FLOOD05435 - Laserfiche WebLink

Home Browse Search

4. Sediment accumulation ln barrel - 80% restricted. street grading operations, particularly in winter when culverts may not be visible, cannot avoiding pushing sand and gravel over the culvert ends. The result i. that the ends become partially blocked and sediment accumulate. in the barrel from trickle flows. Without regular flushing of sediments, the culvert. eventually plug. * NOTES FORTABLE-9 (CONTINUED) 5. Plugged by sediment. 6. Bridge has two barrels 4' wide with present clear wsterway 15"high. ,. The storm circular. Sewer si~e See Plate vanes ,,, hom 4' x 4' to 2\' x 3' to approximate lengths of each 3'x3' to 42" section. Another cause of sedimentation is vegetation growth in the channel. Al- though grass growth is desirable for channel stabilization and aesthetics, it has caused excessive sediment accumulation in Some areas. Downstream from the Elm/Elbert Streets intersection, the dense grass and brush vegetation retards flow snd traps sediment. The sediment has continued to build up until the culvert outlets are being buried. '- Both cutverts are clear. 9. Culvert ~s clear. 10. Plugged by sediment. Baaed On the proposed criteria, it 1S clear that ~he culvert capacities should be increased throughout the town. By contrast, the culverts on the straight sections of the street along Mobile, Paddock, and Tabor Streets are clear of sediment. This is apparently the result of stable ditches, adequate culvert length, and positive outlet drainage. The key points in preventing culvert sedimentation are as follows: Sedimentation 1. Provide culvert lengths which will terminate several feet beyond the street shoulder to prevcnt damage and plugging due to street grading. Deposition of sediment in culverts and channels is a serious problem in Eliza- beth. A number of culverts are partially or totally restricted by sediment. 2. Culvert slopes should be slightly steeper than the inc(lmingchanne 1 slope to maintain equivalent sediment transport. If the culv"r~ sedi- ment transport capacity is less than the channel, the barrel will begin to accumulate sediment snd eventually plug. A 2% slope is a desirable minimum. In our opln~on, the majority of the sediment supply originates from the streets. Each time the streets are graded laos" sand and gravcl are bladed to the street shoulders. Subsequent rains and snowmelt runoff wash ~he sediments into the road- slde drainage ditches. Where flow in the ditches is insufficient to move the sediments, they continue to accumulate and restrict diteh capacities. 3. Provid"r"gularmaintenance for channels upstrellm and downstream of culverts where sediment tends to accumulate. S..'lim..ntstioninculvertsisalsopsrtiallycauS..dbystrt'etgradingoperations. Restricted culverts are located at County Road 136 and at a number of intersections including ehe glm/Xobile and Elm/Elbert Streets intersections. Mo.~ of these colvertshavecommoncauaesforthesedimentacion. First, thecu1vertssrerela- tively short which places the inlets and outlets directly at the shoulder of the street. A number of culvert ends hsve been crushed by ..rrant vehicles. ~e~ond, -57- -51:\-