Before locating and laying out the required cross sections, the engineer should develop an identification or numbering scheme for each potential cross section. All hydraulic programs require that each cross section have a unique identifier, in most cases a numerical value. The most common method is to indicate a section ID or sta-tion as the distance along the channel centerline from the mouth of the river or other prominent downstream point. In the U.S., the identifier is most often river mileage.
For short reaches of small streams, distance in feet may be selected. Stream mileage may already have been completed for the study stream by a governmental agency; for example, in the United States the USACE has established river mileage for all naviga-ble streams. For hydraulic models received from the USACE, the cross-section IDs for the streams are usually the river miles from the mouth of the river. Other agencies may have river mileage developed and published, perhaps for an entire state or prov-ince. Table 5.1 is an excerpt from a State of Illinois publication (Healy, 1979) that sum-marizes mileage for all streams within the state. This report is available to study contractors and identifies drainage areas and mileage from the mouth of every river, creek, and stream for all prominent features within the state, including political boundaries, gages, bridges, dams, and tributaries. Other states/provinces or agencies may maintain similar mileage records.
After an identification scheme is established, required cross sections can be laid out for survey crews. Cross sections typically extend from high ground on one side of the stream valley to high ground on the opposite side. The approximate 100-year water surface elevation is often used as the cross-section beginning and end mark for survey crews. If the floodplain section ends at near-vertical bluffs, the sections could only extend to the toe of the bluff on either side of the valley. The modeler can simply extend the ends of the sections by using topographic maps to add additional elevation and station locations to simulate the valley walls, thus saving survey costs to obtain these beginning and ending points. Similarly, if the floodplain is very wide and only the portion near the channel is effective for conveyance, the engineer might only obtain surveyed cross sections extending through the conveyance area. However, sur-veying only the portion of the floodplain that is effective for conveyance is not ade-quate for quasi-unsteady or full unsteady flow analysis. The full cross section is needed to include both the conveyance and ineffective flow areas (floodplain storage).
Because a steady flow model today may be used for an unsteady flow model in the future, as a general rule the full cross section should be surveyed. In lieu of surveying ineffective flow areas (floodplain storage), the engineer could use topographic maps to complete the rest of each cross section, potentially saving considerable field survey costs. The modeler has to decide if this cost-saving method is acceptable in terms of accuracy of flooded area maps. If aerial contour mapping is available at an adequate contour interval, surveyed cross sections of only the channel, bridge, and culvert crossings should be adequate, with the floodplain geometry taken from the mapping.
Section 5.3 Geometric Data 123
HEC-RAS automatically extends the end of each cross section vertically if the water surface elevation is higher than the elevation of the last point on the cross section.
Normally, this means that the modeler should add additional points to properly model the valley geometry beyond the point of the vertical extension. If there is no significant conveyance outside of the vertical extension, however, the use of the auto-matic vertical extension may be allowable, although it will likely be a source of nega-tive comment during a technical review.
Where possible, the modeler should attempt to place cross sections at locations where access is readily available. When field surveys are used rather than aerial mapping, forcing the survey crew to hack through a mile or more of dense undergrowth to get to a cross-section location significantly increases the data acquisition costs. However, it is important to obtain geometric data at critical locations, regardless of access diffi-culties.
In general, cross sections (actual or interpolated) are desired at the following points:
• All major obstructions to flow, such as bridges and culverts
• Stream gages and highwater marks
• Roadway and railroad embankments across the floodplain Table 5.1 River mileage for Crooked Creek, Illinois (excerpt).
Mileage Description
Drainage Area, m2
Topographic Quad
0.0 at Mouth nr Covington 465 Addieville
3.2 Little Crooked Cr L Addieville
3.2 Area above Little Crooked Cr 348 Addieville
10.1 IL PT 127 Carlyle
10.1 USGS Gage 05593525 nr Posey 366 Carlyle
10.5 Lost Cr R Carlyle
10.5 Area above Lost Cr 265 Carlyle
20.9 Road 526, T IN, R 2W Carlyle
20.9 USGS Gage 05593520 nr Hoffman 254 Carlyle
23.8 Grand Point Cr L Centralia W
23.8 Area above Grand Point Cr 185 Centralia W
24.0 Washington-Clinton Co Centralia W
31.9 IL Rt 161 Centralia W
32.7 Southern RR Centralia W
35.8 Road S 1, T IN, R 1W Centralia W
36.6 Burlington Northern RR Centralia W
37.3 Clinton-Marion Co Ln Centralia W
38.5 Illinois Central RR Centralia W
38.6 US Hwy 51 Centralia W
38.9 Turkey Cr R Centralia W
39.9 Raccoon Cr L Centralia E
39.9 Area above Raccoon Cr 93.2 Centralia E
40.4 Road S 5, T IN, R 1E Centralia E
42.3 Road S 4, T IN, R 1E Centralia E
• Significant increases or decreases in the floodplain width
• Significant geometric changes in the channel
• Significant changes in Manning’s n values in the channel or overbank areas
• At and near levees or other flood damage reduction structures
• Locations just upstream and downstream of significant tributary streams
• Index points where economic-damage information is computed
• Boundaries – start and end points on the main stream and at the ends of any tributaries under study
• Significant changes in stream slope, or at and near control sections where criti-cal depth may occur, such as rapids, drop structures, and dams
Not all of these locations require actual survey data. The engineer must determine the amount of cross-section and/or mapping data that is adequate for the development of accurate profiles. When modeling the floodplain, the engineer can develop additional cross sections by interpolating between two surveyed sections. Typically, a maximum of about one-quarter to one-half of the cross sections in a hydraulic model may be actual surveyed sections, with the balance developed from maps and from cross-section interpolation by the engineer or by the interpolation routine within the HEC-RAS program. For sections generated from digital mapping, all the cross sections could be considered surveyed, except for the portion of the channel that is underwa-ter. Field sections or hydrographic surveys will be necessary for any portion of the channel or overbank geometry that is under water.
There is no hard and fast guidance concerning the appropriate distance between cross sections. In the author’s experience, large rivers (rivers with several thousand square miles of watershed) on low slopes (less than 5 ft/mi, 1 m/km) can have a maximum cross-section spacing of approximately 2500 ft (750 m). On smaller streams with steeper slopes, closer spacing should be the rule. For urban situations, a section every few hundred feet (100 m) or less is often needed. Bridges and culverts require close spacing to properly model flow movement through the openings. Chapters 6 and 7 further address spacing between cross sections at bridges and culverts.
A valuable feature in HEC-RAS is the ability to automatically generate interpolated cross sections. The engineer can ascertain the value of additional sections after com-pleting the initial cross-section input (both the surveyed sections and the user-devel-oped, interpolated cross sections), running the program, and reviewing the computed water surface profile. If the distance between sections exceeds a specified value (such as 500 ft or 150 m), the program is rerun with the instruction to automatically interpo-late additional cross sections. The engineer should review differences in computed water surface elevations with and without the HEC-RAS cross-section interpolations for significant profile changes. If significant differences appear, the engineer could make an additional interpolation with sections no farther apart than 250 ft or 75 m, for example, and again evaluate the results. If the differences between 500 ft and 250 ft spacing are minimal, the 500 ft spacing can be used for the profile analysis. Chapter 8 further discusses using HEC-RAS for cross-section interpolation.
It is not unusual to see differences in water surface profiles of a foot (0.3 m) or more, simply by adding additional cross sections to reduce the length between computation points. HEC (USACE, 1986) found that a significant number of the tested geometric data sets underestimated the water surface profile as compared to tests made with
Section 5.3 Geometric Data 125
additional sections that reduced the maximum distance between sections to 500 ft (150 m). Generally speaking, a reduced length improves the accuracy of the friction loss computation discussed in Chapter 2.
Cross sections used to model channel and floodplain geometry are normally perpen-dicular to flow across the river valley and cross each contour line at a right angle.
However, the section should be modeled to have the velocity vectors intersect each section at right angles. This requirement means that cross sections may be curved, bent, kinked or “dog-legged” to maintain a right angle to flow. Figure 5.5 illustrates the layout of example cross sections incorporating these features.
In addition, each cross section should be representative of the reach for half the dis-tance to each adjacent section. This is necessary because of the way friction losses are computed. Recall from Chapter 2 that friction losses are calculated using a measure of the average friction slope and a discharge-weighted reach length between two cross sections. Features identified in the field survey may be modified or deleted to ensure proper modeling. Figure 5.6 gives an example of section editing, using a cross section from Figure 5.5. As shown in Figure 5.6, the cross section of the pond in the left over-bank and the tributary channel in the right overover-bank were deleted when plotting the cross section, because there is no significant floodplain conveyance in these segments and because these segments are not representative of the reach modeled by the cross section. In fact, the cross section in the survey request would likely have included a note to the survey crew to skip the pond and not include its geometry in cross section 10.9.
Section 10.9 in Figure 5.5 and Figure 5.6 represents surveyed data. In Figure 5.5, the road crossing and sections 5.2 and 7.5 were also surveyed. The other sections on Figure 5.5 could be interpolated from a topographic map and from the surveyed
Figure 5.5 Sample cross-section survey layout.
sections. The engineer may then direct HEC-RAS to interpolate additional sections between those shown. At a minimum, all cross sections should represent the full active flow area, or conveyance, when the maximum water surface elevation is being determined. When both elevation and storage information are to be developed, how-ever, the cross-section data must include both conveyance and storage areas.
Figure 5.7 illustrates this facet, which is further discussed in Chapters 6 and 8.