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 Austin
 Drainage Criteria Manual
 Section 4. INLETS
 Appendix 4.4.0. INLET SYSTEM LAYOUT

  § 4.4.2. Inlet System Design  

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  • Determining the size and location of inlets is largely a trial and error procedure. Based on criteria outlined in sections 2, 3 and 4 of this manual, the following steps will serve as a guide to the procedure to be used.

    Step 1: Delineate the boundaries of the contributing subarea for the assumed location of the inlet at the upstream end of the project and calculate the peak rate of runoff.

    Step 2: Compare the calculated peak rate of runoff to the allowable rate of flow in the street, taking into consideration allowable spread in the street and required clear widths. If the calculated peak rate of runoff is greater than the rate of flow that cannot be exceeded to meet spread and clear width requirements, adjust the inlet location so that the contributing subarea is smaller. If the calculated peak rate of runoff is less than the rate of runoff in the street that cannot be exceeded, adjust the inlet location so that the contributing subarea is larger. Repeat this procedure until the calculated peak rate of runoff is close to, but does not exceed, the flow rate needed to achieve spread and clear width requirements. This is the first point at which a portion of the flow must be removed from the street. The amount of flow to be removed will depend on the clear width requirements (DCM 3.2.0), street cross flow limitations (DCM 3.1.3), and restrictions on the amount of flow allowed through intersections (DCM 3.1.4).

    Step 3: Record the drainage area, time of concentration, runoff coefficient or runoff curve number and calculated peak rate of runoff for the subarea. This information shall be recorded on the drainage map or in tabular format.

    Step 4: If an inlet is proposed to remove flow from the street, determine and record the inlet size, amount of intercepted flow and the amount of bypass flow.

    Step 5: Continue the procedure above, working toward the system low point, until a complete system of inlets has been established. Remember to account for carry over (bypass) flow from one inlet to the next.

    Step 6: Record the information in steps 3 and 4 for all inlets.

    Step 7: After the inlets have been located and sized, the storm drain pipes can be designed (See Section 5).