion are in error and should not be implemented. Deepening this ditch on the east side of Phoenix to their suggested dimensions, would invite and encourage major floodwaters damage to the street, utilities, and resident housing. Keeping the existing drain as is would limit the water flow and damages. The concrete barriers placed on Phoenix, to redirect flooding, would prevent any water going to the east side of the road. See No 3 below. STOPGAP PREVENTATIVE MEASURES The City Street Department must implement the following measures: 1). In flooding conditions, must have personal monitoring the stream on a 24-hour basis. 2). Have excavators on call and ready, that can be dispatched within 30 minutes to remove damming material in channel and culverts. 3). Have stockpiled concrete barriers on hand, that can be rapidly placed in the street at Barwell and Phoenix Rd to redirect floodwater back into its regular channel. This action should be done when ANY floodwater moves onto the street and would limit property damage. 5 6