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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />Grouted sloping boulder and vertical hard basin designs can be adapted for use as check structures. The <br /> <br />analysis steps are the same as for drop structures, with the additional considerations of (1) stable bed <br /> <br />slopes for the unlined trickle or low-flow channel and (2) potential overflow erosion during submergence <br /> <br />of the check structure and where flow converges back from the main channel sides or beiow the check <br /> <br />structure. <br /> <br />The basic design steps for low-flow grade-control check structures include the following: <br /> <br />1. Determine a stable slope and configuration for the low-flow zone. For unlined channels, <br />discharges from full floodplain flow to the dominant discharge should first be considered. The <br />dominant discharge is more fully explained in sediment transport texts (Richardson 1988; Shen <br />1971; Simons 1977; Simons, U and Associates 1982; and Muessetter 1983). It is generally <br />defined as the flow that represents the average or equilibrium conditions controlling the channel <br />bed. In the Denver region, the dominant discharge is typically the 2-year flood. Numerous <br />references (Chow 1959; SCS 1977; and above references) cite information on permissible <br />veloc~ies. <br /> <br />2. The configuration of the low-flow zone and number and placement of the check structures must <br />be reviewed. If the checks are widely spaced, the trickle channel can be quite deep downstream <br />of the check, leading to concentration of higher flows into the trickle channel and the check. A <br />good rule is to have the trickle channel less than 3 feet deep at the crest of the check and less <br />than 5 feet deep below the check structure (relative to the over bank). <br /> <br />3. All hydraulic analysis steps described in Section 2.3 apply, except that the primary design fiow will <br />be the discharge that completely fills the check structure at its crest. <br /> <br />4. The secondary, but also important, design flow is the flow that causes the worst condition for <br />lateral overflow around the abutments and back into the basin or trickle channel below. The goal <br />is to have the check structure survive such an event with minimal or reasonable damage to the <br />floodplain below. The best approach is to estimate unit discharges, velocities, and depths along <br />overflow paths. The unit discharges can be estimated at the crest or critical section for the given <br />total flow. Estimating the overflow path around the check abutment is difficult and requires <br />practical judgment. Slopes can be derived for the anticipated overflow route, and protective <br />measures can be devised such as grouted rock. <br /> <br />5. Seepage control is also important because piping and erosion through or around these structures <br /> <br />can be a problem. It is advisable to provide a cutoff that extends laterally at least 5 to 10 feet into <br /> <br />undisturbed bank at a minimum and has a cutoff depth appropriate to the profile dimensions of <br /> <br />the check structure. <br /> <br />HS-40 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />e <br /> <br />e <br /> <br />e <br />