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4. Bridqes Y. Drop Structures For the purposes of this report, a bridge is defined as a structure in which the top of the structure becomes the actual street surface. If bridges are designed such that the 100 year storm flow will infringe on the structure it shall be securely anchored to prevent movement. Drop structures are designated On portions of channel where a totally new channel cross section must be constructed. Drop structures may be constructed utilizing a gabion basket or concrete core with gabion or riprap protection above and below the drop. The use of sufficient riprap or river mats above and below the drop structure is im- perative to prevent erosion. Drop structures must be of sufficient height to insure that a well defined hydraulic jump will occur below the drop for energy dissipation purposes and so the drop will not be submerged due to the back water from the downstream channel. 5. Culverts For the purposes of this report, culverts are defined as structures with earthen cover between the structure and the road surface. Protective measures are often necessary in channels below culverts due to the increase of velocity through the culvert. 10. CheCk Dams Design of culverts shall be in accordance with the U. S. Department of Commerce, Bureau of PUblic Roads, Hydraulic Engineering Circulars No. 5 and 10 reflecting the effects of inlet and outlet control. Corrugated metal pipe, reinforced concrete pipe, or reinforced concrete box culvert is indicated on the preliminary plans based upon available information of clearances between utilities, design flOWS, and costs. Final design of culverts may often change the type of material so long as the design flow is passed within the allowable head water limits. 6. Conduits Check dams are indicated in all instances where the low flow channel may remain in the location of the existing channel invert. Check dams may be constructed of con- crete or gabion baskets. In either case, they should in- clude a low flow outlet at the grade of the normal low flow channel to prevent serious siltation behind the cheek dam. Sufficient riprap or river mating above and below the check dam is necessary to prevent erosion. Check dams must be sufficiently high to create a well defined hydraulic jump below the check for energy dissipation purposes. Conduits are designed based upon the criteria included in the Storm Sewers and the Major Drainage parts of the Urban Storm Drainage Criteria ~anual. Care must be taken in the des~qn to assure that entrances to conduits are adequate to develop the full capacity of the conduit. 11. Encrgy Dissipators Energy dissipators will be necessary where exccssive vclocities are developed due to high heads behind culverts. These will usually consist of natural rock impact blocks with suitable riprap or gabion protected bottom and side slopes. Formal energy dissipation of formed concrete may be necessary when extreme velocitics arc cncountcred. 7. Irrigation Ditches Crossings of major irrigation ditches are indicated on the master plan as scparated grade crossings. These designs arc offered as recommendations to the irrigation ditch companies involved and are not looked upon as a rpsponsibility of the entity constructing the drainagcway. 12. Detention Storage DetentiOn storage shall be recommended in the plan wherever practical to reduce peak flow rates. 13. Multiple Use 8. lJnn"rrlrains Th" "last..r planning of the gulch"s will h.. clnn.. i.n 11 manner compatible with the following multiple uses and auxiliary uses. Underdrains designed to intercept ground water are reco~ended in areas where the channel invert will be raised significantly above the existing grade. These underdrains should consist of open jointed tile in a gravel envelope. A. Greenuelt and urban open space. B. Reduced street maintenance costs. C, Reduced street construction costs. D. Improved movem~nt of traffic. -58- -8~-